Observation and modeling of loading–unloading hysteresis behavior of unidirectional composites in compression

Abstract

The loading–unloading hysteresis behavior of unidirectional HTS40/PA6 carbon/polyamide laminates under off-axis compression has been examined both experimentally and constitutively in the present study. Monotonic compressive tests are first performed on unidirectional laminates to identify the basic properties of off-axis deformation. Then, five cycles of compressive loading and unloading tests are carried out with a gradually increasing peak stress for each specimen, followed by testing to failure. Significant nonlinear unloading behavior is observed after the material has been loaded into the nonlinear deformation region, and apparent plastic strain is seen after full unloading. The reloading curve also exhibits nonlinearity and conduces to a slightly open hysteresis loop with unloading. An approach is developed to predict the nonlinear hysteresis behavior of unidirectional composites by assuming an anelastic strain component in the nonlinear compressive deformation. The assumption of anelastic strain is a modification of Sun–Chen’s one-parameter plasticity model that considers the nonlinear deformation completely as plastic strain from monotonic tests. The modified model has been validated by the off-axis loading–unloading test results on unidirectional laminates. Results verify that the model accurately captures not only the plastic strain but also the complex nonlinear hysteresis behavior in the observed off-axis loading–unloading stress–strain curves.