A cohesive zone framework for environmentally assisted fatigue

TL;DR

This paper introduces a finite element cohesive zone model for hydrogen-assisted fatigue that captures environmental effects, mechanical behavior, and hydrogen diffusion, aligning well with experimental trends.

Contribution

It develops a comprehensive, physics-based framework integrating hydrogen effects, mechanical deformation, and damage mechanics for modeling environmentally assisted fatigue.

Findings

Model captures fatigue crack growth sensitivity to environment and frequency.

Yield strength and hardening influence crack growth rates.

Additional stress sources needed for quantitative accuracy.

Abstract

We present a compelling finite element framework to model hydrogen assisted fatigue by means of a hydrogen- and cycle-dependent cohesive zone formulation. The model builds upon: (i) appropriate environmental boundary conditions, (ii) a coupled mechanical and hydrogen diffusion response, driven by chemical potential gradients, (iii) a mechanical behavior characterized by finite deformation J2 plasticity, (iv) a phenomenological trapping model, (v) an irreversible cohesive zone formulation for fatigue, grounded on continuum damage mechanics, and (vi) a traction-separation law dependent on hydrogen coverage calculated from first principles. The computations show that the present scheme appropriately captures the main experimental trends; namely, the sensitivity of fatigue crack growth rates to the loading frequency and the environment. The role of yield strength, work hardening, and constraint conditions in enhancing crack growth rates as a function of the frequency is thoroughly investigated. The results reveal the need to incorporate additional sources of stress elevation, such as gradient-enhanced dislocation hardening, to attain a quantitative agreement with the experiments.