Photoelastic relation

Fig. 5-2 Photoelastic stress patterns for these two molded products during the same production run shows that the processing conditions have changed right view relates to why the product fails in service.

Through the photoelastic effect, stress will modify the refractive index of silicon according to the relations... 

It is also useful to be able to study stress distributions experimentally. Some materials, especially glasses and polymers, are photoelastic, i.e., they undergo a change in refractive index under the application of mechanical stress. When viewed under polarized light, meehanical models made from these materials give rise to an optieal pattern that can be related quantitatively to the principal stress distribution in the loaded body. Figure 4.26 shows a phototelastic fringe pattern... 

Photoelastic analysis, one of several related testing techniques, is easy to use and usually a more economical and positive method than computer analysis. From the information it provides, the test can lead to better-designed, lower-cost products. Traditionally used to test the integrity of metal parts, photoelastic analysis is now being used to physically test thermoplastics as well as thermosets. For transparent plastics, the analysis can be made directly on the plastic. For nontransparent plastics, a transparent coating is used. Actual parts and representative models can be tested by a simple procedure. The former may be stressed under actual use conditions, whereas models are tested under simulated conditions. 

An acoustic wave can produce a strain causing a change in the index of refraction via the photoelastic effect (Dixon, 1967). It has been shown (Pinnow, 1970) that the change in the index of refraction is related to the acoustic power Pa by the expression... 

The real possible application is in the area of polarization compensators for the photoelastic analysis of the tension effects in hard bodies. Most of all it is possible to prepare such difficult anisotropic films based on GB-elements, with any size, which stmcturally will be the negative of the tension anisotropic object, i.e., polarization conjugate filter. Similar compensators will give us not only the possibilities of instantly establishing the isochromous and isoclinal effects in the anisotropy of the object, but will be related to the identification of stress in the objects. This is very important in the identification of different faces and objects. 

The photoelastic technique relates to the Brewster s Constant law. It states that the index of refraction in a strained material becomes directional, and the change of the index is proportional to the magnitude of the strain present. Thus a polarized beam in a clear plastic splits into two wave fronts (X and Y directions) that contain vibrations oriented along the directions of principal strains. The index of refi action in these directions is different and the difference (or birefringence) is proportional to the stress level. Result is the colorful patterns seen when stressed plastic are placed between two polarized filters providing qualitative analysis. Observed colors correspond to different levels of retardation at that point, which in turn correspond to stress levels. 

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