Nonlinear Behavior on Fatigue Life Curves of Super Engineering Plastics for Machines at Elevated Temperature

The purpose of this study is to investigate the relationship between nonlinear behavior of fatigue life curves and fatigue life for carbon-fiber-reinforced PEEK and PA 66 under two-step loading above Tg (Glass transition temperature) and below Tg. As a results, the fatigue life curves in the case of PEEK/Below Tg, PA 66/Above Tg and PA 66/Below Ty were found linear. Moreover, the fatigue lives were calculated by linear cumulative damage law under two step loading. On the other hand, the fatigue life curves in the case of PEEK/Above Ty was found nonlinear. Moreover, the fatigue lives were not calculated by linear cumulative damage law under two-step loading. It was found that these factors occurred because of 'Transition phenomenon' which fatigue fracture mechanisms transferred in the range between two fatigue life curves.