13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- furnace-cycled and engine-exposed, respectively. An interface crack formed during furnace cycling is shown in image (c). Figure 11. Correlation dy/dx = f( D, KI = KII ) and D = f( KII max ). From the data in Figure 5 the damage level D for KI = KII and maximum KII are determined. These data are presented in Figure 11. Data in Figure 11 are for interface roughness 10/70, 20/70, 30/70 and 50/70 µm/µm. Series are included for a range of oxide thickness values. For individual kink cracks data are collected for peak-to-valley h, half-wavelength l, distance b and d for crack deflection and interface slope dy/dx at location for crack deflection. Data indicate that the actual location for crack deflection from an interface crack to a deflected kink crack best can be described by a correlation of peak to location for deflection b along the surface normal rather than the projection on a plane parallel to the interface. It is also obvious that the condition for crack deflection best is described by a correlation to the maximum slope of the generalised model interface. Results according to Figure 13 should be interpreted with the stress state at the interface in mind. For short cracks, the stress state is KI dominated. As the crack progresses, the stress state will be increased dominated by shear stresses. Considering crack growth in an isotropic material, the general assumption is that a crack growing in a mixed mode stress field will try to grow in the direction that enables growth under a KI modus. In the case of crack growth at a biomaterial interface, this is not necessarily true. A crack can continue to grow at the interface, if, for instance, the interface acts a weak link. An example of this is the TBC system with a thick thermally grown oxide that acts as a prescribed crack path. In cases where the thermally grown oxide is dense and adherent to the underlying metallic material, the crack should, in principle, be able to deflect away from the interface and into the top coat where crack growth can take place under a KI dominated stress field. Figure 14 visualises the criterion that can be used to determine when the crack should have possibility to deflect into the top coat. For short cracks and low stress intensities, the crack is confined to the interface. As the crack grows longer and the interface oxidizes, the shear stress increases and the conditions for crack deflection can be fulfilled at D=Dm. It is important to notice that not all cracks will deflect. One important factor can be local TGO growth. It is well-known that the oxide thickness is not constant throughout a thermally exposed TBC coating even if the temperature is known and constant over the coating at Tmax. 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0 0,2 0,4 0,6 0,8 d [-] dy/dx [-] 4µ, KI = KII 6µ, KI = KII 8µ, KI = KII 4µ, KII max 6µ, KII max 8µ, KII max
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