A model for calculation of stress corrosion crack growth

As much as 25% of all accidents in process industry reported to the Swedish Plant Inspectorate are caused by stress corrosion crack growth. The situation is believed to be similar in the all industrialised countries. It contributes to the severity of the accidents that they often occurs unexpectedly and at loads that may be far below what normally causes crack growth Experts in the fields of metallurgy, corrosion have treated stress corrosion using mechanical models based on elastic or elastic plastic stress fields where the crack tip is treated as a point. In view of the near tip load distribution this leads to paradoxic results. In the present model corrosion is forming the geometry of the crack tip and is thus itself creating the conditions for strain concentration. We incorporate an interaction between electro-chemical processes and the deformation of the crack tip region in a fracture mechanical theory. The model is based on material dissolution simply being proportional to the surface stretch. No crack growth criteria is used. Computation of the mechanical state is based on a finite element formulation for large strains. The formation of a crack from a surface depression via a pit is studied. Low frequency cyclic load is considered. At the end of a load cycle a metal oxide compound is growing on the crack surface. We assume that there is sufficient time for the chemical process to form a protective film that fully covers the crack surface. This temporarily interrupts the corrosion process. During the application of next load cycle the stretch of the surface breaks the protective film. This creates gaps in the film, which allow dissolution of the underlaying metal. The chemical environment of the crack tip is assumed to be constant and unaffected by the changing geometry as the crack is developing. We assume that there is a linear relationship between strain and corrosion rate, in the sense of removed material per unit of area during each load cycle. The model simulates how dissolution lead to surface roughness, how pits form and grow to become cracks in one continuous process. The resulting natural variation of lengths of the formed cracks causes the cracks to grow with different speeds. During continuation one crack after the other falls into a wake behind a larger crack. Thus, crack tip load of the smaller decreases and finally causes crack arrest: At the end of the simulation only one growing crack remains. The results are compared with experimental observations.