13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- stress state determined by an FE-model is in the correct order of magnitude, and that the location for crack deflection can be described by KII MAX correlated to interface slope. However, it is likely that the stress level should be subject to further investigation. If KII MAX can be used as a criterion for crack deflection and the model prescribes this phenomenon to occur prematurely, the material response causes stresses to be exaggerated. Factors that can play a role in the mathematical modelling can be top coat stiffness, interface geometry, BC constitutive model, oxidation behaviour and thermal/mechanical load. 6. Conclusions The current paper compares experimental findings and modelling results coupled to conditions for when a TBC delamination will change from an interface crack to a kink crack contained partly at the interface, partly in the ceramic top coat. It is shown that the condition for change from interface to kink crack can be fulfilled by comparison of interface slope and interface geometry in terms of asperity height. Comparisons indicate that the interface slope plays an important role ad that experimental and modelling results correlate if the distance from peak to deflection point is taken into consideration. In the comparison, modelling results give the best agreement if the location for KII max is chosen as a criterion for when deflection is to be expected. References [1] G.C. Chang, W. Phucharoen, Behavior of thermal barrier coatings for advanced gas turbine blades, Surf. and Coat. Techn. 30 (1987) 33-38 [2 ]M. Jinnestrand, S. Sjöström, Investigation by 3D FE simulations of delamination crack initiation in TBC caused by alumina growth, Surf. and Coat. Techn. 135 (2001) 188-195 [3] J.W. Hutchinson, Z. Suo, Mixed mode cracking in layered materials, Adv. Appl. Mech. 29 (1990). 63-187 Appendix: Mechanics of the interface crack For a description of the theory of an interface crack see, for instance, [3]. From the FE solution, the energy release rate ܩ and the crack flank displacements ݑ ଵ ሺ௨ሻ, ݑ ଵ ሺሻ, ݑ ଶ ሺ௨ሻ and ݑ ଶ ሺሻat a number of nodes along the crack flank are used. (For definition of materials Nos. 1 and 2 and coordinate directions 1 and 2, see Fig. 4.) The relation between ܩ, ܭ ூ and ܭ ூூ is given by ܩ ൌቆ 1 ܧ ଵ ∗ 1 ܧ ଶ ∗ቇ ⋅ 2cosh 1ଶሺ ߳ߨ ሻ ሺ ܭ ூ ଶ ܭ ூ ଶ ூሻ ሺ1ሻ in which ߳ ൌ 2 1 ߨ ln 1 ߥ ଶ ܧ ଶ ߢ ଵ 1 ߥ ଵ ܧ ଵ ߢ ଶ 1 ߥ ଶ ܧ ଶ 1 ߥ ଵ ܧ ଵ ሺ2ሻ ߢ ൌቐ 3െ4 ߥ 3 െ ; plane strain ߥ 1 ߥ ; plane stress ;݅ ൌ 1,2 ሺ3ሻ
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