Using the ASME and BSI codes to predict crack growth due to repeated thermal shock

Abstract

This paper examines the use of the ASME and British Standard codes to estimate the growth of cracks driven mainly by thermal shocks. 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 high, particularly in the presence of stress concentrations. Repeated application of the thermal shocks may lead to crack initiation and crack growth. The ability to use current codes and standards to describe this type of crack growth is desirable. Unfortunately, thermal shock is a 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 techniques described in the ASME Boiler and Pressure Vessel code Section XI and British Standard BS7910 to predict crack growth rates derived from Monash University experiments. Areas of large conservatism in the methods currently used in industry are identified and possible alternative, less conservative approaches are suggested. If the methods are fully applied, the possibility of crack growth slowing can be captured and the replacement or repair of equipment with thermal shock cracking might be avoided.