Fatigue fracture surface analysis of PAN based carbon fiber

Abstract

Fatigue tests of carbon fiber made from polyacrylonitrile (PAN) were performed to clear S-N properties and discuss the relationships between the fatigue fracture toughness and the pattern of fatigue fracture surface. The fatigue tests were carried out at a frequency of 10Hz with three stress ratios of 0.1, 0.5 and 0.7. It was found that S-N properties of carbon fibers clearly showed the fatigue behavior, depending on the stress ratio R. The fatigue lives of carbon fiber showed a large scatter. Mirror-mist-hackle patterns were clearly observed on the fracture surface of specimen tested under cyclic loading. As outside length on mirror region decreased, the form of the mirror region in each stress ratio changed from a semicircle to a half-ellipse. Relations between K max or AK calculated from their area and fatigue life can be approximated in a straight line. Fatigue crack growth behavior is governed by two parameters such K max and ΔK, and combined parameter K max(1-α) × ΔK α is useful to describe fracture toughness at the different stress ratio. © 2007 The Society of Materials Science.