Constitutive modeling of hydrogen cracking

Hydrogen cracking in multiple-pass weld metals occurs when accumulating hydrogen concentration exceeds a certain critical level. This accumulation via diffusion is a thermally activated, continuing process taking place throughout welding, driven also by the emerging residual stresses. The results of diffusion analyses are difficult to quantify in terms of cracking risk since no reliable coupling between the local hydrogen concentration and material damage has been available. Numerical analyses enabling solution of hydrogen diffusion and local concentration in welds having complex geometries are performed. A predefined temperature solution is used as an input to a finite element transient diffusion analysis. In the mass diffusion problem, a three-dimensional residual stress field is input to describe the pressure stress dependency of the transient diffusion process. The resulting concentration profiles are considered by use of a novel damage mechanics material model in a finite element analysis (FEA) with a cohesive zone modeling framework, which links the local concentration to a continuum mechanics damage description. By use of the damage mechanics constitutive material model, conditions for hydrogen cracking are evaluated for a multipass butt weld. The results of the mass diffusion analysis are in agreement with experimental findings and measured hydrogen concentrations, while the damage mechanics analysis is illustrated to provide means for evaluating the rupture process by use of continuum mechanics.