A Photoelasto-Plastic Experiment by the Combined Use of the Scattered-Light Method

Abstract

The authors have recently reported a new method for the separation of principal stresses, which combines the conventional method with the scattered-light method applied in only one direction. The separation of principal stresses can be carried out by combining the isochromatic fringe order and the isoclinic parameter measured by the conventional method with the fringe order distribution obtained by the scattered-light method. By using the circularly polarized light in the scattered-light method, fractional fringe orders may be determined from the rotated angle of the polarizer, similar to Tardy's procedure for the conventional method.
In this paper, as an extension of the above-mentioned new method of principal stress separation to the plastic range, a photoelasto plastic experimental result is reported. The model material used was nitro-cellulose with good transparency, whose optical and mechanical behaviors, such as the variation of stress sensitivity in the plastic range and the yield criterion, had been examined previously. Clear fringe patterns could be obtained in the scattered-light method by using the ordinary mercury lamp as a light source. As the shape of the specimen, a flat plate with a longitudinal slot was adopted, the ratio of length to width of the slot being varied in four ways. Tensile loads were applied to the specimens in seven steps from the incipient yielding to the penetration of plastic regions. From the experimental results, the principal strains were calculated by using the Prandtl-Rueβ strain incremental equation.
The experimental results are summarized as follows. In the stress distribution along the cross section through the upper edge of the slot, the principal stress σ₁ showed the peak value at some distance from the boundary of the flat plate, due to the strengthening interference between the hole and the free edge. In all the specimens the penetration of plastic regions occurred in the same direction. The load required for the penetration of plastic domains became smaller with the increase in slot length, which was contrary to the tendency at the primary yielding.