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SAFETY-RELATED STRUCTURAL FAILURES IN AUTO COLLISIONS

It is easy to be misled, from descriptions or images, or the results of a collision. Too often we are misled by not asking the simple question "How severe were the actual collision forces?"

In examining many collision-damaged vehicles we have seen that manufacturers often use spot welds to connect two pieces of sheet metal. Generally the spot-welds themselves do not fail. However, the sheet metal fails around the spot weld. This is commonly referred to as a ‘pull-out’ failure. In itself a pull-out failure can be a better consequence than an undeformed separation because as the pullout occurs energy is being dissipated. Consequently, less of the collision forces are transmitted to the occupants. However, we are accustomed to viewing this type of failure as an acceptable result without considering whether the pull-out failure occurred after a reasonable amount of energy was dissipated or whether the failure should have occurred at all.

For example, the extent of over-lap of two joined pieces of sheet metal becomes a significant factor if a spot weld is placed too close to the edge of the sheet. If the forces are favourable this can cause the weld to pullout, in the direction of the edge, at a lower level of force than if an adequate amount of overlap was provided. A manufacturer can reduce the weight and the amount of material used if he can reduce any over-lap of the sheet metal which can be viewed as excess. The extent of over-lap of the sheet metal is generally unnoticed by the public. Yet we have seen instances of separation where the distance between the spot weld and edge of the sheet metal was less than a ¼ of an inch.

The U.S. and Canadian federal governments have developed a set of regulations that are made to protect motor vehicle occupants with respect to the crashworthiness of motor vehicles. The regulations are called the Federal Motor Vehicle Safety Standards (FMVSS) in the U.S. or the Canadian Motor Vehicle Safety Standards (CMVSS) in Canada.

There is no specific safety standard which addresses structural disintegration per se. There are full scale barrier tests conducted at 50 km/h into the front and sides of vehicles. The results of these tests are based on the signals which emanate from a ‘dummy’ occupant. If the forces and accelerations experienced by the dummy do not register above a certain level the car design passes the safety standard.

However, these barrier tests only evaluate the vehicle’s design under the conditions by which the test was performed. Extrapolation to similar circumstances may not prove valid.

For example, the U.S. side impact standard calls for a moveable barrier to be impacted into the central side of a stationary car. The barrier looks like the chassis of a passenger car only it contains some deformable blocks at its frontal surface. An attempt is made to replicate the real-world situation of two moving barrier is travelling at 90 degrees toward the stationary, side-impacted vehicle, the barrier’s front end is pointed 30 degrees to the left of its travel direction. Thus, the initial contact is made by the barrier’s right front corner.

This impact configuration produces a triangular pattern of crush on the side of the struck vehicle. This is deemed to be acceptable because, in a real-world collision, when the two vehicles are in motion, a triangular damage pattern is produced in the side of the struck vehicle (as both vehicles rotate during contact).

Although the same shape of damage pattern is produced in the controlled test as in the real-world collision, the forces which produce the damage are different. In the real-world collision the initial contact to the side of the vehicle causes forces which are applied somewhat from the front progressing toward the rear. In contrast the controlled test produces initial collision forces which are directed from 90 degrees sideways until the struck vehicle rotates.

The direction of the initial impact forces is important because there are several weak points on the sides of vehicles where the structure is liable to separate.

For example, most passenger cars contain a spot welded-joint located directly behind the centre roof post which attaches the front half of the vehicle to the back half.

In a real-life collision, the forces attempt to push the rear half of the vehicle rearward while the front of the vehicle attempts to continue to travel forward. If the vehicle design does not take these forces into consideration the vehicle tears into two pieces along this seam.

Some vehicle separations can prove favourable to the occupant. However, there are several instances where the opposite holds true. We have seen situations where the separated structures, such as doors have been allowed to swing freely into the occupant space resulting in major injuries to occupants.

In the case of a vehicle which has split in half, the outboard floor anchor of a driver’s seat belt is often attached to the rear half of the vehicle while the other parts of the seat belt are attached to the front half. If the driver is wearing his seat belt at the time of separation, then the seat belt webbing will be pressed with tremendous force against the driver’s body resulting in massive injury.

Thus it is necessary to review the results of a collision with a critical eye. The results can be deceiving.

 
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