A Model to Predict the Effect of Pre-Strain on the Fracture Toughness of Line Pipe Steel

New and existing pipelines can be subjected to high plastic strains. Denting a pipeline causes permanent plastic deformation. Onshore pipelines subject to subsidence, frost heave or earthquake loading can experience significant plastic strain during service. Offshore pipelines that are reeled prior to laying, or are laid in deep water, or are operating at high temperatures and high pressures, can experience significant plastic strain both prior to, and during, service. Experimental studies have indicated that pre-strain (permanent plastic deformation) has a detrimental effect on the fracture toughness of steel; it reduces the resistance to crack initiation, reduces the resistance to crack growth, and increases the transition temperature. Consequently, there is a need for a thorough understanding of the effect of pre-strain on the fracture toughness of line pipe. Accordingly, a theoretical model for predicting the effect of tensile pre-strain on the ductile fracture toughness has been developed using the local approach. The effect of pre-strain is expressed in terms of an equation for the ratio of the fracture toughness of the pre-strained material to that of the virgin (not pre-strained) material. The model indicates that the effect of tensile pre-strain on the material’s fracture toughness can be characterised in terms of the effect of pre-strain on the stress-strain characteristics of the material, the critical fracture strain for a stress state corresponding to that during pre-strain, and several parameters that relate to the conditions for ductile fracture (or cleavage fracture). The implications of the model are that it may be possible to estimate the reduction in toughness caused by pre-strain simply from a full stress-strain curve of the virgin material. The model has been validated against the results of crack tip opening displacement (CTOD) tests conducted by Tokyo Gas on two line pipe steels subject to uniaxial tensile pre-strain. It is shown that the predictions and trends of the theoretical model are in broad agreement with the test results.