74 A 3D Cohesive Modelling Approach for Hydrogen Embrittlement in Welded Joints of X70 Pipeline Steel

A 3D finite cohesive element approach has been developed and applied in order to simulate the crack initiation of hydrogen-induced fracture. A single edge notched tension specimen of an API X70 weld heat affected zone was modeled. The results were compared to a similar 2D model and the cohesive parameters were calibrated to fit the experimental results. Under the same levels of global stresses, the three dimensional simulations gave higher results in terms of opening stress at the stress peak, plastic strain levels at the crack tip and hydrogen lattice concentration, as well as faster diffusion, when compared with bi-dimensional simulations. The best fit to the experimental data was obtained for a critical opening of 0.3 mm and initial critical cohesive strength of 1840 MPa and 1620 MPa for the 2D and 3D simulation, respectively.