Crack growth mechanism in precracked torsional fatigue specimens

The mechanists of crack growth of precracked materials was investigated under cyclic torsion loading with and without axial static stress using 4340 steel. During the crack growth, branching of the crack was observed. The length of the crack between the first branching points was dependent on loading conditions. This length was longer when the applied shear amplitude and the static axial stress level were higher. When the crack tips were opened by the tension loads, the crack had the tendency to grow in a shear mode during cyclic torsion, It was found that friction of the crack surfaces prevented shear mode crack growth. Furthermore, at higher stresses the initiation of new microcracks was observed in front of the main crack and their density was dependent on loading conditions. This helped the crack grow in a shear mode before and after branching.