Stress corrosion cracking is a life-limiting factor in many components of nuclear power plant in which failure of structural components presents a substantial hazard to both safety and economic performance. Uncertainties in the kinetics of short crack behaviour can have a strong influence on lifetime prediction, and arise due both to the complexity of the underlying mechanisms and to the difficulties of making experimental observations. This paper reports on an on-going research programme into the dynamics and morphology of intergranular stress corrosion cracking in austenitic stainless steels in simulated light water environments, which makes use of recent advances in high resolution X-ray microtomography. In particular in situ, three dimensional X-ray tomographic images of intergranular stress corrosion crack nucleation and growth in sensitised austenitic stainless steel provide evidence for the development of crack bridging ligaments, caused by the resistance of non-sensitised special grain boundaries. In parallel a simple grain bridging model, introduced to quantify the effect of crack bridging on crack development, has been assessed for thermo-mechanically processed microstructures via statically loaded room temperature simulant solution tests and as well as high temperature/pressure autoclave studies. Thermo-mechanical treatments have been used to modify the grain size, grain boundary character and triple junction distributions, with a consequent effect on crack behaviour. Preliminary three-dimensional finite element models of intergranular crack propagation have been developed, with the aim of investigating the development of crack bridging and its effects on crack propagation and crack coalescence.

International audienceThree-dimensional (3D) short fatigue crack growth behaviour in a cast magnesium alloy, Elektron 21, was studied using a combination of X-ray diffraction contrast tomography (DCT) and microtomography at the European Synchrotron Radiation Facility. A 3D map of grain shapes and crystallographic orientations was produced in a miniature fatigue specimen using DCT. A focused ion beam instrument was used to introduce small notches in selected grains. Synchrotron microtomography was used to study the evolution of fatigue cracks from these notches during interrupted in situ fatigue cycling. Stage I crack growth occurred preferentially on the basal plane of the magnesium hexagonal close packed crystal, with local crack growth rates between 4 and 40 nm cycle 1. Retardations in the local crack growth rate were observed in certain grains and at certain grain boundaries. The observed interactions between the crack and the polycrystalline microstructure can be explained using Schmid factors and a development of the tilt-twist model of Zhai and Wilkinson (Zhai T, Wilkinson AJ, Martin JW. Acta Mater 2000;48;4917)

Two forms of high resolution X-ray tomographic experiments (i.e. synchrotron based X-ray microtomography and desktop microfocus computed X-ray tomography) are demonstrated in the present paper to illustrate the wide application of these techniques for qualitative and quantitative studies of localised corrosion and environmentally assisted cracking. Specifically, synchrotron based X-ray tomography was used to investigate the localised corrosion morphology within aluminium specimens when exposed in situ to a chloride environment while microfocus computed X-ray tomography was used to investigate the morphology and quantify the transition from localised corrosion to stress corrosion cracking in steel specimens exposed ex situ to a simulated corrosive condensate environment.

A three dimensional mechanical model for intergranular crack propagation is presented. The model follows the spirit of existing percolation-like models but offers the inclusion of mechanical effects. This is necessary in order to account more accurately for the crack driving force and the effect of crack bridging ligaments, observed experimentally to be formed by fracture resistant boundaries. The model uses a regular representation of the material's microstructure and a categorisation of grain boundaries as beneficial and detrimental to fracture. This categorisation makes the model applicable to assessing material's resistance to intergranular stress corrosion cracking. The model mechanical behaviour is consistent with experimental observations and demonstrates its capability of simulating the development of bridges in the crack wake as well as crack coalescence. Results show that increasing the fraction of resistant boundaries increases the degree of crack tip shielding developed. This is expected to increase the resistance to stress corrosion crack propagation. The model offers a significant reduction of the computational resources usually needed to simulate intergranular propagation.

A numerical strategy is reported based on the finite element method for simulating the mass transport limited current flowing at macro- and microelectrodes located within a rectangular channel cell. Specifically, simulations of a large planar, microband, and microstrip electrode are presented and the results compared to those predicted using alternative strategies. The finite element method is shown to be a flexible and efficient alternative to the previous strategies employed. The potential to extend finite element simulations to more complex geometries less suited to finite difference simulations is noted.