The goal of this research is to develop constitutive models for tensile deformation and fracture of elastomers at very high strain rates. Specific objectives are to (a) develop an experimental program that would characterize extensive deformation and fracture of elastomers at impact rates (1-103 s-1); (b) derive appropriate hyper-viscoelastic constitutive equations to predict the dynamic response of elastomeric structures when they are subjected to blast or impact loading; and (c) obtain appropriate dynamic fracture parameters for notched specimens under impact tensile loads. A tensile impact apparatus has been designed to obtain both dynamic stress-strain curves of uniaxial strip specimens and force-extension curves of notched panels. The apparatus uses the pendulum of a Charpy impact machine to impart tensile impact forces in two copper cables by connecting the cables to a slider bar that moves suddenly upon impact from the Charpy hammer. The copper cables are connected to opposite ends of the specimen, where there are dynamic load cells and displacement transducers to measure forces and displacements, respectively. Designing the experiment so there are equal grip separation velocities on both sides of the specimen localizes the middle of the specimen so that high-speed video photography can be used to capture fracture and crack propagation in the notched specimens.
The material data and constitutive models that will be developed in this research will be used in protective design of federal buildings and embassies, which are susceptible to terrorist bombings. The Force Protection Branch at the Tyndall Air Force Research Laboratory has recently found that elastomeric coatings on walls offer significant protection to occupants against blast loading. Unfortunately, high strain rate material data on elastomers that fracture under tension and constitutive models that can accurately predict deformation and failure of elastomeric structures under tensile impact are lacking. Most high strain rate data and material modeling for elastomers is related to the design of earthquake isolation bearings and shock absorbers, where the elastomer (rubber) is in compression and shear and stresses are fairly linear with strain and strain rates. Under impact tensile loading, our experiments show that elastomers are highly nonlinear, hyper-viscoelastic materials, sometimes breaking at 300% strain or more. A technological gap therefore exists and we intend to remove this gap with our research so that the Tyndall AFRL can develop standard design criteria in elastomeric coatings for blast mitigation.
Results from this NSF Small Grant for Exploratory Research will be used toward developing a more substantial proposal for NSF consideration. However, the Force Protection Branch at Tyndall Air Force Research Laboratory will loan us equipment, such as a high-speed video camera and piezoelectric load cells, to ensure rapid success of this research under its present funding level. While it is clear that our research addresses one of the strategic goals set by the Homeland Security Department, development of our experimental program and constitutive models has broader impact than this. Future research will be concerned with using viscoelastic energy dissipation of elastomers to mitigate blast and impact loads of other structures, including military ships, aircrafts and helicopters.