Multi-Scale Traction Dynamics in Obliquely Impacted Polymer-Metal Targets

The characterization of polymer behavior at high strain-rates is a criticalarea of research driven by their use as adhesives, structural components, or evenas binders for energetic systems. Current experimentation has been limited to eitherlow strain-rate mechanical testing or plane strain (uniaxial) impact testing.As such, more complicated loading conditions at high strain-rate have remainedunexplored. Particularly of interest include the rate dependencies of polymerstrength as well as interface traction behaviors like adhesion and dynamic frictionphenomena. To investigate these characteristics, fully dense, high durometer,polyurethane (PUR) and epoxy were subjected to combined pressure-shear loadingvia oblique impact experiments. Two distinct configurations of oblique impactexperiments were used to investigate both shear strength and friction behaviors.Oblique impact resulted in high strain-rate (105s��1) combined normal (pressure)and shear stress loading of polymers with magnitudes of approximately 800(pressure) and 120 MPa (shear) respectively, depending on impact velocity and angle.Material response was inferred from free-surface particle velocities measuredusing transverse photon Doppler velocimetry techniques. The impact of a 7075-T6aluminum projectile onto a target consisting of a thin polymer specimen confinedbetween two anvils of the same aluminum allowed for the measurement of polymershear strengths. Strength was found to increase with higher confining normalstress, increasing strength by almost an order of magnitude. This normal stress(or pressure) dependence was observed to have a greater effect on strengtheningthan that of strain-rate alone. The oblique impact of a polymer projectile against a7075-T6 aluminum target provided both a quantification for coefficient of friction,m and additional material strength information. Polyurethane and epoxy werefound to have m values of approximately 0.11 and 0.26 respectively, though withviscoelastic effects distorting the latter. Results are in agreement with previous experimentation.The role of adhesion is discussed and, in agreement with literature,it is speculated that the strength of adhesion is greater than or equal to that of thebulk polymer