Strain-controlled bulge test

A closed-loop approach is adopted to implement strain rate control during the bulge test. Due to the difficulty of measuring strains directly, the technique is based on the conversion of displacement measurements to the corresponding strains using the plane-strain formulation. The necessary temporal evolution of the midpoint displacement of a rectangular diaphragm is derived under the condition of constant strain rate and is imposed as a control criterion. The technique is demonstrated on 500-nm-thick Au diaphragms by applying strain rates ranging from 2 × 10⁻⁶ to 2 × 10⁻⁴ s^–1. By measuring the corresponding yield strength values, a strain rate sensitivity of 0.11 is obtained, which is close to what was previously reported on similar specimens using the microbending test.