Composite failure under combined compression and shear

The effect of a superimposed shear stress on the axial compressive strength of aligned fiber composites is investigated through a combination of experiments and analysis. Experiments were conducted on flat coupons using a custom biaxial testing facility. Shear and compression were found to interact strongly. The interaction failure envelope follows approximately a linear trend along the line joining the critical stress at zero shear on one axis and the shear strength on the other. The 2-D and 3-D micromechanical models used previously to predict the compressive strength are modified to include the shear. In the models the composite has a sinusoidal imperfection which is uniform across the microsection width. The imperfection characteristics are chosen so that the calculated critical stress at zero shear corresponds to the measured strength. The models are shown to capture well the interaction between shear and compression. Calculated failure envelopes are in good agreement with the experimental results. For all combinations of shear and compression considered, deformation localizes into a narrow band of highly bent fibers after the critical state. The band initially is normal to the axial load but broadens and rotates as the solution is followed deeper into the postfailure regime as it did for pure compression. Important aspects of proper testing and modeling are discussed, and recommendations are given for design including a critical review of a simpler model in which the fibers do not deform.