Publications

Contemporary designs of engineering structures strive to minimize the use of material in order to reduce cost and weight. However, the approach taken by focusing on materials selection and on the design of the exterior shape of structures has reached its limits. By contrast, nature implements bottom-up designs based on a multiple-level hierarchy, spanning from nanoscale to macroscale, which evolved over millions of years in an environmentally sustainable manner given limited resources. Natural structures often appear as laminates in wood, bone, plants, exoskeletons, etc., and employ elaborate micro-structural mechanisms to generate simultaneous strength and toughness. One such mechanism, observed in the scorpion cuticle and in the sponge spicule, is the grading (gradual change) of properties like layers thickness, stiffness, strength and toughness. We show that grading is a biological design tradeoff, which optimizes the use of material to enhance survival traits such as endurance against impending detrimental cracks. We found that such design, when applied in a more vulnerable direction of the laminate, has the potential to restrain propagation of hazardous cracks by deflecting or bifurcating them. This is achieved by shifting material from non-critical regions to more critical regions, making the design sustainable in the sense of efficient use of building resources. We investigate how such a mechanism functions in nature and how it can be implemented in synthetic structures, by means of a generic analytical model for crack deflection in a general laminate. Such a mechanical model may help optimize the design of bioinspired structures for specific applications and, eventually, reduce material waste.

The electrospinning of thermoplastic polymers is widely used in applications such as filters and coatings, but has only recently been applied to thermosetting polymers because of their chemical structure and reactivity. Epoxy is a thermosetting polymer which, when combined with a curing agent, chemically reacts to form a crosslinked matrix. In the present study, we demonstrate that to electrospin epoxy and obtain continuous micro and nanofibers, one must precisely control the curing reaction. Epoxy was mixed with triamine curing agent and, to enable electrospinnability, was dissolved in a mixture of tetrahydrofuran and dimethylformamide solvents. We identified a narrow working window wherein a proper solution for electrospinning is close to the gel point, right before the transition from liquid to solid gel state. The solution was characterized by means of (i) Fourier-transform infrared spectroscopy to monitor the extent of reaction, (ii) steady shear viscosity to detect the divergence near the gel point, and (iii) oscillatory loss and storage shear moduli to identify the liquid-to-gel transition. Based on these measurements, it was possible to monitor the chemical transformations that the epoxy solution underwent with time, such as chemical interconnections and gelation, and thus define the working window for electrospinning.

The multiscale structure of biomaterials enables their exceptional mechanical robustness, yet the impact of each constituent at their relevant length scale remains elusive. We used SAXD analysis to expose the intact chitin-fiber architecture within the exoskeleton on a scorpion's claw, revealing varying orientations, including Bouligand and unidirectional regions different from other arthropod species. We uncovered the contribution of individual components’ constituent behavior to its mechanical properties from the micro- to the nanoscale. At the microscale, in-situ micromechanical experiments were used to determine site-specific stiffness, strength, and failure of the biocomposite due to fiber orientation, while metal-crosslinking of proteins is characterized via fluorescence maps. At the constituent level, combined with FEA simulations, we uncovered the behavior of fiber-matrix deformation with fiber diameter

Drawn epoxy fibers possess increasingly high mechanical properties when their diameter is decreased down to a limit of about 10 μm. Smaller diameters with even higher properties can potentially be obtained by electrospinning. Yet, electrospinning of standalone epoxy fibers has not been possible so far, because of the reactivity of epoxy and the fragility of such fibers. These difficulties are overcome in the present study by dissolving the epoxy in methyl ethyl ketone solvent, and achieving suitable electrospinning conditions by controlling the degree of epoxy crosslinking in the solution. The fibers are captured on a net screen, with the positive electrode placed behind it. The resulting electrospun fibers exhibit about 80% higher strength and stiffness compared to bulk epoxy, and striking 900% higher elongation and 1200% higher toughness. Also observed is a size-dependence of the strength on the fiber diameter, which shows a sharp strength rise to more than 300 MPa (compared to about 70 MPa in bulk epoxy) in fibers of diameters down to 3 μm. This rise in properties is likely due to anisotropic molecular rearrangement resulting from the strong stretching forces induced by electrospinning. Highly performant epoxy nanofibers could find applications in diverse fields, including reinforcement in composites, sensors, coatings and electronic devices.

The geometric modification of fibre-matrix interfaces is a promising approach for simultaneous improvement of strength and toughness of composites. In this study, we investigate the effect of discrete epoxy droplets deposited on glass fibres embedded in an epoxy matrix, using single fibre experiments. Pullout tests reveal increases in both the pullout force and work in beaded fibre samples compared to beadless ones. This is somewhat unexpected as the bead and the matrix are made of the same material (cured epoxy) and possess the same mechanical properties. Interestingly, upon pullout, only the fibres are extracted from the matrix whereas the beads remain within the matrix, indicating that failure occurs at the fibre-bead interface. The simultaneous improvement in pullout force and work seen during the pullout of beaded fibres is explained by a conceptual anchoring mechanism based on friction lock, which agrees well with the experimental results. Smaller beads yield higher increases in pullout force and work, leading to the possibility of denser packing of multiple beaded fibres in a practical composite.

We examined the combined effect of crosslinking and dual reinforcement on the thermal and mechanical properties of polyvinyl alcohol (PVA). Glutaric acid (GA) was used as crosslinker while tungsten disulfide nanotubes (WSNT) and carboxylated multiwall carbon nanotubes (f-MWCNT) were used as dual reinforcing agents. Tensile strength and toughness of the hybrid composite, that is, crosslinked PVA reinforced with both f-MWCNT and WSNT, was higher than those of neat PVA, crosslinked PVA, crosslinked PVA reinforced with WSNT and crosslinked PVA reinforced with f-MWCNT. The hybrid composite showed remarkable improvement over neat PVA: tensile strength increased by 138%, Young's modulus increased by 142%, and toughness increased by 246%. Water uptake tendency and thermal stability of the composites were also examined and compared. The advantage of using dual reinforcement was a simultaneous increment in tensile strength and toughness, similar to what is observed in some natural materials like nacre.

Strain sensors are used in varied applications, including personal healthcare monitoring, human-machine interaction, and artificial skin. Here, we report fabrication of a highly sensitive capacitive strain sensor comprising a self-healing polydiacetylene-polyacrylic acid-Cr3+ hydrogel. The dielectric hydrogel medium was prepared through a simple synthesis scheme from readily available ingredients. The elasticity and pronounced sensitivity of the composite hydrogel are attributed to the distinct components of the system. The Cr(H2O)(6)(3+) complexes function as cross linkers, maintaining stability of the hydrogel framework through electrostatic binding to carboxylate moieties within both the polyacrylic acid and polydiacetylene, additionally facilitating incorporation of high concentration of water molecules essential for maintaining hydrogel elasticity. In parallel, polydiacetylene, employed here for the first time as a vehicle for strain sensing, endows the system with high intrinsic capacitance sensitivity to mechanical stimuli, further exhibiting a major contribution towards greater flexibility and resilience under high strains. The sensor exhibits high stretchability of 500%, and high sensitivity exhibiting a gauge factor (GF) of up to 160. Applications of the polydiacetylene-polyacrylic acid-Cr3+ capacitive hydrogel sensor for physiological strain monitoring are presented.

In this work, we investigate the governing fixation mechanism in a polycaprolactone-diol/2,4,2,6-Toluene diisocyanate/Ethylene glycol based shape memory polyurethane (SMPU). In particular, we test whether strain-driven crystallization, known to govern the SMPU fixation state in semi-crystalline SMPUs, is indeed vital for a significant memory capability. As synthesized, the SMPU exhibits semi-crystalline morphology. However, after heating, the synthesized semi-crystalline SMPU becomes amorphous and retains this state for up to 60 h, so that its crystallization is delayed. Consequently, this enables a complete shape-memory cycle without the effect of crystallization over the shape-memory properties. We show that in the amorphous state, the bulk SMPU displays substantial memory-effect capabilities approaching optimal performance, without contribution from crystallization mechanisms. This effect is further enhanced in electrospun SMPU nanofibers, likely as a result of the high molecular orientation induced by extensional flow. We propose that the dominant fixation and recovery mechanism in such SMPUs is the inhibition of molecular chain mobility, imposed by interlocking of the SMPU hard-segments when the polymer network is cooled below the transition temperature.

Summary Epoxy nanocomposites have many promising applications in the fields of aerospace and aeronautics, as well as many others. Achieving tough epoxy nanocomposites remains a great challenge, however. One inspiration for improving the mechanical properties of epoxy nanocomposites is nacre, which has remarkable fracture toughness for its layered “brick-and-mortar” architecture. Inspired by this, we fabricated a lamellar graphene scaffold by the freeze-casting technique. An alternating-layered epoxy-graphene nanocomposite was made by infiltrating epoxy into this graphene scaffold. As our epoxy-graphene nanocomposite consists of ∼99 wt % organic epoxy, in contrast to nacre containing ∼96 wt % inorganic aragonite, we call it an “inverse nacre-like” epoxy-graphene layered nanocomposite. It exhibits exceptional fracture toughness, 3.61 times that of pure epoxy, and demonstrates anisotropic conductivity due to the anisotropic graphene scaffold, which can be used to detect cracks. Our bioinspired strategy provides a promising approach to combine excellent mechanical properties with functional properties to fabricate high-performance nanocomposites.

Epoxy fibers with different diameters were prepared by hot drawing and their mechanical properties were measured under tension. The stiffness, strength, ultimate strain, and toughness revealed substantial scale-dependent effects as they all significantly increased with a decrease in size. Compared to bulk epoxy, an intrinsically brittle material, thin epoxy fibers displayed a highly ductile behavior under tension. A drop in stress observed immediately beyond the yield point was followed by the development of a stable necking region propagating through the entire fiber length, then by strain-hardening up to final rupture. Necked fiber segments tested in tension were found to have even higher strength and modulus compared to the initial as-prepared fibers. Possible reasons for the highly ductile mechanical behavior and the size effects of epoxy fibers are discussed. Size effects for the strength of epoxy can be elucidated in principle either by means of a classical fracture mechanics argument (strength ~ 1/d
^1/2), or via a stochastic model argument (strength ~ 1/d
^1/β, where β is a function of the material and is generally larger than 2). In both models the presence and size of critical defects play a key role. However, defects cannot explain the colossal ductility (plastic deformation) seen in our experiments, nor can the presence of defects justify a size effect in an elastic property, namely Young’s modulus. Only scarce evidence exists in the literature for similar (milder) size effects in epoxy fibers but without any structural justification. We find here that highly cross-linked necked epoxy fibers exhibit partial macromolecular anisotropy which likely explains the observed high mechanical characteristics.

In view of their facile fabrication and recycling, functional materials that are built from small molecules ("molecular plastics") may represent a cost-efficient and sustainable alternative to conventional covalent materials. We show how molecular plastics can be made robust and how their (nano)structure can be tuned via modular construction. For this purpose, we employed binary composites of organic nanocrystals based on a perylene diimide derivative, with graphene oxide (GO), bentonite nanoclay (NC), or hydroxyethyl cellulose (HEC), that both reinforce and enable tailoring the properties of the membranes. The hybrids are prepared via a simple aqueous deposition method, exhibit enhanced mechanical robustness, and can be recycled. We utilized these properties to create separation membranes with tunable porosity that are easy to fabricate and recycle. Hybrids 1/HEC and 1/NC are capable of ultrafiltration, and 1/NC removes heavy metals from water with high efficiency. Hybrid 1/GO shows mechanical properties akin to covalent materials with just 2-10% (by weight) of GO. This hybrid was used as a membrane for immobilizing β-galactosidase that demonstrated long and stable biocatalytic activity. Our findings demonstrate the utility of modular molecular nanoplastics as robust and sustainable materials that enable efficient tuning of structure and function and are based on self-assembly of readily available inexpensive components.

