Roderic Lakes
University of Wisconsin-Madison
Seminar Information
Materials with unusual physical properties and extremely high, even singular values of physical properties are developed. Conceptual, synthesis, and characterization aspects are presented. Negative Poisson's ratio, which entails a transverse expansion on stretching, is considered counter-intuitive; indeed such materials were once thought not to exist or even to be impossible. Indeed, 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. For rubber, Poisson's ratio is close to 0.5; for most other common materials it is between 0.25 and 0.35. We have developed a class of spongy materials with a negative Poisson's ratio. These materials become fatter in cross section when stretched, and thinner when compressed. Negative Poisson's ratio also occurs in phase transitions in ferroelastic solids. 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. Negative material compressibility is claimed to be impossible in some thermodynamic texts but is demonstrated in the laboratory. Negative compressibility is not in fact illegal, it is only unstable in a block with free surfaces. 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 approaching a singularity, high Young's modulus (even greater than that of diamond) or thermal expansion. Such behavior exceeds the usual theoretical bounds. The reason is that assumptions made in deriving the bounds can be relaxed in certain materials and microstructures. Too, materials inspired by (not imitating) hierarchical materials of biological origin, can be designed to exhibit extremely high values of specific strength, energy dissipation, thermal expansion (exceeding the usual bounds), or toughness.
Rod Lakes is a Distinguished Professor in the Department of Engineering Physics at the University of Wisconsin and is also affiliated with the Department of Materials Science. He has taught courses in composite materials, experimental testing of materials, viscoelastic materials, biomechanics, and biomaterials among others. Honors include fellow of the American Association for the Advancement of Science (AAAS) and the American Society of Mechanical Engineers, and several teaching awards. He is the author or coauthor of five books including Viscoelastic Materials, Biomaterials (2nd and 3rd Edition with J. B. Park), and Composites and Metamaterials and is author or co-author of 15 chapters in books and more than 310 articles in archival journals. Research activities have led to the following; some ideas were derived from teaching. Lakes developed the first 3D materials with a negative Poisson's ratio, in later years called auxetic. Lakes has developed the first materials with arbitrarily large magnitudes of positive or negative thermal expansion. Zero thermal expansion is also attainable. Lakes has developed the first extreme materials based on negative stiffness inclusions in composites, developed theoretical framework for elastic chirality, the first experimental measurements of elastic chirality and the first design and making of elastic chiral materials. In recent years, materials with reversed or extreme properties have been called metamaterials. Lakes' research interests also include viscoelastic spectroscopy, development of high performance viscoelastic materials and development of high performance hierarchical materials. Lakes took several mathematics courses at Columbia University while a high school student, then earned the B. S. in Physics at Rensselaer Polytechnic Institute, attended the University of Maryland then returned for a Ph. D. in Physics, at Rensselaer Polytechnic Institute. This was followed by activity as Research Associate at Yale University, and as faculty at Tuskegee Institute and at the University of Iowa prior to moving to the University of Wisconsin.