Time-temperature dependence of tensile strength of unidirectional CFRP

We have observed in our previous works [Miyano, Y., Nakada, M., Kudoh, H. and Muki, R. (1999). Prediction of Tensile Fatigue Life under Temperature Environment for Unidirectional CFRP, Advanced Composite Materials, 8: 235-246; Miyano, Y., Nakada, M., Kudoh, H. and Muki, R. (1999). Determination of Tensile Fatigue Life of Unidirectional CFRP Specimen by Strand Tenting, Mechanics of Time-Dependent Materials, 4: 127-137] that the tensile strength along the longitudinal direction of unidirectional carbon fiber reinforced plastics (CFRP) is highly time-temperature dependent. In this paper, we propose a prediction scheme of the master curve for the tensile strength of CFRP under constant strain rate (CSR) loading; we extend Rosen's analysis [Rosen, B.W. (1964). Tensile Failure of Fibrous Composites, AIAA J., 2: 1985-1994] for tensile failure of unidirectional fibrous composites with elastic matrix to CFRP with viscoelastic matrix by making two modifications. First, we replace the ineffective length with the recovery length over which the interfacial shear stress is uniform. Second, we substitute the value of shear relaxation modulus at the time of failure for shear modulus in Rosen's formula after the first modification. The shear relaxation modulus of polymer in the CFRP was estimated from the tensile storage modulus of CFRP obtained from the dynamic double cantilever beam under various frequencies and temperatures. The tensile test for unidirectional CFRP was carried out at various loading rates and temperatures; a smooth curve was drawn to represent inherently scattered strength data for each temperature. The master curve for CFRP strength together with the shift factors was obtained. The predicted master curve for tensile strength using an assumed value 7.5 for Weibull shape parameter of fiber strength captured the test data for CFRP (T400 and Epikote 828) adequately for small to medium time but poorly for large time.