Modelling of hydrogen assisted stress corrosion cracking

Abstract

Modelling of hydrogen assisted stress corrosion cracking (HASCC) within the framework of mechanics is very important for its control and avoidance. The main focus of this study is to develop suitable approach for modelling and analysis of stable crack growth through high strength steels under HASCC. A new strategy based on combined analytical/numerical solution and finite element based cohesive zone model (CZM) has been developed. This has helped to couple analysis of hydrogen diffusion and crack growth during HASCC. The strategy has been applied to study crack growth in compact tension (CT) specimens. The solution to diffusion process is obtained through either an analytical or a numerical solution to the governing differential equation. The crack growth is analysed by CZM. For the analytical solution, both one- and two-dimensional approximations of the domain have been considered. The new CZM strategy, termed as hydrogen concentration dependent cohesive zone model (HCD-CZM), has been used for both CT and circumferentially notched tensile (CNT) round specimens. The CNT specimen has been employed for the first time to obtain the fracture toughness data of high strength steel under internal and external supply of hydrogen. The experimental scheme involving CNT specimen under slow strain rate loading is demonstrated as a valid experimental procedure for study of HASCC for high strength steels. Both types of HASCC, internal hydrogen assisted cracking (IHAC) and hydrogen environment assisted cracking (HEAC), are found to induce a proportionate drop in fracture toughness under higher hydrogen concentration near the crack tip. The experimentally obtained lowest fracture toughness data compare favourably with lower range of published threshold values for the similar material. The experimental average crack growth rates too agree with the reported data for the material. For CT specimens, both schemes of analysis of diffusion, excluding or including the effect of hydrostatic stress and plastic strain, predict variation of crack opening displacement with crack growth with good accuracy. Diffusion solution based on one- and two-dimensional analyses do not significantly alter the prediction of crack growth. The effect of hydrostatic stress on the distribution of hydrogen concentration is observed to be significant as long as plastic strain is less than 5%. The study has given rise to an important correlation between hydrogen concentration dependent strength reduction and plastic strain rate. A new modelling technique is presented for the CNT specimen with eccentrically placed ligament using two-dimensional finite element approximations; this has considerably simplified analysis of the problem which otherwise would require a three-dimensional solution. For CNT specimens, the HCD-CZM approach employing both analytical and finite difference based diffusion solutions predicted the critical fracture toughness in agreement with experimental results. In this case too, the inclusion of hydrostatic stress in the diffusion analysis has been found to have not so significant influence on the prediction of experimental observations. The K-resistance curve obtained for the case is included. The proposed HCD-CZM has been found to satisfactorily handle variation in specimen geometry, material and source of hydrogen supply. The thesis is divided into six chapters dealing sequentially with introduction, literature review, experiments with CNT specimen, analysis of CT specimens, modelling of CNT specimens and conclusions. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of the Indian Institute of Technology Bombay, India and Monash University, Australia.