applied fracture mechanics

Early Corrosion Fatigue Damage on Stainless Steels Exposed to Tropical Seawater:A Contribution from Sensitive Electrochemical Techniques 249Usually, under conditions of uniaxial load, the geometry of a pit-like crack tends to keep asemicircular shape, whereas for the case of bending load, the surface cracks adopt a semiellipticconfiguration during their propagation [56]. It is for this reason that the direction ofthe applied stress plays an important role in the nucleation and propagation of fatiguecracks from corrosion pits.In this section, the threshold stress intensity factor range (Kth) was evaluated as a functionof the aspect ratio (a/c) and the orientation of the corrosion pit with respect to the directionof the applied stress assuming tilted pits with hemispherical geometry, where in the case offatigue limits based on crack propagation, corrosion pits act as cracks under conditions oflinear elastic behavior [63-65]. The KI, KII, KIII and G were evaluated as a function of theapplied stress direction ( assuming hemispherical pits, the position along the pit contourand also of the aspect ratio (a/c). Figure 17 presents the schematic of the stress element fromwhich the transformed stresses as a function of the pit orientation angle are obtained.Figure 17. Stress state acting on a tilted hemispherical pit-like crack planeRedefining the coordinate system to make the new axes coincide with the pit-like crackorientation, it is possible to resolve the applied stress on normal and tangential components.The relationship between the transformed stresses and the normal applied stress are givenfor equations (13) and (14).xy ' '2x'xcos(13) cossin(14)xwere x’ and x’y’ are the normal and tangential stresses respectively as indicated in figure 18.The stress components associated to the plane x’y’ where the crack is located are x’ y x’y’.The component x’ induces load mode I whereas x’y induces load mode II in the surface andload mode III at the bottom of the pit-like crack. Raju and Newman [66-67] proposed asolution for superficial hemispherical cracks; such a solution was based on finite elementcalculations.