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 J1 = σyδ (where σy 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 J1 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 σdot are obtained from the formula σdot = 2ν (σmax − σmin), where ν is the frequency and σmax, σmin 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.

Experimental results on fatigue crack propagation in PEEK under different load levels, specimen thickness, and environmental temperatures reported in References [1] and [2] are analyzed. When the kinetic data is plotted against J1 = σv δ (where σv is the yield stress of the material and δ is the crack tip opening displacement), the crack speed is practically independent of the load level, specimen thickness, and environmental tempera tures up to 75°C. The experimental data is confronted with a power type kinetic equation and the kinetic equation of the crack layer model. The analysis indicates that both kinetic equations describe well the data under a wide spectrum of loading conditions.

Experimental results on the effects of specimen thickness and environmental temperatures on fatigue fracture behaviour of poly(ether ether ketone) (PEEK) are reported. Low cycle fatigue experiments are conducted on injection moulded single-edge notched specimens of 1.57, 2.70 and 5.42 mm in thickness at ambient temperatures, and on specimens 2.70 mm thick at environmental temperatures of 39, 50, 63, 75 and 100°C. In all the thickness experiments and in the experiments with temperatures of 39 and 50°C, the crack tip profile is initially round. At long crack lengths the crack tip profile changes to a triangular shape. When the test temperature is 63, 75 and 100°C, the crack tip remains round throughout the fracture process. The crack tip angle is primarily dependent upon the test temperature. Examinations of the fracture surfaces and transverse sections indicate that in the thickest specimen, relatively rough fracture surfaces are observed and a few discontinuities (crazes or cracks) underneath the main crack path. Thus, crack propagates in a ‘brittle’ manner. In all other experiments both ‘brittle’ and ‘ductile’ modes of fracture are observed. The point of transition from ‘brittle’ to ‘ductile’ fracture is dependent upon the specimen thickness and test temperature. Fatigue striations are seen throughout the fracture surfaces. Correlation of the striations and the number of cycles indicates a one-cycle crack growth mode. Hysteretic losses during fatigue crack growth are negligible until a few cycles prior to unstable fracture. Crack opening displacements are independent of the specimen thickness and increase with rise in temperature. When crack growth rates are correlated with the elastic energy release rate, they are independent of specimen thickness and increase with increase in temperature.

Results of experimental studies on fatigue fracture behaviour of PEEK are reported. Experiments are conducted on injection moulded single edge notched specimens of 2.68 mm in thickness. At low crack speeds the crack tip is relatively round. At relatively higher crack speeds, the crack tip geometry changes to a triangular shape with an angle of 90° which remains constant until close to unstable fracture. Optical microscopy observations of fracture surfaces and of transverse sections show that at relatively short crack lengths growth occurs in a ‘brittle’ manner. Subsequently necking or lateral contraction of the specimen around the crack tip is observed and is associated with a ‘ductile’ mode of growth. The point of transition from ‘brittle’ to ‘ductile’ fracture is dependent upon the level of the stress. Fatigue striations are observed throughout the fracture surface. Correlation of the striations and the number of cycles indicates a one cycle crack growth mode. The critical crack length is assumed to correspond to the point of minimum thinning which is shortly after the last striation. Hysteretic losses during fatigue crack growth are negligible until a few cycles prior to unstable fracture. Energy release rates evaluated from load displacement curves are practically equal to J1 = δσy (σy is the yield stress of the material and σ is the crack opening displacement). The ‘brittle’ phase of crack propagation is well described by linear elastic fracture mechanics parameters. For the entire phase of propagation and for the same level of J1 (= δσy), the crack speed is practically independent of the stress level.