Photo elasticity

Photoelasticity can be explained as the change in a material’s optical properties when it undergoes mechanical deformation. It is one of the oldest methods used for determining the stress distribution in a material, especially in areas where mathematical methods are inconvenient. It is also used for determining stress points in asymmetrical geometries. This stress analysis method is based on an optomechanical property called birefringence, which is exhibited by several transparent polymers and glass.

Photoelasticity was developed to use optical principles to solve elasticity problems in engineering. The method relies on stress-induced birefringence where the material exhibits birefringence under the influence of an external load. The phenomenon was first observed in glass and then was developed into a viable tool for stress analysis.

Photoelasticity is a handy tool for engineers to visualize areas where a structure might break due to high concentrations of stress. Plastics with birefringence properties reveal areas of strain within a structure in the form of colorful light fringes when viewed under polarized light.

Working Principle of Photoelasticity

In an industry scenario, a model of a structure can be made out of photoelastic plastic, and load can be applied to it. When polarized light is passed through the model, areas with high levels of stress will reveal more colorful light fringes than the other areas. Thus, all the vulnerable breakage areas can be easily detected, and the necessary precautions can be implemented.

A transparent material is said to be birefringent if a ray of light passing through it splits into two refractive indices. This causes a change in the polarization state of the transmitted light, forming interference fringes.

The size of the refractive indices at each point in the material is directly proportional to the size of stresses at each point. Birefringence can be analyzed using a device called polariscope to reveal information such as maximum shear stress and its orientation.

A basic still camera or a video camera is used to record fringe patterns. Isochromatic and isoclinic are two types of pattern that can be obtained. These two patterns indicate the main stress differences and the principal stress directions, respectively.

Photoelastic Coatings

Photoelastic coatings allow the surface analysis of actual components with irregular surfaces. These coatings are usually molded to the surface of an irregular part and thus bonded to it. Light reflected at the coating–component interface propagates twice through the coating thickness and gives an effective path length in the coating.

The coating needs to be thick enough to generate a sufficient number of fringes in response to the strains in the component. At the same time, it should not be so thick that the coating's average strains significantly deviate from the interface strains, and the coating starts to reinforce the component. Sensitive coatings must be used for components that undergo small strains when loaded, and the colors of the isochromatic need to be analyzed very carefully to measure fractional fringe orders.

Advantages & Disadvantages of Photoelasticity

The key benefits of photoelasticity include its ease of use and that it can be adapted for use in static as well as dynamic investigations. Unlike analytical methods of stress determination, photoelasticity provides a more accurate determination of stress distribution, even in irregular materials. It provides consistent full-field values of the difference between the main stresses in the plane of a model. Photoelasticity also provides the value of non-vanishing main stress along perimeters of the model, where stress levels can be the highest. Only a small investment in equipment and materials is required for basic investigations.

However, there are some disadvantages to this technique. Photoelasticity requires the production of a model of the actual part unless photoelastic coatings are used. Moreover, the calculations required to separate the principal stress values at a general interior point are very complicated.  For precise stress analysis in large components, expensive equipment is needed. Also, 3D photoelasticity experiments are very time-consuming and tedious.

Applications

Latest advances in photoelastic analysis offer the opportunity to perform real-time stress monitoring of structures. Time-consuming coating applications and manual reading of fringes have given way to quick cure coatings, fully automated polariscope systems, and high-speed CCD camera that can monitor changing stress patterns in real time. The modern photoelastic technique offers a powerful and unique full-field dynamic stress analysis method.

The following are the key applications of photoelasticity:

• Infra-red photoelasticity
• Dynamic photoelasticity
• Image processing for fringe analysis
• Flaw detection and glass inspection
• Residual stress analysis
• Assembly stress analysis
• FEA model verification
• Impact testing
• Polarimetric fiber optic sensors
• Stereolithography applications
• Research on fracture behavior in materials
• Analyzing strain in denture materials

THE POLARISCOPE

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Components of a Polariscope:

• Polarizer and Analyzer If the polarizer and analyzer are crossed, so that no light exits from the analyzer, the setup is called dark field. Light will only be transmitted due to effects resulting from birefringence in the model. If the polarizer and analyzer are aligned to pass all the light going through the system, the setup is called light field. The effect of the model is to then darken the image. Almost all work is done in the dark field setup. Two types of fringes occur in the image: isochromatics, which identify the magnitude of the maximum shearing stress vector by their color, and isoclinics, which give the direction of the maximum shearing stress vector.

• Two Quarter-Wave Plates The two quarter wave plates are aligned in crossed mode with the fast axis of one aligned with the slow axis of the other. The purpose of the quarter wave plates is to convert the incoming linearized polarized light to circularly polarized light before the light goes through the model, and then convert the light emerging from the model back into linearly polarized light before it passes through the analyzer. Actually, these plates are not necessary, but they remove the isoclinic lines from the pattern so as to produce a clear isochromatic pattern. They can be removed physically, or optically by aligning their axes with the polarizer axes.

• Loading Mechanism Some type of mechanism is required to load the model specimen. It must be placed between the polarizer and quarter-wave pairs.

• Light Source with Filters The light source can either be standard white light or a monochromatic source (usually with a wavelength matching the tint-of- passage, which is also the design wavelength of the quarter-wave plates). White light gives colored fringes, isochromatics, which aid in estimating stresses, but has the disadvantage that only a very few colored fringes can be distinguished easily. Monochromatic light, on the other hand, gives much narrower fringes which can be observed up to much higher order. In either case, the light must be expanded to the size of the model area it is desired to study. The beam can be either from an extended source (fluorescent or incandescent bulbs) diffused by a ground glass, or from a point source (arc lamp or discharge tube) with the light beam expanded and collimated by a collimating lens. Large collimating lenses are expensive, so much work is done with diffuse light sources. In this case however, a telecentric imaging system should be used to select out the light which traveled parallel through the specimen.

• Image Detector This can be a camera film (preferably one with high contrast), a CCD digital camera, or the human eye. Frequently, a ground glass plate is fixed to a camera film plane for visual inspection and alignment, then replaced with film for obtaining a permanent image. In recent times, there has of course been great interest in digital images from a CCD.

• Collimating and Field Lenses The collimating lens is used as described above. The field lens is used to focus the large image exiting from the polariscope down onto an image detector.

Example:

The following image is taken from the Measurements Group Wall Chart and shows the isochromatic fringe pattern obtained when a curved plastic (PSM-1) model of a curved bar is subjected to tensile stress.