Effect of the mechanical state of a photoelastic transducer on its readings

Abstract

Photoelastic transducers in the form of cylinders with axial holes, made from optically sensiUve materials such as glass and used for measuring stresses in a rock mass, are installed in boreholes and glued on the outer side to the borehole wails at a considerable distance from the end face. In the planes normal to the borehole axis, the rock mass may be under conditions close ~o the state of plane stress (for example, near the periphery of the mine workings) or plane strain (further away from the periphery). The photoelastic transducers, which may have different lengths, depending on the required sensitivity and on their weak or firm attachment to the boreholes (which depends on the strength and quality of the glue), may also be in different mechanical states.If the transducer is not attached (or only weakly attached) to the borehole wails, it may take up only radial deformations of the borehole walls. In this case, deformations of the transducer along its axis are not impeded and, whatever the mechanical state of the rock, and for anylength of the transducer, the latter will be in a state of plane stress. In practical work, this is undesirable.If the transducer is completely attached to the borehole and the rock mass is in a state of plane strain or plane stress, the ~ansdueer will operate in a state of plane strain.If the photoelastic transducer is installed in the zone of influence of the end face of the borehole, where the stressed state of the rock is three-dimensional, it is even more difficult to determine the character of interaction of the transducer with the rock mass than in the first two cases, when the transducer is located far from the end face of the borehole.The stresses in the transducer are given by the formulas [2]