Measurement of absolute retardation in photoelasticity using a digital image processing system

A new method for measuring the isoclinics and absolute retardations along the principal stress directions at a point in a loaded photoelastic specimen is presented. In this method, a plane polariscope is used with the digital image processing system. Two images of a loaded specimen are captured at different angular positions of the polarizer and analyzer. Isoclinics and retardations at a point are evaluated from the light intensity values measured on these recorded images. The present method for the measurement of absolute retardations overcomes the difficulties in the measurement of retardations in birefringent specimens using the conventional interferometric methods. To verify the effectiveness of this method, principal stresses are directly evaluated using the values of retardations and these principal stresses are verified with the theoretical solutions. The photoelastic specimens - a circular disk under diametral compression and a simply supported beam with a load at its center - are analyzed.