Poly(butylene succinate) (PBS) is a thermoplastic and biodegradable polyester characterized by high rigidity due to its high crystallinity. However, the use of long-chain biobased monomers to produce segmented copolymers is an effective strategy to tailor the properties of PBS, such as increasing flexibility. In this Article, a series of aliphatic biocopolyesters of poly(butylene succinate-dilinoleic succinate) (PBS-DLS) were successfully synthesized via a direct two-step polycondensation method using a semipilot scale reactor for melt polymerization and titanium dioxide/silicon dioxide coprecipitate catalyst (C-94), an alternative catalytic system. The change in catalyst led to a reduced reaction time and eliminated the distillation step required for the typical organometallic titanium catalyst, thus representing an improvement in energy efficiency. In this study, the thermal and mechanical properties were investigated and compared, focusing on the effect of varying the amount of biobased dilinoleic diol in the structure. With increasing amount of long-chain diol, a decrease in molecular weight, density, and melt flow index was observed. The semicrystalline nature of the copolymers was confirmed using differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) methods. Increasing the soft DLS segment content in the copolymer series resulted in an increase in the elastic behavior of the polymers. The broad range of crystallization temperatures and melt flow index values indicates that a polyester library with customizable properties that spans PBS applications has been successfully obtained.

We present a theoretical analysis of the elastic stresses in a composite reinforced with beaded fibers by extending the classic Cox shear lag theory. The motivation for reinforcing a composite with beaded fibers is to improve both strength and toughness, two often conflicting properties. It is found that owing to their geometry beads intermittently placed on a fiber enhance fiber anchoring in the matrix, and can potentially dissipate energy by deforming the matrix during failure. The composite stiffness is shown to improve compared to a composite with beadless fibers, particularly when the beads are large and stiffer than the surrounding matrix. The stress profiles in the fiber, bead, matrix and along their respective interfaces incur periodic perturbations induced by the beads, modeled by Hill equation. For given elastic constants and bead geometry, these profiles reveal the weakest link loci in the structure, and consequently determine the composite strength and failure mode. A finite element analysis is presented that confirms our results. The bead-fiber and bead-matrix interfaces may be tuned by choice of materials and coatings to achieve desired mechanical properties.

Simultaneous improvement of strength and toughness is a challenge in composite materials, as an improvement in one is generally at the expense of the other. The filler-matrix interface has a crucial role in such improvement. It appears that modification of the interfacial structure/geometry may have wider possibilities and benefits than the classical chemical bonding approach. Using a model glass-epoxy fiber-reinforced composite, we modified the regular cylindrical fiber-matrix interface by applying intermittent epoxy beads along the fiber, taking advantage of the Plateau-Rayleigh liquid instability phenomenon. Under load, the beads serve as fiber anchors in the matrix, thus exploiting the fiber strength to its maximum. During fracture, the pullout of beads through the matrix appears to dissipate more plastic deformation energy compared to the pullout of regular fibers. Fragmentation tests of beaded fibers in epoxy matrix demonstrate these failure mechanisms; single-bead fiber pullout tests with different bead sizes and surface treatments provide strength and toughness data that substantiate this approach. The concept of intermittent beading has ample possibilities for optimization. It is also scalable and therefore practical.

The effect of basalt fibre sizing on the mechanical and interphase properties of fibre-reinforced composites was studied. Two different chemical preparations of the fibre surface (PBT-compliant and PP-compliant) were used. The polymer matrix was prepared from polypropylene/poly(butylene terephthalate) (PP/PBT) immiscible polymer blend and the effect of different compatibilizers on the composite properties was evaluated. SEM hints at improved fibre adhesion to the polymer matrix when a PP-compliant sizing is applied. SEM also reveals improved compatibilization effects when block copolymer instead of multiblock copolymer is used for the PP/PBT blend preparation. The pull-out test was applied to quantitatively evaluate the interface adhesion between the fibres and matrices. It showed a high value of the interfacial shear strength between basalt fibres modified with PP-compliant sizing and polymer blend compatibilized by block copolymer, thus confirming good adhesion. One possible explanation of such good mechanical properties can be related to the chemical interactions between functional groups, mainly maleic anhydride on basalt fibres and the polyolefin component (PP) of the polymer matrix.

We demonstrate a solution-based fabrication of centimeter-size free-standing films assembled from organic nanocrystals based on common organic dyes (perylene diimides, PDIs). These nanostructured films exhibit good mechanical stability, and thermal robustness superior to most plastics, retaining the crystalline microstructure and macroscopic shape upon heating up to 250-300 degrees C. The films show nonlinear optical response and can be used as ultrafiltration membranes. The macroscopic functional materials based on small molecules can be alternative or complementary to materials based on macromolecules.

One major challenge of functional material fabrication is combining flexibility, strength, and toughness. In several biological and artificial systems, these desired mechanical properties are achieved by hierarchical architectures and various forms of anisotropy, as found in bones and nacre. Here, it is reported that crystals of N-capped diphenylalanine, one of the most studied self-assembling systems in nanotechnology, exhibit well-ordered packing and diffraction of sub-angstrom resolution, yet display an exceptionally flexible nature. To explore this flexibility, the mechanical properties of individual crystals are evaluated, assisted by density functional theory calculations. High-resolution scanning electron microscopy reveals that the crystals are composed of layered self-assembled structures. The observed combination of strength, toughness, and flexibility can therefore be explained in terms of weak interactions between rigid layers. These crystals represent a novel class of self-assembled layered materials, which can be utilized for various technological applications, where a combination of usually contradictory mechanical properties is desired.

Purpose: The aim of this study is to determine the effect of variation of the exposure time of near-infrared irradiation on corneal stiffening after a bacteriochlorophyll derivative (WST11) with dextran (WST-D) application.Methods: One hundred four paired eyes of 3-month-old New Zealand White rabbits were included in this study. Fifty-two eyes (ex vivo n = 34, in vivo n = 18) were mechanically deepithelialized, treated topically with WST-D, and irradiated at 10 mW/(c)m(2) using a diode laser at 755 nm for 1, 5, or 30 minutes. Untreated fellow eyes served as controls. Corneoscleral rings were removed immediately after treatment (ex vivo), or 1 month after treatment (in vivo). Corneal strips were cut and underwent biomechanical stress-strain measurements.Results: Ex vivo, the mean tangent elastic modulus was significantly higher in the treatment groups than in the control groups for 1, 5, and 30 minutes of irradiation, respectively, 6.06 MPa, 95% confidence interval (CI, 4.5-7.6) versus 14.02 MPa, 95% CI (10.2-17.8), n = 11, 4.8 MPa, 95% CI (3.9-5.7) versus 15.03 MPa, 95% CI (12-18.1), n = 11, and 7.8 MPa, 95% CI (5.6-10.02) versus 16.2 MPa, 95% CI (13.6-18.9), n = 11; P

PURPOSE. To determine the long-term safety and efficacy of WST-D/near-infrared (NIR) corneal stiffening. METHODS. One eye of 23 New Zealand White rabbits was de-epithelialized mechanically followed by topical application of 2.5 mg/mL WST11, combined with dextran-500 (WST-D) for 20 minutes. Subsequently, samples were irradiated with a NIR (755 nm) laser at 10 mW/cm² for 30 minutes. Untreated fellow eyes served as controls. One week (n = 4), 1 month (n = 6), 4 months (n = 9), or 8 months (n = 4) after treatment rabbits were euthanized. Corneal strips were cut in superior–inferior direction for extensiometry testing (1, 4, and 8 months), and histologic sections were prepared for evaluation of keratocyte distribution (1 week and 8 months). RESULTS. Elastic modulus after treatment was significantly higher than in paired controls (16.0 ± 2.3 MPa versus 9.6 ± 3.6 MPa [P = 0.008], 18.1 ± 4.5 MPa versus 12.6 ± 2.3 MPa [P = 0.003], and 18.6 ± 3.6 MPa versus 14.2 ± 3.6 MPa [P = 0.010], at 1, 4, and 8 months, respectively). A significant decrease in keratocyte count at the anterior stroma was observed directly after treatment (1.5 ± 1.7 vs. 19.0 ± 4.1 [P = 0.002]). At 8 months keratocyte repopulation appeared completed, with similar distribution in treated and untreated corneas (15.9 ± 1.1 vs. 14.5 ± 2.5 [P = 0.562]). Corneal thickness was comparable between treated and untreated corneas at all time points. CONCLUSIONS. WST-D/NIR treatment resulted in significant and persistent long-term increase in corneal stiffness. Initial keratocyte apoptosis in the anterior stroma is followed by repopulation to normal level at 8 months after treatment. The safe nature of NIR light allows treatment of corneas of any thickness without endangering corneal endothelium or deeper ocular structures, potentially benefiting patients deemed unsuitable for riboflavin/UV-A cross-linking.

In this study we explore the materials design principles of the carapace of a Terrapene Carolina box turtle, which possesses a sandwich-like structure consisting of a foam-like interior layer (FIL) enclosed by two dense exterior layers (DEL). A constitutive scheme accounting for the large deformation, plastic yielding and post-yield strain hardening caused by densification of the cells in the foam is developed to model the mechanical properties of the FIL, and a large deformation elastic-plastic model for the DEL is proposed. Computational simulations of the carapace subjected to indentation loading are performed and it is found that the layer sequence plays an essential role in the mechanical properties of the carapace. For the sandwich-like layering, the stiff DEL provides penetration resistance and the FIL contributes to the energy dissipation of the entire structure through plastic deformation, which enables reduction in back-deformations, enhanced penetration resistance and low stresses transmitted to the inner layer. For other layer sequential patterns, the contributions of the DEL and FIL are limited, leading to poorer mechanical performance. Based on these results, we propose that the sandwich-like structure of the carapace of the box turtle is designed to maintain sufficient resistance to penetration deformation, a defeating mechanism, and at the same time to significantly amplify energy dissipation, a defending mechanism. This double function could be used in the development of future human body armor.

The dielectric and electrical characteristics of the semiconductive WS₂ nanotubes/epoxy composites were studied as a function of the nanotubes concentration and the pressure applied during their molding. In addition, the ability of WS₂ nanotubes to serve as stress sensors in epoxy based nanocomposites, for health-monitoring applications, was studied. The nanocomposite elements were loaded in three-point bending configuration. The direct current was monitored simultaneously with stress-strain measurements. It was found that, in nanocomposites, above the percolation concentrations of the nanotubes, the electrical conductivity increases considerably with the applied load and hence WS₂ nanotubes can be potentially used as sensors for health monitoring of structural components.

Functional fillers in multilayered films provide opportunity in tailoring the mechanical properties through chemical cross-linking. In this study, Laponite-graphene oxide co-dispersion was used to incorporate graphene oxide (GO) easily into polyvinyl alcohol (PVA)/Laponite layer-by-layer (LBL) films. The LBL films were found to be uniform and the layer thickness increased linearly with number of depositions. The process was extended to a large number of depositions to investigate the macroscopic mechanical properties of the freestanding films. The LBL films showed remarkable improvements in mechanical properties as compared to neat PVA film. The GO-incorporated LBL films displayed higher enhancements in the tensile strength, ductility, and toughness as compared to that of PVA/Laponite LBL films, upon chemical cross-linking. This suggests the advantageous effects of GO incorporation. Interestingly, cross-linking of LBL films for longer time period (>1 h) and higher temperature (similar to 80 degrees C) was not found to be much beneficial. (C) 2016 Wiley Periodicals, Inc.

The simple process of a liquid wetting a solid surface is controlled by a plethora of factors-surface texture, liquid droplet size and shape, energetics of both liquid and solid surfaces, as well as their interface. Studying these events at the nanoscale provides insights into the molecular basis of wetting. Nanotube wetting studies are particularly challenging due to their unique shape and small size. Nonetheless, the success of nanotubes, particularly inorganic ones, as fillers in composite materials makes it essential to understand how common liquids wet them. Here, we present a comprehensive wetting study of individual tungsten disulfide nanotubes by water. We reveal the nature of interaction at the inert outer wall and show that remarkably high wetting forces are attained on small, open-ended nanotubes due to capillary aspiration into the hollow core. This study provides a theoretical and experimental paradigm for this intricate problem.

The simultaneous sharp rise in stiffness, strength, and toughness of electrospun nanofibers at small diameters is explained here as the result of the molecular orientation induced by the strong stretching of the electrospinning extensional flow. Differing from the common view that this phenomenon is related to the nanofibers size scale, we show by theoretical analysis that it is likely the result of an abrupt transition in polymer chain extension that occurs at high flow strain rates. Consequently, the molecular orientation and mechanical properties experience a matching transition, followed by a linear rise with the strain rate. The model compares well with published experimental data, supporting the assertion that the observed phenomena can be explained as the consequence of electrospinning conditions instead of size dependence. We show how the mechanical properties can be tuned by controlling the process as well as set the goal for future improvement in these properties.

