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Various methods for the measurement of hydrostatic stress i.e. the isotropic component of the stress tensor, are presented. All these methods lead to a whole-field visualization. In two dimensional problems, the hydrostatic stress is obtained by the measurement of the relative variation of thickness, caused by loading the model ; by using the technique of double exposure holographic interferometry. The birefringence effects are eliminated by making the object beam to pass through the model, and in-between the two passes, the beam is subjected to a 900 rotation of its plane of polarization. In the case of three-Aimensional models, the method is based on the observation of the variations in the absolute index of refraction. The frozen-stress technique requires the use of those photoelastic materials which remain compressible at the "freezing" temperature ; but unfortunately, they are unavailable at present. Therefore, a non-destructive optical-slicing method is proposed and it is hoped that it shall give the required information concerning the hydrostatic stress. This method consists of an interferometric comparison of the two superimposed speckle patterns, each scattered from two neighbairing optically isolated light sheets. Further, the measurement of the in-plane strain can be carried out using either holographic or speckle pattern interferometry ; the precision is found to be the same in both cases.
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