When buying a new car, most people think about design, performance, practicality and economy. Safety is rarely the first priority; with all the style and options to think about, most people take safety features for granted. And it’s true that modern cars are much safer than 30 years ago. But what most people don’t know is that behind all these safety advances is a very complex area called Crash Engineering.
The New Car Assessment Program (NCAP) was created in 1979 by NHTSA, the organization responsible for rating vehicles in the United States. Since the organization was founded, car motor vehicle deaths in the US dropped by more than 50% by 2010.
Euro NCAP defines safety ratings as shown:
5 stars: Overall good performance in crash protection. Well equipped with robust crash avoidance technology.
4 stars: Overall good performance in crash protection; additional crash avoidance technology may be present.
3 stars: Average to good occupant protection but lacking crash avoidance technology.
2 stars: Nominal crash protection but lacking crash avoidance technology.
1 star: Marginal crash protection.
The rating system evaluates deceleration values, cabin intrusion and electronic safety aids available on modern cars. The safety institutes perform several crash engineering and stability tests to gather data and the vehicles are rated from 0 to 5 stars for adults and children passengers. Several methods of injury determination are studied and tested. The Head Injury Criteria (HIC) is the most common.
Head Injury Criteria (HIC)
The human body has a limited tolerance for acceleration. Every organ of our body has a limit of acceleration that it can sustain before collapsing. The main focus here is brain damage.
In high speed accelerations the brain behaves like a loose piece of jelly being forced against a rigid wall (your skull). As a result, severe brain damage can occur with sudden accelerations (or decelerations).
When a crash occurs, an impact wave is transmitted through the vehicle’s body to the passengers. When this wave reaches the rearward region of the skull it is reflected, potentially fracturing the skull.
Experiments show that a human’s impact resistance is dependent on the intensity of acceleration and time of exposure. In other words, 10G’s of force can be as severe as 50G’s if it lasts long enough. To define an acceptable region the following equation is used:
The most common HIC acceptance interval is 36ms. In other words, t2-t1 in the formula can’t exceed 36ms.
There are 3 main factors responsible for the increase of safety in automobiles: seat belt, air-bag and chassis construction. They are strongly connected and are designed to work together, and they are designed with one main equation in mind: impact. Impact force is proportional to the speed squared and the inverse of the slow-down distance.
Seat Belt: This item is the most intuitive of the listed equipment, as most people are used to using it. It’s main function is to restrain the passenger in a collision and to absorb the energy of the impact.
Air-bag: With the impact force equation in mind, the airbag is inflated in about 25ms after the sensors detect high deceleration. It is designed to absorb the energy of the impact by increasing the passenger slow-down distance. In other words, the impact is the same, but it’s delivered more smoothly.
An important thing to note is that the airbag is not launched in the direction of the passenger. It basically inflates in position and ‘waits’ until the passenger reaches its fluffy surface. Also, it’s mandatory to measure where the passenger’s head contacts the airbag. Unstable head contact happens when its centre of gravity moves further than the outer edge of the airbag.
If the passenger is not wearing a seat belt, the airbag can actually injure the occupants. The airbag is designed to inflate with the assumption that the passenger will be restrained by a seat belt and will reach the airbag with a certain speed in a certain time.
Figure 1 shows the magnitude of HIC for the same car with and without an airbag. In the upper graph a peak of 100 G’s can be identified for a few milliseconds. With the airbags deployed, however, this peak is filtered and the HIC intensity is reduced by more than half.
Chassis: The chassis construction is the main focus of this text and its construction is the most flexible variable. The approach taken by engineers is to make the survival cell as stiff as possible to avoid cockpit penetration while providing energy absorption through materials deformation in the crumple zone. How is this achieved? As with most engineered components, we look for a trade-off between stiffness and deformability. The chassis can’t be too stiff or it would transmit too much impact to the passengers, but it can’t be too flexible either or it would end up deforming vital parts of the chassis and injuring the passengers.
Having this in mind, the chassis is usually divided into 3 zones: front crumple zone, survival cell and rear crumple zone.
Front crumple zone: This zone is responsible for impact absorption in frontal crashes. The structure is composed of two top hat beams which absorb most of the energy by controlled buckling. The left and right beams are connected by a bumper which must be stiffer to provide even loads for both hat beams.
The beam can be designed in several ways. Normally it has ribbons and holes which are designed to initiate buckling in specific locations. The hat beam design is optimized using computer aided engineering (CAE). Figure 2 shows how simply changing the position of a spot weld can influence the buckling behavior.
