Aluminum and car safety

Modern cars have to carry a large number of strict security requirements, both for drivers and passengers, and pedestrians. The low weight of aluminum makes it possible to increase the safety characteristics of the car and at the same time reduce its weight. The ability of a unit mass of aluminum to absorb energy during its deformation is twice as high, than mild carbon steel and no worse, than the latest high-strength steels, which have been specially designed for this purpose.

Car accident physics

Vehicle speed is a critical factor in all car accidents. This directly follows from the physics of a collision of a car with an obstacle or, even worse, with an oncoming car. The greater the mass of the car and the faster it moves, the more its kinetic energy. Kinetic energy, as we all know from school, proportional to the mass and the square of the velocity of this mass. It means, that when the speed of the car doubles, its kinetic energy quadruples!

When a car collides with an obstacle, its speed drops sharply to zero. This “deceleration rate” physicists call acceleration. In our case, this acceleration has a negative sign, that is, it is deceleration. According to Newton's second law, the magnitude of the force, which occurs on impact, proportional to the mass of the vehicle and this acceleration (deceleration). This acceleration, simplistically, obtained by dividing the speed by the duration of this impact from its beginning to full stop.

The main goal of automotive energy absorption systems is to stretch the duration of the impact as much as possible., reduce, thereby, the resulting acceleration and, Consequently, reduce the magnitude of the force arising from impact.

Contribution of aluminum to vehicle safety

Aluminum contributes to vehicle safety in two ways:

• reduces the total weight of the vehicle;
• aluminum systems reduce acceleration due to the conversion of a large part of the collision energy into the work of plastic deformation of aluminum components.

Safety of body structures

In order to meet various safety requirements, modern cars have a rigid, stable passenger "capsule". It provides the space necessary for survival in the event of an accident. This space is surrounded by crumpled areas, in which the energy of an emergency collision of a car is effectively absorbed without exceeding the accelerations critical for people. When designing a car body, the most important thing is to find an acceptable compromise between structural rigidity, ability to absorb collision energy and its functional characteristics as a vehicle. In this sense, aluminum is an ideal material for solving these, largely contradictory, requirements with the achievement of maximum performance and minimum weight.

Aluminum is three times lighter than steel. The increased rigidity of the aluminum car body structure is a result of the higher thickness of its material. Aluminum structural elements are usually one and a half times thicker, similar steel. An additional opportunity to increase the rigidity of the body is provided by the use of multi-cavity aluminum profiles and large high-strength aluminum castings of a complex design (Figure 1). All this makes it possible to increase the rigidity of the car body while reducing its weight by 40-50 % compared to steel body.

Figure 1 - Bumper bars made of extruded aluminum profiles [1]

Collision energy absorption systems

The collision energy of the vehicle with an obstacle is absorbed mainly by the front and rear systems, and then, due to deformation of the longitudinal beams. Energy absorption systems consist of a bumper bar and two collapsible boxes. These systems are designed to, to minimize vehicle damage at low impact speeds and to absorb maximum energy at high impact speeds (Figures 2 and 3).

Figure 2 - Cross-section of multi-chamber aluminum profiles
for controlled absorption of collision energy

Figure 3 - Controlled axial crushing of a hollow aluminum profile [2]

Aluminum collision energy absorption systems are usually based on aluminum profile structures. The correct choice of alloy for crushable structures ("crash boxes") ensures that, that this system undergoes extensive plastic deformation before, how cracks begin to form (figure 4) . With equal absorption energy, aluminum systems make it possible to reduce the weight of these systems by 40 % compared to steel systems.

Figure 4 - Optimized crash box crumple shape for alloy 6014
compared to conventional alloy 6063

Aluminum protects pedestrians

To protect pedestrians, it is required to ensure that the bumper can be deformed significantly with little effort. This requires material and structures from it., that would provide a low level of impact forces and controlled collision energy absorption characteristics. The correct choice of aluminum alloy and the shape of the profiles makes it possible to fulfill all these requirements.

Weight loss, but not in size

Lower vehicle weight makes it easier to drive and reduce braking distances. These factors are important to prevent accidents.. Research has shown, that size is a decisive factor in vehicle safety, not weight. Reducing vehicle weight reduces forces, which occur on collision, and energy, to be paid off. If this reduces the size of the vehicle, then the inner hard space is also reduced, which is necessary for the survival of the driver and passengers. Therefore, making cars lighter without reducing their size is a positive measure in terms of its safety..

Lighter cars - more safety

An additional factor in improving road safety is the compatibility of different vehicles in an accident.. Big difference in weight, of course, is significantly more dangerous for a lighter vehicle. Therefore, the overall lightening of all vehicles, while maintaining their overall dimensions, could improve the survival rate for all road users.. Aluminum still has a lot of work in this direction..

Sources:

1. The Aluminium Automotive manual. – Applications – Car body – Crash Management Systems, European Aluminium Association, 2013
2. Aluminium in cars – Unlocking the light-weighting potential, European Aluminium Association, 2012.