Luminescent Photoelastic Coating Research Articles

An experimental technique to measure the thickness of luminescent photoelastic coatings

Luminescent photoelastic coating (LPC) methods are used to measure the full-field shear strain and its principal direction. These methods apply a liquid LPC onto the surfaces of the structures to be measured. To correct the effects of coating thickness non-uniformity on the quantitative shear strain measurements, a non-contact method is proposed for coating thickness measurements. The proposed method establishes a linear calibration curve between the thickness and the luminescent signal intensity of the test coating. The results show that the linear calibration curve of the intensity and the thickness of the LPC can be obtained by incorporating an appropriate concentration of the luminescent dye into the photoelastic coating. The correlation coefficient of this linear calibration curve is more than 0.999.

Measurement of refractive index of luminescent photoelastic coating

The refractive index of the coating is one of the essential parameters used for principal strain separation in luminescent photoelastic coating (LPC) method by oblique incidence technique. According to the theoretical analysis of the amplitude of the emitted light exciting the coating, which returns along the same path as the oblique incident excitation light, we propose a new online method of measuring the refractive index of the LPC based on optical Fresnel response of the coating. The refractive index of the coating containing Rhodamine B as luminescent dye, is measured under the excitation at a wavelength of 465 nm and an incident angle of 60. The experimental result proves the feasibility of the new method, and the present method is also applicable to the refractive index measurement of other luminescent coating.

Improvement of the Coating Formulation in Luminescent Photoelastic Coating Method

For overcoming the limitation of a low signal-to-noise ratio (SNR) in practical applications of luminescent photoelastic coating (LPC) method, a new LPC configuration applied in the LPC method is presented. Based on a brief introduction on the principle of molecular luminescence, the intensity formula for the new LPC is derived according to the theory of photoelasticity. Through the comparative analysis with the existing coating configurations (dual-layer coating and single-layer coating), the feasibility of this new LPC used in the LPC method for experimental strain analysis is proved theoretically, and the signal intensities of the LPC method can be increased by over two times.

Factorial Design Experiment to Analyze the Response of a Luminescent Photoelastic Coating

A luminescent photoelastic coating is a relatively new technique to measure full-field surface strain and determine principal strain directions that has many potential applications in engineering. Generally, luminescent photoelastic coating technology uses an in situ strain gage measurement to calibrate the detected optical response to maximum shear strain. This calibration requires the determination of two coating parameters: the polarization efficiency and the coating characteristic. A better understanding and modeling of these two parameters, particularly the polarization efficiency, will enable a simpler a priori calibration and greater measurement accuracy. This paper presents a factorial design experiment that assesses the effect of four factors on the optical strain response of an luminescent photoelastic coating. The factors examined were substrate reflectance, luminescent dye concentration, absorption dye concentration and cure time. Results show the polarization efficiency is strongly correlated with the emission anisotropy of the coating. When the polarization efficiency is modeled as the measured emission anisotropy, the two most significant factors are the substrate reflectance and the luminescent dye concentration. Absorption dye concentration and reflectance-luminescence interaction are minor but significant effects. The most significant tested factors in modeling the coating coefficient are absorption dye concentration and substrate reflectance. The resulting uncertainties for the polarization efficiency and coating characteristic are 1.3 and 3.7%, respectively.

Temperature dependence of the luminescent photoelastic coating technique

This paper presents the optical strain response results of a luminescent photoelastic coating (LPC) at elevated temperatures up to 100 °C. The LPC technique is used to acquire full-field stress and strain measurements on structural components. Differing from traditional reflective photoelastic coatings, an LPC uses a luminescent dye that partially retains the induced polarization changes in the illumination field through the coating and provides a relatively high signal response at oblique incidence angles. These characteristics enable the LPC technique to measure the full-field separated principal strains and their directions using oblique excitation. The strain-dependent response of the coating is affected by several parameters such as the luminescent polarization efficiency, optical sensitivity, coating absorptivity, and effective excitation or emission wavelength. The effect of temperature elevation on these parameters is important to characterize if the technique is extended to high-temperature or cyclic-temperature environments. Temperature sensitivity is documented and measurement uncertainty is addressed. Overall, the measured optical strain response decreased by 77 per cent between 24 °C and 92 °C with a steepest decent of –8 per cent/°C at 65 °C.

