Fatigue crack propagation of materials considering crack tip plasticity was studied. For this, fatigue tests were performed with compact tension (CT) specimens of Inconel 690, Inconel 600, Inconel 718 and Type 304 stainless steel at room temperature. Fatigue test on Inconel 600 was performed to be used as a reference data of Inconel 690. Inconel 718 specimen, which has very high yield strength, was selected to simulate different plasticity at the crack tip in comparison with 304 stainless steel. The effect of specimen thickness on fatigue crack propagation was studied with 304 stainless steel of 3mm-, 6mm- and 25mm-thick specimens.
Inconel 690 has been proposed as a substitute material for Inconel 600 in pressurized water reactor (PWR) steam generator tube application. This alloy was developed to improve the stress corrosion cracking resistance of Inconel 600. Now, it is known that Inconel 690 has better intergranular stress corrosion cracking (IGSCC) property than Inconel 600. But, more data of Inconel 690 about mechanical properties are needed in steam generator design. To investigate the effects of heat treatment on yield strength and fatigue crack propagation of Inconel 690, tensile tests and fatigue tests were performed on heat-treated specimen. From the test results, it is believed that chromium carbide precipitates at the grain boundaries reduce fatigue crack growth rate (FCGR) of Inconel 690 by crack tip blunting as far as the fatigue cracking is intergranular fracture mode.
To investigate the effect of residual stress on fatigue crack propagation, residual stresses were introduced by induction-heat treatment. And, the distribution of residual stresses was measured with 3mm-thick 304 stainless steel by X-ray diffraction (XRD) measurement. From the tests, it was found that FCGR was increased in tensile residual stress region and decreased in compressive region. From the fatigue tests on 304 stainless steel, it was found that FCGR of thick specimen was faster than that of thin specimen, and this result is attributed to the different plasticity at the crack tip region from elastic plastic fracture mechanics (EPFM) analysis. Therefore, for conservative prediction of fatigue crack propagation of materials, it is recommendable that plane strain condition should be used in FCGR test.
A model for fatigue crack propagation has been proposed based on the modified Dugdale model of crack tip plasticity and energy balance approach to stable crack propagation. Results show that the theoretical prediction of fatigue crack propagation agreed well with test results. From this result, it is suggested that the prediction of fatigue crack propagation should take account of the different plasticity related to the variation of specimen thickness, stress states and material's tearing modulus at the crack tip.
When the assumptions in the Paris-Erdogan equation are violated, EPFM parameters are considered in fatigue cracking. In the present study, the possibility of describing FCGR using cyclic plastic zone size was investigated. And, it is thought that cyclic plastic zone size is an ideal EPFM parameter in conjunction with applied stress level, specimen thickness and crack closure phenomena.
