Thermoelastic behavior of carbon fiber/polycarbonate model composites

Model composites of polycarbonate (PC) containing single, multiple and chop ped carbon fibers (CF) with and without and epoxy sizing were prepared by h ot pressing. The thermoelastic behavior of model CF/PC composites was chara cterized by stretching calorimetry at room temperature. For small strains a (i.e., epsilon approximate to 0.01) the specific mechanical work, specific heat effects and specific internal energy changes Delta U were completely reversible in stretching/contraction cycles and quantitatively obeyed the s tandard relationships for elastic solids. Youngs moduli E and Delta U were significantly higher, whereas the linear thermal expansivities alpha(L) wer e lower for model CF/PC composites compared to those for the neat PC. Small er values of the above parameters for composites reinforced with sized CF s uggested weaker CF/PC interfacial interactions. Current theoretical models of thermoelastic properties of composite materials suggest the existence of unusually stiff, highly oriented PC structures in fairly thick boundary la yers around CF. The onset of inelastic deformation, as well as mechanical f ailure in CF/PC model composites at significantly smaller strains compared to the neat PC were tentatively explained by the yield and subsequent plast ic now of the matrix polymer initiated by heat effects of fiber fragmentati on processes, and by higher concentration of microvoids generated in fiber fragmentation/debonding events, respectively.