The long-term goal of this research project is to develop constitutive equations and failure criteria for elastomers under high rates of loading. Elastomers are used to mitigate structural damage caused by impulsive or impact loads because of their low modulus, high damping and large extensibility. Rubber isolation bearings, shock pads and shock absorbers are common examples where elastomers protect structures from impulsive forces, but polyurea coatings for building walls, polyester films in glass windows, and advanced polymeric coatings for ship hulls and army vehicles are more recent examples where elastomers save human lives from bomb blasts. The mechanical behavior of the above-mentioned elastomeric structures is dominated by large strain, high strain rate and nonlinear viscoelastic/viscoplastic material response. Yet testing and modeling of elastomers have been confined mostly to quasi-static deformation, creep, relaxation, and small strain, linear viscoelastic vibration response. No one to date has been able to address the underlying physics behind the high strain rate behavior of elastomers. The intellectual merit of the proposed research is that it offers pioneering contributions in the areas of high speed testing and material modeling of elastomers. Specific objectives of this research are to develop an experimental program that would characterize extensive deformation and fracture of elastomers at impact rates; derive physically-based hyper-viscoelastic constitutive equations; and obtain dynamic failure criteria to predict rupture of polymeric structures. Through a series of tensile impact tests, the PI will elucidate some of the high strain rate phenomena, which cause these materials to be their toughest and strongest at critical values of strain rate. The PI has developed a tensile impact experiment to obtain high strain rate tensile properties of rubber and polyurea under prior NSF funding. This work will be extended by using high speed video photography to obtain more comprehensive deformation and fracture properties of the elastomer. A parametric study on the crosslink density of the polyurea/polyurethane as well as the type and content of carbon fillers in rubbers will allow the PI to develop micro mechanism-driven constitutive laws for these elastomers. These rate-dependent constitutive models are much needed in the design and analysis of elastomeric structures under shock and impact. Thus, the broader impact of this research is in prevention of human injuries and fatalities under impulsive, blast or impact loading. The PI is collaborating with Tyndall Air Force Research Laboratory, where walls are retrofitted with elastomers for blast protection; the Pacific Earthquake Research Center at Berkeley, and Bridgestone/Firestone North American Tires in Akron. These agencies will provide technical consultation and materials, and collaboration with them will allow for effective dissemination of results to the scientific community. The PI will also incorporate some of the experimental program into two senior-level design courses at The University of Akron and apply for additional NSF funding to support more undergraduate students, particularly those from under-represented groups. Students working under this grant will obtain valuable hands-on laboratory experience, academic credit for their work, and a modest stipend for their efforts. The experiments will also become part of a spotlight event used to encourage high school seniors to enroll in engineering.
