A Basic Study on the Construction of Fatigue Design for Carbon-Fiber-Reinforced PEEK at Elevated Temperature. The Problem on Estimation of Fatigue Reliability by Miner's Law at Elevated Temperature above Glass Transition Temperature.

Abstract

The complex deterioration of stress and heat on fatigue reliability was investigated on carbonfiber-reinforced PEEK and PA 66. In particular, this study was focused on the estimation of fatigue life by Miner's law under two-step loading above glass transition temperature. The main results were as follows. (1) Under two-step loading, the damage of PEEK materials could not accumulate, that is, the estimation of fatigue life by Miner's law was not applied to PEEK materials. On the other side, the damage of PA 66 materials could accumulate and the estimation of fatigue life by Miner's law was applied to PA 66 materials. (2) From fractography using SEM (Scanning Electron Microscope) and temperature rising, it was recognized that PEEK materials were more remarkably influenced of stress history of first-stress than PA 66 materials. (3) The mesocrack on surface of specimen of PEEK materials was small compared with that of PA 66 materials. Therefore it could be understood that the estimation of fatigue life by Miner's law was not applied to PEEK materials because of existence of stress relaxation by initiation of minute mesocrack during the fatigue process.