Safety
Materials (and the designs in which they are used) play a critical role in automobile
crashworthiness. The general concept of vehicle design for crash energy management involves
two aspects. The first is that the front and rear of the vehicle are intended to be collapse/crush
zones. Their main function in a crash is to provide maximum absorption of the vehicle kinetic
energy. In the crush zone, the ideal structure collapses progressively in a predetermined mode,
while avoiding instability and buckling. In a frontal crash against a fixed barrier at 35 mph, the
crush distance is typically 20 to 35 inches.^15 The resistance of the structure to crush forces
(sometimes called vehicle “stiffness”) should be such that during the crush, the forces transmitted
to the passenger compartment remain constant, just below the tolerance level for passenger injury.
This defines the most efficient use of crush space.The second principle of sound crash design is that the passenger compartment should maintain
its structural integrity, to minimize intrusion into the passenger space. As a rule, high-strength
materials are required, especially in the side structure, where there is relatively little space
between the passenger and the door.Currently, sheet steel products constitute the principal material used in the automobile chassis
and body structure. Considerable experience has been derived over the years in modeling the
behavior of sheet steel structures in crash situations, and designers have confidence in their ability
to predict this behavior. Alternative materials, such as aluminum or composites, offer some
potential advantages in crash energy management over steel, but have far to go to match the
comfort level designers have with steel.
One advantage of aluminum is its high specific energy absorption (energy absorbed divided by
density). Pound for pound, aluminum structures have a 50 percent higher energy absorption than
identical steel structures.l6 Recent crash tests suggest that weight savings of 40 percent or more
can be achieved in aluminum structures with a comparable or even an increased crash
performance compared with steel.^17 Automakers interviewed by the Office of Technology
Assessment (OTA) expressed a surprisingly high comfort level with the crash performance of
aluminum-intensive test vehicles. A concern, however, is that while an aluminum vehicle may
perform well in a crash test against a fixed barrier, it will be at a disadvantage in a crash with a
heavier steel vehicle, owing to the transfer of momentum from the heavier to the lighter vehicle.
This may mean that lighter aluminum vehicles will have to be designed with additional crush zone
space or other safety features to compensate.Several studies have now shown that composite structures can have an energy absorption
potential comparable to, and in some cases better than, that of metal structures.^18 The difference
between metal and composite structures is that the metal structures collapse by plastic buckling,15 A. Paluszny, "State-of-the-Art Review of Automobile Structural Crashworthiness,”report prepared for the American Iron and Steel Institute,
June 1992.(^16) V. K. Banthia et al., "lightweighting of Cars with Aluminum for Better Crashworthiness,” paper presented at the SAE International Congress
and Exposition, Detroit, MI, Mar. 1-5, 1993 (SAE Technical Paper Series number 930494).17Ibid
(^18) P. H. Thornton and R. A. Jeryan, "Crash Energy Management in Composite Automotive Structures," International Journal of Impact
Engineering, vol. 7, No. 2, 1988, pp. 167-180.