Optical Strain Gauges

There are three types of optical strain gauges: photoelastic, Moire interferometry, and holographic interferometry.

Photoelastic strain gauges subject an elastomeric material to a load and illuminate the material with polarized light from the measurement instrumentation, a reflection polariscope. The patterns of color which appear are directly proportional to the stresses and strains within the material. As stress increases, the sequence of colors is black (zero stress), yellow, red, blue-green, yellow, red, blue-green, yellow, red, etc. The transition lines between the red and green bands are known as fringes. The stresses in the material increase proportionally as the number of fringes increases. Closely-spaced fringes indicate a steeper stress gradient. Uniform color represents a uniformly-stressed area. Therefore, overall stress distribution can be studied by observing the numerical order and spacing of the fringes. Furthermore, a quantitative analysis of the direction and magnitude of the strain at any point on the coated surface can be performed with the reflection polariscope and a digital strain indicator.

Moire interferometry is another method used by optical strain gagues. This optical technique uses coherent laser light to produce a high contrast, two-beam optical interference pattern. Moire interferometry reveals planar displacement fields on a part's surface. These fields are caused by external loading or other source deformation. Moire interferometry responds only to geometric changes of the specimen, and is effective for diverse engineering materials. Contour maps of planar deformation fields can be generated from x and y components of displacements.

Optical strain gauges that use holographic interferometry allow the evaluation of strain, rotation, bending, and torsion of an object in three dimensions. Because holography is sensitive to the surface effects of an opaque body, extrapolation into the interior of the body is possible in some circumstances. In one or more double-exposure holograms, changes in the object are recorded. From the fringe patterns in the reconstructed image of the object, the interference phase-shift for different sensitivity vectors are measured. A computer is then used to calculate the strain and other deformations.