J. Botsis et al., THE EFFECTS OF STRESS LEVEL ON STEADY CRACK-GROWTH IN AN EPOXY REINFORCED WITH LONG ALIGNED GLASS-FIBERS, Polymer composites, 16(1), 1995, pp. 29-37
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.