Influence of Material Heterogeneity on the Strain Capacity of Pipelines

The strain capacity of pipes under combined loading is an important research topic within strain-based design. While strain capacity equations assume homogeneous pipe material properties, a realistic pipeline shows scatter in mechanical properties. Test and simulation programs have indicated that this “pipe heterogeneity” may reduce the tensile strain capacity under uniaxial loading by a factor up to two. To date, its effect in other scenarios (compressive; combined internal pressure and axial plastic deformation) has received little attention. To investigate these scenarios and compare them with uniaxial tensile loading, Europipe, Salzgitter Mannesmann Forschung (SZMF) and Soete Laboratory, UGent have set up a large-scale test program on UOE pipe X70 (OD = 1219 mm, WT = 17.5 mm), comprising a full-scale pressurized bend test and curved wide plate tension tests. All tested welds joined pipes with nominally equal pipe grade from the same pipeline project, with strongly different actual properties. Optical full field strain measurements by means of digital image correlation reveal local effects of pipe heterogeneity on the strain distribution in the vicinity of the girth weld. All tests showed pronounced degrees of non-uniformity in strain development, in some cases even inhibiting plastic deformation in the stronger material as its weaker counterpart collapses. Implications of the observations with respect to strain-based design are discussed.