The development of composite materials that are simultaneously strong and tough is one of the most active topics of current material science. Observations of biological structural materials show that adequate introduction of reinforcements and interfaces, or interphases, at different scales usually improves toughness, without reduction in strength. The prospect of interphase properties tuning may lead to further increases in material toughness. Here we use evaporation-driven self-assembly (EDSA) to deposit a thin network of multi-wall carbon nanotubes on ceramic surfaces, thereby generating an interphase reinforcing layer in a multiscale laminated ceramic composite. Both strength and toughness are improved by up to 90%, while keeping the overall volume fraction of nanotubes in a composite below 0.012%, making it a most effective toughening and reinforcement technique.

Defects in crystalline structure are commonly believed to degrade the ideal strength of carbon nanotubes. However, the fracture mechanisms induced by such defects, as well as the validity of solid mechanics theories at the nanoscale, are still under debate. We show that the fracture toughness of single-walled nanotubes (SWNTs) conforms to the classic theory of fracture mechanics, even for the smallest possible vacancy defect (∼2). By simulating tension of SWNTs containing common types of defects, we demonstrate how stress concentration at the defect boundary leads to brittle (unstable) fracturing at a relatively low strain, degrading the ideal strength of SWNTs by up to 60%. We find that, owing to the SWNT's truss-like structure, defects at this scale are not sharp and stress concentrations are finite and low. Moreover, stress concentration, a geometric property at the macroscale, is interrelated with the SWNT fracture toughness, a material property. The resulting SWNT fracture toughness is 2.7 MPa m0.5, typical of moderately brittle materials and applicable also to graphene.

The aim was to develop a hybrid three-dimensional-tissue engineering construct for chondrogenesis. The hypothesis was that they support chondrogenesis. A biodegradable, highly porous polycaprolactone-grate was produced by solid freeform fabrication. The polycaprolactone support was coated with a chitosan/polyethylene oxide nanofibre sheet produced by electrospinning. Transforming growth factor-β3-induced chondrogenesis was followed using the following markers: sex determining region Y/-box 9, runt-related transcription factor 2 and collagen II and X in quantitative real-time polymerase chain reaction, histology and immunostaining. A polycaprolactone-grate and an optimized chitosan/polyethylene oxide nanofibre sheet supported cellular aggregation, chondrogenesis and matrix formation. In tissue engineering constructs, the sheets were seeded first with mesenchymal stem cells and then piled up according to the lasagne principle. The advantages of such a construct are (1) the cells do not need to migrate to the tissue engineering construct and therefore pore size and interconnectivity problems are omitted and (2) the cell-tight nanofibre sheet and collagen-fibre network mimic a cell culture platform for mesenchymal stem cells/chondrocytes (preventing escape) and hinders in-growth of fibroblasts and fibrous scarring (preventing capture). This allows time for the slowly progressing, multiphase true cartilage regeneration.

A synergetic effect between graphene nanoplatelets (GNP) and single-walled carbon nanotubes (SWCNT) leading to an improvement of the electrical conductivity of poly(trimethylene terephthalate-. block-poly(tetramethylene oxide) (PTT-PTMO) based nanocomposites is demonstrated. PTT-PTMO based nanocomposites were prepared with varying concentration of SWCNT and GNP as conducting fillers, and their electrical conductivity and morphology were evaluated using Dielectric and Raman spectroscopies respectively. It has been shown that the addition of SWCNT and GNP enhanced the electrical conductivity of composites, particularly in the case of composites with 0.3 wt. % SWCNT and 0.1 wt. % GNP nanoparticles. These results suggest the existence of synergy arising from the combination of two conducting fillers with different geometrical shapes and aspect ratios as well as different dispersion characteristics in the PTT-PTMO thermoplastic elastomer matrix.

Biological structures consisting of strong boney elements interconnected by compliant but tough collagenous sutures are abundantly found in skulls and shells of, among others, armadillos, alligators, turtles and more. In the turtle shell, a unique arrangement of alternating rigid (rib) and flexible (suture) elements gives rise to superior mechanical performance when subjected to low and high strain-rate loadings. However, the resistance to repeated load cycling - fatigue - of the turtle shell has yet to be examined. Such repeated loading could approximately simulate the consecutive high-stress bending loads exerted during (a predator) biting or clawing. In the present study flexural high-stress cyclic loads were applied to rib and suture specimens, taken from the top dorsal part of the red-eared slider turtle shell, termed carapace. Subsequently, to obtain a more complete and integrated fatigue behavior of the carapace, specimens containing a complex alternating rib-suture-rib-suture-rib configuration were tested as well. Although the sutures were found to be the least resistant to repeated loads, a synergistic effect was observed for the complex specimens, displaying improved fatigue durability compared to the individual (suture or even rib) constituents. This study may assist in the design of future high-stress fatigue-resistant materials incorporating complex assemblies of rigid and flexible elements.

Ceramic composites found in nature, such as bone, nacre, and sponge spicule, often provide an effective resolution to a wellknown conflict between materials' strength and toughness. This arises, on the one hand, from their high ceramic content that ensures high strength of the material. On the other hand, various pathways are provided for stress dissipation, and thus toughness, due to their intricate hierarchical architectures. Such pathways include crack bridging, crack deflection, and delamination in the case of layered structures. On the basis of these inspiring ideas, we attempted here to create simultaneously strong and tough laminated alumina composite with high ceramic content. Composites were prepared from highgrade commercial alumina with spin-coated interlayers of ductile polymers (PMMA and PVA). The specimens' ultimate properties (strength, fracture toughness, and work of fracture) were measured by a four-point bending method. In some cases, fracture toughness of the composites was increased by up to an order of magnitude, reminiscent of the natural layered composites. It is proposed that this increase may be attributed to an interlocking mechanism, often encountered in biological composites. The significance of sample architecture and the role of the interfacial and bulk properties of the interlayer material are discussed.

Extremely fast growth of carbon nanotubes (CNTs), within just 5 s, was achieved by a facile microwave (MW) assisted heating technique, under ambient conditions. A mixture of graphite, ferrocene and carbon fiber was used as the precursor material. Growth characteristics were examined using different compositions of the precursor and MW powers. Both graphite and carbon fiber were found to be necessary for achieving fast growth of CNTs. MW heating of the precursor mixture at 100% power (1800 W) resulted in a yield of CNTs (26 +/- 5 wt%). Conversion efficiency of carbon from ferrocene to CNT at 100% MW power is about 82 wt% on average. When relative proportion of graphite in the mixture is high, yield decreases but conversion efficiency of carbon increases. Base growth mode is proposed as dominant growth process of the CNTs. Driven by the capillary effect of CNTs, catalyst particles are found to be encapsulated inside the tubes at different locations along their length. Solvent free, instant, easy, and cost effective growth process proposed here using ordinary MW oven is likely to be amenable to scale-up for industrial production of CNTs. (C) 2014 Elsevier Ltd. All rights reserved.

A number of biological armors, such as turtle shells, consist of a strong exoskeleton covered with a thin keratin coating. The mechanical role upon impact of this keratin coating has surprisingly not been investigated thus far. Low-velocity impact tests on the turtle shell reveal a unique toughening phenomenon attributed to the thin covering keratin layer, the presence of which noticeably improves the fracture energy and shell integrity. Synthetic substrate/coating analogues were subsequently prepared and exhibit an impact behavior similar to the biological ones. The results of the present study may improve our understanding, and even future designs, of impact-tolerant structures.

In the present work, MoS2 nanoparticles with fullerene-like structure, and most particularly those doped with minute amounts of rhenium atoms, are used as additive to medical gels in order to facilitate their entry into constricted openings of soft material rings. This procedure is used to mimic the entry of endoscopes to constricted openings of the human body, like urethra, etc. It is shown that the Re-doped nanoparticles reduce the traction force used to retrieve the metallic lead of the endoscope from the soft ring by a factor close to three times with respect to the original gel. The mechanism of the mitigation of both friction and adhesion forces in these systems by the nanoparticles is discussed.

In this study, a molecular dynamic model was employed to simulate oblique extraction of a multi-walled carbon nanotube (MWNT) from resin and to calculate the carbon nanotube's (CNTs) deformation and stress distribution. Numerical results reveal the occurrence of elongation and necking in a CNT prior to the movement of its embedded end and of local buckling in a segment near where a CNT exits resin under large angle oblique extraction. Numerical results for the 0, 15, 30, 45, 60 and 75 oblique extractions reveal the effects of the oblique angle on the force-displacement curve and the tensile stress distribution over selected CNT cross-sections. Based on the simulation results, it is noted that CNT breakage in large-angle oblique extraction is mainly attributed to stress concentration caused by local buckling and it has a detrimental effect on the CNT toughening capacity.

The utilization of highly branched polymer (e.g., epoxy resins) in engineering applications is often limited by their brittle nature (low fracture toughness). Loading the polymer matrix by fillers such as individual nanotubes is a promising alternative to enhance fracture toughness without compromising other mechanical properties. However, to fully understand the nanotubes toughening role and correctly characterize the nanocomposite failure mechanisms, a complete exfoliation of the nanotubes aggregates into individual nanotubes is essential. In this work, we embed only individual nanotubes in the polymer matrix using a novel dispersion method. The individual nanotube concentration in the composite is accurately determined. We achieve a record fracture toughness enhancement and, for the first time, demonstrate a coherent quantitative correlation between the fracture toughness and the surface roughness. Finally, comprehensive statistical investigation of the nanotube failure mechanisms shows that carbon nanotubes fail via fracture mechanism, while tungsten di-sulfide nanotubes via pullout mechanism. The failure mechanism could be predicted by the slope of the surface roughness vs. fracture toughness curve.

Insertion of endoscopes and other medical devices into the human body are ubiquitous, especially among aged males. The applied force for the insertion/extraction of the device from the urethra must overcome endoscope-surface-human-tissue interactions. In daily practice a gel is applied on the endoscope surface, in order to facilitate its entry into the urethra, providing also for local anesthesia. In the present work, a new solid-state lubricant has been added to the gel, in order to reduce the metal-urethra interaction and alleviate the potential damage to the epithelial tissue. For that purpose, a urethra model was designed and fabricated, which allowed a quantitative assessment of the applied force for extraction of the endoscope from a soft polymer-based ring. It is shown that the addition of MoS₂ nanoparticles with fullerene-like structure (IF-MoS₂) and in particular rhenium-doped nanoparticles (Re:IF-MoS₂) to Esracain gel applied on the metal-lead reduced the friction substantially. The Re:IF-MoS ₂ showed better results than the undoped fullerene-like nanoparticles and both performed better than the gel alone. The mechanism of friction reduction is attributed to fullerenes' ability to roll and act as a separator between the active parts of the model.

Growing carbon nanotubes (CNTs) on the surface of fibers has the potential to modify fiber-matrix interfacial adhesion, enhance composite delamination resistance, and possibly improve toughness. In the present study, aligned CNTs were grown upon carbon fabric via chemical vapor deposition. Continuously monitored single-fiber composite fragmentation tests were performed on pristine and CNT-grafted fibers embedded in epoxy, and single-laminate compact-tension specimens were tested for fracture behavior. A significant increase (up to 20 %) was observed in the interfacial adhesion, at the cost of a decrease in the fiber tensile strength. As a result, the maximum load of the composite was decreased, but its residual load-bearing capacity more than doubled. The likely sources of these effects are discussed, as well as their implications.

We quantify nanocomposite toughness through a reanalysis for nanotubes of the Cottrell-Kelly-Tyson (CKT) model, of the definition of critical length, and of the energy dissipation model for pull-out. The effect of the hollow cylindrical geometry of nanotubes is discussed, followed by an examination of proper ways to compare energy dissipation at the nano and micro levels.

The mechanical and structural properties of the sublayers of osteonal lamellae were studied. Young's modulus (E) of adjacent individual lamellae was measured by nanoindentation of parallel slices every 1-3 μm, in planes parallel and perpendicular to the osteon axis (OA). In planes parallel to the OA, the modulus of a lamella could vary significantly between sequential slices. Significant modulus variations were also sometimes found on opposing sides of the osteonal canal for the same lamella. These results are rationalized by considerations involving the microstructural organization of the collagen fibrils in the lamellae. Scanning electron microscope imaging of freeze fractured surfaces revealed that the substructure of a single lamella can vary significantly on the opposing sides of the osteonal axis. Using a serial surface view method, parallel planes were exposed every 8-10 nm using a dual-beam microscope. Analysis of the orientations of fibrils revealed that the structure is rotated plywood like, consisting of unidirectional sublayers of fibrils of several orientations, with occasional randomly oriented sublayers. The dependence of the measured mechanical properties of the lamellae on the indentation location may be explained by the observed structure, as well as by the curvature of the osteonal lamellae through simple geometrical-structural considerations. Mechanical advantages arising from the curved laminate structure are discussed.

