Relation between Fracture Toughness and Sample Thickness of High Strength X140 Casing Steel

Fracture toughness KIQ of high strength X140 casing steel sample with three different thicknesses was measured according to the national standard GB/T 4161-2007. Based on the theoretical formulas on the relation between fracture toughness and sample thickness, the material constants ξ and κ were got by using least square method. A new analytical formula of fracture toughness, which was related to the sample thickness, was proposed, and the values of KIC of different thickness samples were calculated. J resistance curve test was conducted to verify the accuracy of the new formula. The results show that, for X140 steel, calculated fracture toughness agreed well with the result from J test. With the increase of the thickness,fracture toughness increased at first, then decreased and stabilized finally.