The focus of this study is to improve the operation of a biaxial tensile test fixture for use in characterizing hyperelastic materials. The test fixture in this study was constructed based on a design developed by Brieu, et al. [1]. Due to a combination of manufacturing and design issues the original test fixture was not able to provide acceptably accurate stressstrain data. Based on visual inspection of the machine during operation, it was hypothesized that the as-built system over-constrained certain components, which resulted in binding of the specimen grips. This binding made large displacement tests inaccurate and added to the error introduced in obtaining a 1:1 equibiaxial load ratio on a test specimen. A minimum constraint design (MCD) analysis was conducted in order to establish the degree to which the original design was over-constrained, and to ensure that the newly proposed system would function properly. Due to symmetry, the design analysis of the test fixture was simplified to one half of the machine. A modified design of the test fixture was subsequently developed and built to satisfy the minimum constraint design theory. Modifications include an alignment shaft, spherical bushings, and a planar alignment plate. The new fixture design exhibits no binding and an improved biaxial load ratio. Previously, only load ratios of 0.85:1 had been achieved, but the new design achieves the desired equibiaxial loading. Tighter component tolerances have also reduced backlash when reversing from tension to compression during a test, improving the ability to obtain load-displacement data for both the load and unload paths of a specimen.

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