Making cars that are lightweight and crash-safe

Sep 04, 2013
This crash test is designed to establish how much a sample component deforms under a defined bending stress. Credit: Fraunhofer IWS

Lightweight or crash-safe – must it always be a trade-off for auto makers? The answer is no. With a new lightweight construction technology, researchers are making it possible to do both. The result is less fuel consumption and lower manufacturing costs.

The auto industry needs to have a rethink: having turned out ever heavier cars year on year, in future vehicles will have to be lighter with lower and CO2 emissions. If do not dramatically reduce the average CO2 emissions of their cars, they will face hefty fines. That was determined by the European Commission in a new piece of legislation. One way to achieve a major cut in fuel consumption is through lightweight construction – in other words, cars have to slim down. But this must not jeopardize the safety of vehicle occupants – and that is a big challenge for auto designers, who are faced with the task of fulfilling these contrasting requirements. Vehicle bodies until now have consisted largely of a homogenous sheet steel structure with constant component sheet thicknesses. Components that are subject to particularly strong local stresses are often oversized, because the wall strength has to be designed to withstand the highest local stress point. This means that the sheet thickness is greater than needed in areas that are subject to less stress, making components unnecessarily heavy. Moreover, automakers use lots of expensive, high-strength steel sheets. At present, then, compromises are constantly being made between component weight, component cost, and crash safety.

Now researchers at the Fraunhofer Institute for Material and Beam Technology IWS in Dresden have developed a lightweight construction technology that makes it possible to reduce vehicle weight while ensuring adequate crash safety. "Safety and need not contradict each other," says Markus Wagner, a scientist at the IWS. In order to match the characteristics of body components more precisely to the stresses that act on them, the engineer and his colleagues are pursuing an exciting new approach called "local laser reinforcement". This approach involves using low-cost, low-strength with minimized wall thickness and reinforcing them locally only in those areas that are subject to strong stresses. To do this, the experts guide a focused laser beam over the surface of the unprocessed sheet. The zones treated in this way heat up or even begin to melt, before solidifying again. The heat dissipates quickly into the adjacent cold material, causing the track to cool down rapidly. This produces hard phases and the material is significantly strengthened. "We obtain strengths of up to 1,500 MPa (megapascals). That's roughly twice the strength of the unreinforced basic material," says Wagner. "This enables us to optimize the weights and stresses above all in the design of the front and rear bumper beams, the B-pillar, and various stiffeners."

Components that bend only half as much

Crash stresses create complex high-speed deformations in components. By means of local laser reinforcement, scientists are striving to obtain greater resistance to deformation. The less the car body part bends, the greater protection the driver has. At the same time, failure behavior can be influenced by predetermining the position of the first plastic deformation. For this to work, the researchers have to determine the optimum position and geometry of the reinforcement tracks. Should the tracks be pointed? Slanted? Should they run lengthwise? What should the material's composition be to optimize how difficult it is to deform the reinforcement zone? The researchers can find the answers to all these questions via simulation tests on the computer. "With our simulations, we are able to model field tests. The results obtained from trials and simulations deviate just a few millimeters from each other," says Wagner.

With the aid of numerical simulation, the scientist and his team have developed a crash-optimized track design for bending stress such as might arise when a car collides head-on with a tree or is hit by another car from the side. The track design was transferred onto real components using a laser. "We managed to halve the deflection of a locally laser reinforced pipe profile compared to the reference part, even though we locally reinforced only three percent of the component volume. In other words, we doubled its crash performance," explains Wagner.

The IWS researchers have already applied the method to various crash profiles and seat components on behalf of customers. Thanks to the new, stress-specific design, they are able to significantly reduce wall strengths and thereby make components up to 20 percent lighter, all without neglecting crash safety. As the next stage, Wagner and his colleagues want to perfect their technology by means of an automated optimization of track geometry.

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antialias_physorg
5 / 5 (2) Sep 04, 2013
Cool.

Now if one could do this on the fly (during a crash) we'd already be halfway towards 'structural integrity fields' from science fiction.
Eikka
1 / 5 (1) Sep 04, 2013
Light and crash-safe is possible, but add durability to the equation and it gets more complex.

It's entirely possible to make a box beam that is both light, stiff, and collapses in a controlled fashion during a crash, but what happens when you add ten years of corrosion, aging and stress fractures? Or how much mass you have to add to coat everything so it doesn't corrode away?

It's not very environmentally friendly to scrap cars earlier and earlier either. The average age of a vehicle is around a decade, and a great many of them must last twice as long because people just can't afford to buy new cars.
antialias_physorg
5 / 5 (1) Sep 04, 2013
Or how much mass you have to add to coat everything so it doesn't corrode away?

Depends on how you do your corrosion resistance.

Corrosion is driven by an electrochemical process. If you up the potential of your chassis then it won't corrode. One way is coating it with a less noble material. But you don't really need to do this over the entire material.

