Description of Fatigue Crack Growth under Modes II, III and II+III in Terms of J-integral

AbstractMode II and mode III fatigue crack growth rates and the related intrinsic stress intensity factor range thresholds ΔKIIeff,th and ΔKIIIeff,th were for a long time expected to be equal. However, there are many theoretical and experimental indications of a slower crack growth rate and a higher threshold for mode III than that for mode II for various metallic materials. In this article, a description of shear-mode fatigue crack growth for specimens made of ARMCO iron in terms of a diagram da/dN vs. ΔJ is presented and compared with that based on ΔK. For higher applied loads the description in terms of ΔK is no more valid which turned out from a precise assessment of plastic zone size for a wide range of shear-mode loading levels in these specimens. This numerical elasto-plastic analysis revealed that the small-scale yielding conditions were only fulfilled in the near-threshold region and, for higher loading levels, the ΔJ approach should be utilized. The measured curves plotted for loading modes II, III and II+III showed higher effective mode II threshold than that of mode III but approximately equal crack growth rates for both modes II and III. The mode II cracks in metals usually propagate faster than the mode III ones because the “factory-roof” morphology created under mode III loading produces a high friction, thus reducing the effective crack driving force significantly. The unusual behavior of ARMCO iron can be explained here by a coplanar crack propagation without forming the factory roof morphology