High strength pipeline steels have different plastic properties in each direction. This anisotropy is developed by TMCP processes in a heavy plate mill. The process may also lead to anisotropic ductility and toughness of the plate. The purpose of this study is to develop a constitutive model integrating anisotropic behavior and ductile damage for a high strength pipeline steel. The model is based on a set of experiments on various smooth, notched and cracked specimens and on a careful fractographic examination of the damage mechanisms. The model is also based on an extension of GTN model which includes plastic anisotropy. Provided brittle delamination is not triggered, the developed model can accurately describe the plastic and damage behavior of the steel. The developed model is then used as a numerical tool to investigate the effect of plastic anisotropy on ductile crack extension. It is shown in particular that it is not possible to obtain a unified description of rupture properties for notched and cracked specimens tested along different directions without accounting for plastic anisotropy.
Economic studies have shown that development of oil and gas transportation over long distances requires the use of high grade steels whose mechanical properties allow to substantially increase the internal pressure for a given pipe thickness. Research projects have then been focused on the development of API grades X80 and X100 (Hillenbrand et al., 2004; Okatsu et al., 2002) and more recently to grades X120 (Hillenbrand et al., 2004). Their mechanical behaviour needs to be characterized both in terms of plastic behaviour and crack growth resistance. In particular resistance to ductile crack initiation and longitudinal propagation needs to be evaluated to assess the high-grade pipelines structural integrity. In practice, standards recommend the use of Charpy-V or drop weight tear tests in relation with semi-empirical correlations to predict the outcome of full-scale burst tests of pipelines (Civallero et al., 1981; Maxey, 1981; Wiedenhoff et al., 1984). These correlations have been established on lower grade steels. Results of recent full-scale testing campaigns (Vogt et al., 1993; Demofonti et al., 2003) have shown that these correlations no longer hold for the new grades.
