Modelling and experimental studies on microstructural effects on hydrogen-assisted cracking of a high-strength steel

Hydrogen-assisted cracking (HAC) is a well-known threat to high-strength structural steels. The presence of hydrogen in the steels may cause premature and catastrophic failure of structures. Quantified information of the influence of hydrogen on fracture toughness is useful to adopt/design suitable strategies to prevent any failure. The thesis presents certain experimental and numerical methods to assess the microstructural effects on hydrogen-induced damage to the fracture resistance of a high-strength steel.Microstructure is a significant factor that influences HAC susceptibility of a steel. To understand the fracture behaviour in situations such as HAC of steel welds, it is essential to understand the HAC response of various microstructures of a susceptible steel. Three microstructures (as-received, quenched and, quenched + tempered) of AISI 4340 steel were experimentally investigated to determine their susceptibilities to HAC through fracture toughness testing using circumferentially notched tensile (CNT) specimens. The study provides a conservative critical stress intensity factor (~10 MPa.m0.5) for AISI 4340 in HAC prone situations. Moreover, the relatively unexplored CNT specimen testing technique proves to be a viable option for assessing the role of microstructural variations in the context of HAC of high-strength steels.Numerical analysis of HAC is performed using finite element (FE)-based cohesive zone modelling. A cohesive zone model for HAC typically has two sub-models – the hydrogen transport model and the crack propagation model, which are suitably coupled during the analysis. Three models are presented in the thesis; a 2-D model for the compact tension (CT) specimen and two models (3-D and 2-D axisymmetric) for the CNT specimen. The 2-D model for the CT specimen incorporates an analytical solution for hydrogen diffusion, coupled with the FE-based crack propagation model. A more efficient fully FE-based approach is developed and employed for the other two models. The 3-D model is able to capture the influence of nonuniform stress field at the crack front on hydrogen diffusion, suggesting a slightly higher hydrogen concentration near the point of the deepest pre-crack depth. The 3-D model can be applied for HAC analysis in specimens or geometries that possess 3-D features or loading. The axisymmetric analysis of the CNT specimen established that the typically observed eccentricity of the pre-cracks can be ignored for the modelling purpose. The axisymmetric model proved to be computationally more efficient than the 3-D model and equally acceptable for the CNT specimen geometry. By using the axisymmetric model, the threshold stress intensity factors (SIF) of the three microstructures of AISI 4340 were successfully predicted through simulations of rising displacement tests and constant load tests of CNT specimens under hydrogen charging conditions.