The elasti region is ideal because the material will return to its original

The elasti region is ideal because the material will

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manufacturers can see a material’s elastic limit, yield stress, and ultimate stress. The elastic region is ideal because the material will return to its original shape without permanent deformation, meaning the applied force will not cause any material failure due to yielding or necking. 4
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2 Theory Photo elasticity describes changes in the optical properties of a material under mechanical deformation. It is a property of all dielectric media and is often used to experimentally determine the stress distribution in a material, where it gives a picture of stress distributions around discontinuities in materials. Photo elastic experiments (also informally referred to as photoelasticity are an important tool for determining critical stress points in a material, and are used for determining stress concentration in irregular geometries. In its simplest form, the photoelastic polariscope consists of a light source, a polarizing device called the polarizer. The photoelastic model and a second polarizing device called the analyzer. When the model is stressed the photoelastic stress pattern becomes visible on the ground glass or viewing screen of the photoelastic polariscope. 5
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Figure 1 Figure 1 is a typical photoelastic fringe patterns, Light field and dark field. These light waves or vibrations are transverse to the rays of light. In general, they vibrate in all directions, perpendicular to the light ray, in a manner similar to the vibrations of a violin string. A particular type of light, called plane polarized light, is produced when the light vibrations are all oriented in one direction. The polarizing device in most common use is the commercial product, Polaroid. All other light vibration components are absorbed or filtered by the polaroid. The emitted light is plane-polarized. In the simple polariscope, the light can be imagined as emerging from the first Polaroid disk or polarizer and vibrating in a vertical direction. The second Polaroid or analyzer is “crossed” with respect to the first. This means that it is oriented to pass light that is vibrating in a horizontal direction. When Polaroid disks are “crossed” the first Polaroid absorbs or filters all except the vertical components of the light ray, and the second polaroid absorbs or filters all except the horizontal components. All light is therefore absorbed, and the “crossed” Polaroids, each of which is transparent, appear 6
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black when viewed together. The two ray components will have different velocities in the two planes; therefore, while passing through the model, the wave front of one ray will pass the wave front of the other ray a small distance. This distance is called the relative optical displacement of the two ray components. If the two components are combined after leaving the stressed plate by passing them through a second polarizer so that they will vibrate again in the same plane, interference will take place due to the relative optical displacement, 6 Complete interference will occur when 6 = 0, 1, 2, 3, . . . , N, wave lengths; thus furnishing a means of measuring the relative optical displacement. It has been shown theoretically sand
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  • Fall '17
  • YI
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