Using S–N curves to analyse cracking due to repeated thermal shock

Thermal shock loading of operating pressure equipment is a common occurrence, particularly in thermal power stations. The tensile stresses that are produced at the surface of a heated component exposed to a rapid thermal down shock can be very high, particularly in the presence of stress concentrations such as an abrupt change in geometry. Repeated application of the thermal shocks may eventually lead to crack initiation and crack growth. In some cases these cracks lead to component failure while in other cases the cracks arrest at a harmless depth. The ability to use current codes and standards to describe this type of crack growth is particularly desirable. Unfortunately, thermal shock is a very complex transient situation with highly non-linear stress distributions and environmental effects that are not well described by some codes. This paper describes attempts to use the stress versus cycles curve (S–N curves) described in the ASME Boiler and Pressure Vessel code to predict crack initiation in a flat plate carbon steel specimen exposed to repeated thermal shock from temperatures below the creep range. The issue considered in this paper is how to estimate stresses in this complex situation for notched specimens. Modifications to the code are suggested where necessary to achieve the desired accuracy.