Internal Stress Measurement

Operating Principle

The modification of the polarization of light by an optically transparent object with internal stress is known as photo-elasticity. By measuring the difference between the polarization state of the incident light, and the polarization of the light transmitted through an object, the internal stress of the object can be determined.
To measure the polarization of light a filter known as a polarizer is used. The polarizer has the characteristic of only allowing light waves oscillating in one direction to pass through it, blocking or partially blocking light waves oscillating in other directions. The intensity of a given polarization can be determined, by measuring the intensity of the light that is passed through the polarizer filter at a particular angle.

For example, in the case of two polarizers with orthogonal axes light passing through the first filter is blocked by the second (Figure 1).
When a transparent object without internal stress is introduced between the polarizers no light passes through the second polarizer (Figure 2).
However, when a sample with internal stress is placed between the polarizers the polarization is modified and some light is able to pass through the second polarizer (Figure 3).

The amount of optical stress can be determined from the intensity of light and the direction of the stress from its polarization.
Current methods rely upon the skill of the operator to determine the amount of stress qualitatively.
The PA-110 solves this problem using photonic crystal technology and an original algorithm, making it possible to instantaneously image the internal stress distribution over a 2D region. This image is analyzed making it possible to obtain quantitative data easily.