Role of crack-tip hydrostatic stress in environmental assisted fatigue cracking

Abstract During service, environmentally assisted cracking of light weight high strength Al-alloys is a common cause of failure. For a given alloy, the important parameters associated with environmental cracking are: the maximum load and load range, the aggressiveness of environment, and the frequency of loading. These parameters relate to damages due to cyclic loading (mechanical fatigue) and time (environment). In general, two causes are commonly attributed to overall damage: (1) hydrogen embrittlement, dominant as the yield strength of the alloy increases and (2) anodic dissolution. The mechanical stress due to cyclic loading can develop a high triaxial stress state in notched and cracked components which can help both the crack initiation and propagation stages of fatigue. Cracks can originate from corrosion pits (as stress concentration sites) produced by local anodic dissolution process. In the case of hydrogen embrittlement process, the triaxial stress state at the notch/crack-tip may add to the increase in local hydrogen concentration. In this paper, we attempt to analyze the effects of the hydrostatic stress state, SH, at the crack-tip on fatigue crack growth in aqueous environment where hydrogen embrittlement process can occur. This is done by means of finite element modelling under plane stress and plane strain conditions for both notched and cracked specimens assuming an elastic perfect-plastic or Ramberg-Osgood material behaviours. In an aqueous environment, a passive film formation at the crack tip may induce additional tensile stresses which could enhance the local damage process. The effect of tensile stresses induced by the passive film is simulated by applying constant nodal forces at the surface nodes in tangential direction to the notch-tip curvature. The present computations explore the hypothesis that a different stiffness in compression should be assigned to the material elements having high hydrostatic stress (SH>0.9SHmax). This is based on the premise that the element with high hydrostatic stress SH would lead to a collection of high hydrogen (H) concentration. In particular, it is assumed that the elements with high H concentration (or high SH) would exhibit restrivtive reversed plasticity (in a limiting case no yielding is assumed in compression). The results indicate that the load displacement curve during unloading is strongly affected by the different stiffness of the elements assigned with high H concentrations. Also significant differences between plane strain and plane stress in terms of hydrostatic stresses are demonstrated. In the case of plane strain the peak value and the gradient of the hydrostatic stress distribution are much higher than in the case of plane stress. Finally, the overall trends in the numerical results are supported by the experimental data taken from the literature on high strength Al-alloys.