Seminar: Extreme & Reversed Physical Property Values in Heterogeneous Solids
April 5, 2011
Dr. Roderic LakesProfessor, Department of Engineering Physics
Engineering Mechanics Program
University of Wisconsin, Madison
Friday, April 15, 2011, at 3:00 PM
Warnock Engineering Bldg. Rm. 2230
Reception to follow at 4:00 PM
Normal elastic materials resist both shape changes and volume changes. Rubbery materials, which easily change shape but not volume, become thinner in cross-section when stretched. Poisson’s ratio, a measure of this thinning, is defined as the transverse constriction strain divided by the longitudinal extension strain during stretching. For rubber, Poisson’s ratio is close to 0.5; for most other common materials it is between 0.25 and 0.35.
Negative Poisson’s ratio, which entails a transverse expansion on stretching, is consid- ered counter-intuitive; indeed such materials were once thought not to exist or even to be impossible. We have developed in our laboratory a class of spongy materials with a negative Poisson’s ratio. These materials, by now familiar, become fatter in cross section when stretched, and thinner when compressed.
Negative stiffness entails a reversal in the usual assumed direction between forces and resulting deformations. Negative structural stiffness is known to occur in buckled structural elements, including buckled tubes and buckled single cells of foam. Materials with negative compressibility may be stabilized by constraining them externally or by embedding them in a composite.
Materials with designed heterogeneity including inclusions of negative compressibility can exhibit extremely high values of viscoelastic damping, Young’s modulus (even greater than that of diamond) or thermal expansion.
About Dr. Lakes
Dr. Lakes is a Distinguished Professor in the departments of Engineering Physics and Biomedical Engineering at the University of Wisconsin, Madison.