impact wrench is a device designed to impart torque impacts to bolts to tighten or loosen them. At normal speeds, the forces applied to the bolt would be dispersed, via friction, to the mating threads. However, at impact speeds, the forces act on the bolt to move it before they can be dispersed. In ballistics, bullets utilize impact forces to puncture surfaces that could otherwise resist substantial forces. A rubber sheet, for example, behaves more like glass at typical bullet speeds. That is, it fractures, and does not stretch or vibrate. The field of applications of impact theory ranges from the optimization of material processing, impact testing, dynamics of granular media to medical applications related to the biomechanics of the human body, especially the hip- and knee-joints. Also, it has vast applications in the automotive and military industries. Impacts causing damage Road traffic accidents usually involve impact loading, such as when a car hits a traffic bollard, water hydrant or tree, the damage being localized to the impact zone. When vehicles collide, the damage increases with the relative velocity of the vehicles, the damage increasing as the square of the velocity since it is the impact kinetic energy (1/2 mv2) which is the variable of importance. Much design effort is made to improve the impact resistance of cars so as to minimize user injury. It can be achieved in several ways: by enclosing the driver and passengers in a safety cell for example. The cell is reinforced so it will survive in high speed crashes, and so protect the users. Parts of the body shell outside the cell are designed to crumple progressively, absorbing most of the kinetic energy which must be dissipated by the impact. Various impact test are used to assess the effects of high loading, both on products and standard slabs of material. The Charpy test and Izod test are two examples of standardized methods which are used widely for testing materials. Ball or projectile drop tests are used for assessing product impacts. The Columbia disaster was caused by impact damage when a chunk of polyurethane foam impacted the carbon fibre composite wing of the Space Shuttle. Although tests had been conducted before the disaster, the test chunks were much smaller than the chunk that fell away from the booster rocket and hit the exposed wing. When fragile items are shipped, impacts and drops can cause product