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The Energy Absorbed in Impact Testing

Many people, particularly those with families, look at crash test results conducted by the National Highway Traffic Safety Administration and Insurance Institute for Highway Safety as a deciding factor when purchasing a new vehicle. To the casual observer, these tests are kind of fun to watch -- five seconds of imminent doom and a half-second of carnage combine for some interesting viewing. It is, however, the 10 seconds of quiet contemplation which follows that illustrates the crash test's true value.

  1. US-NCAP vs. IIHS Testing

    • The federal government's United States New Car Assessment Program -- or US-NCAP for short -- and the National Highway has changed little since being created in 1978. The NCAP test procedures use a sled of a given weight to simulate the effects of a crash, which provides consistent results. However, the newer Insurance Institute of Highway Safety (IIHS) test -- based on a European evolution of the NCAP instituted in 1992 -- is a far more thorough simulation of real world events. The IIHS includes all of the data that the NCAP does, but its updated testing procedures place more emphasis on what happens to the passengers instead of just what happens to the car.

    The Basics of Force

    • Force calculations aren't a fixed thing based solely upon a given object's speed and mass. The time it takes for a weight to decelerate plays a crucial role in determining how much force actually goes into the event. This is why engineers build crumple zones into cars. These zones act like cushions to slow the impact and thus reduce the amount of force that the vehicle experiences over a given period of time. Express that force in Newtons by multiplying the vehicle's mass in kilograms by its speed in meters per second. Divide that figure by the time in seconds from the moment of impact to the end. Finally, convert the metric back into pounds of force by multiplying the Newtons of force by 0.2248.

    Impact Tests

    • Acceleration impacts consist of a car hitting a stationary object at a given speed. These tests include the Full-Width Frontal where the car hits a solid wall head-on; the Frontal Offset where the car hits a wall covered with a deformable honeycomb material offset to one side or the other; and the Pole Impact. The forces here vary by how heavy the car is and how long it takes to slow down. Speed for impact remains a constant 40 mph for all tests. The US-NCAP side impact test consists ramming a 3,015-pound trolley with a deformable aluminum face into a car at 30 mph and at a 27-degree angle from the front. The IIHS test and Euro-NCAP tests are the same except that the impact is at a dead-on 90 degree angle to the side of the car.

    Impact Forces from a Side Collision

    • These are fairly easy to calculate, since the trolley weight and speed are known constants. The only variable is the duration of the impact, which is exactly what the test aims to calculate. Using the formula above and converting to metric, you can calculate that the trolley's momentum is equal to 18,373.344 (13.44 meters per second times 13.70 kilograms) under all conditions. A heavy vehicle with a really poor side impact duration of 0.05 seconds from beginning of impact to the end, equates to 367,466 Newtons -- or 82,606.55 pounds of impact force. The impact duration for a lighter vehicle expands to a full 0.40 seconds. This drops the force exerted to 45,932 Newtons, or a more reasonable 10,325 pounds of force. For a frame of reference, that's about 20 times the force of a baseball bat hitting the car at 90 mph, or about the same as a 10-pound shell fired at 500 mph from the main gun of a tank.

    Frontal Collision Forces

    • These are bit more complicated, since impact force varies by vehicle weight. Speed remains a constant 40 mph, so the calculation starts from there. A 6,000-pound SUV with no crumple zones and very little impact time -- for example, say about 0.05 seconds -- will experience about 975,175 Newtons during a full frontal collision. This averages to around 219,220 pounds at impact. That's about the same force as a 75-pound golden retriever hitting a wall at slightly less than the speed of sound. Keep that disturbing visual in mind the next time you opt not to wear your seat belt. A 2,500-pound vehicle with an impact time of 0.5 seconds would experience 9,135 pounds of force. This is barely enough to squish a grape. The human skull which be crushed flat at slightly more than 15 pounds per square inch of pressure.

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