Dynamic Impact Characterization of Al+Fe 2 O 3 +30% Epoxy Composites Using Time Synchronized High-Speed Camera and VISAR
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Dynamic Impact Characterization of Al+Fe2O3+30% Epoxy Composites Using Time Synchronized High-Speed Camera and VISAR Measurements Louis Ferranti, Jr. and Naresh N. Thadhani School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, U.S.A. ABSTRACT Reverse Taylor anvil-on-rod impact experiments were conducted on Al+Fe2O3+30% epoxy composites to measure their viscoelastic and fracture response to dynamic loading. Impact velocities ranged from 80 to 200 m/s. High-speed camera images capturing transient deformation reveal these materials exhibit significant elastic recovery in both the longitudinal and radial directions. Images were time synchronized with free surface velocity measurements, using VISAR, to track elastic/plastic wave interactions attributed to the material’s dynamic loading response. Some specimens underwent brittle fracture once a critical areal strain was exceeded while the axial strain response appeared unaltered. INTRODUCTION The well known one-dimensional wave theory of Taylor [1] for the mushrooming of flatended metallic projectile upon impact with a rigid anvil is based on momentum conservation across the plastic wave front. Knowing the original length and density of the projectile, and impact velocity, Taylor was able to estimate the dynamic yield strength based on the overall length change of a recovered deformed rod. The theory of Taylor assumed an ideally rigidplastic material model that does not account for elastic wave effects. Taylor impact experiments have commonly been performed on metallic systems, although some work has been carried out on polymers such as high density polyethylene [2,3] and low density polyethylene [3], thermoplastic polyether ether ketone (PEEK) [4], and more recently particle reinforced epoxy composites [5]. However, for the case of polymeric material impacts, elastic strains in general are not negligible compared to plastic strains and the rigid-plastic material behavior assumed by Taylor, which inherently neglects elastic strains, cannot be applied. Briscoe and Hutchings [2] studied the impact yielding of a high density polyethylene using Taylor’s analysis and found uncharacteristically high values of flow stress. Large elastic strains are not accounted for in the theory, which results in over estimate of flow stress. In the present work, instrumented reverse Taylor anvil-on-rod impact tests were performed on Al+Fe2O3+30% epoxy composites at impact velocities ranging from 80 to 200 m/s. Time synchronized VISAR (Velocity Interferometer System for Any Reflector) and high-speed digital images were used to measure the material’s dynamic strain response to impact loading. EXPERIMENTAL DETAILS Cast Al+Fe2O3+30% epoxy specimens were prepared by dispersing a stoichiometric mixture of aluminum (spherical particles 2 µm) and iron-oxide (sub-micron platelet shaped) powder in an epoxy binder. This particular composition was chosen because it represented the minimum amount of epoxy that could be combined with the powders
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