Protection materials in defense applications must withstand impacts that produce incredible forces and high strain rate deformations, yet remain light enough for use in personal or vehicle armor applications. Understanding the failure processes of these materials is crucial to improving their protection capacity, but experiments to probe these processes are complex. The conventional methods using explosive, gas or gunpowder-driven experiments are dangerous, expensive, and difficult, requiring large-scale facilities where experimental throughput is low. In this thesis, we attempt to achieve similar loading conditions (e.g. strain rates, shock stresses, energy density, etc.) with a high throughput apparatus: a laser-driven micro-flyer plate launcher.
Laser-driven micro-flyer plate (LDMFP) facilities use a short duration pulsed laser with high peak power to launch small metal foil flyers at velocities of several km/s by generating an ablation pressure behind the flyer. Here we describe the Hopkins Extreme Materials Institute LDMFP facility, including the launcher configuration, expected velocity envelope, and photon Doppler velocimetry (PDV) diagnostics. We interrogate the failure of magnesium alloys and boron carbide using the facility.
The widely available AZ31B Mg alloy has a potential application as a low-weight vehicle protection material. We use the LDMFP facility to drive incipient spall failure in AZ31B foils. In spall, shockwave interactions from the impact loading generate high tensile stresses within the target specimen, leading to failure through void growth, coalescence and fracture. Our experiments show an increase in spall strength when compared to lower strain rate spall experiments on the same alloy, and also show differences in strength based on the level of deformation in the as-received microstructure. The LDMFP apparatus facilitates specimen recovery by imparting little kinetic energy, so we perform micro-computed tomography scans of the preserved shocked specimens to learn the void distribution within.
Next, we demonstrate the LDMFP facility capability for high experimental throughput to learn the orientation dependent strength of a Mg-9 wt. % Al binary alloy. The binary alloy is prepared without second phase particles when fully solutionized, and with lath precipitates when warm-aged. The large number of experiments, coupled with numerical simulations, indicate a lack of orientation dependent strength in the solutionized sample, and significant orientation dependent strength in the precipitate-laden microstructure.
Finally, we use the LDMFP facility to examine brittle fragmentation of boron carbide, a lightweight ceramic used in personal body armor. We design the micro-flyer experiment to have a similar energy density as conventional ballistic experiments, and compare the resulting fragmentation statistics. The results suggest that fragment sizes from projectile impact are related to microstructural length scales for both ballistic and laser-driven loading conditions
