Thursday, January 31, 2013
Mechanical and Civil Engineering Seminar
On Measuring the Dynamic Response of Soft Materials
Rodney Clifton, Professor, Engineering, Brown University
Measuring the dynamic response of soft materials, such as elastomers and biological tissues, presents some special challenges. This talk will describe how some of these challenges are being met by pressure-shear plate impact (PSPI) experiments for studying the response of an elastomer (polyurea) at high pressures and high strain rates as well as by torsional wave experiments (TWE) for measuring the viscoelastic properties of biological tissues (vocal folds) at phonation frequencies. Brief mention will be made of the extension of the latter work to measuring the viscoelastic properties of heart valves. For the PSPI experiments, plane wave reverberations through the thickness of thin polyurea samples bounded by high impedance elastic plates are used to determine the isentrope for this material. Unloading waves are used to extend the isentrope into the tensile regime and determine a failure stress for tension under uniaxial strain conditions. Shear waves propagating through the thin polyurea samples are used to measure the shearing resistance of polyurea at high pressures (up to 9 GPa) and high shear rates (up to ). For the TWE experiments, thin circular disks of a biological tissue, or a mechanically similar synthetic material, are sandwiched between a coaxial bottom disk driven by a galvanometer and a coaxial top disk that adheres to the top face of the tissue sample. The frequency dependence of the amplification factor relating the rotation of the top plate to that of the bottom plate is used to determine the complex shear modulus of the sample. Suggestions will be made for modifications to both the PSPI experiments and the TWE experiments in order to address additional challenges in the measurement of the dynamic response of soft materials.