H-Enhanced Deformation and Fracture in the Crack Tip Process Zone

Mechanism-based crack tip damage models that quantitatively predict material-environment dependent H cracking properties are necessary for prognosis of structural life-cycle performance in hydrogen service. Models are based on understanding the elements that intensify H localization and control damage in the crack tip process zone, including: (a) very high stress and localized plasticity dictated by continuum mechanics with strain gradient plasticity as well as microstructure, (b) reversible H trapping at nano precipitates and interfaces in open or closed-concentration systems, (c) H-impacted dislocation structure that elevates local stress and trapped-H concentration to condition interfaces for H decohesion, (d) special crystallographic character and 3-dimensional connectivity of the H-crack path, (e) interface purity, (f) a decohesion failure criterion, and (g) size and physical basis for the critical distance. High resolution probes of trapped H concentration, plasticity, damage, boundary crystallography, and crack path provide important inputs. These elements are integrated in micromechanical models of the threshold stress intensity and H-diffusion-limited rate of crack propagation; as validated by a broad data base. Model-leveraged properties are applied, using stress intensity and H concentration similitude, in a damage tolerant code for probabilistic structural performance simulation, constituting a modern multi-scale approach to manage H cracking.