Abstract
Advanced high-strength steels (AHSS) possess a great combination of ductility, strength and crash performance. Hence, these steels provide new possibilities for steel-based lightweight design of modern cars. However, with increasing strength AHSS become prone to hydrogen embrittlement. Very low hydrogen contents of about 0.1 wppm may even cause time-delayed fracture. Exact measurement of low local hydrogen contents inside of specimens using conventional sensors is challenging, as these contents are always averaged over large bulk volumes. Nevertheless, local hydrogen distribution can influence the macroscopic evaluation of hydrogen embrittlement, e.g., by using slow strain rate testing (SSRT). Therefore, this work presents a diffusion-mechanical 3D model for simulating SSRT of smooth and notched AHSS specimens. This model considers a variety of phenomena including lattice hydrogen diffusion, multiple hydrogen trapping and grain boundary diffusion. Coupling of mechanical analysis and diffusivity analysis enables predicting the influences of stress-driven hydrogen diffusion and of multiplication of trapping sites due to plastic straining. Boundary conditions for considering the influence of mechanical stresses on hydrogen desorption under atmospheric conditions are presented. Linear elastic-plastic material behaviour and isotropic material hardening are considered. The diffusion-mechanical model was parametrized using (i) properties of an industrial dual phase (DP) steel, (ii) results from thermal desorption spectroscopy (TDS), (iii) results from uniaxial tensile testing of smooth specimens, and (iv) ab-initio calculated parameters from literature. The diffusion model was validated by additional desorption experiments performed under atmospheric conditions, and the mechanical model was validated by additional tensile testing of notched specimens with different strain rates. Finally, SSRT with different strain rates was performed using hydrogen-charged smooth and notched specimens. The experimental results were compared with numerical results for monitoring the local hydrogen distribution during SSRT. It is demonstrated that model-based approaches may support investigations on the roles of local critical hydrogen content, plastic strain, stress and strain rate on fracture initiation of hydrogen-charged specimens.