Fatigue studies on piping components including fatigue-ratchetting interaction

A component test programme was initiated to address the applicability of leak-before-break to Indian PHWRs. After completing the tests with constant amplitude loading under air environment, the interactive nature of fatigue was addressed. The parameters considered the present studies are:

• Water environment and constant amplitude loading
• Loading due to bending and torsion
• Fatigue ratchetting

This paper describes the results of the effect of above parameters on the fatigue crack growth behavior of the piping components of Indian Power Reactors. Water environment accelerates the fatigue crack growth rate in case of both carbon steel and stainless steel. Similarly, the presence of torsion has the effect of inducing higher crack growth rate. But the final aspect ratio is not affected by these interactions and the leak-before-break argument remains valid. The results of the fatigue-ratchetting tests address a new phenomenon. The test data has necessitated a modification to the Coffin-Basquin equation to account for the triaxiality and the ductility exhaustion due to strain accumulation resulting from ratchetting. The modified equation gave a very good match to the fatigue life observed during the tests.