*3.1. Methods Used to Induce Failure*

The ultrasonic tester used for this procedure was a Sonomechanics (New York, NY, USA) 1200 W ultrasonic liquid processor shown in Figure 10. The equipment consists of a generator that provides electric signal input to the piezoelectric transducer, a water-cooled barbell horn and a protective enclosure. The samples were placed in a solvent (water or ethanol) and the beaker containing such dispersion was placed in a second beaker, which was filled with iced water and a copper coil that was also circulating cold water to maintain the bath at a constant temperature. The generator produced a constant frequency of 20 kHz at the horn tip; however diverse samples were exposed to the ultrasonic waves during different conditions: the amplitude was varied between 20% and 100% (which correspond to 17 to 81 pp, respectively) and times of exposure extended from 3600 to 10,800 s.

**Figure 10.** Ultrasonic horn setup. Note that a cooling bath was used to maintain a constant temperature, however the bath that included the cooling coil and the sample in the liquid media were not in contact. The latter were placed in a second beaker. The blue foam was used to keep the inner beaker in place.

A shock tube was used for creating a shock wave that propagated and impinged on the sample. A Kevlar layer was positioned on the surface of the particles to prevent their dispersion inside the shock tube. The system uses a pressurized driver section and evacuated driven section separated by a pair of diaphragms to create a supersonic wave that propagates along the tube and impinges on a test holder at the end of the device. Pressure transducers are located along the tube allow for monitoring of the shock and measuring its speed and strength (pressure sensors 1, 2, and 3 in Figure 11). Detailed theory can be found in [26].

The material to be tested was attached to the holder and bolted onto the end section where pressure transducer cable was connected. The shock tube was then loaded with two heat-treated copper diaphragms 0.025 inches thick scored to a depth of 0.013 inches. Vacuum was then drawn to an absolute pressure of approximately 2 mm Hg in the driven section through the use of a roughing pump. When this was complete, gas was added in a controlled manner until the driven section reached 25 mm Hg. The vacuum gauge was isolated and then disconnected to prevent damage to it during creation of the pressure event. The driver section was then loaded with either Nitrogen or Helium (depending on the desired Mach number) to 720 psig. When the pressure gauge reached 350 psig, the

mid-section was isolated. The shock tube was then fired by opening the firing valve. As pressurized gas entered the diaphragm section the pressure differential across the second diaphragm caused it to burst creating a wave through the driven section. The subsequent drop in pressure in the diaphragm section caused the first diaphragm to burst and the formation of a second, faster wave. The second wave overtook the first and the two coalesced into a single wave that traveled along the driven section and to the sample holder.

**Figure 11.** Shock tube setup (**top**) and image of the sample contained with Kevlar (**bottom**).

For the impact with military rounds the IF powder sample was dispersed in between Kevlar layers and the layers introduced into a nylon pouch. The later was then positioned in front of a clay bed and a 7.62 mm NATO round was then fired at it as indicated in Figure 12. The IF-WS2 particles located close to the hole left by the penetrator were then collected and analyzed by the methods described in the next section.

**Figure 12.** Sample testing setup (**left**) and orifice left by penetrator in a nylon bag that contained the sample in between Kevlar layers (**right**).