We recently discovered that minute additions of sodium chloride (NaCl) during the preparation of electrospun polymethyl methacrylate (ES PMMA) nanofibers significantly raise their mechanical properties. In the present work, twisted yarns made of such ES PMMA nanofibers were prepared and the effects of NaCl mediation and yarn structure and twist on the resulting mechanical properties were studied. Improvements in these properties due to NaCl mediation are again observed for yarns, as they were for single ES fibers. Furthermore, both Young's modulus and the ultimate strength of the yarns show an increase with increasing angle of twist up to a maximum, followed by a decrease at higher twist angles.

Purpose. We evaluated the efficacy and safety of photochemical corneal stiffening by palladium bacteriochlorin 13′-(2-sulfoethyl)amide dipotassium salt (WST11) and near infrared (NIR) illumination, using ex vivo and in vivo rabbit eye models. Methods. Corneas of post mortem rabbits and living rabbits were pretreated topically with 2.5 mg/mL WST11 in saline or in 20% dextran T-500 (WST-D), washed and illuminated with an NIR diode laser (755 nm, 10 mW/cm2. Studies with corneas of untreated fellow eyes served as controls. Tensile strength measurements, histopathology, electron spin resonance, and optical spectroscopy and fluorescence microscopy were used to assess treatment effects. Comparative studies were performed with standard riboflavin/ultraviolet-A light (UVA) treatment. Results. WST11/NIR treatment significantly increased corneal stiffness following ex vivo or in vivo treatment, compared to untreated contralateral eyes. The incremental ultimate stress and Young's modulus of treated corneas increased by 45, 113, 115%, and 10, 79, and 174% following 10, 20, and 30 minutes of incubation with WST11, respectively. WST-D/NIR had a similar stiffening effect, but markedly reduced post-treatment edema and shorter time of epithelial healing. WST11/NIR and WST-D/NIR generate hydroxyl and superoxide radicals, but no singlet oxygen in the cornea. Histology demonstrated a reduction in the keratocyte population in the anterior half of the corneal stroma, without damage to the endothelium. Conclusions. Treatment of rabbit corneas, with either WST11/NIR or WST-D/NIR, increases their biomechanical strength through a mechanism that does not involve singlet oxygen. The WST-D/NIR treatment showed less adverse effects, demonstrating a new potential for clinical use in keratoconus and corneal ectasia after refractive surgery.

Dentin and enamel are viewed here as multi-scale composites comprising a staggered micro-structure made of stiff platelets embedded in a more compliant matrix, and further assembled into macroscopic composite-like structures. Mechanical models are formulated for both tissues and their effective moduli are evaluated analytically. The resulting predictions are in very good agreement with Finite Elements (FE) simulations and experimental data from the literature. The models developed in this study demonstrate the possibility, in certain cases, to generate special mechanical effects linked to the structural complexity of these tissues.

In this article, Multi-Walled Carbon Nanotubes (MWCNTs) of varying diameters, both untreated and polycarboxylated, were dispersed at constant weight percentage in an epoxy matrix, and resulting fracture toughnesses (K _Ic) were measured in each case. We show that changing the MWCNT diameter has two effects on the composite fracture toughness: (i) a small MWCNT diameter enables larger interfacial surface for adhesion maximization, which increases toughness; (ii) at the same time, it limits the available pull-out energy and reduces the MWCNT ability to homogeneously disperse in the matrix due to this same large active surface: this decreases toughness. Most commercially available MWCNTs have a length range of several μm, thus an optimal diameter exists which depends on MWCNT wall thickness and surface treatment. Such optimal diameter maximizes pull-out energy and thus composite fracture toughness. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 Carbon nanotubes (CNTs), when sufficiently dispersed in a polymer matrix, offer exceptional mechanical properties as reinforcement fillers in nanocomposite materials. In addition to homogeneous dispersion, optimization of the interfacial bonding between both contacting phases is essential for CNT incorporation to reach full potential. In this study, multiwalled CNTs of various diameters, both untreated pristine and polycarboxylated, were dispersed at equal weight percentage in an epoxy matrix, and resulting fracture toughnesses have been measured. An optimal diameter exists depending on CNT thickness and surface treatment.

Variations in Young's modulus of individual lamellae around a single bone osteon have been measured in three orthogonal planes by nanoindentation. The objective of these measurements was to establish a correlation between the mechanical properties and the microstructure of the osteonal lamellae. When indentation was performed in a plane perpendicular to the osteon axis (OA), the modulus of the lamella closest to the canal appears to be higher than the modulus of all other lamellae. No such difference was observed in planes parallel to the OA. However, in the parallel planes, an unexpected asymmetry in modulus was detected on opposing sides of the canal, potentially supporting the validity of the rotated plywood structure model of bone lamellae. Finally, based on the experimentally measured Young's modulus values, most osteonal lamellae appear to exhibit structural anisotropy.

The effective moduli of a multi-scale composite are evaluated by a bottom-up (hierarchical) modeling approach. We focus on a two-scale structure in which the small scale includes a platelet array inside a matrix, and the large scale contains fibers inside a composite matrix. We demonstrate that the principal moduli of the multi-scale composite can be fine-tuned by the platelet arrangement and orientation. As a case study, we consider the phenomenon of fiber micro-buckling within the multi-scale composite. It is found that the compressive micro-buckling strength can be considerably increased for specific platelet orientations. The multi-scale design approach presented here can be used to generate novel families of composite materials with tunable mechanical properties.

This work aims at evaluating the elastic modulus of hard biological tissues by considering their staggered platelet micro-structure. An analytical expression for the effective modulus along the stagger direction is formulated using three non-dimensional structural variables. Structures with a single staggered hierarchy (e.g. collagen fibril) are first studied and predictions are compared with the experimental results and finite element simulations from the literature. A more complicated configuration, such as an array of fibrils, is analyzed next. Finally, a mechanical model is proposed for tooth dentin, in which variations in the multi-scale structural hierarchy are shown to significantly affect the macroscopic mechanical properties.

Inorganic nanoparticles of layered [two-dimensional (2D)] compounds with hollow polyhedral structure, known as fullerenelike nanoparticles (IF), were found to have excellent lubricating properties. This behavior can be explained by superposition of three main mechanisms: rolling, sliding, and exfoliation-material transfer (third body). In order to elucidate the tribological mechanism of individual nanoparticles in different regimes, in situ axial nanocompression and shearing forces were applied to individual nanoparticles using a high resolution scanning electron microscope. Gold nanoparticles deposited onto the IF nanoparticles surface served as markers, delineating the motion of individual IF nanoparticle. It can be concluded from these experiments that rolling is an important lubrication mechanism for IF-WS ₂ in the relatively low range of normal stress (0.96±0.38 GPa). Sliding is shown to be relevant under slightly higher normal stress, where the spacing between the two mating surfaces does not permit free rolling of the nanoparticles. Exfoliation of the IF nanoparticles becomes the dominant mechanism at the high end of normal stress; above 1.2 GPa and (slow) shear; i.e., boundary lubrication conditions. It is argued that the modus operandi of the nanoparticles depends on their degree of crystallinity (defects); sizes; shape, and their mechanical characteristics. This study suggests that the rolling mechanism, which leads to low friction and wear, could be attained by improving the sphericity of the IF nanoparticle, the dispersion (deagglomeration) of the nanoparticles, and the smoothness of the mating surfaces.

It is now possible to synthesize inorganic nanotubes (INT) of WS2 in a pure phase and substantial amounts. Due to their crystalline perfection, they are characterized by excellent mechanical behavior. This study is dedicated to the investigation of the effect of INT-WS2 on the mechanical, thermal, adhesion and tribological properties of epoxy based nanocomposites. Various concentrations up to 1.0 wt% of the INT were added to the epoxy matrix. First a method was developed to mix the nanotubes in the epoxy resin matrix. A combination of both magnetic stirring and ultrasonic mixing was used. The adhesion, fracture toughness and strain energy release rate were studied. The INT-WS2 were found to significantly improve all these properties. The wear of the nanotubes-reinforced epoxy was eight-times lower than that of pure epoxy. These results suggest numerous applications.

A generic mechanical model for bio-composites, including stiff platelets arranged in a staggered order inside a homogeneous soft matrix, is proposed. Equations are formulated in terms of displacements and are characterized by a set of non-dimensional parameters. The displacements, stress fields and effective modulus of the composite are formulated. Two analytical models are proposed, one which includes the shear deformations along the entire medium and another simplified model, which is applicable to a slender geometry and yields a compact expression for the effective modulus. The results from the models are validated by numerical finite element simulations and found to be compatible with each other for a wide range of geometrical and material properties. Finally, the models are solved for two bio-structures, nacre and a collagen fibril, and their solutions are discussed.

Over the past 30 years Raman spectroscopy has transformed the field of composite micromechanics and is being successfully applied to the field of nanotube-reinforced polymers. Under the effects of stress, careful study of the Raman spectrum of carbon nanotubes provides a unique insight into various physical phenomena in nanocomposites. This chapter demonstrates the power of the Raman signature of carbon nanotubes as a detector of bulk matrix defects, the occurrence of polymer phase transitions, and the nanotubes' own orientation change with respect to applied stress, stress profiles from Raman-insensitive fibers, and dual information about improvements in the stress transfer ability and about nanotube wall structure degradation due to the surface treatment itself. Remaining challenges are described.

Extracellular matrix (ECM) is an important component of stem cell niche areas, which provide residence, regulate stem cell pool size and control stem cell mobilization. ECM is a complex interlinked composite of collagenous molecules, non-collagenous molecules and water-rich mucopolysaccharide ground substance. Cells are integrated to their matrix via integrin and non-integrin receptors, which are utilized in the control of adhesion, migration, division, growth, anoikis, transdifferentiation and other cellular behaviour. ECM provides architecture and strength, but also growth factor deposits, which proteinases as signalling scissors can release in a site- and -process-specific manner. Several processes, like wounds, cartilage, fractures, myocardial infarctions and tumor growth are used to exemplify regenerative processes.

Inorganic layered materials can form hollow multilayered polyhedral nanoparticles. The size of these multi-wall quasi-spherical structures varies from 4 to 300 nm. These materials exhibit excellent tribological and wear-resisting properties. Measuring and evaluating the stiffness of individual nanoparticle is a non-trivial problem. The current paper presents an in situ technique for stiffness measurements of individual WS₂ nanoparticles which are 80 nm or larger using a high resolution scanning electron microscope (HRSEM). Conducting the experiments in the HRSEM allows elucidation of the compression failure strength and the elastic behavior of such nanoparticles under uniaxial compression.

Outer surfaces of spacecraft in orbit are exposed to hypervelocity impact originating from micro-meteoroids and space debris. The structural composite materials are integral parts of the spacecraft envelope. We studied the impact response of structural micro-composites containing Kevlar 29, spectra 1000 and oxygen RF (Radio Frequency) plasma surface-treated spectra 1000 fibers of 27-μm diameter, embedded in 100-μm epoxy resin films, in a series of planar impact experiments. The composites were loaded by 50-μm aluminum and polycarbonate impactors having velocities ranging from 400 to 550 m/s. The velocity of the free surface of the composite samples was continuously monitored by VISAR (Velocity Interferometer System for Any Reflector). The dynamic tensile (spall) strength of the micro-composites was calculated on the basis of the recorded free surface velocity profiles. Correlations were found between the spall strength and the separately measured: (i) fiber/matrix interfacial adhesion, (ii) tensile strengths of the fibers, of the matrix and of the micro-composites, and (iii) internal residual stresses. The spall strength of surface-treated spectra fibers micro-composites was found to be lower than that of both pristine spectra fibers micro-composites, and the pure epoxy film. The epoxy film reinforced by Kevlar fibers was found to have the highest spall strength.

