Residual strength prediction for unidirectional CFRP using a nonlinear viscoelastic constitutive equation considering entropy damage

We propose a viscoelastic/plastic constitutive equation for viscoelastic media based on irreversible thermodynamics and the viscoelastic theory. Uniaxial tensile tests on a TriA-X polyimide resin are performed at different temperatures and strain rates to determine the material parameters for the constitutive equation. In the numerical analysis, the FORTRAN programming language is used to apply the proposed equation to the matrix resin. Transverse tensile failure analysis is performed on the unidirectional carbon fiber reinforced plastic (CFRP) in constant strain rate condition after holding constant strain. In this condition, the failure strength is lower than the static tensile strength. The same tendency is observed in the three-point bending fracture test of the unidirectional CFRP. The proposed model expresses the accumulation of resin damage while holding a constant strain and predicts the residual strength and lifetime of the unidirectional CFRP.