QUASI-CYCLIC FRACTURE STUDIES ON NARROW GAP WELDED STAINLESS STEEL STRAIGHT PIPES

摘要

In order to study the crack growth and cyclic tearing instability, which are required for realistic assessment of Leak Before Break applicability, experimental investigations were carried out on five numbers of narrow gap welded stainless steel straight pipes under quasi-cyclic loading. This paper gives details of the test set-up arrangement and test procedure and discusses the test results. All the pipes had circumferential through-wall notch in the weld. The material used in the studies was SA 312 Gr. TP 304LN stainless steel conforming to ASTM A 312/ A 312 M - 09 standard. The specimens were initially fatigue pre-cracked under four point bending in order to produce a sharp crack front. Subsequently, fracture tests were conducted under quasi-cyclic loading. Out of the five specimens, one specimen was tested under a combination of load- and displacement-controlled loading and the remaining four specimens were tested under load-control. During the fracture tests, load, load-line displacement, crack mouth opening displacement, deflection of the pipe (at five locations), and the surface crack growth were continuously monitored. Based on the load-controlled experimental results, material specific plot between cyclic load amplitude (as a percentage of maximum load carrying capacity of a specimen under monotonic fracture) and number of cycles to failure was obtained. The test results indicate that the piping components subjected to quasi-cyclic loading may fail in very less number of loading cycles even when the load amplitude is sufficiently below the monotonic fracture/collapse load. These studies will be helpful in realistic assessment of Leak Before Break applicability in NPP piping components subjected to cyclic loading.