We study the strain-induced shift of the D* Raman band of single-wall carbon nanotubes in polyvinyl alcohol-nanotube composite fibers. If embedded in structural components, such strain-sensitive fibers may be considered for potential applications as strain or stress sensors. Due to improved interfacial adhesion, stronger shifts of the D* Raman band are observed when carboxylic functional groups are present at the nanotube surface. This indicates that nanotube carboxylation would yield better efficacy for future sensing applications. However, we also observe that the improvements of interfacial adhesion do not lead to substantially better mechanical properties of the fibers. This effect is discussed by considering possible degradation of nanotubes during surface functionalization.

The probability that debris will collide with a spacecraft in low Earth orbit is a rising concern of the space community. The most exposed materials are composite materials and polymers that serve as structural materials. Studying the fracture mechanisms of fiber/matrix microcomposites due to hypervelocity impact is the main purpose of this work. A laser-driven flyer-plate technique was used to simulate the debris impact, and microcomposite specimens of Spectra 1000 or Kevlar 29 fibers and epoxy resin as a matrix were prepared. In addition to studying the micromechanical response of hypervelocity impact on pristine microcomposites samples, the response of irradiated microcomposites was also examined. Pristine microcomposites and samples after exposure to gamma radiation were impacted by the aluminum laser-driven flyers at 0.9 and 1:7 km=s. The micromechanical response was correlated to the fibers and fiber/matrix interface properties.

Investigation of composite materials response to hypervelocity impact by space debris has been carried out. In order to simulate hypervelocity impact, a unique laser driven flyer plate (LDFP) system was used, generating hypervelocity debris with velocities of up to 3 km/s. The materials studied in this research were Kevlar 29/epoxy and Spectra1000/epoxy thin film micro-composites (thickness of about 100 μm). Both Spectra and Kevlar fibers are used in long-duration spacecraft outer wall shielding to reduce the perforation threat. The micro-mechanical response of different composites was studied and correlated to the fiber, the matrix and the fiber/matrix interface properties. Visual and microscopic examinations of the damaged area identified fiber debonding as the prevailing failure mechanism. On the basis of a simple energy balance model it can be stated that for Spectra/epoxy composite the dominant mechanism is new surface creation, whereas for Spectra surface-treated fibers/epoxy the fiber pull out is the dominant mechanism. For Kevlar/epoxy fiber, pull out mechanism plays an important role.

Bone is mechanically and structurally anisotropic with oriented collagen fibrils and nanometer-sized mineral particles aggregating into lamellar or woven bone.^[1] Direct measurements of anisotropic mechanical properties of sublamellar tissue constituents are complicated by the existence of an intrinsic hierarchical architecture. Methods such as nanoindentation provide insight into effective modulus values; however, bulk material properties cannot sufficiently be characterized since such measurements represent properties of near-surface volumes and are partially averaged over fibril orientations. ^[2-5] In this study, we focus on the material properties of bone at one single level of hierarchy. By measuring properties of individual parallelfibered units of fibrollamellar bone under tension under controlled humidity conditions, an unusually high anisotropy is found. Here, we clearly demonstrate ratios as large as 1:20 in elastic modulus and 1:15 in tensile strength between orientations perpendicular and parallel to the main collagen fiber orientation in native wet bone; these ratios reduce to 1:8 and 1:7, respectively, under dry conditions. This extreme anisotropy appears to be caused by the existence of periodic, weak interfaces at the mesoscopic length scale. These interfaces are thought to be relevant to the proper mechanical and physiological performance of bone.

The structural toughness arising from conventional energy dissipation mechanisms has the potential to be much more significant in nanocomposites than in composites containing conventional micron-size fibers. This is illustrated and discussed here using nanotube pull-out as an example of such dissipation. While somewhat challenging conventional scaling arguments, this appears to concur with current experimental observations.

Geoinspired synthetic chrysotile nanotubes both stoichiometric and 0.67 wt % Fe doped were characterized by transmission electron microscopy and electron diffraction. Bending tests of the synthetic chrysotile nanotubes were performed using the atomic force microscope. The nanotubes were found to exhibit elastic behaviour at small deformations (below ca. 20 nm). Young's modulus values of (159±125) GPa and (279±260) GPa were obtained from the force-deflection curves using the bending equation for a clamped beam under a concentrated load, for the stoichiometric and the Fe doped chrysotile nanotubes, respectively. The structural modifications induced by Fe doping altered the mechanical properties, with an apparent dependence of the latter on the number of constituting walls of the nanotubes.

The lamellar bone's strength is mainly affected by the organization of its mineralized collagen fibers and material composition. In the present study, Raman microspectroscopic and imaging analyses were employed to study a normal human femoral midshaft bone cube-like specimen with a spatial resolution of similar to 1-2 mu m. Identical bone lamellae in both longitudinal and transverse directions were analyzed, which allowed us to separate out orientation and composition dependent Raman lines, depending on the polarization directions. This approach gives information about lamellar bone orientation and variation in bone composition. It is shown that the nu(1) PO4 to amide I ratio mainly displays lamellar bone orientation; and nu(2) PO4 to amide III and CO3 to nu(2) PO4 ratios display variation in bone composition. The nu(2) PO4 to amide III ratio is higher in the interstitial bone region, whereas the CO3 to nu(2) PO4 ratio has lower values in the same region. The present study provides fresh insights into the organization of a lamellar bone tissue from two orthogonal orientations. (C) 2007 Elsevier Inc. All rights reserved.

A process for melt mixing of carbon nanotubes in polyacrylonitrile, which is aided by addition of plasticizer is proposed and employed to extrude/draw filaments at various draw ratios and test them for a range of properties. Microscopic observations show that the nanotubes are evenly dispersed throughout the filament and that they are preferentially aligned along its axis. The extrusion/drawing process results in two fold higher draw ratios of the nanocomposite filament compared with the unfilled control, and the first is shown by wide-angle X-ray diffraction and by polarized Raman spectroscopy to possess higher crystalline and morphological orientation. Thermal analysis and measurements of electric conductivity and mechanical properties attest to the condition that the carbon nanotubes are engaged in π-π interactions with the nitrile groups and/or with their polymerized conjugated imine system. These interactions interfere with and reduce the mutual dipole interactions of the nitrile groups of the polyacrylonitrile, allowing the nanocomposite filaments to be stretched to higher draw ratios. The potential reinforcing effect of the carbon nanotubes is cancelled out by the loss of the original dipole interactions, resulting in lower mechanical properties. However, the interaction of the carbon nanotubes with the conjugated imine system offers potentially interesting electric properties.

The surface of multi-wall carbon nanotubes (MWNTs) was functionalized by covalent linking of long alkyl chains. Such functionalization led to a much better tube dispersion in organic solvents than pristine nanotubes, favored the formation of homogenous nanocomposite films, and yielded good interfacial bonding between the nanotubes and two polymer matrices: a thermo-set (Epon 828/T-403) and a thermoplastic (PMMA). Tensile tests indicated, however, that the reinforcement was greatly affected by the type of polymer matrix used. Relative to pure PMMA, a 32% improvement in tensile modulus and a 28% increase in tensile strength were observed in PMMA-based nanocomposites using 1.0 wt% nanotube filler. Contrasting with this, no improvement in mechanical properties was observed in epoxy-based nanocomposites. The poorer mechanical performance of the latter system can be explained by a decrease of the crosslinking density of the epoxy matrix in the nanocomposites, relative to pure epoxy. Indeed we demonstrate that the presence of nanotubes promotes an increase in the activation energy of the curing reaction in epoxy, and a decrease of the degree of curing.

Fibrous arrays and composites are among the strongest structures created by man or found in nature. Such materials often fail in a slow, cumulative fashion, suppressing the sudden occurrence of rapid structural collapse. Our classical understanding of the statistical tensile strength and failure of unidirectional composites is usually based on a stochastic model where the key predictor is the size (N*) of a critical cluster of adjacent broken fibres, which inevitably leads to final composite failure. Here we show, via direct measurements using high-resolution synchrotron X-ray tomography, that in a quartz-epoxy composite the classical stochastic theory underpredicts - by a factor 3-5 - the size N* of the critical 'failed-fibre' cluster. A simple fracture mechanics argument which relates the critical fibre cluster size to the material strength is proposed to account for our data.

The nanoscale deformation and fracture mechanisms of parallel fibered bone are investigated using a novel combination of in-situ tensile testing to failure combined with high brilliance synchrotron X-ray scattering. The technique enables the simultaneous measurement of strain at two length scales - in the mineralized collagen fibrils (∼100 nm diameter) along with the macroscopic strain (∼1 mm diameter). Under constant rate tensile loading, we find that fibril strain saturates beyond the macroscopic yield point of bone at ∼0.5%, providing a correlation between the failure mechanisms at the nanoscale and the bulk structural properties. When bone stretched beyond the yield point is unloaded back to zero stress, the fibrils are contracted relative to their original state. We examine the findings in the context of a fiber - matrix shearing model at the nanometer level.

Microscale aggregate formation, resulting from high intrinsic filler attractions, is one of the major issues in nanocomposite preparation and processing. Herein, the dispersive effects achieved by a wide range of surface-active agents, as well as surface oxidation and functionalization, are investigated. The aim of our research is to form a uniform, multiwalled carbon nanotube (MWNT) distribution in water-soluble (poly(ethylene glycol)) and water-insoluble (polypropylene) polymers. In order to understand the surface-charge-related stability of the treated nanotubes solutions, zeta-potential measurements are applied. Quantification of the state of the MWNT dispersion is derived from particle-size analysis, while visual characterization is based on optical and electron microscopy. To estimate the nucleating ability of the surface-modified carbon nanotubes, the temperature of crystallization and the degree of crystallinity are calculated from differential scanning thermograms. Finally, we suggest general guidelines to produce uniform MWNT dispersions using a dispersive agent and/or surface treatment in water-soluble and water-insoluble polymers.

The mechanical properties of materials and particularly the strength are greatly affected by the presence of defects; therefore, the theoretical strength (≈10% of the Young's modulus) is not generally achievable for macroscopic objects. On the contrary, nanotubes, which are almost defect-free, should achieve the theoretical strength that would be reflected in superior mechanical properties. In this study, both tensile tests and buckling experiments of individual WS₂ nanotubes were carried out in a high-resolution scanning electron microscope. Tensile tests of MoS₂ nanotubes were simulated by means of a density-functional tight-binding-based molecular dynamics scheme as well. The combination of these studies provides a microscopic picture of the nature of the fracture process, giving insight to the strength and flexibility of the WS₂ nanotubes (tensile strength of ≈16 GPa). Fracture analysis with recently proposed models indicates that the strength of such nanotubes is governed by a small number of defects. A fraction of the nanotubes attained the theoretical strength indicating absence of defects.

The thermo-mechanical properties of epoxy-based nanocomposites based on low weight fractions (from 0.01 to 0.5 wt%) of randomly oriented single- and multi-walled carbon nanotubes were examined. Preparation methods for the nanocomposites, using two types of epoxy resins, were developed and good dispersion was generally achieved. The mechanical properties examined were the tensile Young's modulus by Dynamic Mechanical Thermal Analysis and the toughness under tensile impact using notched specimens. Moderate Young's modulus improvements of nanocomposites were observed with respect to the pure matrix material. A particularly significant enhancement of the tensile impact toughness was obtained for specific nanocomposites, using only minute nanotube weight fractions. No significant change in the glass transition temperature of SWCNT/epoxy nanocomposites was observed, compared to that of the epoxy matrix. The elastic modulus of the SWNT-based nanocomposites was found to be slightly higher than the value predicted by the Krenchel model for short-fiber composites with random orientation.

Deformation mechanisms in bone matrix at the nanoscale control its exceptional mechanical properties, but the detailed nature of these processes is as yet unknown. In situ tensile testing with synchrotron X-ray scattering allowed us to study directly and quantitatively the deformation mechanisms at the nanometer level. We find that bone deformation is not homogeneous but distributed between a tensile deformation of the fibrils and a shearing in the interfibrillar matrix between them.

Nanocomposites based on semi-crystalline poly(vinyl alcohol) (PVA) and well-dispersed chemically functionalized single-walled carbon nanotubes are combined through simple mixing. The interaction between the nanotubes and the polymer matrix is studied using optical and thermal methods. Significant enhancement of the mechanical properties is obtained for the functionalized-nanotube-based composites. These results imply that promoting nanotube dispersion and strong interfacial bonding through adequate functionalization of nanotubes improves the load transfer from the matrix to the reinforcing phase.

