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Here we will see the famous crash tests, how to improve the car chassis to better withstand impacts and how to interpret a crash test.
How do crash tests work?
On the one hand there are the crash tests that are used to obtain a certification and on the other hand are the tests carried out by the manufacturer. For certifications, the tests are carried out mainly by two entities: The National Highway Traffic Safety Administration (NHTSA) and the Insurance Institute for Highway Safety (IIHS).
For the development of a vehicle, like any other test, the manufacturer can decide whether to carry it out at its own or third-party facilities. Through the crash test we can study those passive safety elements. That is, the seat belts, airbags, headrest and above all, the structure of the vehicle itself.
Mannequins called "dummies" are used, which are equipped with a multitude of sensors to interpret the personal injury that a person would suffer in the collision. These sensors are wired to a screwed connection box in the trunk of the vehicle or other key location.
They are extremely expensive, and can cost between € 120,000 and € 300,000 each. For this reason, we always rely on computer simulations as much as possible before going through a real physical test, however, any large manufacturer performs up to 400 crash tests a year. These dummies are mounted on the car that will be crashed on a semi-rigid wall, at speeds of up to 80km / h.
Although the standard dummy has the measures of the middle percentile of the population, car manufacturers today already use dummies of different sizes while testing. The crash process is captured by high-speed cameras that record the impact at a rate of 1,000 frames per second.
This, together with the crashed car, will later be used to assess the damage, as well as to detect areas for improvement.
There is not only the frontal collision, but all kinds of impacts are made, all of them are perfectly defined in conditions of speeds, impact angles, distances, etc.
For example, impacts against posts or other dummies, as well as side impacts where a special platform hits the vehicle at 50km / h.
At the certification level, the same tests are not carried out in all geographical areas. As a general rule, roof crash tests, rollover resistance test and frontal offset tests are also required.
Autonomous Emergency Braking (AEB) and collision warning sensors are also tested. In addition, the components of car's cabin that may be dangerous for the occupants are studied.
Not all tests are carried out with a vehicle, but other tests are carried out with a guided specimen platform (sled) in which the car seats are located, with or without other car components like the dashboard. This lowers costs and allows Crash tests to be carried out when the prototype is not yet fully developed.
Keys to Body Design
How to design a car body to be safe?
We are going to start by giving one of the keys to the design of vehicle bodies:
Most of the vehicles use a self-supporting body, and we can divide it into three modules: front, central and rear modules.
The frontal area, in addition to housing the motor system, protects the occupants from a collision, "absorbing" as much energy as possible as the vehicle deforms in the event of an impact. We just mentioned the key to body design right now, but it probably went unnoticed, right? Don't worry, we'll explain it below:
Imagine that, as automotive designers, a supplier presents us with a non-deformable material with which to protect the entire front of the vehicle. In addition, it would be a super-light and inexpensive material. Sounds good. So, Would we be interested in using this material on the front? If so, we would get "indestructible" cars. Let's think about it for a moment: This car would be a bestseller. We would have incredible advertising videos of the car hitting a wall without the car being damaged.
So, would you purchase this innovative material?
The correct answer is "No". The front of the car needs to be deformed to absorb the maximum energy of the impact, otherwise this energy would pass instantly to the passenger cabin, where the passengers are, causing them very serious damage. So, since the safety of people is first, cars must be able to deform in those areas where there are no people. A crashed car is better than an injured person.
To do this, brands create a programmed deformation zone or crumple zone (also called crash zone or crush zones), that is, the car is designed so that the vehicle deforms in a specific way at strategic points of the structure. All this has to be done while also avoiding the detachment of the elements of the bodywork as far as possible.
Most sources voice that what is sought in a crumple zone is to absorb as much energy as possible so that less energy is transferred to the occupants. It's a very visual concept, but it's a misconception. Being precise, what is sought is to spread the total force and lengthen the impact time so that the peak force imparted is lower, reducing the probability of injuries.
Therefore, the central module is what interests us to be as rigid and non-deformable as possible, since it is where the cabin is. We are interested in the fact that the front and rear modules can gradually deform during an impact, thus protecting the central module. With this, we already know one of the most important keys in car design in terms of safety. We can already give good reasons why it isn't interesting to fabricate an indestructible car.
If we look at the image below, we can see that the self-supporting chassis has numerous perforations, for example, inside the red circle. Why are the holes put in the self-supporting chassis?
It's true that a lighter chassis makes the car a safer vehicle, since the lower the body mass, the less energy it will have to dissipate for the same speed. But the holes that are in the red circle are for a very different reason than to lighten weight. So why do manufacturers put those holes in the car? The answer is related to the design key that we discussed earlier.
Some of these holes are made for safety reasons, instead of reinforcing the structure the opposite is sought. This seems to make no sense, but once again, we are looking to deform the front part so that as little energy as possible reaches the cabin area. That is why various "geometric tricks" are performed to facilitate deformation at points of interest.
With the holes, we "weaken" that area of the structure, creating a concentration of stresses that would make the structure even weaker in that area. It isn't intended that the elements of the car break easily, but if it does, it is much better to do it in the areas furthest from the passengers and to absorb as much energy as possible. In this way, we lengthen the deceleration time for vehicle occupants by reducing the forces acting on the occupants in an impact. This concept has evolved to achieve an intelligent and gradual deformation.
So now, in addition to being able to design aesthetic vehicles according to market trends, we are already able to obtain technical criteria to know how the car structure should be. Something that will not appear in any automotive design course, unless it's excessively technical.
