At this particular load, which is a little shy of the failure load, the strain plots offered information about the crack initiation site and conceivably the damage in EBC during loading. It should be noted that the current DIC unit used in this experi- mental study has a resolution of 16 MP higher-resolution DIC will certainly allow better observation of the crack initiation and the progression to final failure. For surface cracks and damages of the specimen tested at room temperature, the technique applied in this effort works reasonably well for pre- dicting such anomalies. It is reasonable to mention that using DIC as an NDE technique for detecting subsurface cracks and propagation of through-the-thickness cracks may be relatively difficult. DIC is considered as a line-of-sight for measuring strain and displacement. Figure 10 shows the specimen failure site as observed by DIC along with the actual failed dog-bone shape illustrating the crack site location. Results and Conclusions DIC, along with other NDE techniques that included SEM and computed tomography, were applied to investigate the dura- bility of a BSAS-coated SiC/SiC composite dog-bone specimen. The ultimate goal was to determine initial cracking stress and progressive failure of an EBC. The specimen was loaded incre- mentally to a specified fraction of the ultimate tensile strength and then unloaded. During loading, the axial strain was monitored by DIC. Upon unloading, the gauge section of the Extensometers Case 9, 3 kN 1226 pixels 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 Damage or localized strain at 3000 N 1226 pixels 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 x104 Marker, metal fiber Test no. 13, 3.4 kN 1226 pixels 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 More obvious damage progression at 3400 N 1226 pixels 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 x104 Figure 9. Variation of displacement and strain fringe plots with increasing load. Figure 10. Specimen failure observation via DIC and the failed specimen crack site (from Abdul-Aziz et al. 2015). Primary crack resulting in failure at 140 MPa Crack located very close to failure ME | DIGITALIMAGECORRELATION 66 M AT E R I A L S E V A L U AT I O N • N O V E M B E R 2 0 2 2 Estrain xx X-axis displacement (pixels)

specimen was analyzed by optical microscopy, SEM, and CT. The key findings are as follows: Ñ Optical microscopy did not offer much detail concerning the matrix cracks formed on the surface of the EBC due to the surface roughness and poor depth of focus of the optical microscope. Ñ CT also did not identify the cracks due to the high atomic contrast of the EBC layers, in addition to the technical limita- tions of CT in resolving cracks less than 4 mm. Ñ The DIC strain plots offered a successful demonstration of the primary crack initiation on the EBC’s top surface that led to the final fracture of the specimen. Ñ The initiation of secondary cracks on the EBC was also noticed on the strain plots at stresses close to ultimate tensile strength. However, the presence of these cracks was not confirmed by SEM they were only noticed in the matrix. The DIC data was to some extent effective in monitoring and identifying crack initiation sites compared to the optical and CT techniques applied. 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