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Fatigue may be defined as the reduction in mechanical properties during continued cyclic loading. In these experiments, a tensile sample is stressed to a pre-defined limit and released to zero tension repeatedly at a given frequency using a square waveform. After a certain number of cycles, samples undergo either brittle failure or plastic deformation. The failure mechanism is often dependent on the extent of localized heating that occurs within the sample during testing and has been shown to vary with frequency.
Figure 13 shows the maximum number of cycles that natural PEEK polymer materials can withstand under fatigue stress at ambient temperatures.
Figure 13 - Fatigue Stress Versus Cycles to Failure for PEEK Polymer Materials at 73°F (23°C) at 5-20 Hz
Figure 13 clearly shows that the excellent fatigue resistance of 450G is enhanced by both glass and carbon fiber reinforcement. Independent studies have shown that these compounds feature the optimum level of reinforcement for improved fatigue and mechanical performance.
The fatigue performance of composite materials is a function of both the fibers (aspect ratio, sizing) and the mechanical properties of the matrix. Figure 14 compares the fatigue performance of PEEK polymer with other engineering thermoplastics containing the same amount and type of glass fibers.
Figure 14 - Fatigue Stress Versus Cycles to Failure for Various High Performance Engineering Thermoplastic Matrices Containing the Same Reinforcement
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