Local thermal effect on the growth rate of fatigue crack

The retardation of fatigue crack caused by crack tip local heating was studied. Steady fatigue crack growth under constant amplitude loading would be disturbed by the sudden change in plastic zone size resulting from a single overload. However, mechanical properties of metal, such as yielding stress, Young's modulus, etc., may vary with increasing temperature. From this consideration, the fatigue crack tip plastic zone size can be altered by locally changing the mechanical properties of this area. In order to understand the thermal effect on the retardation of fatigue crack, constant amplitude fatigue propagation tests were carried out using A5083P-O aluminum alloy specimens. Tests were performed under identical applied load, but with different levels of heat input. The a-N curves were recorded to compare the degree of retardation, and the variation in crack tip opening was recorded with CCD camera. The results show that the crack growth rate was reduced, when the level of heat input to crack tip was increased. Additionally, the distribution of stress during loading and heating was calculated by FE methodology. The result shows that temperature increase at the crack tip would cause the residual compressive stress, which is believed to be responsible for the retardation of fatigue growth.