To investigate the nonlinear viscoelastic response of E-glass fiber reinforced epoxy resin matrix composite

Schapery's model was used to describe the nonlinear viscoelasticity of composites

and the tensile creep/creep-recovery tests were carried out for [90]16 transverse specimens and [±45]4S shear specimens at different stress levels. According to the observed residual phenomena

a viscoplastic strain component was introduced into the Schapery's model and a method based on the analytical procedure for identifying the nonlinear viscoelastic parameters was proposed. The Schapery's nonlinear parameters g0

g1

g2 and aσ were calculated

which are functions of the applied stress. The results show that the E-glass/epoxy composite exhibits obvious nonlinear viscoelasticity if the applied stress is higher than 53% of the tensile strength in transverse direction and 31% of the shear strength in the in-plan shear direction. In the transverse direction

the nonlinear parameters g0T and g2T are independent of stress

while g1T increases linearly with stress

and aσT decreases linearly with stress. In the in-plane shear direction

the nonlinear parameters g0S and g2S increase exponentially with stress

while g1S increases linearly with stress

and aσS decreases linearly with stress. Compared with the traditional nonlinear parameter identification method

the proposed method can obtain the relationship between nonlinear parameters and the applied stress conveniently and accurately.