Strain Separation on a Nonplanar Object Using a Luminescent Photoelastic Coating

This paper presents the theoretical modeling and corresponding experimental results of the oblique incidence response of a luminescent photoelastic coating (LPC) applied to a cylinder under load. LPC is a measurement technique to acquire full-field maximum shear strain and its principal strain direction. The technique uses an absorption dye and a luminescent dye within a photoelastic coating, and the coating is applied on the surface of the specimen using conventional aerosol techniques. On 3D objects, the response of the emission field is dependent on the excitation orientation due to the surface inclination of the structural component and the out-of-plane strain component within the coating. Full-field strain separated results have been previously demonstrated on a 2D specimen. The extension of the strain separation technique to a 3D specimen—a cylinder in bending—is the focus of this investigation. Two different responses were obtained from normal and oblique excitation. As a result, the principal strain was separated over ±56° of circumference of the cylinder with RMS error relative to the theoretical result of 87 μɛ for maximum principal strain and 78 μɛ for minimum.

Coupled Strain and Fresnel Response of Photoelastic Coatings at Oblique Incidence

This paper presents a theoretical model and corresponding experimental results of the oblique-incidence response of a luminescent photoelastic coating (LPC). LPCs use a luminescent dye that both partially preserves the stress-modified polarization state and provides high emission signal strength at oblique surface orientations. These characteristics enable the technique to acquire full-field strain separated measurements and principal strain directions, potentially on complex three-dimensional geometries, without the use of supplemental experimental or analytical techniques. Results of a single-layer LPC on a disk in diametral compression are presented to assess a theoretical model and evaluate the measurement sensitivity.

Load and Boundary Condition Calibration Using Full-field Strain Measurement

For critical load bearing structures, it is often necessary to experimentally determine the load distribution on the structure so that accurate finite element models can be developed for stress and fatigue life predictions. An inverse problem approach is presented here for computing or calibrating the loads and boundary conditions acting on a structure. This enables the creation of more accurate finite element models, especially for structures that have complicated load distribution and compliant boundary conditions. The method presented here involves minimizing the least square error between the strains computed using the finite element model and the strains and displacements obtained experimentally. The nodal loads and the compliance at fixed boundaries are treated as the variables in the optimization problem. The compliance is modeled as springs attached at the nodes that are on the boundary where the structure is restrained. The method is verified by computing the loads and boundary conditions when displacements, maximum shear strain or both are available at large number of points on the surface of the structure. The experimental data set was generated using the luminescent photoelastic coating (LPC) technique.

Characterization of a new luminescent photoelastic coating

The luminescent photoelastic coating (LPC) technique measures the full-field shear strain and its principal direction on the surface of complex three-dimensional components. The measured optical strain response is also dependent on the coating thickness. Achieving uniform coating thickness is difficult, and thus requires thickness correction for accurate quantitative strain measurements. The original formulations of LPC employed a dual-layer coating containing luminescent dyes to transmit both strain and thickness information. This paper will document (theory and experiment) a new strategy: a single-layer coating that incorporates both a luminescent dye and an absorption dye. Dependent on the concentration of the absorption and luminescent dyes, the solution is sprayed onto the object of interest to a minimum threshold thickness that corresponds to a predefined penetration depth. Advantages of the single-layer coating are the elimination of thickness dependency, the elimination of compliance and adhesion issues between multiple layers, simpler data acquisition and post-processing methods, and easier and faster coating preparation and application.

Characterization of a New Luminescent Photoelastic Coating

Characterization of a New Luminescent Photoelastic Coating

Luminescent Strain-Sensitive Coatings

The use of novel luminescent coatings and digital imaging to map the in-plane strain field on structural components under static load is described. The technology, referred to as strain-sensitive skin by Visteon Corporation, employs two different approaches: the first uses a luminescent brittle coating (LBC), and the second uses a luminescent photoelastic coating (LPC). A coating consisting of a binder, generally polymeric in nature, and luminescent dye is applied to the surface of a test part by conventional aerosol techniques. The LBC is excited with incoherent ultraviolet or blue illumination, and the corresponding emission is imaged via a digital camera. The relative change in emission intensity is related to the in-plane volumetric strain response for moderate strain levels. The I,PC is excited by the same sources after being conditioned with polarizing and retarding optics to create circularly polarized light. The relative change in emission ellipticity, both in magnitude and phase as measured after passing through an analyzing polarizer, are related to the in-plane shear strain and its corresponding principal direction. These techniques offer quantitative, repeatable, and high spatial resolution measurements. Additionally, they are applicable to complex three-dimensional geometries, cost efficient to implement, and suitable to be integrated in the product design cycle in conjunction with finite element analysis tools. Results from a test conducted on an automobile suspension control arm under static loads are presented and discussed.