Fatigue strength of PAN-based carbon fiber under cyclic loading

Abstract

Load-controlled fatigue tests were carried out on PAN-based monofilament carbon fibers under cyclic tensile load conditions at a frequency of 10Hz. Fatigue strength data are obtained under pre-determined maximum load Pmax with three different stress ratios of A (= Pmix/Pmax) = 0.1, 0.5 and 0.7. The maximum load Pmax is not an appropriate parameter to evaluate the fatigue strength behavior because the cross-sectional area of monofilament carbon fiber is not constant and varies along the longitudinal direction. This fact results in a large scatter of fatigue lives when the data are plotted on Pmax-Nf diagram. In order to evaluate the fatigue strength behavior more precisely, the fatigue strength data must be plotted on S-N diagram by using the maximum tensile stress σmax determined from the cross-sectional area of the fracture surface. It is found that S-N properties of monofilament carbon fibers clearly show the fatigue behavior, depending on the stress ratio R. It is also found that the fatigue strength of monofilament carbon fiber is governed by two parameters such as the maximum stress σmax and the stress amplitude σa, and that the combined stress parameter σmax(1-α)×σaα is useful to describe the fatigue strength behaviors of the different stress ratios.