Effect of residual stresses and complex loadings on the fatigue behavior of underclad cracks

Abstract

The objective of this paper is to show that the fatigue crack propagation of cold underclad cracks in reactor vessel nozzles is much less than predicted by the usual conservative safety assessment methods, and that it is possible to estimate real margins more accurately. The crack propagation of underclad cracks was assessed by numerical methods with conservative safety hypotheses: • - the upper bounds of fatigue crack propagation laws in an air environment have been used for both the base metal and the cladding, • - fatigue crack initiation on the actual underclad defects has not been taken into account, • - the crack lengths have been corrected using Irwin's correction, which accounts for plastic-zone sizes at crack tips, • - elliptical defects have been considered to be strip defects, • - the lower bounds of the yield strength and thoughness of the materials have been used. An extensive experimental fatigue program was also performed on large specimens in several laboratories. The specimens were taken out of a full-size nozzle (SA 508 Cl.3 steel) containing cracks in the heat-affected zone under the three layers of cladding. The experimental observations are as follows: • - a significant fraction of the lifetime is spent in crack initiation, • - the crack growth rate is lower because of a non-aggressive (inert) environment in the defects, • - the residual stresses at 300°C can be neglected, • - the residual stresses at room temperature are significant. The numerical calculations of crack propagation, taking into account the residual stresses, load ratio and temperature effects, give a fair estimate of the behavior of the underclad cracks observed during testing. Combining the experimental and numerical results provides a good knowledge of the real margins between the actual underclad crack propagation and the propagation that was calculated in previous safety assessments. It can be concluded that the actual propagations of underclad cracks will be very small or even negligible during the specified lifetime of the components.