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This paper presents results from a thermal sensitivity study of a luminescent photoelastic coating. The investigation is
part of larger research program to integrate luminescence sensing for strain measurement and health monitoring in civil,
automotive and aerodynamic applications. Luminescent photoelasticity is a new approach to measure mechanically
induced stress and/or strain, and corresponding principal directions on structural components. The technique
incorporates a luminescent dye that partially preserves the stress-modified polarization state within a birefringent coating
and provides high emission signal strength at oblique surface orientations.
The optical strain response of the coating is a nonlinear function of the maximum shear strain in the plane perpendicular
to the propagation of light. Several parameters may affect the strain response including luminescent polarization
efficiency, optical sensitivity, coating absorptivity and effective excitation-emission wavelength. The temperature
dependency of these parameters is important to characterize if the technique is extended to high-temperature or cyclic-temperature
environments. A small thermal chamber was constructed with open optical access to test coated cantilevered
specimens under a linearly varying bending stress. Results show that the optical strain response decreases as temperature
increases. Over a temperature range spanning from 24 °C to 92 °C, the optical strain response decreased by 77%. Most
of the percentage drop occurred between 35 °C and 80 °C, with relatively constant response for temperatures lower and
higher. The primary source of the temperature dependency is the coating sensitivity coefficient which is a function of the
modulus of elasticity. Higher strains tended to delay the transition, indicating strain-temperature coupling in the optical
sensitivity coefficient.
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