Numerical analysis of surface cracks at regions of curvature in oxide scales

Finite element simulations are used to examine surface cracks at regions of local curvature (corners or convolutions) in protective oxide scales. Stresses are generated during cooling from oxide formation temperatures. Three different modeling approaches are employed, since each adds some insight to crack behavior. For the first, a series of standard static analyses with varying crack lengths is used to approximate crack motion. Next, a simple node-release technique is used, permitting dynamic crack growth along an assumed path. Finally, a model based on an arbitrary crack path is employed, wherein the crack path is included as an unknown and is part of the solution. To quantify geometric effects, three different ratios of corner radii to scale thickness are considered. Further, the influence of the substrate material is investigated by considering both perfectly-plastic and work-hardening behavior. The computed stress-intensity factor at the crack tip is compared to the fracture toughness of the scale material to predict crack growth. Simulations indicate that sharper corners and lower substrate yield strengths increase crack growth potential. Reductions in the stress-intensity factor with increasing crack length are observed that result from the constraining effects of the substrate. Predictions of crack trajectory indicate initial crack motion perpendicular to the free surface of the scale, followed by a near 90δ turn, resulting in a crack path nearly parallel to the free surface.