The role of crack tip strain rate in hydrogen assisted cracking

In this paper a simple procedure is proposed to estimate the local strain rate at a crack tip, the relevant variable in environmentally assisted cracking processes. The estimation is experimentally checked by applying a kinematic fracture criterion for high strength steel under hydrogen embrittlement environmental conditions. Calculation of local strain rate is carried out on the basis of the elastic displacement distribution in the vicinity of a crack tip in conditions of plain strain. A local reference length is chosen next to the crack tip, at a distance estimated from the fractographic results of the tests which show a zone microscopically affected by hydrogen. The other relevant variable in hydrogen assisted cracking is the depth of the maximum hydrostatic stress point, which is obtained by using approximate stress distributions in the vicinity of the crack tip in the elastic-plastic regime, or calculated as the asymptotic depth (for quasi-static tests) of the hydrogen affected area. Model estimations agree fairly well with previous modelling of hydrogen embrittlement phenomena on notched samples.