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Inmechanics, animpact is when two bodiescollide. During this collision, both bodies decelerate. The deceleration causes a highforce orshock, applied over a short time period. A high force, over a short duration, usually causes more damage to both bodies than a lower force applied over a proportionally longer duration.
At normal speeds, during a perfectlyinelastic collision, an object struck by a projectile willdeform, and this deformation will absorb most or all of the force of the collision. Viewed from aconservation of energy perspective, thekinetic energy of the projectile is changed into heat and sound energy, as a result of the deformations and vibrations induced in the struck object. However, these deformations and vibrations cannot occur instantaneously. A high-velocity collision (an impact) does not provide sufficient time for these deformations and vibrations to occur. Thus, the struck material behaves as if it were morebrittle than it would otherwise be, and the majority of the applied force goes intofracturing the material. Or, another way to look at it is that materials actually are more brittle on short time scales than on long time scales: this is related totime-temperature superposition.Impact resistance decreases with an increase in themodulus of elasticity, which means thatstiffer materials will have less impact resistance. Resilient materials will have better impact resistance.
Different materials can behave in quite different ways in impact when compared with static loading conditions.Ductile materials likesteel tend to become more brittle at high loading rates, andspalling may occur on the reverse side to the impact if penetration doesn't occur. The way in which the kinetic energy is distributed through the section is also important in determining its response. Projectiles apply aHertzian contact stress at the point of impact to a solid body, withcompression stresses under the point, but withbending loads a short distance away. Since most materials are weaker intension than compression, this is the zone where cracks tend to form and grow.
Anail is pounded with a series of impacts, each by a singlehammer blow. These high velocity impacts overcome thestatic friction between the nail and the substrate. Apile driver achieves the same end, although on a much larger scale, the method being commonly used during civil construction projects to make building and bridge foundations. Animpact 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. Inballistics, 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.[1] Also, it has vast applications in the automotive and military industries.[2]
Road traffic accidents usually involve impact loading, such as when a car hits a trafficbollard, waterhydrant or tree, the damage being localized to the impact zone. When vehicles collide, the damage increases with the relativevelocity of the vehicles, the damage increasing as the square of the velocity since it is the impactkinetic 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. TheCharpy test andIzod 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.
TheColumbia disaster was caused by impact damage when a chunk ofpolyurethane foam impacted thecarbon fibrecomposite wing of theSpace 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 damage. Protectivepackaging andcushioning help reduce the peak acceleration by extending the duration of the shock or impact.[3]
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