The development of photo-stimulated luminescence spectroscopy for three-dimensional stress measurements

The development of photo-stimulated luminescence spectroscopy (PSLS) for stress measurement in chromium-doped alumina has been motivated by the need for a nondestructive technique to establish the integrity of the thermally grown oxide (TGO) layer on turbine blades of jet engines. Applications of PSLS have been based on the R-line peak positions and their stress dependence has been fully exploited for planar measurements. However, factors associated with the oxide layer, such as its undulated nature and growth stresses, emphasize the need to measure other stress components unavailable with the planar model in use today. Our new findings on the piezospectroscopic nature of vibronic bands in the optical spectrum of chromium-doped alumina pave the way for a 3-D model of stress measurement never before established. The focus of this research is on developing the PSLS technique to measure 3-D stresses in chromium-doped alumina by studying the behavior of these less explored parameters and the experimental factors that affect their measurement. The development of a spectral analysis methodology based on genetic algorithms for optimization of the curve fitting was instrumental in detecting and monitoring the shifts of the peaks within the vibronic bands with stress. The stress measurement model was applied to polycrystalline alumina and thermal barrier coating specimens and data for future validation was obtained using synchrotron x-ray diffraction. This work leads to an improved stress-prediction method in non-destructive testing of the TGO and eventual life prediction of the thermal barrier coatings of aircraft engine turbine blades.