Stress analysis, photoelastic

Photoelastic stress analysis helps the processor to determine why a part broke and how to prevent similar failures in the future. Parts ranging in size from structural composites to tiny thermoplastic heat valves can all be 

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. 

Although theoretical analytical methods such as finite element analysis offer one a chance to solve complex stress problems, there are many causes of strain in parts that cannot be reliably tested by these expensive computer-oriented techniques. For instance, strains due to the assembly of components and those caused during processing are extremely difficult problems to analyze without physically testing the part. 

The degree of strain is indicated by a fringe order, which is simply a collection of black bands appearing in close proximity to each other between colors in the stress pattern. As the stress configuration increases, so do the number of black bands in a fringe order. 

Plastics provide the processor with materials that are useful, meet product requirements, produce simple to complex shapes, and are economically beneficial (Fig. 11-1). They can be made to have a long life, to resist corrosive environments, to be degradable, to be recyclable, and to meet prac- 

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Other interference-produced colors falling into this section include doubly refracting materials such as anisotropic crystals and strained isotropic media between polarizers, as in photoelastic stress analysis and in the petrological microscope. 

This photoelastic stress analysis is a technique for the nondestructive determination of stress and strain components at any point in a stressed product by viewing a transparent plastic product. If not transparent, a plastic coating is used such as certain epoxy, polycarbonate, or acrylic plastics. This test method measures residual strains using an automated electro-optical system. 

This high-speed (El nearly 1000) color Film has high resolution and eliminates the need for a minus-blue filter on the camera. Schlieren and photoelastic stress analysis can conceivably gain in contrast with this new film. In addition, it should be less affected by smoke and other explosive debris which scatters the shorter wavelength light (blue) more than red light... 

Photoelastic stress analysis is a powerful full-field technique where a product part is first covered with a thin film of special, transparent plastic. The layer must be bonded on with a reflective adhesive. The part is then deformed under static conditions and illuminated with polarized light. Viewing the part with a special polarizing optic system... 

In basic mechanical testing, the mechanical characteristics that can be tested include expansion, penetration, extension, flexure, and compressive compliance. Photoelastic-stress analysis allows the stress distribution to be visually displayed, and strain gauging allows the stress distribution to be approximated. Residual stress, also known as molded-in stress, can be measured by a variety of techniques (1). 

Cross-linked or thermosetting polymers are typically used for photoelastic stress analysis. Thus, it is clear that a certain amount of crystallinity can be induced by stresses in network polymers but the degree of crystallinity is necessarily very small. 

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