Wisconsin Distinguished Professor,
Department of Engineering Physics, Engineering Mechanics Program, Department of Materials Science, Rheology Research Center, Department of Biomedical Engineering.
University of Wisconsin
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We study materials with heterogeneous structure, including natural viscoelastic composites such as bone, ligament, tendon and wood, as well as synthetic composites, biomaterials, and cellular solids with structural hierarchy. We investigate the freedom of natural and synthesized materials to behave in ways not anticipated in elementary continuum representations, to ameliorate stress concentrations, and to attain physical properties of much higher magnitude than anticipated from standard theories. Viscoelastic materials are of particular interest as high performance damping materials and as practical materials which undergo creep in industrial settings. We determine viscoelastic properties including internal friction, dependence on strain rate, and creep over eleven orders of magnitude of frequency and time, with no need for temperature shifts.
In our laboratory we synthesize and characterize materials with extreme and unusual physical properties. Materials which undergo phase transformation are of interest in the context of viscoelastic damping and of negative stiffness. We have developed new materials with reversed properties, including negative Poisson's ratio (auxetic), negative stiffness, and negative thermal expansion. Designed materials can have thermal expansion or piezoelectric sensitivity of arbitrarily large magnitude. Designed lattices have zero thermal expansion. Composite materials stiffer than diamond over a temperature range have been demonstrated in the lab.
We also study practical composite materials such as dental composites for tooth restorations and aircraft composites in the context of damage resistance and damage tolerance as well as moisture ingression. Piezoelectric composites and lattices, chiral elastic lattices, as well as thermoelastic composites and lattices are presently of particular current interest. We pursue basic research as well as applied research for industry. Industrial research has included high temperature performance studies of alloys for small engines, improved shoe insoles, and advanced dampers.