Physicists say to measure something it need change something into it. To measure temperature the body it need heat a thermometer and cool the body. To measure weight of the body it need embed a scales between the support and the body. What do it need do to measure a tension into the body? Do it need destroy the body? No. It is possible measuring without mechanical interaction with body. It is possible with photoelasticity method.
The photoelasticity method is a non-destroying method of defining stresses in transparent, optically isotropic in unstressed bodies (amorphous bodies and crystals). The objective for the method application is plastic, glass and crystals of cubic symmetry of their crystal lattice. The important advantage of photoelasticity method is its ability to determine residual stresses, i.e. the stresses present in a body without any external influence on it.
What is photoelasticity?Photoelasticity is a full-field technique used in stress analysis testing. Initial observation of a photoelastic pattern provides quick qualitative analysis of the overall stress distribution, and then accurate quantitative data at any selected point is easily obtained using straightforward measurement techniques and modern optical instrumentation.
When a photoelastic material is subjected to load and viewed with polarized light, colorful patterns are seen which are directly proportional to the stresses and strains in the material. The photoelastic color sequence observed with increasing stress is: black (zero), then yellow, red, blue-green, yellow, red, green, yellow, red, green, etc. The transition line between the red to green colors is defined as a "fringe" (from the phenomenon of birefringence). As the number of fringes increases, the stress increases proportionally. When these color bands, or fringes, are closely spaced, the stress gradient becomes steeper. Conversely, the appearance of a uniform color represents a uniformly stressed area. In brief, the overall stress distribution can easily be studied by recognizing fringes, their numerical order, and spacing with respect to one another.
Photoelasticity embraces three broad categories: 1) Photostress, or photoelastic coating; 2) two-dimensional model analysis; 3) three-dimensional model analysis.
With PhotoStress, a specially formulated plastic coating is bonded to the actual test part or structure. Then, as test or service loads are applied, the coating is illuminated with polarized light from the measurement instrumentation, in this case called a reflection polariscope. Quantitative analysis of both the strain direction and magnitude at any point on the coated surface is easily performed with the reflection polariscope and its companion digital strain indicator.
For two-dimensional model analysis, design geometries and cross sections are made from flat sheets of photoelastic material and analyzed in a transmission polariscope. Preliminary studies with two-dimensional models offer an expedient and cost-effective approach for obtaining stress information early on en the design process.
As a design crystallizes, three-dimensional model testing, using the stress-freezing prodess, may be appropreate. With this technique, a scaled three-dimensional model is made from a specially formulated epoxy plastic. The model is then placed in an oven and, with forces applied, subjected to a carefully prescribed heating-cooling cycle to "freeze" the deformations resulting from stresses. After stress-freezing, the model is sectioned to permit removal of slices from various planes of interest. Slice examination takes place in a specially configured transmission polariscope to reveal and measure the complete stress distribution in the plane of the slice.
The primary benefits and selection criteria for use of these photoelastic testing methods are described on the facing page.
What troubles have the photoelasticity method?
Photoelasticity method is an inverse problem. The primal problem is stress searching when there are photoelasticity measurements. It is a piezooptic effect. Primal and inverse problem is usually very strong different problems about unstability. Primal problem is usually stability one. Inverse problem is unstable. Piezooptic effect is a primal and linear problem therefore is stable. Photoelasticity method is inverse and unstable problem. Problems such this are most often implicit. As a rule any of inverse problems is ill-posed problem. Photoelasticity method is just so problem. Solving of unstable and implicit problem is very difficult process.
Here you can make some calculation of the tension
In later versions
of this page I'll provide more information about the photoelasticity method.
