Abstract
In order to characterize small mode III crack growth torsional fatigue tests were done using circumferentially notched prefatigue-cracked specimens. The change of the shape of hysteresis loop during crack growth was monitored in terms of two wire strain gauges mounted on the surface perpendicularly across the notch. Then, the change of compliance during fatigue crack growth was evaluated in terms of the hysteresis loop. Regarding to characteristic growth behavior of the small mode III fatigue crack, the small crack first grew in the coaxial plane. Then it gradually changed growth plane out of that coaxial plane. Finally the crack was propagated in the direction 45° inclined to the cross sectional plane. That means, the change of three dimensional crack plane was found to correspond to so-called factory roof pattern, common at the first stage of fatigue crack growth from the notch root in torsional fatigue condition. The fatigue crack growth rate varied in accordance with that mode change of small crack growth. In addition, the crack growth rate was considerably affected by the friction force acting on the crack surface. Increasing crack length in mode III crack growth condition decelerated the crack growth, which reflected increase in friction force. The change of compliance during crack growth was found suitable for estimating the effect of the friction force on crack growth rate. In lower ΔKeff region, the friction force was successfully characterized using a parameter of compliance. In case of large scale yielding condition, ΔJeff parameter including the effect of friction force-generated increase of torque was found to be a suitable parameter to estimate crack growth rate. The friction force acting on the unit area of crack plane was constant, irrespective of crack length and loading condition.