For an engineer this part is key, but a conceptual or styling designer will never be able to calculate these deformations. Even so, this knowledge gives him a great competitive advantage over other designers.
Keys to Body Design
How do we design safer cars?
In the previous post of this transport design course we have seen one of the car design secrets, and now we have just seen another one.
At this point, we are going to reveal some of the key points to achieve safer chassis based solely on their design and construction, that is, without adding elements of active and passive safety that we all know.
Crush initiators: We have already seen the use of perforations in the self-supporting chassis. This controlled deformation can also be achieved with grooves and folds, as well as all kinds of geometric "tricks." The most notable are the Crush initiators, which are grooves, grimaces or grooves. We find them in a previous image within the yellow circles, it is simply an incision of any type in the metal so that it deforms or breaks (collapses) in that area.
Sheet characteristics: Another easy way to make the structure deform in specific areas is to reduce the thickness or section in the desired zone. Therefore, designing the car body is not about making it as resistant as possible, but in having a series of tricks and an ace under the sleeve to apply them in specific areas so that the structure deforms in the desired way during an impact. For this, steels of different strengths and densities are also used, as well as the use of specific heat treatments to reinforce certain areas and leave others without consciously reinforcing.
The aim is to disperse that energy to the surrounding points of the car interior, where there are no passengers, for example, towards the underside of the car or towards the A and C pillars of the vehicle. The structure is designed to redirect the greatest amount of energy outside the car cabin. For this, it's important to reinforce the doors with high-resistance steel bars so they can redirect the crash energy as much as possible. In any case, the energy in a side impact is the most critical one, since the impact is made directly in the passenger cabin. The entire deformable zone is called: Impact Energy Absorption (IEA) structure. Therefore, the IEA in the front and rear module is quite "deep", but in the doors this depth is much less.
Bumper support: In the previous image, we can see a central bar that exceeds and stands out from the rest: the bumper support. It can be placed both in the front and in the rear area and is specially designed to absorb as much energy as possible and lengthen the impact time. It's made up of different sections and with different materials. Deformable elements are also often added to further increase absorption.
As a design trend, the use of rigid structures in pedestrian impact areas such as bumpers is increasingly being reduced. In the event of an impact at low speed, these areas should be damaged as little as possible to be easily repairable (so-called Low speed impact absorption). For this, crash tubes or crash boxes are used for mounting bumpers, which is a hollow steel tube. It's one of the few cases in which the car prevails over the driver, since an impact at low speed, around 20km / h, doesn't put drivers at risk. Although the main thing in this case is always the pedestrian.
Beams: All the elements that go from the front to the rear of the vehicle have a certain conicity, slightly increasing their section. In this way, during a frontal impact, they will progressively deform. In addition, these beams are bifurcated to distribute the energy at the different points of interest.
Naval ram: In ancient times, warships were armed with naval rams to puncture, sink or disable the enemy ship. This same idea, but in a more civic way, is transferred to the car structure. The front is designed in such a way that it moves the impacted object against the side of the vehicle, reducing the effects of a frontal impact. If this weren't enough, engineers design the car so that in the event of an offset frontal collision, the other vehicle or object helps absorb some of the energy. Sorry man, this is the war.
The pity is that you cannot put naval rams on the cars, so the effect will have to be subtly recreated using the crumple zone.
We can find an example of a crumple zone in the steering column of the steering wheel. At present they are always collapsible, that is, they deform in the event of an impact. But in the early days of the car this didn't happen: the steering columns were made of solid metal, so they were totally rigid and non-deformable, in addition, they were attached directly to the steering wheel. Therefore, sometimes, a low speed impact ended up being a fatal accident, since the bar was projected against the body of the person. Starting in 1939, the collapsible steering column began to be used, saving countless lives.
In conclusion, the automotive engineer will try to use all possible resources to protect the cabin, some of them, such as the collapsible steering column, are mandatory elements today. This part of the design is purely engineering, but in essence, the important lesson is to know the key to design: front and rear deformable, and rigid cabin.
An advantage for us is that we can carry out simulations of these elements with any computer and with a CAD program that has a FEM module (Finite Element Analysis, which is considered within the CAE). If you don't know what this is, you can see the post where we explain it. Although simulating a crash is complicated and requires more powerful computers, it's quite easy to simulate a beam, make grimaces and holes, and study the different behaviors of the material against the applied forces. To do this we can go to websites like GrabCad and look for these elements, for example, by typing: "Body in White" or "BIW".
OW TO UNDERSTAND A CRASH TEST?
How to interpret a crash test?
Many of us will have seen a crash test, but do we really know what we are seeing? Do we know how to understand a crash test? Engineers have a multitude of high-speed cameras and sensors to collect data. Then they study the parts and the chassis once it's deformed. But it is not necessary to collect all this data to obtain a previous analysis of the impact.
If the A-pillar or the driver's door undergoes any deformation on impact, we can consider that the impact energy hasn't been properly absorbed and it will pass to the driver's area, which is the dangerous area. The greater the deformation, the greater the danger.
If in a crash test, the front of the car is completely destroyed, but the aforementioned elements are in perfect condition (front door and A-pillar), we can interpret that most of the energy has been absorbed, so that the damage suffered in the passenger compartment is much smaller.
We can also pay attention to other details:
This is not enough to do a technical analysis of a crash test, but it gives us a very good idea to know what to look for in a crash test when we see a video of a frontal crash test.
It should be noted that computer programs are increasingly being used to simulate crash tests, but real tests will always have to be carried out. In turn, more and more different types of collisions are being added. Although they are not only to certify the vehicle, but for the brand to obtain its own knowledge on how to improve the safety.