Collisions are of two basic types, elastic collisions, in which the kinetic energy of the colliding objects is conserved (though it may be redistributed between them), and inelastic collisions, in which part or all of the kinetic energy is absorbed converted into some other form of energy รป ultimately, usually, into heat.
The classic familiar example of elastic collision is that between two billiards balls. One ball may stop, transferring its kinetic energy to the other, but the combined kinetic energy of the two is (nearly) conserved. High-speed photography and precise measurement would show the balls slightly deforming at the moment of impact, then returning to their original shape. A typical inelastic collision is that of a snowball against a wall. It goes splat, is permanently deformed, and slightly heated.
In an ideal world we might want the collision of cars to be elastic (Yusuf et al. 2). The cars would bounce off each other (or a single car would bounce off a wall), momentarily deform, then return to its original shape, undamaged. In practice, automobile collision at anything above very low speeds involve too much energy for this to be practical. Moreover, human beings are not elastic. A bouncing-off collision might leave structures undamanged, but cause severe injuries to the driver and passengers.
Thus, in practice, collision safety design, including bumper design, centers on reducing the rate of deceleration in a collision, thus reducing the g-forces to which occupants are exposed (Farr 1). Modern bumpers may be elastic in very low-speed collisions (e.g., backing into another car while parking). In higher-speed collisions, the bumper is designed to absorb the collision energy through (relatively) gradual deformation, essentially a controlled crumpling process.
In moderate speed collisions (a few miles per hour), the bumper sustains damage,
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