Individual multiwalled carbon nanotubes were controllably wetted by polyethylene glycol, glycerol, and water. A Wilhelmy force balance approach was used to calculate contact angles at the nanotube-polyethylene glycol and nanotube-glycerol interfaces, allowing examination of the contact angle dependence on the nanotube diameter. Water, however, exhibited a significantly larger interaction with the nanotube, which could only be explained by allowing for internal wetting of the open carbon nanotube structure. This internal wetting angle is smaller than the external one.

Materials and material development are fundamental to our very culture. We even ascribe major historical periods of our society to materials such as the stone age, bronze age, iron age, steel age (the industrial revolution), polymer age, silicon age, and silica age (the telecoms revolution). This reflects how important materials are to us. We have, and always will, strive to understand and modify the world around us and the stuff of which it is made. As the 21st century unfolds, it is becoming more apparent that the next technological frontiers will be opened not through a better understanding and application of a particular material, but rather by understanding and optimizing material combinations and their synergistic function, hence blurring the distinction between a material and a functional device comprised of distinct materials.

Pullout experiments were performed at the nanoscale using an atomic force microscope, to assess the interfacial adhesion between multi-walled carbon nanotubes and a matrix of polyethylene-butene. Fracture energy for the nanotube-polymer interface was calculated from the measured pullout forces and embedded lengths. The results suggest the existence of a relatively strong interface, with higher fracture energy for smaller diameter nanotubes.

Stepped conical structures have been produced at the surface of poly(ethylene glycol) by contacting a single, relatively short carbon nanotube attached to an AFM tip with the molten polymer surface, followed by polymer cooling. Cooling of the polymer melt in the nanotube vicinity is the most likely mechanism for these ziggurat-like structural formations. Simple heat transfer calculations confirm the effect of the nanotube length on the propensity for local solidification of the polymer.

The feasibility of using Raman spectroscopy to map strain fields in model composites is demonstrated by means of two experiments. (1) The mapping of the stress concentration in the vicinity of a break in a strained E-glass fiber, using the Raman spectrum of single-wall nanotubes dispersed in the polymer around the fiber. (2) The mapping of strain using the Raman response of a strained high modulus carbon fiber and, simultaneously, from the surrounding polymer matrix using single-wall nanotubes dispersed in the polymer. The size and shape of the 'zone of influence' resulting from the stress concentration effects around the fiber breaks are evaluated in both cases. Model experiments of this type provide fundamental design information about the fiber-matrix stress transfer mechanisms in composites.

The measurement of the force required to separate a carbon nanotube from a solid polymer matrix was discussed. The reproducible nanopullout experiments were performed by using atomic force microscopy during the analysis. The results showed that the polymer matrix in close vicinity of the carbon nanotube was able to withstand stresses, causing considerable yield in a bulk polymer specimen.

An experimental technique for probing the adhesion of carbon nanotubes to a polymer matrix was described. The nanotube-polymer interaction was quantified by detaching individual single-walled carbon nanotube bundles and multiwalled-carbon nanotube from an epoxy matrix using a scanning probe microscope tips. These experiments were used for the calculation of the nanotube-polymer interfacial shear strength.

The interfacial shear strength in single-wall nanotube-polymer composites is calculated using a traditional force balance approach, modified for a hollow tube, and the effect of varying some of the model parameters is examined and discussed. It is shown that high values of the interfacial shear strength (compared to those in current advanced fiber-based polymer composites) are in principle attainable. Defects in the hexagonal structure of a nanotube, which technically is a 'perfect' material, are expected to strongly reduce its strength and the model predicts that, as a consequence, a large variability should be experimentally observed in either the interfacial strength or the critical length of apparently identical nanotubes.

The existence of a size effect is an integral part of the physical phenomenon of brittle fracture. The literature dealing with the influence of fiber size on its ultimate strength has mainly focused on specimens whose failure is governed by a single category of flaw. In this work, we are discussing the size effect from a statistical point of view for fibers exhibiting multiple failure modes.

The D^* Raman band of single-wall carbon nanotubes aligned by shear flow in a polymer matrix has been measured as a function of tensile strain. The Raman intensity varies with the optical polarization direction, an effect which is used here to assess the degree of tube alignment. The strain dependence of the Raman shift depends strongly on the nanotube orientation and the polarization direction. We show that, using polarized light, unoriented nanotubes can be used as strain sensors so that no tube alignment is necessary and the strain can be measured in all directions in a single sample.

When a single-fibre composite test is performed to obtain information about the interfacial adhesion in a composite, a gradual strain increase often causes an opaque (black) cylinder to nucleate at, and grow from, the fibre failure sites. The nature of the opaque cylinder is difficult to ascertain using optical microscopy. This is the subject of the present note. To study the inside of the opaque cylinder we use several experimental methods based on imaging the failed region: optical microscopy, laser Raman spectroscopy and scanning electron microscopy. Mechanical models, including FEM analysis and analytic equations based on the shear-lag approach, are used to discuss the experimental work. The nature and growth mechanism of the opaque cylinder are of importance in defining the parameters and/or contributions that appear in both the force balance and the energy balance schemes.

The interfacial properties for E-glass/epoxy composites were measured using push-out tests and single fiber fragmentation tests. Theoretical models for both stress-based and energy-based criteria were used to interpret the experimental results. Fibers treated with γ-aminopropyl-triethoxysilane (γ-APS) showed higher bond strength (∼1.7 times higher) and interfacial toughness (∼1.9 times higher) than those of unsized E-glass based composites. However, the average interfacial toughness obtained from fragmentation tests was about six times higher than that obtained from push-out tests. Considering the analytical frameworks employed to interpret the values measured in the present work, the fragmentation test is a more appropriate method to obtain interfacial energy for polymeric composites, but both methods are appropriate for relative measurements of interface strength.

Raman spectral shifts of single-wall carbon nanotubes embedded in polymer systems were used to measure transitions in polymers. Glass-transition temperatures and secondary transitions were observed, and Raman spectroscopic data were compared with dynamic mechanical tests for a thermosetting and a thermoplastic polymer. The data confirm that the Raman spectral response of carbon nanotubes embedded in polymers is sensitive to polymer transitions.

We report for the first time details of the morphology of alpha isotactic polypropylene transcrystallinity induced by aramid fibers as determined by high spatial resolution X-ray diffraction. We suggest that the parent lamellae nucleate at the fiber surface with the crystallite a*-axes parallel to the fiber axis, twist one quarter turn about the parent a axis within an approximate distance of 25 mu -m and then continue to grow without further twisting. This result is unexpected since lamellar twisting has never been observed in pure a spherulitic polypropylene.

The relation between the mechanical properties and the lamellar morphology of the α-isotactic polypropylene transcrystalline interphase in a fiber composite was investigated by means of nanometric shear and directional indentation measurements, using scanning force microscopy. Measurements were performed along directions parallel and perpendicular to the transcrystalline growth direction. A key finding is the inversion of the shear modulus anisotropy ratio from 2.3 to 0.5 as the crystal grows away from the fiber. On the basis of this finding, an improved view of the hierarchical lamellar and molecular order of the transcrystalline layer is proposed.

Small amounts of single-wall carbon nanotubes embedded in a polymer matrix were used to sense the mechanical response of the polymer using microRaman spectral data. A flow orientation method was applied to align the nanotubes in the matrix. The Raman spectra obtained for specimens cut both parallel and perpendicular to the flow direction were found to be significantly different, as a function of mechanical strain. Thus Raman spectroscopy combined with mechanical testing provides a way to probe the alignment of nanotubes in composites. The Raman shift-strain response for samples loaded perpendicular to the flow direction suggests that nanotube reorientation is achieved upon straining the polymer beyond its yield point. Our data suggest that the adhesion between the nanotubes and the polymer exceeds the shear yield strength of the matrix. We show that a stress-strain curve for the polymer may be produced directly by means of Raman spectroscopy.

The components that contribute to Raman spectral shifts of single-wall carbon nanotubes (SWNT's) embedded in polymer systems have been identified. The temperature dependence of the Raman shift can be separated into the temperature dependence of the nanotubes, the cohesive energy density of the polymer, and the buildup of thermal strain. Discounting all components apart from the thermal strain from the Raman shift-temperature data, it is shown that the mechanical response of single-wall carbon nanotubes in tension and compression are identical. The stress-strain response of SWNT's can explain recent experimental data for carbon nanotube-composite systems.

Specific peaks of the Raman spectrum of single-wall nanotubes shift significantly upon immersion of the tubes in a liquid, relative to the corresponding peaks in air. This observation means that nanotubes are sensitive to molecular forces, and is interpreted by relating the corresponding molecular strain to a thermodynamic parameter, the cohesive energy density (or more loosely, the surface tension) for a range of liquids. We find that nanotubes deform by a different amount for each liquid. Calibration of this phenomenon enables the construction of a compressive stress-strain curve for carbon nanotubes.

Lamellar bone is common among primates, either in the form of extended planar circumferential arrays, or as cylindrically shaped osteons. Osteonal bone generally replaces circumferential lamellar bone with time, and it is therefore of much interest to compare the mechanical properties and fracture behavior of these two forms of lamellar bone. This is, however, difficult as natural specimens of circumferential lamellar bone large enough for standard mechanical tests are not available. We found that as a result of treatment with large doses of alendronate, the lateral sides of the diaphyses of baboon tibia contained fairly extensive regions of circumferential lamellar bone, the structure of which appears to be indistinguishable from untreated lamellar bone. Three-point bending tests were used to determine the elastic and ultimate properties of almost pure circumferential lamellar bone and osteonal bone in four different orientations relative to the tibia long axis. After taking into account the differences in porosity and extent of mineralization of the two bone types, the flexural modulus, bending strength, fracture strain and nominal work-to-fracture properties were similar for the same orientations, with some exceptions. This implies that it is the lamellar structure itself that is mainly responsible for these mechanical properties. The fracture behavior and morphologies of the fracture surfaces varied significantly with orientation in both types of bone. This is related to the microstructure of lamellar bone. Osteonal bone exhibited quite different damage-related behavior during fracture as compared to circumferential lamellar bone. Following fracture the two halves of osteonal bone remained attached whereas in circumferential lamellar bone they separated. These differences could well provide significant adaptive advantages to osteonal bone function. (C) 2000 Kluwer Academic Publishers.

Why does a bone fracture? We should be able to answer that in a nontrivial manner, but we cannot. This is an embarrassing admission in an age when treatments are available that can reverse the tendency toward bone loss and thus lessen the danger of fracture. Furthermore, if we knew much more about the relation between the material bone and its fracture properties, we might be able to optimize treatments to improve the quality of the material, or at least predict when the chance of a fracture increases, taking into account also the state of the material. We have been working on aspects of this problem for 15 years. We were recently much humbled by some observations we made,2 which illustrate just how little is still known about the relations between bone microstructure and its mechanical properties. As these results are probably of interest to many in the bone community, and especially researchers focussing on osteoporosis, we thought it would be appropriate to briefly summarize the results and highlight major open questions. [First paragraph]

High hydrostatic pressures were applied to single-wall carbon nanotubes by means of a diamond anvil cell (DAC), and micro-Raman spectroscopy was simultaneously used to monitor the pressure-induced shift of various nanotube bands. The data confirm recent results independently obtained from internal pressure experiments with various liquids, where the peak shifts were considered to arise from compressive forces imposed by the liquids on the nanotubes. It is also shown that the nanotube peak at 1580 cm^-1 (the G band) shifts linearly with pressure up to 20 000 atm and deviates from linearity at higher pressure. This deviation is found to be coincident with a drop in Raman intensity for the disorder-induced peak at 2610 cm^-1 (the overtone of the D* band), possibly corresponding to the occurrence of reversible flattening of the nanotubes. The independent results presented here confirm the potential of nanotubes as molecular sensors.

The flexural modulus and work-to-fracture properties of circumferential lamellar bone from baboon tibia are presented, based on experiments with miniature cantilever bending specimens. Data is provided for specimens in three orthogonal directions that show marked anisotropy. The advantages of such miniature specimens (about 150 μm in diameter and 2 mm long) include the possibility of sampling very small volumes within a heterogeneous structure such as osteonal bone, or studying biological materials that are not available in large enough volumes for conventional mechanical analysis.

