Controlling the local compliances of cruciform samples to probe equibiaxial failure

Multiaxial loading states are known to develop in common design structures such as corners, joints, and thin-walled shells. Despite the prevalence of multiaxial stress states in design, the characterization of multiaxial behavior in soft polymers and gels has lagged behind that for stiff materials where standards have been developed to perform such measurements. Given the lack of standardization, determining an appropriate geometry and method to probe the multiaxial mechanical response of soft materials falls under the purview of the individual researcher. Herein cruciform samples capable of quantifying the failure behavior of soft polymers under biaxial tension are designed. Using digital image correlation to quantify the local deformations, it is found that controlling the relative compliances of the legs to the center square is key to observing multiaxial failure. Further, controlling the transverse stiffness of the legs is found to significantly impact the uniformity of the deformation state that develops in the center square. Finally, the failure stresses measured in cruciform samples with varied corner geometry are found to be in reasonable agreement with independent measurements of the failure stress from uniaxial extension and equibiaxial inflation. These findings have strong implications for the design of structures where multiaxial stress states develop during regular use.