Mode III and Mode I fatigue crack propagation behaviour under torsional loading

Fatigue crack growth rates have been measured under cyclic torsional loading (R = −1, 1 Hz loading frequency) in AISI 4340 steel tempered at 650° C, with circumferentiallynotched specimens (12.7 mm specimen diameter). The Mode III fatigue crack growth curve for macroscopically flat fracture surface — being obtained by an extrapolation procedure which eliminates the “Mode III crack closure” influence — has higher crack growth rates and shows a greater slope than Mode I results in the stress intensity range of ΔKIIIeff = 18 to 50 MPa m1/2. In the range of ΔKIIIeff < 18 MPa m1/2, the fracture surface has “factory roof” morphology (Mode I). The difference between fatigue crack growth behaviour in Mode III and Mode I as well as mechanisms that can lead to a fracture mode change are discussed. A comparison of the Mode III crack closure influence for specimen diameters of 12.7 mm (this study) and of 24.5 mm (an earlier study) shows that this influence is not only dependent on the depth of the crack and applied torque but also on the specimen diameter. The extrapolated crack growth rates show good agreement with measurements for various specimen diameters.