The US, for example, keeps its iron/steel frame seaports corrosion free by simply having them connected to a power supply that keeps them under a constant, low voltage.
In germany we use 'sacrifice anodes' (which are basically blocks of less noble material every few hundred meters connected to long distance water pipes). The anode corrodes (and has to be replaced periodically) while the pipe remains corrosion free. A similar system could easily be deployed in a car.
The zinc coating on your car is exactly that, but as noted: there's no reason to coat everything. A block of zinc connected to the chassis would do just as well
Humpty
1 / 5 (5) Sep 04, 2013
Hmm my main gripe of many a year is that (stupid) people are MARKETING, designing, and building carriages that weigh thousands of kilograms, for carcass's that only weigh 100Kg.

I have never been able to get a grip on the sheer inefficiency of this equation.

Carriages mass and power ought to be proportional to their occupancy.

Eikka
not rated yet Sep 05, 2013
The zinc coating on your car is exactly that, but as noted: there's no reason to coat everything. A block of zinc connected to the chassis would do just as well


Sacrifical anodes only work if there's a galvanic connection between the sacrifical metal and the part that is being corroded, so it forms an electric circuit. This works well with water pipes and buildings, because the ground/water is more or less conductive, but not in cars where you may get a splash of water somewhere but not elsewhere, or some door seal leaks water or you get condensation to the inside of the car where it remains while the outside dries up.

That's why the entire chassis is dipped in zinc, and not just some parts of it. Of course, non-essential frame parts don't necessarily need to be coated. It's just that once your door handles rust off, the car is effectively done for anyways.
Eikka
5 / 5 (2) Sep 05, 2013
I have never been able to get a grip on the sheer inefficiency of this equation.


That's because the mass of the vehicle has relatively little to do with how much energy it takes to move it through air at speed. Rolling resistance is directly proportional to speed, while power lost to air resistance is proportional to the cube of speed.

antialias_physorg
not rated yet Sep 05, 2013
or some door seal leaks water or you get condensation to the inside of the car where it remains while the outside dries up.
As you say: As long as there is a connection (anywhere) to the metal part the water is in contact with to your sacrificial anode you're all good.

But the zinc passivation is really not an issue for what is described here - as the procedure they propose doesn't affect the amount of zinc needed one bit.

It's particular interesting with the upcoming corrosion resistant molybdenum chassis.
Eikka
not rated yet Sep 06, 2013
As you say: As long as there is a connection (anywhere) to the metal part the water is in contact with to your sacrificial anode you're all good.


No. You need a circuit - a loop for the ions to flow through to have a galvanic reaction. If you just tie a piece of zinc to a steel rod and then dip the other end in water, it will rust and the zinc will do nothing. You need to have both the zinc and the steel touching the same water at the same time before the electrochemical reaction will corrode the zinc instead of the steel.

Even if you have a thin layer of water on top of the steel connecting the zinc block, the resistance of that film limits the ion transfer rate, and the steel will still rust. This is no problem if the entire structure to be protected is submerged in water or covered in moist soil, which is why it works on buildings and boats, but not on cars.
Eikka
5 / 5 (1) Sep 06, 2013
as the procedure they propose doesn't affect the amount of zinc needed one bit.


It does. You might have noticed how manufacturers only guarantee their rust-proofing for 5-12 years. The amount of zinc needed is calculated by the fact that it eventually runs out at the worst corroded spots and then the frame itself starts to rust. Then it's a matter of time before the frame starts to fail because of the corrosion, but it's typically years before it really affects the structural integrity of the part.

If the frame cannot take any rust without failure, because it's been deliberately weakened, then the whole thing now rests on the zinc not running out, so the safety margin they have for corrosion now depends on the amount of zinc, which you need more of to deliver the same safety margins and longevity for the frame.
Newbeak
1 / 5 (2) Sep 07, 2013
I had an optional electronic anti-rust module installed in my car purchased new in 2009 which warranties against corrosion for 10 years.
Newbeak
1 / 5 (1) Sep 08, 2013
How are insurance premiums going to be affected by these high tech chassis? I had a SL2 Saturn,and the premium on my car insurance was higher than similar cars.When I asked,I was told repairs are more expensive than for all metal bodied cars.
Eikka
not rated yet Sep 09, 2013
How are insurance premiums going to be affected by these high tech chassis? I had a SL2 Saturn,and the premium on my car insurance was higher than similar cars.When I asked,I was told repairs are more expensive than for all metal bodied cars.


Well, you obviously can't just take a piece of steel and weld it in place of the broken one, or strip the chassis and straighten it out in a press like you can do with most cars. Composite self-supporting frames are single use only; they have to be replaced as one unit because the composite fails if you cut it into pieces.

So yeah, if you crash it, it's totaled. Not worth much after the fact.