Micromechanical analysis of the off-axis rate-dependent inelastic behavior of unidirectional AS4/PEEK at high temperature

The rate-dependent nonlinear stress–strain behavior of a unidirectional carbon fiber-reinforced composite AS4/PEEK at high temperature has been examined under displacement- and load-controlled conditions, together with the effects of unloading on the off-axis response. The monotonic tensile stress–strain curves at high temperature show marked nonlinearity for the off-axis angles in the range 10°<θ<60°, regardless of loading rate and control mode. A permanent strain clearly appears after unloading for all off-axis angles except 0°. The hysteretic response to consecutive unloading and reloading is of the Masing type, and the width of the hysteresis loop depends on off-axis angle. The off-axis flow stress in the subsequent nonlinear regime, as well as the initial slope of the stress–strain relationship, depends on loading rate. These observations reveal that the off-axis deformation of the unidirectional AS4/PEEK shows viscoelastic and viscoplastic natures. To predict the off-axis rate-dependent nonlinear stress–strain behavior of AS4/PEEK, a simple micromechanics model has been developed using the Aboudiʹs method of cells for composite homogenization and the Chaboche model for description of the rate-dependent inelastic behavior of the PEEK matrix. The micromechanics model has favorably reproduced the rate-dependence and nonlinearity of the off-axis tensile behavior of AS4/PEEK at high temperature.