Basic Physical Properties Relied upon in the Frozen Stress Technique

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Abstract
The “Stress Freezing” method is a useful means of solving problems of three-dimensional stress by photo-elasticity. The method makes use of the fact that some thermo-setting resins having photo-elastic properties possess considerable residual thermo-plasticity. When softened by heat they obey Hooke's law, though the Young's modulus is, of course, very much reduced. At a given temperature, the birefringence, as well as the strain, has been shown to be proportional to the stress. Both strain and birefringence persist after cooling and unloading, and even after slicing the model. This behaviour is illustrated by means of a simplified concept. In the U.S.A., Glyptal resin is used (Bakelite B.T. 61 893), but this material cannot be had in thicknesses greater than 1 inch. Glass-clear phenol-formaldehyde resin (Catalin 800), which gives a higher stress-optical coefficient, can be had in larger sizes. The properties of both materials are compared, and troubles such as “rind effect” are discussed. Some experimental results obtained with Catalin 800 are given by way of illustration. The suitability of this material has been under investigation for some time at the Royal Aircraft Establishment, Farnborough, and careful tests have shown that, when suitably cured, Catalin 800 can give reliable results, but that “rind effect” is still a serious drawback.

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