On front-engine vehicles, the engine is usually responsible for 20% of energy absorption in frontal crashes. Top-class vehicles have chassis made of Aluminium or carbon fiber for higher stiffness and light weight, while the crumple zone is made of Aluminium for high energy absorption.
Several vehicles have an Aluminium/Aluminium honeycomb crash box (shown in yellow in Figure 4), fitted between the front bumper and the longitudinal beam. The objective here is to reduce repair costs by ‘’sacrificing’’ this component in a minor crash. The expensive longitudinal beams remain intact and only the crash box and bumper are replaced.
Survival Cell: The objective is to leave the driver/passenger compartment as intact as possible in a crash. To achieve this, the cockpit is designed to be very stiff with high yield resistance. On road cars, this compartment is usually made of ultra-high strength steel. Aluminum and carbon fiber are used for special vehicles.
For side crash safety, the system responsible for the occupant’s safety is the B-pillar (the red column between the yellow A-pillar and red C-pillar in Figure 7). This is an ultra-resistant component designed for minimum displacement. It also supports the vehicle doors which have a cross-beam reinforced by ultra- deformation resistant steel. Here, there is not much room for energy absorption and the door is designed to avoid cabin penetration.
Another important goal of the survival cell is to avoid deformation in case of a rollover. The vehicle’s roof is allowed a minimum acceptable displacement. For convertibles and racing cars, a rollover hoop is placed above the driver’s head for protection.
Racing categories have a set of rules defining the design of the roll-hoop. Usually there is a hoop above the steering wheel and another above the driver`s head. A common way of defining the size of the those structures is to determinate a minimum distance from the driver`s head to an imaginary line passing through both hoops.
Rear crumple zone: Rear collisions are usually less aggressive to the driver. However, the principle is similar to the front structure. There are two beams connected by a bumper which absorb energy by controlled buckling. Again, the structure has to be stiff enough to support the suspension loads and engine mounts (in case of middle and rear engine cars).
Summary of Crash Engineering
The Corvette Stingray, shown in Figure 9, shows the current trend for sports cars. The chassis is constructed with welded or bonded and riveted aluminum.
The survival cell (light blue) has higher grade aluminum alloys with reinforced extrusion walls. This section transfers most of the load into the chassis and keeps the driver intact in case of a collision – high stiffness and strength is necessary.
The crumple zone (orange part in the extremities), has the objective of absorbing as much energy as possible in the shortest possible distance. The extrusion is designed and optimized through several iterations to find the best combination of high energy absorbency, low cost and ease of manufacture. However, the material must be strong enough to avoid excessive deformation but not too stiff – low/medium-low strength necessary.
Older cars were thought of as rock solid and people thought that was a good thing. ‘’Cars today are weak and made of plastic; I wish they were as strong as my first car in 1970’’. This may be a common statement, but the reason modern cars are designed to ‘’break’’ in a collision is to prevent the driver from absorbing higher loads and deceleration.
And finally, the bumper. Although it is responsible for only about 5% of energy absorption, this component is very important as it ensures that the top hat beams buckle as expected. Again, a ‘’rigid’’ bumper will submit its extremities to similar loads and both beams will buckle in a synchronized way – medium strength and stiffness necessary.
These understandings about safety and reliability come from the field of crash engineering. By doing in- depth mechanical analysis on the failure modes and effects, engineers are able to better design cars for the safety of the passengers.
References – Crash engineering articles and papers
- EuroNCAP (http://www.euroncap.com)
- Henn, H. Crash Tests and the Head Injury Criterion, Teaching Mathematics and its Applications, Vol 17, No 4, 1998.
- General Motors Media Website (http://media.gm.com/media/intl/en/chevrolet)
- Gurdjian, E.S.; Lissner H.R.; Patrick, L.M. (1963) Concussion-mechanism and pathology. Proc. 7th
- Stapp Car Crash Conference, pp. 470-482.
- NHTSA (http://www.nhtsa.gov)
- Mercedes-Benz Media Website (http://media.daimler.com)
- Volvo Cars (http://www.volvocars.com)
- White M. D. and Jones N. (1999), A theoretical analysis for the dynamic crushing of top hat and double top hat thin walled section, Proc. Instn. Mech. Engnrs., Vol 213, Part D.
- Witteman W. J. (1999), Improved Vehicle Crashworthiness Design by Control of the Energy Absorption for Different Collision Situations, PhD Thesis, University TU Eindhoven