Three-dimensional nonlinear constitutive model for composites

A three-dimensional nonlinear material model for fiber reinforced polymers is developed. The constitutive behavior is formulated based on mechanisms to allow for a minimum of material parameters. As a result, the model is able to capture complex nonlinear deformations under arbitrary loading conditions. Mechanisms like fiber reorientation induced by deformation and the hydrostatic sensitivity of the resin are considered. The constitutive model includes two independent non-associative flow rules to describe yielding due to transverse and longitudinal shear loading, respectively. In order to verify the capabilities of the model, transverse and off-axis compression tests superimposed with various hydrostatic pressures are simulated. The parameters controlling the hydrostatic sensitivity on the material response are discussed. Analyzes of angle-ply tension tests under atmospheric conditions show, that considering hydrostatic sensitivity is not relevant to simulate their response. Neglecting hydrostatic sensitivity results in a different characterization of material parameters, which compensate its influence. Uniaxial tests with plane stress states are not suitable to derive the hydrostatic sensitivity parameters. Therefore, tests under high hydrostatic pressure are mandatory.