FATIGUE FRACTURE-BEHAVIOR OF PEEK .1. EFFECTS OF LOAD LEVEL

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-degrees 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 = delta-sigma-y (sigma-y is the yield stress of the material and delta 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 (= delta-sigma-y), the crack speed is practically independent of the stress level.