Oil and gas pipelines may be subject to high plastic strains, either intentionally as a result of the method of installation, or the requirements of the design and operation, or accidentally (due to mechanical damage), before they enter service (transportation, construction/installation, etc.) and during operation. Pre-strain is introduced by denting, cold bending, land slides, subsidence, frost heave, ice gouging, earthquake induced ground movement, reeling, installation in deep water, and wrinkling or buckling. Material subjected to pre-strain will have different material properties to that of the virgin material. Previous experimental studies have indicated that pre-strain 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. To investigate the effect of pre-strain on the fracture toughness of line pipe steel a programme of tests and numerical analyses has been undertaken. The results of tensile, notched tensile, fracture toughness (J-integral and CTOD) and Charpy V-notch impact tests of virgin (not pre-strained) material, prestrained material and artificially strain aged material are reported. It is shown that the effect of pre-strain can be simulated numerically using a finite element model incorporating the influence of material damage through a Gurson-Tvergaard constitutive model. The properties of the virgin material that influence the effect of pre-strain on toughness are discussed. The role of material damage (void nucleation and growth, etc.) during the introduction of pre-strain is shown to be less significant than the changes to the tensile properties and ductility caused by pre-strain. The effect of tensile pre-strain on 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, and several parameters that relate to the conditions for ductile fracture (or cleavage fracture). A simple, engineering approximation to the effect of pre-strain on fracture toughness for application to pipeline design and fitness-for-purpose assessment is proposed in terms of the true strain at the tensile strength of the virgin material.

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