Unstable fracture from stable fibrous crack growth -3rd report

Abstract

Stress corrosion cracks are often found in chemical or nuclear power plant piping systems. One of the concepts that assess the safety of these piping systems is the Leak Before Break (LBB) concept. Namely, even if ductile mode crack initiates and grows, the plant is thought to be safe, when the growth of the crack is stable. The purpose of this paper is to establish the LBB assessment method for piping systems with cracks.A criterion is proposed which estimates the tearing instability at the onset of penetration from a surface cracked plate in tension. The criterion is based on Crack Tip Opening Angle (CTOA)-R curve concept.Two kinds of experiments were carried out. By stable carck growth tests, the specimen geometry dependence of J-and CTOA-R curve was investigated. J-R curve was found to be dependent on specimen geometry. On the other hand, CTOA-R curve was relatively insensitive to specimen geometry, in addition, it could be estimated from surface notched specimens.Unstable fracture tests were carried out in order to investigate the validity of the criterion proposed here. Two kinds of fractures were observed from the tests using surface notched plates with highly compliant loading system, i. e., instability at the onset of penetration and instability after stable crack growth. And it was found that the proposed criterion could predict whether the instability occurred at the onset of penetration or not.