Applicability of instrumented indentation for assessing allowable flaw sizes of circumferentially flawed stainless steel piping

河 侑成  ; Negyesi, M.*; 長谷川 邦夫; Lacroix, V.*

Ha, Yoosung; Negyesi, M.*; Hasegawa, Kunio; Lacroix, V.*

Structural integrity assessment of flawed pipes, as required by ASME B&PV Code Section XI, relies on failure stress calculated by the Limit Load Criterion (LLC). The failure stress is derived from flow stress, which depends on yield and ultimate tensile strengths. Instrumented indentation technique (IIT) is a nondestructive alternative to the conventional tensile testing. A large amount of studies reported that IIT is capable to assess the yield strength and the ultimate tensile strength with 10 % accuracy compared to the tensile testing. By employing IIT, the tensile properties are expected to be obtained nondestructively and directly from the actual pipe material. The benefit of IIT for the analysis of allowable flaw sizes is critically assessed in this study based on experimental data and Code calculations. Intrinsic standard deviations of yield and ultimate tensile strengths for Type 304 stainless steels are discussed in this paper. As a result, IIT method is capable to estimate the flow stress for Type 304 stainless steel within 3.2 % deviation from the result of conventional tensile tests. Then, the allowable circumferential flaw sizes for pipes subjected to tensile loading are obtained based on strength properties assessed by IIT. In addition, the allowable flaw sizes are compared to the allowable flaw sizes derived from the flow stress tabulated in the ASME B&PV Code Section II, Materials. The conclusion is drawn that IIT yields sufficiently accurate estimates of the flow stress for austenitic stainless steel pipes. Moreover, the flow stress determined from the tabulated values in ASME B&PV Code Section II, Materials, is much lower than the actual measured value, resulting in more conservative estimates of allowable flaw sizes. Therefore, IIT is a beneficial method for the LLC analysis.