Nonlinear multiaxial behavior and failure of fiber-reinforced composites

The objective of this study was to examine the nonlinear, three-dimensional behavior of a fiber-reinforced polymer. The material that was considered f or this study was the carbon fiber composite, AS4/PEEK, graphite fibers in a semi-crystalline thermoplastic matrix. Arcan specimens were chosen for th is study because the specimens are compact and biaxial stress states can be easily obtained. Strains were measured using geometric moire, and specimen s were subjected to shear and biaxial loading up to failure. The nonlinear behavior as examined in both the longitudinal and transverse planes of the composite. The effects of shear on the axial stress-strain behavior were ob tained, as well as the effects of axial stress on the shear stress-strain b ehavior. A one-parameter plasticity model was used to characterize the obtained nonl inear stress-strain curves. The original model was unable to characterize t he nonlinear behavior. The effect of two-dimensional mean stress on the she er yield surface was then added to the: model and a significant improvement was achieved. The work potential model with only one internal state variab le was also used for characterizing the nonlinear behavior. It could not pr edict the observed difference of the nonlinear behavior of the composite un der shear plus tension and shear plus compression.