Applicability of Instrumented Indentation for Assessing Allowable Flaw Sizes of Circumferentially Flawed Stainless Steel Piping

Abstract 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 conventional tensile testing. A large number of studies related to IIT reported that IIT is capable of assessing the yield strength and the ultimate tensile strength with 10% accuracy compared to the tensile testing. 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 article. As a result, the IIT method is capable of estimating 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 calculated 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 a sufficiently accurate estimate of the flow stress on austenitic stainless steel piping. Therefore, IIT is a beneficial method for LLC.