One method of analyzing stressed conditions in rocks is by photoelasticity. By this method, a photoelastic coating of birefringent material with a reflective backing is cemented to the surface of the rock specimen. When the rock is loaded or destressed, the strain field at its surface is transmitted to the photoelastic coating. This produces a birefringent effect which can be evaluated quantitatively, usually in terms of principal strains difference and their directions. In many cases, this amount of information is not sufficient and may lead to serious errors. To fully describe the state of plane strain, a knowledge of both principal strains and their directions is required. To obtain this information, a method that employs a perforated photoelastic coating is proposed. The photoelastic coating is perforated with circular holes. Around each hole the coating is protected against cementing to the rock surface. The theoretical background for the method is presented and the response of the perforated photoelastic coating to any biaxial strain field is discussed. As an example of how this technique can be applied, this paper briefly describes its use to measure the time-dependent recovery of well cores to establish in situ stress conditions.
Stressed conditions in rocks can be evaluated by photoelastic coatings. Through proper selection of the coating material and its thickness this method can be used to study a wide spectrum of problems which vary from measuring elastic responses of isotropic material to solving complex nonlinear strain-analysis that involve the time-dependent plastic and anisotropic properties of materials. The photo-elastic coatings, when cemented to the surface of the structure, convert the deformation of the structure to a linear output of photoelastic data. The complete analysis of surface strain requires the separation of principal strains which is very difficult to achieve in practice. Usually two measurements of a fringe order at a point or within its vicinity are required to separate principal strains. It is proposed that the accuracy and usefulness of a particular method be evaluated by examining the term
(Equation in full paper)
A new method has been developed by the authors for the measurement of principal strains and their directions in photoelastic coatings. This method employes a coating perforated with circular holes. The backside of the photoelastic coating in the vicinity of the holes is protected against cementing to specimen surface so that the photoelastic material around the hole will act as a biaxial "gage" that is exposed to the average strains throughout the diameter of the protective layer. The proposed procedure covers the whole range of strain changes, i.e. C = e2/e1 (e1>e2) assuming values from -1 to +1.
Since the perforation disturbs the continuity of the photoelastic coating, the principal strains that can be evaluated by this method at the “points” can be at least 6 to 7 diameters of the hole. Thus, the photoelastic coatings can be applied even when there is a small change in strain over the distance between the holes.