Exploring the Role of Residual Stress in Correlating Yield Strength From Indentation-Based Nondestructive Measurements and Destructive Tensile Testing

Abstract Nondestructive determination of the yield strength (YS) for in-ditch materials verification of line pipe is routinely accomplished using indentation methods such as the instrumented indentation test (IIT). While the IIT-predicted YS (YS(IIT)) is generally within ±10 % of the tensile test value (YS(TT)), it occasionally differs by as much as 25 %. This work explores the possible contribution of residual hoop stresses introduced during forming to this aspect of the IIT performance. While in-ditch inspections are rarely informed by prior knowledge of residual stress levels, a better understanding of the potential contribution from manufacturing-induced residual stresses may improve the ability to forecast when measured YS(IIT) values are most likely to accurately reflect benchmark YS(TT) values. The current work compares results of IIT performed on 20 line pipes and on smaller coupons extracted from those same pipes. The results are interpreted based on the assumption that residual hoop stresses introduced during pipe forming are relieved in the coupons when they are extracted from the larger pipe sections. FEA simulations were performed for two of the pipes to explore the potential effect of residual stress and to guide analysis of the experimental data. The analysis revealed probable contributions from residual tensile hoop stresses in half of the pipes. Additional analyses identified limitations in using the Hollomon power law to model the plastic deformation and predict the yield strength from the indentation process. The results indicate that the Hollomon power law tends to underpredict yield strength in materials where the strain hardening exponent is below approximately 0.130 and overpredict it when it is above approximately 0.180.