Compressive fiber fragmentation in carbon/polypropylene composites:: Effects of residual thermal stresses and transcrystallinity

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.