Experimental investigations of fatigue crack growth and behaviour on stainless steel elbows

Abstract

Straight pipes, Tees and elbows form the primary heat transport piping system of nuclear power plants. Leak-before-break (LBB) design philosophy is used in the design of such piping systems. Leak-before-break analyses assume that the flaw becomes a through-wall crack, and that the through-wall crack can be detected by leakage at normal operating loads in advance of a catastrophic failure. Hence, systematic experimental investigations were carried out to study the fatigue crack initiation and growth behaviour of healthy (without notch) elbows and notched elbows having circumferential outer surface notch at one crown, extrados and intrados used in primary heat transport systems of nuclear power plants. Tests have been carried out on eight numbers of 90 degrees radius, 168 mm outer diameter and 15.1 mm nominal thickness elbows made of SA403 type 304LN stainless steel. The aspect ratios 2C/a (notch length to notch depth) and a/t (notch depth to thickness) of the notches were 1.5 to 4.5 and 0.18 to 0.28. The initial notch depth was varied from 3 to 4 mm. The fatigue tests were conducted at stress ratio (R) of 0.1 under load and displacement control. Fatigue crack detection and sizing were carried out using alternating current potential difference (ACPD) and Ultrasonic techniques. Fatigue crack growth curves and crack profile development curves were generated based on experimental data. Number of cycles to crack initiation and through-wall crack under in-plane bending moment were determined. Multiple cracks were observed in the elbow at various locations during fatigue crack growth studies. The two elbows with axial notch at crown location have shown less fatigue life compared to that of one elbow with circumferential notch at extrados and one elbow with circumferential notch at intrados.