Implementation of an unloading compliance procedure for measurement of crack growth in pipeline steel

Abstract

As the demand for fossil fuels increases, pipelines are constructed in inhospitable areas. Underthese conditions, not only the strength but also the deformability of the pipelines becomes crucial. A strainbased design (SBD) procedure needs to be established. Traditional stress based approaches to evaluatedefect tolerance lead to conservative predictions. There is a need to accurately define the fracturetoughness of the pipeline steel and assess the criticality of weld defects under strain based conditions. Thispaper focuses on the implementation of the unloading compliance method to determine stable crackgrowth. The standardized test procedure described in ASTM E1820 is applied. This method is a handy toolto obtain the J-resistance curves which can forecast ductile failure in pipeline girth welds. Preliminaryexperiments have been performed on Single Edge Notch Bend (SENB) specimens of plain pipe metal.Using the implemented procedure, it was possible to obtain a good fit between calculated and measuredcrack size. The most important result is the smoothness of the calculated crack growth and the rathermonotonic increase of crack size. Since testing on SENB is well known to provide conservativemeasurements, Single Edge Notch Tension (SENT) specimens will be evaluated in future work