Controlling Local Compliance to Probe the Biaxial Failure of Soft Elastomers in a Cruciform Geometry

Characterizing the multiaxial mechanical behavior of soft solids is a critical step in understanding their response in real world applications where complex stress states can arise during loading. During multiaxial deformation, aspects of the mechanical behavior of soft solids including the large strain constitutive response, yield stress, and ultimate failure point will all be sensitive to the stress state that develops in the material. While this is well-established in the mechanics of stiff solids such as metals and glassy or semi-crystalline polymers, multiaxial deformations are underexplored in rubbery soft polymers. Here we explore the use of cruciform samples to produce biaxial loadings to the point of failure in silicone networks. Cruciform samples contain a center square and four "legs" that connect that square to a biaxial stretching instrument. While a controlled biaxial state of stress develops in the center square each leg is subjected to a predominately uniaxial stress state. Due to this, it is found that probing the biaxial failure of the material requires controlling the local compliance of the center square to be greater than the compliance of the connecting legs. Otherwise, sample failure ends up being driven by the uniaxial deformation that occurs in the legs. Further, it is found that the onset of failure is influenced by the corner radius at the junction between the center square and the legs. These findings are important to understanding multiaxial behavior of soft solids and will enable rigorous characterization of the failure of soft materials subjected to controlled biaxial stress states.