Mean strain effects on cyclic deformation and fatigue behavior of polyether ether ketone (PEEK)

The fatigue behavior of polyether ether ketone (PEEK) thermoplastic was investigated under uniaxial strain-controlled cyclic loading condition with various mean strains (R-epsilon = 0, 0.2, and 0.25). The mean stress responses of PEEK from all mean strain fatigue tests were found to be fully-relaxed. As a result, minimum mean strain effect on fatigue behavior of PEEK was observed. Fractography analyses utilizing a scanning electron microscope were performed on PEEK specimens under tensile mean strain cyclic loading. Two distinct stages of fatigue, crack initiation and physically small crack growth, were observed on all the fracture surfaces. Moreover, two types of inclusions, unmelted particles and void-like defects, were identified to be the cause of crack initiation. Void-like defects were found to have more detrimental effect on fatigue life of PEEK when compared to particles of similar size. The effect of inclusions on fatigue behavior of PEEK was observed to be more significant than that of the tensile mean strain. (C) 2016 Elsevier Ltd. All rights reserved.