Publications

Geomechanics of penetration :laboratory analog experiments using a modified split hopkinson pressure bar/impact testing procedure

Gettemy, Glen L.; Holcomb, David J.; Bronowski, David R.

This research continues previous efforts to re-focus the question of penetrability away from the behavior of the penetrator itself and toward understanding the dynamic, possibly strain-rate dependent, behavior of the affected materials. A modified split Hopkinson pressure bar technique is prototyped to determine the value of reproducing the stress states, and mechanical responses, of geomaterials observed in actual penetrator tests within a laboratory setting. Conceptually, this technique simulates the passage of the penetrator surface past any fixed point in the penetrator trajectory by allowing for a controlled stress-time function to be transmitted into a sample, thereby mimicking the 1D radial projection inherent to analyses of the cavity expansion problem. Test results from a suite of weak (unconfined compressive strength, or UCS, of 22 MPa) concrete samples, with incident strain rates of 100-250 s{sup -1}, show that the complex mechanical response includes both plastic and anelastic wave propagation, and is critically dependent on incident particle velocity and saturation state. For instance, examination of the transmitted stress-time data, and post-test volumetric measurements of pulverized material, provide independent estimates of the plasticized zone length (1-2 cm) formed for incident particle velocity of {approx}16.7 m/s. The results also shed light on the elastic or energy propagation property changes that occur in the concrete. For example, the pre- and post-test zero-stress elastic wave propagation velocities show that the Young's modulus drops from {approx}19 GPa to <8 GPa for material within the first centimeter from the plastic transition front, while the Young's modulus of the dynamically confined, axially-stressed (in 6-18 MPa range) plasticized material drops to 0.5-0.6 GPa. The data also suggest that the critical particle velocity for formation of a plastic zone in the weak concrete is 13-15 m/s, with increased saturation tending to increase the critical particle velocity limit. Overall, the data produced from these experiments suggests that further pursuit of this approach is warranted for penetration research but also as a potential new method for dynamic testing of materials.