Abstract
The mechanisms of crack growth of pre-cracked materials under cyclic torsion with static axial load were investigated using 4340 steel, 1050 aluminum and polycarbonate. In the case of 4340 steel, branching of the main crack was observed. The length of the crack between the first branching points was dependent on the loading conditions, and it was found that friction between the crack surfaces prevented shear mode crack growth. Micro-cracks, which initiated in front of the main crack, were a factor in branching of the main crack. The initiation of micro-cracks was also observed in 1050 aluminum, though the main crack did not branch. The main crack in 1050 aluminum propagated by shear mode in all stages of crack propagation. It is well known that the crack growth behavior under cyclic load is related to the slip systems of the materials. To confirm the effect of the crystal structure on the crack growth mechanisms, the crack propagation behavior of a polycarbonate specimen, which is a non-crystalline material, was also investigated. From these observations, it was found that shear mode crack propagation is influenced by not only the crystal structure, but also the friction between crack surfaces.
