Nonlinear viscoelastic behavior of rubber-toughened carbon and glass/epoxycomposites

A previously-developed constitutive equation for nonlinear viscoelastic mat erials is first used to characterize mathematically the ply-level stress-st rain behavior of a rubber-toughened carbon/epoxy composite. Emphasized are some practical aspects of carrying out such a characterization. Constant st ress rate tests are used to derive first-loading material response in the d ry state, at room temperature. It is found for a large portion of the loadi ng curve that the shear and transverse compliances can be described by a qu asi-elastic model expressed in terms of a single scalar function of stress state, a ratio of compliances, a time or rate exponent, and two elastic ter ms. This model offers a simple way to incorporate nonlinearity and time dep endence in a material model for limited loading conditions. Results are the n compared to those for a glass/epoxy composite with the same rubber-toughe ned resin system. Special considerations for using off-axis coupons are exa mined and unique tabbing is employed to derive material properties out to h igh stress Levels. The consistency cf the carbon/epoxy characterization is checked by comparing experimental stress-strain response to results not use d in the characterization. Behavior of the two composite systems is compare d and checked for consistency using a micromechanical model.