Micromechanics based modeling of nonlinear viscoplastic response of unidirectional composite

Schaperyʹs nonlinear viscoelastic, viscoplastic material model is used to characterize the inelastic response of glass fiber epoxy composite (Vicotex NVE 913/28%/192/EC9756 300MM produced by HEXCEL Composites Ltd.). Since the viscoelastic response is found to be linear, the nonlinearity is included only in viscoplastic terms. The micromechanical Hashinʹs Concentric Cylinder Assembly model in viscoelastic formulation is used to predict the viscoplastic behavior of the composite using matrix viscoelasticity. The time and stress dependence of viscoplastic strains is obtained from off-axis creep tests as difference between experimental measured creep strains and predicted linear viscoelastic strains. Viscoplastic shear strain in material symmetry axes in identified as the main source of viscoplasticity. Two master curves are determined that describe the viscoplastic shear strain dependence on time and stress correspondingly. The developed material model is validated in creep tests on [0n90m]s and [±45n]s specimens.