FLAW ALIGNMENT CRITERIA BASED ON LIMIT LOAD SOLUTIONS FOR NON-ALIGNED THROUGH-WALL FLAWS

摘要

Multiple flaws are frequently replaced with simpler flaws according to the flaw alignment rules for the purpose of engineering assessment. Such rules typically compare the flaw separation distance with the characteristic dimension, such as flaw length or flaw depth. For configurations involving flaws on parallel offset planes, current ASME Boiler code, Section XI, Article IWA-3300 suggests a fixed distance of 12.4 mm between the offset flaw planes as a flaw alignment rule. Recent experimental work of Hasegawa et al [4] showed this criteria is not always conservative for ductile fracture. In this paper the analytical and computational methods are used to derive plastic collapse load for plates with twin non-aligned through-wall flaws. Results show that in a multiple flaw geometry the global plastic collapse load is independent of the flaw separation when horizontal and vertical separations between flaw tips are equal. Such geometries are of most interest, since experimental work shows that flaws in such geometries interact over distances several times larger than the characteristic distance (flaw length or thickness). A criteria for flaw alignment based on the ratio of a local to global limit load is derived. A local limit load is defined as the load to cause collapse of the ligament between the flaw tips. It is shown that the separation between parallel flaws should be more than three times the flaw length to maintain interaction below 15 per cent and ensure a conservative assessment.