Strain gradient plasticity-based modeling of hydrogen environment assisted cracking

Abstract

Finite element analysis of stress about a blunt crack tip, emphasizing finite strain and phenomenological and mechanism-based strain gradient plasticity (SGP) formulations, is integrated with electrochemical assessment of occluded-crack tip hydrogen (H) solubility and two H-decohesion models to predict hydrogen environment assisted crack growth properties.SGP elevates crack tip geometrically necessary dislocation density and flow stress, with enhancement declining with increasing alloy strength. Elevated hydrostatic stress promotes high-trapped H concentration for crack tip damage; it is imperative to account for SGP in H cracking models. Predictions of the threshold stress intensity factor and H-diffusion limited Stage II crack growth rate agree with experimental data for a high strength austenitic Ni-Cu superalloy (Monel K-500) and two modern ultra-high strength martensitic steels (AerMet TM 100 and Ferrium TM M54) stressed in 0.6M NaCl solution over a range of applied potential. For Monel K-500, KTH is accurately predicted versus cathodic potential using either classical or gradient-modified formulations; however, Stage II growth rate is best predicted by a SGP description of crack tip stress that justifies a critical distance of 1 µm. For steel, threshold and growth rate are best predicted using high-hydrostatic stress that exceeds 6 to 8 times alloy yield strength and extends 1 μm ahead of the crack tip. This stress is nearly achieved with a three-length phenomenological SGP formulation, but additional stress enhancement is needed, perhaps due to tip geometry or slip-microstructure.

On the suitability of slow strain rate tensile testing for assessing hydrogen embrittlement susceptibility

“…It is important to note that the value of χ will likely be affected by other modeling assumptions, such as accounting for the increase in apparent solubility due to hydrostatic stresses [97][98][99] or the use of strain gradient plasticity to resolve micro-scale deformation [6,100]. Moreover, physical interpretation of the values of χ is hindered by the phenomenological, macroscopic approach adopted, where both the elastic and plastic parts of the strain energy density contribute to fracture and where the damage micromechanisms are not explicitly resolved.…”

Gradient plasticity crack tip characterization by means of the extended finite element method

“…allows to overcome such numerical difficulties and can therefore enable crack tip characterization in a wide range of load levels. This could be particularly useful in environmentally assisted cracking, where GNDs have proven to play a fundamental role [22].…”

“…Very recently, Martínez-Pañeda et al [20,21] have identified and quantified the relation between material parameters and the physical length over which gradient effects prominently enhance crack tip stresses. Their results have revealed the important influence of strain gradients on a wide range of fracture problems, being particularly relevant in hydrogen assisted cracking modeling due to the central role that the stress field close to the crack tip plays on both hydrogen diffusion and interface decohesion [22,23].…”

Strain Gradient Plasticity-Based Modeling of Damage and Fracture

“…This necessarily implies that gradient effects lead to a much lower SSDs density with respect to conventional plasticity predictions. However, the same argument cannot be used for c t ; as shown in [33,142], SGP models predict large values of ρ in the vicinity of the crack, as ρ G largely dominates the total dislocation density. Therefore, further research and critical experiments are needed to quantitatively elucidate the role of GNDs in hydrogen trapping and other embrittlement mechanisms.…”