Development and use of an instrumented alternative to the clay box

Roma Clay #1 is frequently used in ballistic testing of body armour systems. It allows the residual depth of the clay indent to be measured, which standards limit for system approval. TNO is developing experimental methods that get more data from these tests. This data can be used to improve the correlation to injury and lethality. An instrumented clay box was developed that allows real time measurement of local pressure, local accelerations, as well as the indent depth using an ultrasound system. The positioning of the sensors in the box was aided by the use of finite element simulation of several bullet to armour systems with a clay backing. Using ultrasound pulses from the rear of the clay box, the formation of the bullet impact induced clay indent could be measured real time as well. This system also provides the maximal dynamic indent in the clay and its elastic recovery.
Experiments with the instrumented clay box on hard and soft body armour systems using various ballistic threats has given insight on the repeatability of the measurements. The use of RP1 required a special repair routine for the clay between each shot to maintain the sensitivity of the ultrasound measurement system. This significantly reduced the cycle time between shots. A second concept was developed where a transparent synthetic gelatin backing material was used which does not require repairs between shots and allows for high speed videos to verify the ultrasound measurements. The signals of the embedded sensors and the indent formation process can be used to compare and validate computer simulation results. The instrumented box allows parameters like pressure, acceleration and velocity of the clay to be measured to assess their possible correlation with injury levels.