Stress Analysis

Different nondestructive techniques are used to evaluate the stress level in products. They can predict or relate to potential problems. There is the popular electrical resistance strain gauges bonded on the surface of the product. This method identifies external and internal stresses. The various configurations of gauges are made to identify stresses in different directions. This technique has been extensively used for over half a century on very small to very large products such as toys to airplanes and missiles.

There is the optical strain measurement system that is based on the principles of optical interference. It uses Moire, laser, or holographic interferometry. Another very popular method is using solvents that actually attack the product. It works only with those plastics that can be attacked by a specific solvent. Immersed products in a temperature controlled solvent for a specific time period identifies external and internal stresses. After longer time periods products will self-destruct. Stress and crack formations can be calibrated using different samples subjected to different loads.

With the brittle coating system that is applied on the surface of a product one identifies conditions such as stressed levels, cracks, etc. A lacquer coating is applied, usually sprayed on the surface of the product. It provides experimental quantitative stress-strain measurement data. As the product is subjected to a load simulating the load that would be encountered in service, cracks begin to appear in the coating. The extent of cracks is noted for each increment of load. Prior to this action, the coating is calibrated by applying the coating on a simple beam and observing the strain at which cracks appear and relating them to the stress behavior of the beam.

Photoelastic measurement is a popular and useful method for identifying stress in transparent plastics. Quantitative stress measurement is possible with a polarimeter equipped with a calibrated compensator. It makes stresses visible. The optical property of the index of refraction will change with the level of stress (strain). When the photoelastic material is stressed, the plastic becomes birefringent identifying the different levels of stress via color patterns.

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.

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 refraction 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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