Flaw Proximity Rules for Parallel Surface Cracks Based on Elastic, Elastic-Plastic Fracture Mechanics and Limit Load Analyses

When multiple cracks approach one another, the stress intensity factor, J integral value of cracks and limit load of cracked component are likely to change due to the interaction of the stress field. Since the changes in these parameters are not always conservative in structural reliability evaluations, the interaction between multiple cracks must be taken into account. Section XI of the ASME Boiler and Pressure Vessel Code provides a flaw characterization rule for interacting multiple cracks. In Sec. XI, adjacent cracks are replaced by a coalesced single combined crack if they are located within the distance of half depth of deeper crack. However, the criterion for offset distance is given by an absolute value, although magnitude of the interaction is inevitably dependent on the crack size. In current study, an alternative criterion for the offset distance is examined. The linear-elastic fracture, elastic-plastic fracture and limit load analyses are performed for interacting semi-circular and semi-elliptical surface cracks by using the finite element method under a tensile and bending load. Based on the relationship between fracture strength and relative position of the two cracks, the allowable condition for the offset distance is discussed.