In this work, the initiation of the first failure event in unidirectional polymer composites subjected to different loading conditions is studied. Two energy based point failure criteria – critical dilatational and critical distortional energy densities – are considered. Local stress fields are calculated by finite element models using micromechanical simulations to evaluate damage initiation in the matrix by a brittle failure mechanism (cavitation) and by an inelastic (yielding) process.
The disorder in the fiber distribution induced by manufacturing is quantified by defining the degree of nonuniformity (NU) of fiber distribution in the composite cross section. An algorithm to create simulations of the nonuniform distribution of fibers at different overall fiber volume fractions (FVFs) and different degrees of nonuniformity is developed. Representative volume elements (RVEs), with their minimum size determined on the basis of statistical analysis of interfiber distances are established for different fiber volume fractions.
Under tension applied normal to the fibers, brittle cavitation is found to occur before yielding in the matrix. This first failure event is assumed to induce local debonding of the fiber/matrix interface. Effects of variables such as the degree of nonuniformity and the fiber volume fraction, as well as the ratio of matrix to fiber stiffness modulus, on the initiation of brittle cavitation are studied.
Combined loading consisting of transverse tension and axial shear applied to unidirectional composites is studied next. It is found that under certain axial shear/transverse tensile stress ratios brittle cavitation requirements are fulfilled. When only axial shear is imposed, cavitation requirements are not satisfied. Instead, matrix yielding is found to occur.
A parametric study of the matrix/fiber stiffness ratio shows that the mechanical strain to onset of cavitation under transverse tension increases as this ratio increases and approaches a constant value at high ratios under given fiber volume fractions and degree of nonuniformity. Also, under these conditions and under axial shear, the mechanical strain at which yielding initiates is found to increase with increasing the matrix/fiber stiffness ratio.
A limited study of the effects of matrix voids was also conducted and the preliminary results showed that the presence of voids affects the initiation of brittle cavitation by altering the local stress fields near the fiber/matrix interfaces
