The objective of this study was to examine the nonlinear, three-dimensional behavior of a fiber-reinforced polymer. The material that was considered for this study was the carbon fiber composite, AS4/PEEK, graphite fibers in a semi-crystalline thermoplastic matrix. Arcan specimens were chosen for this study because the specimens are compact and biaxial stress states can be easily obtained. Strains were measured using geometric moiré, and specimens were subjected to shear and biaxial loading up to failure. The nonlinear behavior was examined in both the longitudinal and transverse planes of the composite. The effects of shear on the axial stress-strain behavior were obtained, as well as the effects of axial stress on the shear stress-strain behavior.
A one-parameter plasticity model was used to characterize the obtained nonlinear stress-strain curves. The original model was unable to characterize the nonlinear behavior. The effect of two-dimensional mean stress on the shear yield surface was then added to the model and a significant improvement was achieved. The work potential model with only one internal state variable was also used for characterizing the nonlinear behavior. It could not predict the observed difference of the nonlinear behavior of the composite under shear plus tension and shear plus compression.