1948年应用数学家Ronald Rivlin提出理论当对一块厚的弹性材料施加合适的拉力时，它将突然断裂成平整的薄片，然而过去近70年来未曾有试验验证过这种理论，直到最近哈弗大学的研究人员试验验证该理论是正确的。参考文献：Tensile Instability in a Thick Elastic Body；Johannes T. B. Overvelde, David M. J. Dykstra, Rijk de Rooij, James Weaver, and Katia Bertoldi；Phys. Rev. Lett. 117, 094301 – Published 24 August 2016.
（redit: Johannes T.B. Overvelde/Harvard SEAS）
A range of instabilities can occur in soft bodies that undergo large deformation. While most of them arise under compressive forces, it has previously been shown analytically that a tensile instability can occur in an elastic block subjected to equitriaxial tension. Guided by this result, we conducted centimeter-scale experiments on thick elastomeric samples under generalized plane strain conditions and observed for the first time this elastic tensile instability. We found that equibiaxial stretching leads to the formation of a wavy pattern, as regions of the sample alternatively flatten and extend in the out-of-plane direction. Our work uncovers a new type of instability that can be triggered in elastic bodies, enlarging the design space for smart structures that harness instabilities to enhance their functionality.
Imagine pulling or compressing a block of soft material—like rubber—equally in all directions. You wouldn’t expect the block to deform much because of the nature of the material. However, in 1948, an applied mathematician named Ronald Rivlin predicted that with the right amount of tensile force, a thick cube of soft material would suddenly deform into a thin, flat plate.
For almost 70 years, this prediction remained purely theoretical. Materials scientists, hoping to add the instability to the pantheon of material functionality, were unable to prove the theory experimentally.
Recently, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) demonstrated for the first time experimentally that Rivlin was right. Using tensile—stretching—forces, the team triggered this instability in a centimeters-thick elastomer block, deforming it into a flat surface.
“We knew that this instability existed but no one was able to show it,” said Katia Bertoldi, John L. Loeb Associate Professor of the Natural Sciences at SEAS and senior author of the paper. “We were able to identify a configuration that can be tested experimentally.”
“This research uncovers a type of instability that can be triggered in soft, elastic bodies, and widens the design space for new architected materials that use instabilities to change or enhance their functionality,” said Johannes T. B. Overvelde, first author of the paper and former graduate student at SEAS. “With this instability, we can create materials that can suddenly switch between behaviors by using simple triggers to change their geometry.”