Profilometry‐Based Indentation Plastometry and Uniaxial Testing of Pipelines: Detection of In‐Plane Anisotropy and Potential for Simplification of Test Procedures

Abstract

This article concerns anisotropy in stress–strain relationships exhibited by steel pipelines. Since the stress from internal pressurization is highest in the hoop direction, standard industrial practice to measure these properties has focussed on tensile testing in this direction, requiring a prior flattening operation. The associated plastic deformation may affect properties, typically causing a (poorly‐defined) degree of hardening and the creation of inhomogeneity. Field testing of such pipelines, requiring in situ measurement of tensile properties (yield stress (YS) and ultimate tensile stress (UTS)), is based on indentation testing of the outer surface. One such test (profilometry‐based indentation plastometry (PIP)), not only gives the stress–strain curve, but also allows detection of any (in‐plane) anisotropy, with a high sensitivity. Both PIP testing and compression testing (with and without prior flattening) have confirmed that none of the eight pipes examined in the current work exhibited any such anisotropy, although the effects of flattening did tend to generate apparent anisotropy in the tensile test outcomes. It may therefore be appropriate to switch the focus of tensile testing to the axial direction, such that no flattening would be required.