Short-beam shear fatigue behavior of round curved pultruded composite

Short-beam shear test has been used to evaluate the fatigue strength of round curved pultruded carbon fiber - epoxy matrix composite specimens in three point bending test of a convex-beam. The specimens were cyclically loaded at peak stresses from 50 to 80% of the quasi-static short-beam strength, and load ratios of 0.1 to 0.5, and damage evolution was monitored using the apparent stiffness method. Basquins' law was used to fit the experimental S-N data. The results showed that the fatigue strength of curved pultruded composites is lower than for conventional thin flat unidirectional laminates, and less sensitive to load-ratio effects. Residual strength of pre-fatigued specimens was determined by quasi-static short-beam tests, and it sometimes exceeded the pristine material performance due to fatigue-induced strengthening phenomenon. Stress redistribution and residual manufacturing-induced stress relief due to the previous cyclic loading were found the main responsible for mitigating damage growth under quasi-static loading. Fractographic survey indicated that stress redistribution and stress relief governed the strengthening process under high-cycle and low-cycle fatigue regime, respectively.

Automated summary

Fatigue strength of curved pultruded composites is lower than conventional thin flat unidirectional laminates, but sometimes the residual strength of pre-fatigued specimens exceeds the pristine material performance due to fatigue-induced strengthening phenomenon. Residual manufacturing-induced stress relief and stress redistribution play key roles in mitigating damage growth under quasi-static loading. Stress redistribution and stress relief govern the strengthening process under high-cycle and low-cycle fatigue regime, respectively.