A contribution to mode III fatigue crack propagation

Abstract Fatigue crack growth behaviour under mode III (antiplane shear) reversed loading (R = −1; loading frequency, 1 Hz) is reported for AISI 4340 steel specimens of 24.5 mm diameter and with circumferential notches. Measurements with ΔKIII = constant showed that fatigue cracks grow in a macroscopically flat (mode III) fracture mode at nominal ΔKIII values between 30 and 60 MPa m 1 2 and that the crack growth rates decrease monotonically with increasing crack length. The reason for this is the friction between the fracture surfaces which reduces the stress intensity at the crack tip with increasing crack length to a smaller effective value. At a crack growth rate of approximately 5 × 10−5mm cycle−1 the fracture mode changes from macroscopically flat (mode III) to a “factory roof” (mode I) type consisting of fracture surfaces inclined at 45°. This changes in the fracture mode may be considered as the “threshold value” for mode III crack propagation as smaller crack growth rates with mode III fracture mode obviously cannot be obtained. The mode III fatigue crack growth curve, which is obtained by an extrapolation procedure that eliminates the influence of friction (the nominal ΔKIII equals the effective ΔKIII), is higher at ΔKIII values between 30 and 60 MPa m 1 2 than the corresponding mode I curve. The mode III curve is also steeper than the corresponding mode I curve. Thus the mode III curve approaches the mode I curve at smaller ΔKIII values and eventually becomes lower. Mode III loading generates mode I branch cracks. If mode I crack propagation predominates during mode III loading (at low loads), mode I branch cracks grow quicker than mode III cracks and a factory roof fracture is formed. At high mode III loads, however, mode III crack propagation prevails and macroscopically flat fracture surfaces develop. At ΔCTDIII values of 5 × 10−3 − 3 × 10−2mm the crack growth rates are about 0.04–0.4 times the ΔCTDIII values.