The mode III fatigue crack growth threshold for mild steel

摘要: Fatigue cracks grow perpendicular to the maximum principal applied tensile stress, or put more precisely into fracture mechanics terms, in mode I. Like most generalizations this one has its exceptions, but it does mean [I] that fatigue crack growth data can conveniently be analyzed terms of range I stress intensity factor, AK A threshold value AKI, AKIt must exceeded before a will propagate. Various techniques used determine ; simple approach is define limit cracked specimens. Conventional specimens behaviour have initial oriented stress. In practice components fail by from crack-like flaws are not necessarily at right angles and general plane crack. Definition extends naturally such combined situations; for example, AKIIIt may defined as critical , shear factor necessary cause with leads failure, even though It recently been found [2] thresold mild steel pure II loadings depends on two separate factors: firstly, whether branch forms near precrack tip under loading being applied, secondly, sufficient continued branch. If easily, already present, controlled some reason formation suppressed, higher apparent values obtained. follows that, understanding Ill behaviour, III required. Two dimensional cases involving small extensively discussed, [3]. The problem difficult because three nature. Approximate solutions branched problems case obtained [3] solving an application prior traction branch, merely ensuring "extended branch" (Fig. I) free. Application method gives KIII (branch) = [main), independent angle @. For element 2), K [branch) (main), geometrical incompatabilities prevent assembly these elements Consideration various possible configurations suggests [4]