Investigation of a Non-Destructive Method to Characterize Material Mechanical Properties of Pipelines in Service

In the frame of Pipelines Integrity Management, a better knowledge of actual materials properties of pipelines in service can save money by avoiding the cut and interruption of gas transit thanks to a more realistic defect assessment at failure. Different ways have to be explored to improve this knowledge on materials. One is based on the development of adapted correlations, well validated, to get from the poor material data available, the material properties which are required to analyse the defect behaviour. Another one is to use directly on field a non-destructive mechanical characterization tool. This paper focuses on non-destructive characterization by indenter method. This method is a succession of micro-indentations made by a spherical tungstene carbide indenter. From the ball displacement and stress applied during the load phase, tensile mechanical properties and toughness of the material are estimated. As the depth of the indenter displacement in steel is about 300 μm with no creation of micro-cracks, the test is considered as non-destructive. Hence, measurement can be made on operating pipelines without cut or interruption of gas transit. Gaz de France’s R&D Division has led laboratory and field-testing to evaluate the accuracy of the indenter tool and the ease and reliability to use it in-ditch conditions. Results on determination of actual tensile properties and toughness by indentation are presented, compared with results from destructive tests on standard specimens. Residual stresses and anisotropic effect on the indenter results are slightly investigated. Finally, all results are discussed to estimate the relevancy of this method for the Pipeline Integrity Management and some perspectives are given.