MATRIX EFFECTS ON LIFETIME STATISTICS FOR CARBON-FIBER EPOXY MICROCOMPOSITES IN CREEP-RUPTURE

Experimental results are presented for the strength and lifetime in creep rupture of carbon-epoxy microcomposites consisting of seven carbon fibres (Hercules IM6) within an epoxy matrix (Dow DER 332 epoxy with Texaco Jeffamine T403 curing agent) in an approximately hexagonal configuration. Special attention was paid to clamping, specimen alignment, shock isolation and accurate lifetime measurement. The results were analysed using a previously developed model, which involves a Weibull distribution for fibre strength and micromechanical stress redistribution around fibre breaks where the matrix creeps in shear following a power law. The model yields Weibull distributions for both microcomposite strength and lifetime where the respective shape and scale parameters depend on model parameters such as the Weibull shape parameter for fibre strength, the exponent for matrix creep, and the effective load transfer length and critical cluster size for failed fibres. Experimental results were consistent with the theory, though a fractographic study suggested time-dependent debonding along the fibre-matrix interface as being a key mechanism. Arguments were given to suggest, however, that the overall analytical forms would essentially be preserved. The results were compared with earlier results using a different epoxy system (Dow DER 331 epoxy with DEH 26 curing agent). Values for the matrix creep exponent and the effective load transfer length were about double and triple respectively the values from the earlier study, leading to slightly reduced strength, about one-half the variability in lifetime, but almost one-half the value of the exponent for the power law relating microcomposite lifetime to stress level.