Evaluation of Subsurface Crack Propagation Under Fatigue

Abstract

There is a proximity rule to calculate residual fatigue life of components in nuclear power plants. It is easy to evaluate soundness of a structure member by using this rule. If a subsurface crack is located near free surface of the structure, this real subsurface crack is transformed to surface crack. The condition to transform subsurface crack to surface crack is defined by relationship between crack size and the distance from crack tip to free surface. However, many organizations proposed proximity rules which differ from each other. It is advisable to verify which rule is preferable in these rules by experiment, but it is difficult to introduce subsurface crack at an optional position. Therefore, numerical simulation is needed for this purpose. Especially, S-version FEM is very useful for as much as model of subsurface crack is independent of global structure, and crack growth is easily simulated. Both subsurface and transformed surface crack growths are simulated by S-FEM. Subsurface crack grows toward free surface. When subsurface crack tip was touched to free surface, this crack was converted into surface crack by using stress intensity factor calculated at this time. In this way, crack growth behavior from subsurface to surface flaw is represented. By comparing the crack growth rate of surface to subsurface flaw with that of surface flaw transformed by each proximity rule, proximity rules can be verified by numerical simulation. Authors had proposed the proximity rule at the ASME PVP 2013 conference [1]. However, new rule was proposed by numerical simulation only under cyclic tensile load. In addition, only two proximity rules were studied at the last conference. In this study, the number of proximity rules is increased, and this problem is simulated under other loading condition such as cyclic bending load. New proximity rule gives reasonable and conservative results in numerical simulation.