STRENGTH CHARACTERISTICS AND FATIGUE-CRACK GROWTH IN A COMPOSITE WITHLONG ALIGNED FIBERS

Strength values and fatigue crack growth on a specially made composite material are reported. The composite specimen consisted of an epoxy m atrix and one layer of long aligned glass fibers that were equally spa ced. The results on strength showed that for a range of fiber spacing lambda, the composite's strength sigma(c), scaled with a fiber spacing in the form of sigma(c) root lambda = kappa. Based on dimensional arg uments the constant kappa was found proportional to the fracture tough ness of the matrix. Fatigue crack propagation experiments were perform ed on specimens with different fiber spacing and applied loads. The cr ack speed reached a steady mode of propagation in specimens where rela tion sigma(c) root lambda = kappa was satisfied. The same mode of prop agation was reached for the debonding along fibers in the bridging zon e as well as the crack opening displacement. The crack opening displac ement at a fiber location and the corresponding debonding were linearl y related. Within the resolution of the observations, no fiber fractur e was seen in the bridging zone. Using a standard Green's function, st ress intensity factor simulations were carried out for different types of tractions on the fibers in the bridging zone. When the fibers in t he bridging zone were under a uniform load, the total stress intensity factor K-t, at the crack tip, was found constant at the steady state and proportional to sigma(infinity)root lambda. Assuming that K-t is c onstant during steady crack growth, the results of the simulations wer e used to correlate steady crack speed in three sets of data. Dimensio nal analysis of the steady crack speed was carried out as an attempt t o identify important parameters and the role of the fiber spacing in t he fracture of the composite specimens. The steady crack speeds were c orrelated with the total stress intensity factor for each fiber spacin g. The resulting exponents were found to be about 20 percent different . Assuming that at steady state the energy release rate for an interfa cial crack is proportional tot t(2)r, where t is the stress carried by a fiber [20], a power expression for the rate of debonding with t roo t r was found to have an exponent approximately equal to that for the steady crack speed.