Investigation of Crack Tip Opening Displacement Behavior for Cryogenic Steel Considering Strength Mismatch Effects

Fracture toughness is an important parameter in the design of welded structures. Especially, reliable fracture toughness values are essential for engineering critical assessment of structures with defects. Representative fracture toughness parameters include stress intensity factor, J-integral, and crack tip opening displacement (CTOD). Among them, CTOD is widely adopted in elastic-plastic fracture mechanics. ISO 12135, provided by the International Organization for Standardization (ISO), is a standardized method for evaluating CTOD. This standard incorporates yield to tensile strength ratio into the CTOD calculation formula, ensuing reasonable CTOD value for materials like carbon-manganese steel. However, this CTOD calculation formula provides conservative CTOD values for cryogenic steels, which involve significant plastic deformation and have a relatively low yield to tensile strength ratio. To address this phenomenon, this study investigates the crack tip opening behavior in cryogenic steel, focusing on the actual mechanical properties. Finite element analysis (FEA) was conducted on common cryogenic materials, including nickel steel, high manganese steel, and stainless steel. The findings provide insights into the fracture behavior of cryogenic steels and highlight the limitations of current CTOD calculations formula for these materials. This study aims to contribute to a better understanding of CTOD behavior in cryogenic steel and to propose improved methods for CTOD evaluation.