Polariscope For Dynamic Stress Analysis

One of the more useful tools in mechanical research is the photoelastic polariscope for stress analysis. The science of photoelastic stress analysis is based on the property of certain transparent materials which causes them to exhibit, when strained, a degree of optical double-refraction exactly proportional to the net stress. Such materials, under stress, are no longer isotropic to light, a ray of which, entering such a stressed material, will travel faster in one plane than in the plane at right angles to it. Further, the retardation in the "slow" plane is a direct function of the causative stress.

Monochromatic polarized light traversing such a stressed material is resolved into two vector components, fast and slow respectively, the effect of —v which, through cancellation and reinforcement depending upon the relative delay, is visible as a pattern of light and dark bands when the model under stress is viewed through a polarizing analyzer.

Since the relative delay is a function of the net stress, the number of bands or "fringes" at a given point indicates net stress at that point. Furthermore, stresses parallel to the direction of plane polarization produce no effect under plane polarized light, so that unaffected areas represent the locus of all stresses parallel to the plane of polarization. Such clear areas are called isoclynics and may be used to determine the direction of the principal stresses. Under circularly polarized light the isoclynics disappear, since circularly polarized light represents a plane polarized light, whose plane of polarization is rotating at the frequency of the monochromatic light employed, thus blurring out the isoclynics. Thus both the magnitude (under circular polarization) and direction (under plane polarization) of the net stresses can be derived from observation of a photoelastic model in the polariscope.*

*M. M. Frocht, "Photo-Elasticity," John Wiley and Sons, N. Y.

Figure 1. View of complete polariscope with incandescent light source and camera.

Figure 1. View of complete polariscope with incandescent light source and camera.

Polariscope Stress Analysis

A polariscope, basically, requires a source of monochromatic light, or a light source and means for effectively securing monochromatism; a polarizer and analyzer, both capable of plane and circular polarization with means for determining the inclination of the axis of plane polarization; and a means of visual or photographic observation of the stress patterns exhibited by a strained photo-elastic model interposed in the light beam between polarizer and analyzer.

The fullest employment of photoelas-tic methods of stress analysis has, in the past, been hampered by the cumbersome and costly nature of the equipment required for visual or photographic observation. If a sizable field for easily manipulated models was required, the optical requirements imposed by the use of available monochromatic light sources, natural polarizing media, and model substances of relatively low photoelastic sensitivity could only be met by expensive, bulky apparatus with relatively weak illumination. Photographic exposures ran into minutes, so that the "bench," as such a setup is termed, had to be carefully designed to minimize vibration or any relative motion be tween its multiple parts. Bench assemblies weighing many tons are commonplace in photoelastic laboratories. Sensitivity of model materials was still further reduced by the choice of monochromatic light of long wavelength, due to the low radiation of blue-violet by most artificial light sources adaptable to the purpose.

Many of the earlier limitations were removed by the appearance of synthetic polarizing materials in sheet form. These relatively inexpensive materials replaced costly natural media and simultaneously reduced the optical problem by removing the narrow dimensional limits imposed by natural polarizers. Nevertheless, photographic exposure times were still sufficiently long to require stability, attained through bulk; and the cost reduction, while appreciable, was not great enough to bring equipment into more general use.

The General Radio Type 1534-A Polariscope removes most of the remaining obstacles to wider use of photoelastic techniques, through the use of the Type 1532-A Strobolume as a light source. The intense, short flash of the Strobolume reduces the exposure time

Figure 2. View of polariscope with Strobolume flash source.

Figure 2. View of polariscope with Strobolume flash source.

Continue reading here: Strobolume Power Unit

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