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for Mechanics of Deformation-Triggered Pattern Transformations and Superelastic Behavior in Periodic Elastomeric Structures

Mary Boyce, MIT

Mechanics of Deformation-Triggered Pattern Transformations and Superelastic Behavior in Periodic Elastomeric Structures

Periodic microstructures abound in nature and provide numerous interesting and unique mechanical, photonic, phononic and hydrophobic properties. Here, novel and uniform deformation-induced pattern transformations have been found in periodic elastomeric cellular solids upon reaching a critical level of mechanical load. This behavior is accompanied by a superelastic stress-strain behavior where, after reaching the critical load, the material deforms elastically to large deformations at nearly constant stress. Numerical simulations utilizing Bloch wave analysis clearly show the mechanism of the pattern switch to be a form of local microstructural elastic instability, giving reversible and repeatable transformation events as confirmed by experiments. The nature of the instability and accompanying pattern transformation depend on the initial periodic pattern. The deformation-triggered transformations have been physically realized for several periodic elastomeric structures including square and oblique arrays of circular holes as well as rectangular arrays of elliptical holes, each subjected to axial compression. A ligament buckling instability was found to trigger the transformations in the square and rectangular lattices and a shear instability triggered the transformation in the oblique lattice. Post-deformation transformation is observed to accentuate the new pattern and is found to be elastic and to occur at nearly constant stress, resulting in superelastic behavior. 
Furthermore, periodic microstructures are a known method to control wave propagation and create materials with tailored band gap structures. Here, we calculate the acoustic band structures of different periodic elastomers at different levels of deformation. We demonstrate the ability to use deformation to transform phononic band gaps. The elastomeric nature of the material makes the transformation in both structural pattern and phononic band gap a reversible and repeatable process, creating a phononic band gap switch.

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