The Optical Method of Reflected Caustics Applied for a Plate with a Central Hole: Critical Points and Limitations

Some critical points related to the applicability of the optical method of reflected caustics in the case of a plate with a central hole are explored in the present paper. The study is carried out in a combined experimental and numerical procedure: poly-methyl-meth-acrylate plates with a central hole are subjected to uniaxial tension and the diameter of the caustics developed is measured for various load steps from a series of successive photographs with the aid of suitable software. The remote stress is then calculated according to the method of caustics and the results are compared with those of the load cell. In addition, the finite element method is employed for the determination of the stress field and the shape and magnitude of the plastic zones around the hole. Conclusions are drawn concerning the range of the external load for which the results of the method of caustics are valid as a function of the relative dimension of the width of the plate versus the diameter of the hole. In addition, the factors influencing the results of the method are considered and the sensitivity of the results to the even the slightest misalignments of the experimental set-up, to the kind of the optical setup preferred and to the slightest deviation from flatness of the lateral faces of the plate is pointed out. This sensitivity of the method was proved to be responsible for discrepancies between theory and experiment, which sometimes exceed 100%.