PHOTOELASTIC TECHNIQUES FOR DETERMINING STRESS DISTRIBUTION
The mathematical analysis of stress distribution in anisotropic substances, such as are found in the earth's crust, are often very difficult and time consuming, and three-dimensional problems may be practically intractable. The techniques offered by photoelastic analysis enables problems of stress distribution to be solved very rapidly and with a high degree of accuracy. The technique consists of making models of the structure from certain transparent celluloid, plastic, or gelatin materials, subjecting them to stress, and observing the models bv transmitted polarized light. The model materials which are normally isotropic become optically active and doubly refracting. When viewed through a polariscope a series of dark lines (isoclinic lines) are seen in the model from which the orientation of the principal stresses and their values may be determined. (For details of this technique see Coker and Filon, 1931 ; Filon, 1936; Frocht, 1948, 1963). For many years this method has been employed by engineers for determining the distribution of the stress in beams and frames of complex shape, and around holes and notches. It has been used for discovering the stresses in some geological structnres (e.g., Currie, Patnode and Trump, 1962), and it may well become an important tool for future research. For the investigations of stress distribution in three dimensions the normal photoelastic technique outlined above is modified. Normally, when the load is removed from the model, the optical activity disappears as the elastic strains are recovered. If, however, the model is loaded and then heated and allowed to cool while under load, the stress patterns disappear as the material internally adjusts itself to the load. If now the model is cooled and the load removed, the model becomes optically active. The patterns of the isoclinic lines in this frozen stress pattern are identical to those produced normally by loading. The model can be sliced up, and the stress distribution in the slices is unaltered. This technique would appear to have a considerable potential for investigating the stress in complex three-dimensional structures. For further details the reader is referred to the work of Hetenyi (1950).