Small fatigue crack propagation in notched components under combined torsional and axial loading

Abstract

The present paper deals with two specific subjects related to the fatigue strength and life of notched bars under combined torsional and axial loading. The first subject is the fatigue thresholds of materials with small defects. The fatigue threshold of materials with small defects or sharp notches is not controlled by the initiation of fatigue cracks, but by their propagation. The R-curve method is very useful to predict the fatigue thresholds of notched components. Small cracks nucleated at the notch root becomes nonpropagating when the applied stress intensity factor drops below the resistance of the material. It is important that the R curve is independent of loading conditions and only the applied stress intensity factor depends on loading conditions. In the present paper, the R-curve method was successfully applied to predict the fatigue thresholds of holed tubes made of carbon steels under in-phase and outof-phase combinations of cyclic torsion and axial loading.The second subject is an anomalous phenomenon of notch strengthening found in torsional fatigue of circumferentially notched round bars of austenitic stainless steel. In torsional fatigue of circumferentially notched bars of austenitic stainless steel, the fatigue life of notched bars was found to be longer than that of smooth bars and to increase with increasing stress concentration under the same amplitude of the nominal torsional stress. On the basis of the electrical potential monitoring of the initiation and propagation of small cracks at the notch root, the crack initiation life decreased with increasing stress concentration, while the crack propagation life increased. The anomalous behavior of the notch-strengthening effect was ascribed to the larger retardation of fatigue crack propagation by crack surface contact for the cases of sharper notches. The superposition of static tension reduced the retardation due to the smaller amount of crack surface contact, which gave rise well-known notch-weakening of the fatigue strength. The crack initiation life is controlled not by the peak strain amplitude at the notch root, but by the strain amplitude about 0.1mm distant from the notch root where the strain distribution was calculated by the elasticplastic finite element method using cyclic stress-strain relation. The propagation rate of cracks can be predicted using the J-integral range when the static tension is superposed on cyclic torsion. The shielding by crack face sliding contact greatly reduced the crack propagation rate under cyclic torsion without static tension.