Mechanism of crack path morphology and branching from small fatigue cracks under mixed loading

Abstract

ABSTRACTIn order to investigate the crack path in materials containing a small crack under mixed mode loading, reversed torsion and combined push–pull/torsion fatigue tests were carried out on 0.47% carbon steel specimens containing an initial small crack. The initial small cracks were introduced by a preliminary push–pull fatigue test using a specimen that contained an artificial small hole of 40 μm diameter/depth. Push–pull fatigue tests followed by reversed torsion, and reversed torsion followed by push–pull were carried out. Fatigue tests of combined push–pull/torsion followed by push–pull were also carried out to examine the effect of crack geometry, such as branching and kinking, on cumulative fatigue damage. Different crack growth behaviours due to different loading modes and sequences complicatedly influenced the fatigue crack path and eventually the cumulative fatigue damage. Thus, existing fatigue damage theories cannot be applied to the cases presented in this study. A single factory roof morphology was formed on the fracture surface of the specimen having an initial semielliptical surface crack under cyclic torsion. Taking the result into consideration, torsional fatigue tests of circumferentially cracked specimens were carried out to investigate the Mode III crack growth threshold and the formation mechanism of the factory roof. It was found that the factory‐roof pattern was formed by Mode I crack branching from small semielliptical cracks nucleated ahead of the circumferential crack tip. Thus, stress intensity factors at the semielliptical crack tip ahead of the circumferential crack tip were analysed to make the mechanism of crack branching from the semielliptical crack clear. The analysis shows that the Mode III stress intensity factor, KIII, at the deepest point of the semielliptical crack decreases with increase in the depth of semielliptical crack. It follows that as the shear crack growth becomes difficult at the semielliptical crack tip, crack branching starts at the major axis ends in the direction for Mode I growth, forming factory roof, due to ΔKIth < ΔKIIIth.