Biaxial Tension Test Fixture
Mechanical characterization of soft materials is challenging - something I learned firsthand as I built out accurate constitutive models for elastomers. Most have heard of uniaxial tensile testing (or “dogbone” testing), which is sufficient to characterize most linear elastic materials.
For more complex material models that describe the behavior of hyperelastic solids, we often need to gather more extensive characterization data: enter the biaxial test, where a sample is stretched in four directions at once.
I designed this linkage-based pseudo-equi-biaxial strain fixture to translate a single input degree of freedom (the downward motion of the top crossbar) into four nearly-orthogonal motions at crossheads where a sample could be gripped.
This design uses commodity motion components, hardware, and 3D printed pieces, bringing the cost below $100. Importantly, I considered loading direction of each part and its print orientation, in order to maximize strength.
I implemented a linkage solver to study the motion of this design before fabricating. This analysis is important because it quantifies how perfectly biaxial the motion of the crossheads is, as they move in arcs and not straight lines.
I also performed a kinetics analysis to study force transmission through the structure, ensuring that the fixture is stiff enough to accurately measure compliance in soft materials.