13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- For the fracture-mechanical computations, a least representative cell has been defined, consisting of a substrate+bond coat+TGO+top coat cell, having the width ܮ ( = half the period of the idealised cosine wave of Fig. 3). This cell has been given boundary conditions corresponding to symmetry along the left-hand vertical boundary. The model bases on an idealised (sinusoidal) bond coat (BC)/top coat (TC) interface profile, in which the thermally grown oxide (TGO) is assumed to grow. See Fig. 3. Equal cracks grow symmetrically from all profile tops, i.e., when D = a/L = 1.0, cracks extend along the whole interface, leading to complete failure. Further essential model assumptions: Plane strain Stress-free TBC system at the end of the high-temperature part of the cycle The maximum damage-driving energy release rate G and stress intensity factors KI and KII appear after cooling down from the maximum temperature (low thermal expansion of the TC) The material behaviour of all components of the TBC aggregate during the cooling down from maximum temperature is assumed to be linearly elastic G, KI and KII are computed by a virtual crack extension method and by interface crack theory (see [3] and appendix) Fig 4 shows examples of the FE geometry and of the stress pattern in the interface region for 4 typical ݄ ܮ/ cases Figure 4. Examples of the FE geometry and of the stress pattern in the interface region for 4 typical h/L cases. Most of the FE computations so far have been done in an in-house code. We have, however, moved over to the commercial code Abaqus for a planned continuation, which will, among other things, inh L Material No. 1 Material No. 2 1 2
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