Over the past few years, the luminescent photoelastic coating technique has been extended to acquire principal strain
measurements on static, three-dimensional structural components under load. The approach uses oblique incident
excitation and digital imaging of the luminescence; subsequent analysis is performed on a three-dimension grid
compatible with finite element analysis. This paper discusses the initial efforts to extend the technique for dynamically
loaded specimens in which the excitation is strobed in synchronization with the load application cycle.
The luminescent photoelastic coating technique is an optical technique to measure the full-field strain on 3-D structural components. A luminescent dye within a photoelastic binder is excited with circular polarized light, and the corresponding coating emission intensity is detected via a digital camera for loaded and unloaded states of the specimen to which the coating is applied. Images are processed to find the relative change in emission with respect to camera analyzer position and subsequently analyzed to determine maximum in-plane shear strain and the principal strain directions. For 3-D structures with moderate movement or deflection in the field of view, especially when implementing an oblique excitation approach to separate the principal strains while accounting for non-strain-related polarization changes due to surface inclination, the image analysis is preferably performed on a 3-D grid. This study describes such an approach and discusses the analysis procedures to separate the principal strains and to obtain full-field strain distribution. The theoretical results are compared to experimental data from both a 2-D and a 3-D test specimen.
The luminescent photoelastic coating (LPC) technique is an optical technique to measure the full-field strain on
three-dimensional (3D) structural components. A luminescent dye within a photoelastic binder is excited with circular
polarized light, and the corresponding coating emission intensity is detected via a digital camera for loaded and unloaded
states of the specimen to which the coating is applied. Images are processed to find the relative change in emission with
respect to camera analyzer position, and, subsequently, analyzed to determine maximum in-plane shear strain and the
principal strain directions. For 3D structures with moderate movement or deflection in the field-of-view, especially when
implementing an oblique excitation approach to separate the principal strains while accounting for non-strain related
polarization changes due to surface inclination, the image analysis is preferably performed on a 3D grid. This study
describes such an approach and discusses the analysis procedures to separate the principal strains and to obtain full-field
strain distribution. The theoretical results are compared to experimental data from a 3D test specimen.
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.
Luminescent photoelasticity is a new approach to measure mechanically induced strain 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. These characteristics can
facilitate determining the principle strains and directions on complex geometries, without additional experimental or
analytical techniques, by exploiting the angle dependent emission from multiple camera and/or illumination orientations.
This paper presents a brief overview of the technique and a newly initiated research program sponsored by the NSF.
Resulting advancements of the research are expected to integrate luminescence sensing for strain measurement into time-demanding
product design cycles, validate complimentary computational methods, and provide a foundation to expand
into critical need areas such as structural health monitoring in civil, automotive and aerodynamic applications.
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