Fenrikree A new method for studies of clinical mechanics in prosthetic dentistry Dental Materials,pp. Upon the application of stresses, photoelastic materials exhibit the property of birefringence, and the polariscole of the refractive indices at each point in the material is directly related to the state of stresses at that point. The two basic kinds of setup used are plane polariscope and circular polariscope. Photoelasticity The isoclinics change with the orientation of the polariscope while there is no change in the isochromatics.
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Such an arrangement produces what is called a circular polariscope, because the effect of the quarter-wave plates is to produce circularly polarized light between the polarizers. Assuming that the polarizers are crossed to produce a dark field, the polariscope is then described as a circular dark-field polariscope.
Referring to the photoelastic equation given previously the observed intensity is given by Both of these equations refer to a linear polariscope; if quarter-wave plates are now inserted, the first sine term vanishes refer to discussion in Dally and Riley, Experimental Stress Analysis, pp. Another benefit of using quarter-wave plates is that they allow the use of interpolation methods such as null-order compensation and Tardy compensation to determine fringe orders more accurately.
Specifically, the Tardy compensation method is commonly used to obtain fractional fringe orders to within a few hundredths of a fringe. Tardy Compensation A plane polarizer quarter-wave plates removed physically or optically is first used to find the directions of the principal stresses at any selected point. The system is now in the standard circular dark-field polariscope condition. In rotating the analyzer up to 90 degrees, the polariscope is changing from a dark-field configuration to a light-field configuration.
This change will cause a fringe to move a half-order. Many polariscopes index a half-circle scale in terms of divisions, so that the fractional order can be read off directly from the scale. Scaling Finally, it must be specified how the stress values obtained from observing a plastic model under one load can be scaled up to give the proper results for a real prototype under a different load. This is discussed in the textbook on page 99, under section 5. Basically, the result is where subscripts m and p refer to model and prototype, P is the load scale factor, s is shearing stress factor, a is the dimensional scale factor, and d is the thickness scale factor.
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By studying the fringe pattern one can determine the state of stress at various points in the material. For materials that do not show photoelastic behavior, it is still possible to study the stress distribution. The first step is to build a model, using photoelastic materials, which has geometry similar to the real structure under investigation. The loading is then applied in the same way to ensure that the stress distribution in the model is similar to the stress in the real structure. Isoclinics and isochromatics[ edit ] Isoclinics are the loci of the points in the specimen along which the principal stresses are in the same direction.
Photoelasticity Circular Polariscope