Nonlinear viscoelastic, viscoplastic characterization of unidirectional GF/EP composite

Viscoplastic strains of unidirectional continuous fiber composite (HEXCEL GF/EP prepreg system) are studied experimentally and theoretically. Creep and strain recovery tests are used. Schapery's nonlinear viscoelastic viscoplastic constitutive equations are used and generalized to describe inelastic behavior of unidirectional composite under isothermal creep and strain recovery conditions. The methodology to quantify the viscoplastic strains with respect to applied stress is proposed. Viscoplastic strains of composite are described by plastic shear strain in material symmetry axis. Assumptions has been used and validated that the function describing the stress and time dependence of viscoplastic strain can be presented as a product of two, time and correspondingly stress dependent, master curves.