FATIGUE-CRACK GROWTH ANALYSIS IN PEEK

Experimental results on fatigue crack propagation in polyether-etherke tone (PEEK) under different load levels, specimen thicknesses, and env ironmental temperatures reported in Refs 1, 2 are analysed. Energy rel ease rates evaluated from load displacement curves in the 'ductile' ph ase of crack growth are nearly equal to J1 = sigma(y)delta (where sigm a(y) is the yield stress of the material and delta is the crack tip op ening displacement). Thus the material around the crack tip behaves as an elastic-perfectly plastic material under plane stress. When the ki netic data are plotted against J1 the crack speed is practically indep endent of the load level, specimen thickness, and of environmental tem peratures up to 75-degrees-C. Both cycle- and time-dependent crack gro wth are observed when the environmental temperature is 100-degrees-C. The effects of stress rate are investigated by keeping the stress leve ls of the fatigue cycle the same and changing the frequency. Values of the stress rate sigma are obtained from the formula sigma = 2nu (sigm a(max) - sigma(min) where nu is the frequency and sigma(max), sigma(mi n) are the maximum and minimum stresses of the fatigue cycle. The resu lts show that when the kinetic data are plotted as DELTAl/DELTAt again st the crack length, an increase in crack speed is observed with stres s rate. If the same data are treated as DELTAl/DELTAN against the crac k length a decrease in DELTAl/DELTAN with test frequency is seen. Crac k growth data under different thicknesses and temperatures are correla ted with a power relation and the kinetic equation of a crack layer. T he present analysis indicates that both a power relation and the kinet ic equation of the crack layer model describe well crack growth data u nder a wide spectrum of loading conditions.