Effect of High Tensile Pre-Strain on the Torsional Fatigue Properties of a Structural Steel

This study is developed to investigate the effect of monotonic plastic deformation on the torsional fatigue properties of a structural steel. Five different kinds of tensile pre-strain, 2%, 5%, 8%, 12% and 22%, were applied to the specimens, respectively. And the maximum pre-strain value is near to the necking strain of the test material. The effects of tensile pre-strain on surface hardness, fatigue crack initiation and propagation behaviors, and the behavior of non-propagating cracks. The main results obtained are: The fatigue limits are 145, 160,175, 200 and 215MPa for specimens with tensile pre-strain of 2% 5%, 8%, 12% and 22%, which are improved to 104%, 114%, 125%, 143% and 153% of the fatigue limit for non-pre-strained specimens, respectively. The torsional fatigue limit increases with the tensile pre-strain increasing, until the pre-strain value being near to the necking strain ratio. However, the fatigue limit increase becomes more slowly for high tensile pre-strained specimens than the lower ones. The fatigue cracks of the tensile pre-strained specimens initiated earlier than that of the non-pre-strained specimens, and the propagation is also accelerated, but there is no effect on the fatigue crack initiation point and the branch point. Non-propagating crack length becomes shorter with increasing of tensile pre-strain until the value near the necking strain, and the quantity of non-propagating crack increases at the same time.