Comparison of Stress Intensity Factors of Two Flaws and a Combined Flaw Due to Combination Rules

Abstract

If the flaws detected during in-service inspection are multiple discrete flaws that are in close proximity to one another, the flaws are evaluated as to whether they are combined or not, in accordance with combination rules in the ASME Code. The combination rules require that multiple flaws shall be treated as a single combined flaw if the distance between the adjacent flaws is equal to or less than the dimension of the flaw depth. After the coalescence of the multiple flaws, the flaw length becomes larger and then the stress intensity factor of the combined flaw would be expected to be significantly larger. Stress intensity factors for two surface flaws and a combined flaw under membrane stress and bending stress were analyzed using influence function method. From the calculation results of the stress intensity factors for two flaws and the combined flaw, it is shown that less conservative combination rules are appropriate, as compared to the existing combination rules in the ASME Code.