Implementation of a finite element framework coupling chemo‐mechanics and the Gurson‐Tvergaard‐Needleman model

Abstract

Hydrogen embrittlement in ductile metals, such as steel, is a significant concern, for instance, in the safety assessment of existing pipeline infrastructure intended for hydrogen transport. The ductile damage mechanism in steels is characterized by the nucleation, growth, and coalescence of microvoids, which is further enhanced by the presence of hydrogen. This leads to material damage and premature failure in components. Mechanisms contributing to hydrogen‐induced reduction of strength in steels include hydrogen‐enhanced decohesion (HEDE), hydrogen‐enhanced local plasticity (HELP), and hydrogen‐enhanced strain‐induced vacancies (HESIV). The HEDE mechanism leads to a principal stress‐controlled brittle failure mode. Conversely, the HELP and the HESIV mechanisms, which are dominated by plastic deformation, alter the ductile damage behavior as they lead to accelerated void growth and coalescence. Furthermore, interstitial diffusion of hydrogen leading to lattice expansion, commonly referred to as swelling, also contributes to hydrogen‐induced embrittlement. Hydrogen embrittlement is therefore a stress‐diffusion process that involves chemo‐mechanical coupling, where hydrogen atoms primarily diffuse toward areas with high hydrostatic stress. In this regard, we propose a framework using the finite element method and combining coupled chemo‐mechanics and the well‐known Gurson‐Tvergaard‐Needleman (GTN) damage model. The framework is motivated by mixed rate‐type potentials, that account for the influence of hydrogen concentration on the damage behavior. An additional dependence of the fracture strain and evolution of the void volume fraction on hydrogen is included. A comparison of the fully‐coupled model to simplified versions is conducted to individually assess the role of hydrogen concentration on damage evolution and the stress state on diffusion.