Determination of the strain distribution in a circular plate under uniform pressure for biaxial stress tests

A simple apparatus is described for generating a biaxial stress at the surface of a plastics testpiece in the form of a circular plate. A theory is developed for the calculation of the strain distribution in situations where the deflection of the centre of the plate is comparable with its thickness. Theoretical predictions are compared with strain values determined by a moiré fringe method and using strain gauges. These comparisons are made on two plates of different thickness loaded to pressure levels which produce similar maximum strains in the two plates but different deflection-to-thickness ratios. Conclusions are drawn on the applicability of the theory, and recommendations are made for the testing of plastics plates for failure studies.