THE EFFECTS OF STRESS LEVEL ON STEADY CRACK-GROWTH IN AN EPOXY REINFORCED WITH LONG ALIGNED GLASS-FIBERS

Results are reported of fatigue crack propagation experiments on an ep oxy reinforced with long aligned glass fibers. The composite was prepa red in such a way that fiber spacing was approximately the same in eac h specimen and the reinforcing fibers were sufficiently stronger than the matrix For a number of experiments with the same fiber spacing, fa tigued under various stress levels, the crack speed as well as the rat e of debonding reached steady values, i.e., independent of the crack l ength. In addition, the evolution of debonding followed a self-similar growth pattern. The data implied that the applied load was a controll ing parameter of the steady growth. Within the resolution of observati ons, no fiber fracture was observed in the bridging zone. Fiber debond ing seemed to be the dominant mechanism of energy dissipation. Moreove r, the crack front was not straight. Instead, it consisted of two bran ches growing on the specimen's surfaces first and then through the thi ckness of the specimen with a highly curved front. Stress intensity fa ctor calculations showed that when the fibers in the bridging zone wer e under a uniform load, the total stress intensity factor was proporti onal to sigma root lambda (where sigma is the applied stress and lambd a 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 resul ting power equations were found to have the same exponent.