INTERFACIAL ADHESION IN MICRO-COMPOSITES:
EFFECTS OF TRANSCRYSTALLINITY AND THERMAL STRESSES

Thesis for the degree of

Master of Science

by

Andras Gati

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Table of Contents Table of Figures List of References

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Abstract

The main motivation behind the present study is to resolve the existing controversy in the literature regarding the effect transcrystalline interphases may have on the macroscopic properties of semi-crystalline based composites. The examination of the effects of transcrystallinity also lead us to the investigation and utilisation of compressive fiber fragmentation due thermal residual stresses.

In the first part of this research we focus on the effect a transcrystalline interphase in semi-crystalline based composite materials may have on the efficiency of stress transfer to the fibers. Using a reliably large set of experimental data with carefully prepared poly (caprolactone) (PCL) microdroplets spread on polyaramid (Kevlar 149) fibers, we demonstrate that the presence of a transcrystalline phase does not significantly influence the level of fiber-matrix adhesion. Evidence for the absence and presence of such a transcrystalline layer is provided by optical microscopy of microtomed droplet cross sections.

In the second part, the effects of fiber volume fraction, substrate, and transcrystallinity in single fiber composites, on 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. Thermoelastic constants of the interlayer are predicted to have a severe effect on the residual stresses generated in the fiber, interphase, and the matrix. There is, therefore, a definite need for direct experimental measurements of these constants.