Load history and sequence effects on cyclic deformation and fatigue behavior of a thermoplastic polymer

The effect of load history and sequence on the cyclic deformation and fatigue behavior of polyether ether ketone (PEEK) thermoplastic was experimentally investigated. Various uniaxial strain-controlled multi-block fatigue tests were conducted, including (1) fully-reversed (Rε = −1) two-, three- and four-block loadings with adjusted frequency to maintain a nominal temperature rise on the specimen gage surface, (2) fully-reversed two-block loading to study the frequency effect, and (3) pulsating tension (Rε = 0) two-block loading to investigate the combine effect of pre-loading and mean stress/strain on cyclic deformation and fatigue behavior. For all fatigue tests under fully-reversed (Rε = −1) multi-block loading, cyclic stability of PEEK specimens was achieved at approximately the same reversal and with similar stress amplitudes as those without pre-loading under identical test conditions. Additionally, for tests with Rε = −1, pre-loading was found to have a significant beneficial effect on PEEK fatigue resistance, irrespective of the load sequence (i.e. high-low or low-high). However, no obvious load history and sequence effect on fatigue behavior of PEEK was observed for pulsating tension (Rε = 0) block loading tests. Experimental results also revealed that increasing test frequency generally leads to higher fatigue lives.