In the process of fracture of fiber-reinforced composites, the stress concentrations caused by the redistribution of stress from a failed fiber to its intact adjacent neighboring fibers play a substantial role, as they determine the overall failure pattern of the composites. In the present article we propose a new model for the stress concentration factors (SCFs) in two-dimensional unidirectional composites. This model is based on an earlier shear-lag scheme which is here modified in several respects: a new shear-lag constant β(N) (proposed by Nayfeh) is included (which replaces the classical shear-lag constant of Cox, β(C)), and the effects of fiber/matrix debonding and fiber/matrix interfacial friction are incorporated. We find that the proposed model is in good agreement with recently published Raman spectroscopy results. When the fibers are in close contact, the SCF is found to possess a maximum value of 1.33, identical with the classical result of Hedgepeth and Van Dyke. However, unlike the scheme of Hedgepeth and Van Dyke, the SCF decreases with increasing interfiber distance, similar to more recent models, but in closer agreement than these models with the micro-Raman spectroscopy data.

We report the first observation of the formation of damage doublets in adjacent carbon nanotubes embedded in a polymer matrix. Such damage clusters are comparable to those arising in fiber-reinforced composite materials as a result of the redistribution of stress from a failed fiber to its unfailed adjacent neighbors. This observation may help shed light on fracture nucleation and growth in nanotube-based composites.

Some aspects of the fragmentation phenomenon in materials are briefly reviewed, and its various interpretations are discussed. In particular, we show that fragmentation may occur under a variety of conditions and at a wide range of scales. Depending on the fragmentation configuration and type, several types of physical properties of the material may be measured or inferred. We describe some of the implications of the recent observation of the tensile and compressive fragmentation of thin-walled carbon nanotubes in a polymeric matrix.

Single-fiber composite experiments designed to assess the stress transfer ability of the interface between a commercial epoxy resin and optical fibers have been conducted. Unconventional fragmentation patterns were observed, which demonstrate the presence of dynamic energy release processes. Such dynamic contributions, which are not easily seen (if at all) in fragmentation tests with thinner fibers (such as carbon, glass, and kevlar), may have to be accounted for in the theoretical modeling of the test.

The term bone refers to a family of materials, all of which are built up of mineralized collagen fibrils. They have highly complex structures, described in terms of up to 7 hierarchical levels of organization. These materials have evolved to fulfill a variety of mechanical functions, for which the structures are presumably fine-tuned. Matching structure to function is a challenge. Here we review the structure-mechanical relations at each of the hierarchical levels of organization, highlighting wherever possible both underlying strategies and gaps in our knowledge. The insights gained from the study of these fascinating materials are not only important biologically, but may well provide novel ideas that can be applied to the design of synthetic materials.

Well-aligned bundles of single-wall carbon nanotubes under tensile stresses were observed to fracture in real-time by transmission electron microscopy. The expansion of elliptical holes in the polymer matrix results in a tensile force in bridging nanotubes. The polymer matrix at both ends of the bundles deforms extensively under the tension force, and fracture of the nanotubes occurs in tension within the polymer hole region rather than in shear within the gripping polymer region at the ends of the bundles. This provides evidence of significant polymer-nanotube wetting and interfacial adhesion.

Experimental observations of various deformation and fracture modes under compression of single multiwalled carbon nanotubes, obtained as a result of embedment within a polymeric film, are reported. Based on a combination of experimental measurements and the theory of elastic stability, the compressive strengths of thin- and thick-walled nanotubes are found to be about 2 orders of magnitude higher than the compressive strength of any known fiber.

Unidirectional plane-parallel (UPP) platelet-or ribbon-reinforced composites have three orthogonal planes of symmetry. Their elastic behavior is characteristic of three-dimensional (3D) orthotropic materials, i.e. they possess nine independent elastic constants. Unlike the situation in unidirectional fiber composites, in which extreme behavior is observed along two orthogonal directions, the elastic constants (such as Young's modulus) along three orthogonal directions can differ significantly. An approach is proposed for the calculation of the Young's moduli of such 3D composite materials. The specific cases of unidirectional platelet- and ribbon-reinforced composites are presented as examples of particular relevance to advanced composite structures of present-day interest. While fiber-reinforced composites show lower specific strength and stiffness properties in directions perpendicular to the oriented fiber direction, UPP composites reinforced with platelets or ribbons offer superior stiffness properties in the plane of the lamina.

A theoretical model for built-in residual thermal stresses in three concentric, transversely isotropic, cylinders is presented. The scheme is an extension of earlier work by Nairn, who considered the case where only the central cylinder possesses such particular form of orthotropy, the two other (external) cylinders being isotropic. The generalization proposed here enables study of the thermal stresses in fiber-reinforced composite materials containing transversely isotropic interphases and matrices. In particular, the longitudinal thermal stress present in the fiber prior to a single-fiber fragmentation experiment is found to be compressive in nature, and in some cases is high enough to induce extensive fiber fragmentation. The number of such pre-existing breaks varies strongly as a function of the Weibull shape parameter of the fiber compressive strength distribution, and of the fiber volume fraction. Alternatively, the fiber Weibull shape parameter in compression may be predicted from fitting procedures, using experimental results that include the number of fiber breaks versus either the thermal decrement or the fiber volume fraction. General implications for high-fiber-content composites are discussed. The effect of interface thickness on the residual thermal stresses is discussed for the particular case of transcrystalline interphases, with assumed morphologies for α and β isotactic polypropylene.

A shear-lag analysis based on energy is used to predict the amount of debonding that occurs when a fiber fragment breaks into two fragments. The shear-lag analysis reproduces all features of more sophisticated analyses. A drawback of the shear-lag analysis, however, is that it depends on an unknown parameter which can be expressed in terms of an effective fiber volume fraction. If the effective fiber volume fraction can be determined (by experiments or by advanced stress analyses), the shear-lag model can be used to interpret debonding experiments.

The residual stresses in both thermosetting and thermoplastic single-fibre composites have been experimentally evaluated by means of an original technique based on the continuous monitoring of the fragmentation test performed at various temperatures. The difference between the strain at the break of a single fibre in air and one embedded in a polymeric matrix has been measured as a function of temperature. By considering the compressive fibre modulus this strain difference has been converted into fibre compressive stresses related to the matrix thermal shrinkage after curing of the samples. In fact, as the test temperature increased, the thermal compressive stresses decreased until a zero value was obtained, corresponding to a so called "stress free temperature", equal to the curing temperature for amorphous thermosetting matrix composites or equal to the matrix melting temperature for semicrystalline-thermoplastic matrix composites. The experimental results have been compared with data obtained from a theoretical model and a good agreement was found especially if the temperature dependence of the matrix Young's modulus and matrix thermal expansion coefficient are accounted for in the computation.

The effects of fiber volume fraction and transcrystallinity in single fiber composites, on the phenomenon of compressive fiber fragmentation due to residual thermal stresses, are studied. A concentric cylinder model is used, jointly with experimental data, to predict the Weibull shape parameter of the compressive strength distribution of pitch-based high and medium modulus (HM and MM) carbon fibers, with isotactic polypropylene as the semi-crystalline embedding matrix. A severe effect of the fiber content on the thermal residual stress in the fiber and. thus, on the fiber break density, is predicted and experimentally confirmed. The effect of the presence of isothermally grown polypropylene transcrystalline interlayers (using pitch-based HM carbon fibers as a substrate) on the compressive stresses induced upon subsequent quenching is investigated, both experimentally and theoretically. Cooling rate results are also presented. The thermoelastic constants of the interlayer are predicted to have a severe effect on the residual stresses generated in the fiber, the interphase, and the matrix. There is therefore, a definite need for direct experimental measurements of these constants.

A continuous fragmentation test, using thermal stresses, has been developed to determine the compressive strengths and Weibull parameters required to characterize the strength-length dependence of carbon fibres from a single test procedure. The onset of thermal stress in the fibre is determined in sit u and for amorphous systems is commensurate with the glass transition temperature of the microcomposite matrix. It is shown that the compressive strengths are considerably lower than the associated tensile strengths for all the fibres tested and that an electrochemical oxidation surface treatment and a polytetrafluoroethylene coating do not significantly affect the compressive strengths or compressive Weibull shape parameters with respect to the unmodified fibres. The mechanisms of stress transfer have been investigated and a compressive stress profile has been proposed that can determine the interfacial shear strength from fundamental scientific principles. The temperature dependence of the interfacial shear strength is investigated for carbon-fibre-polycarbonate microcomposites and the values obtained are concordant with a system that has weak interfacial bonding.

Existing models for built-in residual stresses in composite materials are reviewed and discussed. In particular, the thermal longitudinal stress present in the fiber prior to a single-fiber fragmentation experiment is studied using various model composite data. It is found that this stress is typically compressive in nature and that, quantitatively, it depends on the fiber content, the degree of undercooling, and the thermoelastic constants of the fiber and the matrix. In the case of single-fiber composites (or microcomposites), the thermal longitudinal stress present in the fiber is high enough to either induce fiber sinewave buckling (such as in E-glass/epoxy), or extensive fiber fragmentation (such as in graphite HM/polypropylene) that may then be used to measure the dependence of compressive fiber strength upon length. This has to be accounted for in quantitative models that calculate interfacial adhesion parameters using single-fiber tests, such as the fragmentation test or the microbond test. Implications for high fiber content composites (or macrocomposites) are discussed.

By coating glass fibers with the appropriate nucleating agent, transcrystallinity can be generated in polypropylene/glass composities. Transcrystallinity can consist either of the alpha (monoclinic) or beta (hexagonal) crystal structure. Through the use of directional solidification, the transcrystalline morphology can be duplicated in polypropylene films on a level large enough for mechanical and morphological study. Permanganic etching and subsequent electron microscopy reveals that lamellar orientation in alpha transcrystallinity differs significantly from the beta form. Alpha transcrystallinity consists of lamellae which are edge-on relative to the polypropylene film thickness, while beta transcrystallinity consists of lamellae which are primarily flat-on. This difference in morphology results in significant variations in mechanical properties and damage mechanisms.

Functionalized polypropenes have been synthesized to be used as coupling agents at the glass fiber/polypropene interface. The cocrystallization of this coupling agent and the matrix is studied by DSC and WAXS and in an other hand by a Monte Carlo simulation. The required characteristics of the functionalized polypropene cocrystalliize with the PP matrix are discussed. In a second part, the ability of the selected polymers to be grafted on a glass substrate is studied. The influence of the introduction of such coupling agents at the interface is studied in microcomposites.

A model has been proposed that represents the compressive fragmentation phenomenon and can evaluate the interfacial shear strength without recourse to complicated stress transfer models. The temperature dependence of the interfacial shear strength is investigated for carbon fibre-polycarbonate microcomposites and the values obtained are applicable to a system that has weak interfacial bonding.

The effect of particulate volume fraction v_p and diameter d _p on the composite Young's modulus E_c is studied both experimentally, using a silica bead/epoxy system, as well as with the help of computer simulations. The experimental and simulation results show that for a given particulate size, the overall E_c vs v_p curve displays a concave upward shape and not a linear shape. This superlinear trend of the data implies that the average strain normalized to the applied strain λ=ε̄_p/ε_c transferred to the particulates increases with volume fraction. The above finding is explained in terms of a mean-field picture, where a single particle interacts with an effective medium consisting of the remaining particles embedded in the matrix. As the modulus of the effective medium surrounding a reference particle increases with v_p, the modulus mismatch between the reference particulate and the medium is consequently reduced. This leads to an overall increase in the normalized average strain λ transferred to each particulate as v_p is increased. The experimental results using silica particulates with various sizes d_p, as well as the simulation results, show that smaller particulates provide an increased composite modulus as compared to larger particulates, at constant v_p. General equations are developed, which relate the composite modulus to the average particle stress or strain, given only information about the volume fraction and the Young's modulus of each of the phases present. Through the application of these relations, it is found that smaller particulates display a greater amount of normalized average strain λ transferred than larger particulates. The effect of particulate Young's modulus E_p in combination with particulate size on the resulting E_c is also studied using simulations only. It is found that for a low particulate to matrix modulus ratio E_p/E_m, the particulate size has very little influence on E_c. Moreover, the shape of the E_c vs v_p curve can be well approximated by a straight line up to large values of v_p. On the other hand, as the ratio E_p/E_m is increased, the superlinear trend of the composite modulus E_c vs v_p data is more apparent. This results in a smaller range of the E_c vs v _p curve, which can be approximated by a linear function. It is also found that the extent of this linear region also decreases with particle size.

