Optical Strain Gauges

• Photoelastic
• Moire
• Holographic interferometry

Photoelastic Strain Gauges

When a photo elastic material is subjected to a load and illuminated with polarized light from the measurement instrumentation (called a reflection polariscope), patterns of color appear which are directly proportional to the stresses and strains within the material. The sequence of colors observed as stress increases is: black (zero stress), yellow, red, blue-green, yellow, red, blue-green, yellow, red, etc. The transition lines seen 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 means a steeper stress gradient, and uniform color represents a uniformly stressed area. Hence, the overall stress distribution can easily 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.

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Moire Interferometry Strain Gauges

Moire interferometry is an optical technique that 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, which is caused by external loading or other source deformation. It 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.

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Holographic Interferometry Strain Gauges

Holographic interferometry allows the evaluation of strain, rotation, bending, and torsion of an object in three dimensions. Since 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.