Numerical simulation of hydrogen embrittlement and local triaxiality effects in notched specimens

Abstract

Metals and alloys, especially high-strength steels and nickel-based alloys, are damaged by the phenomenon known as hydrogen embrittlement. Although these types of hydrogen-assisted fractures have been well documented, micro-mechanisms are not yet completely understood. Strain localization and decohesion seem to be usually accelerated by hydrogen, which might cause brittle fracture. With the aim of predicting those failures, the local interaction between notch fracture mechanics and hydrogen diffusion is analysed. As the stress-strain state affects hydrogen diffusion, a great hydrogen concentration is expected in the vicinity of a notch. Additionally, the level of constraint affects plastic localization. In the present paper the effect of the notch radius as an indicator of triaxiality and stress concentration on hydrogen diffusion is studied. Moreover, damage evolution is implemented with cohesive elements along the notch tip plane using the Traction – Separation Law proposed by Park, Paulino and Roesler (PPR). Cohesive energy and critical separation depend on hydrogen content but also on triaxiality, i.e. on the notch radius. In order to reproduce a brittle behaviour at the macroscopic scale, tensile tests of Notched Round Bars with different notch radii are simulated in a high-pressure hydrogen environment. The competition and/or synergy between constraint and hydrogen embrittlement are discussed.