The theory and design of photoelastic load gauges incorporating glass element transducers

Abstract

A glass cylinder compressed across a diameter can be used as an effective load transducer by utilizing the birefringent properties of the glass. Calibration of applied load in terms of fringe order can readily be obtained by using a suitable polariscope. Incorporating the basic cylindrical transducer into a steel body greatly extends the load-measuring range, enabling a flexible load-gauge system to be developed. Stresses applied to the glass and steel components are kept to well below their respective elastic limits which, combined with the absence of moving parts, gives tise to gauges of excellent long-term stability. The various factors affecting the design and performance of this type of load gauge are discussed herein. The stress distribution and photoelastic effects within the glass cylinder are considered, together with its mode of deformation under diametral compression. Attention is also directed towards determining the stresses and displacements in various gauge components; in this connexion, an analytical solution is given for the deformation of a circular hole in a uniaxially loaded strip of finite width. A range of theoretical calibration curves for column-type and ring-type gauges is given, and further aspects such as temperature susceptibility and long-term stability are examined. An example of the design and application of a photoelastic load gauge which has been used in practice is also included.