Effect of Compressive Part of Cyclic Stress Range on Crack Propagation Behavior

Abstract

It has been considered that the crack propagation rate is controlled by the tensile part of stress range but not by the compressive part. However, according to the experimental results carried out by the present authors for a plane bending stress condition, the compressive part of stress range affected the crack propagation behavior and the accelerated crack propagation rate was observed, causing shorter fatigue life than that without a compressive part.
To clarify the effect of the compressive stress range of cyclic amplitude on the crack propagation, push-pull fatigue tests with a constant stress amplitude two step multi-fold stresses were carried out for plane sheet specimens (CCT) of low carbon steel (the ultimate tensile strength is 517MPa). The crack opening stress level was measured by the unloading elastic compliance method. The conclusions obtained are summarized as follows:
(1) When the crack propagation rate was high, the compressive part of stress range accelerated the propagation, due to the lower crack opening stress level.
(2) When the compressive part of stress range, that is the absolute value of minimum stress in cyclic stress range, increased beyond a certain value, the acceleration of crack propagation rate appeared. The compressive part of stress range larger than this value showed the same acceleration effect on the crack propagation behavior.
(3) The acceleration effect of the compressive part of stress range on the crack propagation rate was observed in the tests where the stress ratio R₁ in one step was 0 and R₂ in another step was -0.33.
(4) When the crack propagation rate was low, the comppressive part of cyclic stress range decelerated the crack propagation rate because of the oxide induced crack closure.