Editorial Feature

What is Photoelasticity?

Jun 20 2014

Image Credit: martinnovacek.cz/Shutterstock.com

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: