Use of polarization in imaging the residual stresses in polycrystalline alumina films

Residual and applied stresses modify the characteristic R-line luminescence from trace amounts of Cr3+ incorporated in aluminum oxide. This piezospectroscopic effect has been extensively used to measure, and image, stress distributions in a variety of aluminum oxides, including those formed by the thermal oxidation of aluminum-containing alloys. One drawback until now has been that the shift in the luminescence frequency is dependent on the mean stress rather than the individual stress components. In this work, we show that the principal stress directions and their magnitude can be obtained by combining the ratio of the R2 and R1 peaks as a function of collection polarization with the measured R2 frequency shift. This is illustrated with images recorded from regions of a thermally grown oxide in which a narrow strip of the oxide has been isolated to produce regions of varying principal stresses and directions.