Statistics for the strength and lifetime in creep-rupture of model carbon/epoxy composites

Abstract

A theoretical model and experimental data are presented for the strength and lifetime in creep-rupture of unidirectional, carbon fiber/epoxy matrix microcomposites at ambient conditions. The model ‘microcomposites’ consisted of seven parallel carbon fibers (Hercules IM-6) embedded in an epoxy matrix (Dow DER 331 epoxy/No. 26 hardener) and forming an approximately hexagonal array. The results are interpreted by means of the model which involves Weibull distributions for fiber strength, micromechanical stress-redistribution, and power-law, matrix creep around noncatastrophic fiber breaks from which the creep-rupture originates. For the microcomposites, the model yields approximate Weibull strength and lifetime distributions with parameters depending on the various model parameters. Also obtained is a power-law relationship between stress level and lifetime whose exponent depends on the Weibull shape parameter for fiber strength, the creep exponent for the matrix, and the critical cluster size for failed fibers in the microcomposite. The experimental results agree quite well with theoretical predictions though time-dependent debonding appeared to be part of the failure process; this debonding was observed in independent experiments.