PSI - Issue 13

Marcel Adam et al. / Procedia Structural Integrity 13 (2018) 1226–1231 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

1230

5

The evolution of the out-of-plane strains and their impact on coating damage has been studied by correlating the measured strains and the corresponding AE-with the results obtained by the digital image correlation (DIC) measurements. Fig. 4 displays the out-of-plane displacement (radial displacement of TBC surface) for three different loading levels resp. identified damage events, which have been identified by the AE measurement and analysis (see again Fig. 2 (b)).

Fig. 4. (a) overview of the total captured picture; (b) evaluated out-of-plane displacement for the area of interest and for three different loading levels.

Load level 1 in Fig. 4 (b) and Fig. 2 (b) indicates the state after pre-loading. Load level 2 indicates the global strain level just after the threshold value has been exceeded. Load level 3 indicates an axial strain, where a first, pronounced increase in AE signal could be observed. Regarding the corresponding out-of-plane displacements, a uniform “global” displacement can be observed for load level 1, which is in good agreement with the calculated radial displacements caused by the radial dilatation of a cylinder under uniform axial compression. Please note that within the DIC evaluation, all deformations are referenced to the pre-loaded condition prior. Once exceeding the AE threshold value at load level 2, local narrow areas of intense out-of-plane deformation (red spots) can be observed. Also, an increased in out-of-plane displacement can be observed at the top edge of the investigated surface area, which might be caused by the multiaxial stress state at the free edge. Once the axial load is increased to load level 3, the locally displaced areas increase in intensity and size, suggesting local delaminations within the coating or the coating-substrate interface to occur. Point 4 in Fig. 2 (d) indicates visible through-cracking of YSZ and GZO layers and thus a total failure of the coating system, see Fig. 5 (a).

Fig. 5. (a) visible trough-cracking at load level 4; (b) axial path of the resulting out-of-plane deformation for load level 2 measured by means of DIC.