Fatigue Behavior and Modeling of Polyether Ether Ketone under Variable Amplitude and Multiaxial Loading
Type of DegreePhD Dissertation
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Experimental and analytical roadmap to thoroughly understand and establish benchmark material properties for unfilled polyether ether ketone (PEEK) under monotonic tension and compression, as well as uniaxial and multiaxial fatigue loading conditions along with the obtained results are documented in the present dissertation. A systematic experimental program was designed to study the effects of various loading characteristics, including strain amplitude, test frequency (i.e. cyclic strain rate), control modes (i.e. strain-controlled versus force-controlled), self-heating, mean stress/strains, pre-loadings, and several multiaxial load paths. Knowledge gained from investigating the cyclic deformation behavior was then used to determine a suitable fatigue life model capable of correlating fatigue life of the PEEK polymer under various types of loading conditions. Monotonic tension and compression tests revealed that the quasi-static tensile and compressive properties of PEEK polymers were significantly affected by the applied strain rate. Similarly, under fatigue loading, the applied test frequency not only affected the rise in temperature on the gage section of the specimen, but it also had an impact on the resulting fatigue life. Cyclic deformation behavior represented by the stress responses under strain-controlled loading revealed significant cyclic softening under fully-reversed loadings, while stress relaxation was evident under tensile mean strain loadings. Variations in loading condition were also seen to affect the resulting deformation response, which was represented by the hysteresis loops. Under variable amplitude block loading tests, the beneficial effect of load history was observed to be significant and irrespective to the loading sequence. Characterizing PEEK polymer under more realistic multiaxial loading revealed that the fatigue lives were dependent on the applied loading path, among which the non-proportional loading was observed to be the most detrimental compared to proportional and uniaxial loading. It was also realized that due to the complex failure mechanisms in polymeric materials, which were significantly different to those in the metallic materials, traditional fatigue life models were not able to accurately correlate the fatigue data of PEEK thermoplastic. Furthermore, reasonably well correlations of fatigue data under different types of uniaxial and multiaxial loading conditions were obtained using an energy based model that was capable of capturing the effect of cyclic deformation on the fatigue behavior.