Fiber orientation dependence of continuous carbon/epoxy composites nonlinear viscoelastic behavior

The effect of fiber orientation on the nonlinear viscoelastic response of a unidirectional fiber polymeric system is examined. Based on Schapery’s nonlinear viscoelastic equation and a model for the prediction of the nonlinear parameters, the nonlinear behavior of carbon/epoxy composite off-axis specimens is studied in order to achieve a complete characterization of an orthotropic system. The stress dependence of the parameters of the nonlinearity, introduced in the model is achieved and applied through a generic function developed and already confirmed [Compos. Sci. Technol. 58 (6) (1998) 883; Composites Part A 30 (7) (1999) 839; Progress in Durability Analysis in Composites Systems Conference Proceedings, DURACOSYS 97, Blacksburg, Virginia, USA, 14–17 September, 1997, Balkema Publishers, pp. 271–278; Compos. Sci. Technol. 59 (9) (1999) 1311] in the isotropic case. Verification of the model in the case of orthotropy is performed through a series of creep tests in composite specimens at various fiber orientations. The estimated values indicate an increase in the nonlinearity as the applied stress increases, while satisfactory predictions of the nonlinear parameters can be achieved, using the proposed model.