On the failure and post-failure of fiber composites in compression

The failure and post-failure behavior of an AS4/PEEK composite in axial compression was studied through experiments and analysis. Compressive stresses cause shearing of the matrix in regions of fiber misaligned. Increasing stresses yield the matrix, resulting in fiber collapse and failure of the composite. The strength is strongly imperfection sensitive. After failure, deformation localizes into an inclined region of rotated fibers called a kink band.; Compressive failure was investigated using 2-D and 3-D finite element models and three simpler models. When properly calibrated, their strength predictions are quite similar, especially for larger fiber misalignments. However, the FE models allow examination of post-failure events leading to kink band formation. The rate dependence of compressive strength was examined experimentally and with a modified version of a simple model.; The addition of a far-field shear stress significantly reduces the compressive strength. Failure predictions from modified versions of the 2-D model and the simple models agree well with experimental data. Post-failure events are very similar to those observed for pure compression loading, except that failure can occur in a controlled manner for certain loading paths.; It has been discovered that the kink bands formed during compressive failure can propagate axially in a steady-state manner at a constant stress level termed the propagation stress. This propagation stress, a new characteristic stress of the material, is approximately 40% of the compressive strength for AS4/PEEK. Steadystate propagation can give “ductility” to a material normally considered quite brittle. The propagation has been observed directly, revealing the mechanism of propagation as well as new details about fiber rotation in the band. A 3-D numerical model was developed which simulates the steady-state propagation. The propagation stress and deformation are predicted reasonably well.; As a basis for modeling of failure and post-failure, the non-linear rate-dependent behavior of the composite was investigated experimentally using a custom-built test fixture. Transverse compression and shear tests at various loading rates show the composite is rate-dependent at room temperature. Thirteen biaxial tests were conducted along three distinct loading paths. Significant path dependence was observed, as was unusual yielding behavior.