Transcrystalline microstructures are normally not observed at the interface between E-glass fibers and an isotactic polypropylene matrix, unless mechanical translation is applied to the fiber while it is in the supercooled polymer melt. We demonstrate here that transcrystallinity can form at the surface of E-glass fibers if appropriate nucleating agents are used to coat the fibers. These agents can nucleate either the α (monoclinic) or β (hexagonal) crystal forms of polypropylene. Single-fiber composite experiments were performed to assess the effect of transcrystallinity on matrix deformation. The preliminary results presented here reveal the occurrence of a previously unreported damage mechanism by which interlamellar fractures form preferentially at the interface well before any bulk matrix damage occurs. The density of this damage zone is higher in transcrystallinity of the β crystal form than of the α form, although it was found that in the α form the damage can propagate into the matrix. The occurrence of this damage mechanism suggests that toughness increases may potentially be obtained by careful design of the interfacial transcrystalline region in E-glass/polypropylene composites.

A round-robin programme has been undertaken to assess the compatibility in the micromechanical techniques used to evaluate the interfacial shear strength of the fibre/matrix bond in composite materials. The tests selected for evaluation were the single-fibre pull-out test, the microdebond test, the fragmentation test and the micro-indentation test. Twelve laboratories were invited to participate in this programme. Each laboratory was supplied with Caurtaulds XA fibre in the untreated condition and with a standard surface treatment, and a quantity of epoxy resin, hardener and catalyst, all from the same batch. Some laboratories were supplied with composite bars made with the same materials. A common cure cycle was chosen for sample preparation. Each laboratory conducted the tests to its own procedures. The results showed that the scatter within each laboratory was acceptable but the scatter between laboratories for a particular test was high. The results are discussed and possible explanations are presented for these observations. The indications are that the fundamental procedures used in each laboratory are sound. The results also suggest that there is great potential for achieving standard procedures and reducing the inter-laboratory scatter. A further round-robin programme is proposed to generate test protocols.

Experimental data are presented for the fiber/matrix interfacial adhesion in unsized and sized E-glass/epoxy microcomposites. A comparison of the results obtained by the microbond and the single-fiber composite tests is presented and discussed. The adequacy of specific micromechanical adhesion tests for a given fiber/matrix system is discussed. It is conjectured that the “true” value of the adhesion strength falls in-between the results from the microbond and the single-fiber composite tests.

This study deals with the effect of a transcrystalline LLDPE (linear low-density polyethylene) layer grown on Spectra 1000 UHMWPE (ultrahigh molecular weight polyethylene) fibres. Chemical similarity between the fibre and the surrounding melt does not promote transcrystallinity as no transcrystalline microstructure appears from the surface of as-received Spectra 1000 UHMWPE fibres. However, oxygen plasma treatment of the UHMWPE fibres yields a degree of surface roughness that appears to promote easy nucleation and growth of LLDPE transcrystallinity. The kinetics of transcrystalline growth were investigated quantitatively. The growth rate increased by a factor of about 12 for a 10°C increase in supercooling, and at 105°C the maximum observed thickness of the transcrystalline layer was about one fibre diameter. The induction time was found to decrease as the crystallization isotherm increased. We discuss the possibility of using surface energy parameters to define a better criterion for the nucleation of transcrystallinity from the UHMWPE fibre substrate. Preliminary data were generated for the interfacial mechanical shear strength by means of the microbond test. It is conjectured that the combined effects of a thermal treatment and the presence/absence of a transcrystalline layer might produce significant changes in the interfacial shear strength, as illustrated here by a 43% increase observed with specimens subjected to different thermal treatments.

The work presented in the present paper focuses on the interface in Kevlar 49-epoxy composites. Experimental results for the interfacial shear strength obtained using the microbond test by means of different loading configurations (parallel plate loading, parallel conical plate loading, circular loading), are presented and compared. Contrasting with recent finite-element predictions proposed in the literature, the interfacial shear strength is found to be altogether insensitive to the type of microbond loading configuration. A comparison of the results obtained using two micromechanical tests (microbond and fragmentation) is performed. The interfacial shear strength results obtained by means of the fragmentation test are found to be higher by a factor of about 50% than those obtained by means of the microbond test. A possible explanation for this difference is proposed and discussed, and the value of the "true" interfacial shear strength is conjectured to fall between the values measured by these two tests. The effect of fibre surface chemistry modification (surface desizing) is probed by surface-sensitive techniques (XPS-ESCA and contact angle measurements from droplets) and by micromechanical testing techniques. Surface-sensitive techniques and micromechanical testing provide compatible information for the Kevlar-epoxy system studied here, and the knowledge of the chemical characteristics of the fibre surface can therefore be used as a means of predicting the interfacial shear strength.

A recent study of bone structure shows that the plate-shaped carbonate apatite crystals in individual lamellae are arranged in layers across the lamellae, and that the orientation of these layers are different in alternate lamellae. Based on these findings, a new micromechanical model for the Young's modulus of bone is proposed, which accounts for the anisotropy and geometrical characteristics of the material. The model incorporates the platelet-like geometry of the basic reinforcing unit, the presence of alternating thin and thick lamellae, and the orientations of the crystal platelets in the lamellae. The thin and thick lamellae are modeled as orthotropic composite layers made up of thin rectangular apatite platelets within a collagen matrix, and classical orthotropic elasticity theory is used to calculate the Young's modulus of the lamellae. Bone is viewed as an assembly of such orthotropic lamellae bent into cylindrical structures, and having a constant, alternating angle between successive lamellae. The micromechanical model employs a modified rule-of-mixtures to account for the two types of lamellae. The model provides a curve similar to the published experimental data on the angular dependence of Young's modulus, including a local maximum at an angle between 0 and 90°. A rigorous testing of the model awaits additional experimental data.

Carbon/J-polymer single fibre composite samples were tested under tensile conditions with the fibre direction perpendicular to the tensile loading axis. The Poisson ratio effect induced a compression strain field in the fibre, resulting in a fragmentation phenomenon similar to that observed in a fibre subjected to tensile loading. This observation introduces a novel technique for the measurement of the compressive strength of single fibres, calculated either from the stress at first break, or from the Weibull scale parameter obtained from the fragmentation data produced at various stress levels. The special sample loading configuration used here also provides the first measurement of the effect of the length of the fibre on its compressive strength value.

This paper demonstrates the usefulness of careful experimental work with model composite materials, such as thin polymeric films in which single fibers are accurately positioned, in at least two respects: to assess the validity of a theory for a given physical property, and to accurately probe the effects of various parameters on the behavior of composites. Working with such model composites has obvious advantages, such as the full control of experimental parameters, the possibility of introducing perturbative effects in a controlled way, and the possibility of verifying theoretical models in the range of low fiber content. Indeed, macroscopic composite materials contain various types of defects and perturbative effects, such as fiber misalignment or slack, fiber‐poor regions, voids, etc., which bias any quantitative assessment of mechanical and physical properties, and preclude the accurate verification of theoretical schemes. One difficulty in working with microcomposite models, also recalled here, is the need for an appropriate “scaling‐up” procedure to the level of macroscopic composites.

A brief reply to the Comment is provided. Both the procedure I suggested and the tables presented by Carroll [Textile Res. J. 47, 56 (1977)] allow easy and fast estimation of contact angles.

The characterization of the physicochemical nature of interfaces is a key problem in the field of advanced fibrous composites. The macroscopic regime contact angle, which reflects the energetics of wetting at the solid-liquid interface, is difficult to measure by usual methods in the case of very thin cylindrical fibers, but it may be calculated from the shape of a liquid droplet spread onto a cylindrical monofilament using a method developed by Yamaki and Katayama [J. Appl. Polym. Sci. 19, 2897 (1975)], and B. J. Carroll [J. Coll. Interf. Sci. 57, 488 (1976)]. Unfortunately, measurements of the contact angle based on this method are, so far, unable to provide an accuracy of better than about 5°. In the present article two simple extensions of the method of Yamaki and Katayama and Carroll, are presented, from which highly accurate values of the contact angle may be obtained. This is demonstrated experimentally from the spreading of glycerol droplets on carbon fibers and epoxy droplets on aramid fibers.

This work presents preliminary investigations of the fatigue behaviour of hybrid composite materials in flexure by three methods; examination of the hybrid effects of S—N data as a function of the fibre types and structure of the material; study of the rate-dependent flexural behaviour and its correlation with the fatigue life; and by incremental loading tests to simulate fatigue. Aramid fibre/carbon fibre hybrids with aramid skin and carbon core exhibited a positive hybrid effect on flexural strength. Their properties were superior to those with carbon fibre skin and aramid fibre core. A possible explanation for the positive effect in the ACA hybrids may lie in the strain rate dependence of these materials. The reasons for the high strain rate dependence are discussed.

First results are presented concerning the elastic and ultimate mechanical behaviour of p(MMA) bone cement reinforced with as-received and surface-modified Spectra 900 polyethylene fibres. Even though the surface chemistry and reactivity of the fibres was modified, the surface oxidation and surface grafting treatments of the polyethylene fibres apparently did not significantly affect the mechanical properties of the polyethylene-reinforced p(MMA) bone cement or improve the interfacial bonding. This may be attributed to the rather unfavourable area-to-volume ratio of PE fibres for such treatments, as well as to the necessarily low content of PE fibres in the bone cement which does not allow a clear differentiation between the various samples.

Some recent developments in the study of fibre and composite material fracture are reviewed and analyzed using statistical and probabilistic concepts. Short-term strength and long-term lifetime are considered and an attempt is made to show how statistical kinetics may provide some physical basis for the molecular fracture of polymeric fibres. Transversal and longitudinal size effects in the mechanical strength of various types of highly oriented fibres typically used in composites are thoroughly discussed and a comparison of fracture mechanics and probabilistic approaches is performed. Some currently unsolved problems inherent to the Weibull distribution as a model for strength are discussed and, using maximum likelihood estimation techniques, conjectures are made regarding the relative roles of defects distributed over the fibre boundary and in the bulk. Theoretical and experimental results regarding the fracture of unidirectional fibrous composites are reviewed, and, in particular, the importance of the fibre bundle theory is emphasized. Problems for future research are suggested.

Using polymeric fibre strength data from the literature and from our own laboratory tests, we show in the first part of the present paper that simple modifications of Poisson/Weibull concepts, resulting in a new failure probability function, can be used as a modeling scheme for the study of diameter effects on strength in a way at least as satisfactory as previously used LEFM-based schemes. In the second part of the paper a new experimental approach for the study of composite failure is presented. Specially prepared composite monolayer models were tested in simple tension under an optical microscope equipped with crossed polarizers and with a video camera. The potential usefulness of this approach for the characterisation of basic failure modes in fibre-reinforced composites, and as a probe of existing strength theories in such materials, is demonstrated.

Heterophase or composite materials possessing a fibrillar structure arise frequently in nature as well as in a wealth of today's man-made objects and structures. The degree of adhesion between the fibrous component and the support matrix is known to be an important factor regarding the mechanical response of the material. Well-bonded highly fibrous heterophase materials possess improved elastic properties, as compared to the matrix, which are modeled by various theoretical treatments. For less frequently used weakly bonded composite materials, the theories are inadequate. Here we present a simple, approximate theoretical scheme for the case of low fibre content ( ≦ 20 pc by volume) composites with a weak interface among the constituents. Such composites are sometimes utilized for the fixation of prostheses in joint surgery. The proposed theoretical scheme appears to be appropriate as confirmed by experimental data.

A study of the fracture behaviour of Kevlar 29 reinforced dental cement is undertaken using both linear elastic and nonlinear elastic fracture mechanics techniques. Results from both approaches-of which the nonlinear elastic is believed to be more appropriate-indicate that a reinforcing effect is obtained for the fracture toughness even at very low fibre content. The flexural strength and modulus are apparently not improved, however, by the incorporation of Kevlar 29 fibres in the PMMA cement, probably because of the presence of voids, the poor fibre/matrix interfacial bonding and unsatisfying cement mixing practice. When compared to other PMMA composite cements, the present system appears to be probably more effective than carbon/PMMA, for example, in terms of fracture toughness. More experimental and analytical work is needed so as to optimize the mechanical properties with respect to structural parameters and cement preparation technique.

The flexural modulus of graphite/glass-reinforced hybrids exhibits deviations from the rule-of-mixtures base-line. The deviation increases as the segregation of the glass and graphite layers increases, and it reaches a maximum when the two fibre types are arranged in 3 layers. When the stiffer fibres (graphite) are in the outer layers the deviation is positive but it is negative with the less stiff fibres (glass) on the outside. The extent of deviation is shown to be predictable by the analysis.