Fatigue crack growth analysis in PEEK

Abstract

Experimental results on fatigue crack propagation in polyether-etherketone (PEEK) under different load levels, specimen thicknesses, and environmental temperatures reported in Refs 1, 2 are analysed. Energy release rates evaluated from load displacement curves in the ‘ductile’ phase of crack growth are nearly equal to J 1 = σ y δ (where σ y is the yield stress of the material and δ is the crack tip opening displacement). Thus the material around the crack tip behaves as an elastic-perfectly plastic material under plane stress. When the kinetic data are plotted against J 1 the crack speed is practically independent of the load level, specimen thickness, and of environmental temperatures up to 75 °C. Both cycle- and time-dependent crack growth are observed when the environmental temperature is 100 °C. The effects of stress rate are investigated by keeping the stress levels of the fatigue cycle the same and changing the frequency. Values of the stress rate σ dot are obtained from the formula σ dot = 2ν ( σ max − σ min), where ν is the frequency and σ max, σ min are the maximum and minimum stresses of the fatigue cycle. The results show that when the kinetic data are plotted as Δl/ Δt against the crack length, an increase in crack speed is observed with stress rate. If the same data are treated as Δl/ ΔN against the crack length a decrease in Δl/ ΔN with test frequency is seen. Crack growth data under different thicknesses and temperatures are correlated with a power relation and the kinetic equation of a crack layer. The present analysis indicates that both a power relation and the kinetic equation of the crack layer model describe well crack growth data under a wide spectrum of loading conditions.