A Cohesive Zone Modeling Approach to Hydrogen Induced Stress Cracking in 25%Cr Duplex Stainless Steel

Abstract

Hydrogen influenced cohesive zone elements are implemented in finite element (FE) models of rectangular U and V notched tensile specimens. The material description outside the cohesive zone is representative of a fine grained 25% Cr duplex stainless steel, UNS32760-S. A three step procedure consisting of conventional elastic plastic stress analysis, stress driven diffusion analysis and finally cohesive zone fracture initiation analysis makes the basis for the presented work. The applied boundary conditions are representative of mechanical stresses and environmental loads on an oil and gas pipeline in subsea conditions. A linear traction separation law gives reasonably good fit with experimental results for gross stress levels of 0.85–0.9 times the material yield stress. Hydrogen concentration of 40 ppm at the surface and 1 ppm in bulk always gives crack initiation at the surface despite the peak normal stress localized in front of the notch tip.