Nonlinear viscoelastic behavior of paper fiber composites

Anisotropic material model for composites made from phenol-formaldehyde impregnated paper is developed starting from Schapery's nonlinear viscoelastic and nonlinear viscoplastic constitutive law derived from thermodynamics. Methodology for determination of the included stress-dependent functions has been developed and used to determine properties of two composites in two directions related to material symmetry. Creep with following strain recovery tests have been performed at several levels of stress. Data reduction is based on two-step load application methodology to include effect of finite time of load increase to the plateau value and its removal. No stiffness reducing micro damage was detected and the plastic deformations were considered negligible or accounted for in a simplified manner. The material model was used to simulate a constant stress rate loading and unloading test. Model is validated comparing simulations with results from test.