The effects of stress level on steady crack growth in an epoxy reinforced with long aligned glass fibers

Abstract Results are reported of fatigue crack propagation experiments on an epoxy reinforced with long aligned glass fibers. The composite was prepared in such a way that fiber spacing was approximately the same in each specimen and the reinforcing fibers were sufficiently stronger than matrix. For a number of experiments with the same fiber spacing, fatigued under various stress levels, the crack speed as well as the rate of debonding reached steady values, i.e., independent of the crack length. In addition, the evolution of debonding followed a self‐similar growth pattern. The data implied that the applied load was a controlling parameter of the steady growth. Within the resolution of observations, no fiber fracture was observed in the bridging zone. Fiber debonding seemed to be the dominant mechanism of energy dissipation. Moreover, the crack front was not straight. Instead, it consisted of two branches growing on the specimen's surfaces first and then through the thickness of the specimen with a highly curved front. Stress intensity factor calculations showed that when the fibers in the bridging zone were under a uniform load, the total stress intensity factor was proportional to \documentclass{article}\pagestyle{empty}\begin{document}$ \sigma \sqrt \lambda $\end{document} (where σ is the applied stress and λ is the fiber spacing) and constant within the regime of steady crack growth. The steady values of crack growth and rate of debonding were correlated with the stress level, spacing, and fiber radius. The resulting power equations were found to have the same exponent.