Effect of thickness on the fracture toughness of high strength steel for gas well casings

Abstract

Abstract The fracture toughness (KIC) of high strength casing steel (1130 MPa) has been determined by a three-point-bend specimen. Three specimens with thicknesses of 5 mm, 8 mm and 10 mm, respectively have been used to observe the effect of thickness on the fracture toughness of steel. The analytical formula of the relationship between the fracture toughness and material thickness of high strength casing (1130 MPa) is proposed based on the energy theory and on linear elastic mechanics. The values of fracture toughness, K, measured by three thicknesses of parent metals are combined with the quantitative model of the relationship between fracture toughness and specimen thickness. The material constant (and κ) are calculated by using the least squares method. Furthermore, an analytic formula for the relationship between fracture toughness and specimen thickness is proposed which allows for the calculation of the KIC value. Plane-strain fracture toughness (KIC = 133.94 MPa × m1/2) is a fixed value only if the thickness of the specimen exceeds 43 mm. Fracture toughness values of different thicknesses of high strength casing are derived by fracture toughness tests of several thicknesses based on the model in question in order to save manpower and material resources. This method is also significant for measuring the fracture toughness of other OCTG, since their thicknesses do not meet the requirement of the standard test methods. For the present, it provides general methods and reference rules for obtaining the KIC of other metallic OCTG.