Aluminum, cars, and fatigue limits

"BaconSandwich is tasty." (baconsandwich) 08/14/2015 at 12:13 • Filed to: None

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Lately I’ve been doing a bit of reading about aluminum. When compared to steel, even if it is stressed below its fatigue limit, it eventually weakens. (See: !!!error: Indecipherable SUB-paragraph formatting!!! So what does this mean when it is used in a car, especially a component that is going to receive continual stresses? I remember reading a forum post a long while ago about a guy with a Lotus Exige. From what I remember, the upper strut mounts were aluminum (among other things). I’d think an area like that would be under frequent stress - essentially every time the vehicle goes over a bump.

So I’m curious - what does this mean about the longevity of aluminum based vehicles, such as the newer Audi sedans and the new Ford F-150? Are they engineered to essentially only last so long?

ChooChooMotherFudger > BaconSandwich is tasty. 08/14/2015 at 12:32

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Engineering student here, so no means an expert, but I know a bit.

Its true that they will wear out eventually. A cycle here is full stress on and then full stress off (or full one way then full another way) a suspension mount won’t really be in a full on full off that often. And the body of the Ford should never be under any significant amount of stress, as I understand it the frame and chassis is still steel.

With steal if it can withstand a million cycles it will last forever as seen in picture you attached. Aluminum won’t but it can still last very long (if a million is a lot ten over even a hundred million is very much more so) if you mange the stress right.

There is a need in the current industry to lower weight, hence why the use of aluminum. Yes Aluminum may fatigue at some point where steel wouldn’t, but it will (if done right) still last a frigging long time, while being lighter, stiffer and/or stronger.

Azrek > BaconSandwich is tasty. 08/14/2015 at 13:04

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The way I look at technology like this is to check into professional racing. They tend to be 5-10 years ahead of the curve on some technology going into cars. They have been using aluminum (pronounced the British way), carbon fiber and magnesium for awhile now. What are their results?

Then I’d speculate they have a good grasp on what the materials can handle and for how long.

Santiago of Escuderia Boricua > BaconSandwich is tasty. 08/14/2015 at 13:18

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It’ll be fine. The rest of the car should break first

RamblinRover Luxury-Yacht > BaconSandwich is tasty. 08/14/2015 at 13:22

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Like others have said, it’s as much a question of full load as number of cycles. The above has a majority high-copper aluminum alloy body which is more prone to fatigue than many modern alloys, yet can be driven to over 250,000 miles with relatively limited cracking (front fender edge several places, spot welds to rear “face” panel at the bottom, etc.

luvMeSome142 & some Lincoln! > BaconSandwich is tasty. 08/14/2015 at 13:36

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When designing stuff out of steel if there is any sort of higher frequency stress cycling you want to keep the stress under the endurance limit. It doesn’t take long to rack up a million cycles.

A couple of examples:

A crankshaft. For simplicity, let’s assume the stress is fully reversing and happens once every rotation. It takes around 3 hours to rack up a million cycles at 5,500 rpm. So, for something that will see millions of cycles, it’s prudent to use steel and make sure the stress is below the endurance limit.

For something like the strut mounts the engineer would look at a number of scenarios and work out the stress and number of cycles for the of each scenario (over the design life of the product*). The scenarios might be bad potholes, rutted roads, speed bumps. Then a calculation is done to see if it meets the minimum criteria.

*in answer to your last question. Yes.

BaconSandwich is tasty. > luvMeSome142 & some Lincoln! 08/14/2015 at 13:58

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That certainly makes sense. I suppose parts that are going to be stressed frequently could either be reinforced or made from steel rather than aluminum.

BaconSandwich is tasty. > RamblinRover Luxury-Yacht 08/14/2015 at 13:59

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I recall seeing several old buses still in service when I was living in Edmonton, which had aluminum body work. Despite that, I’m sure they still had a steel chassis underneath. I’m kind of curious to know how an entirely aluminum car holds up.

BaconSandwich is tasty. > ChooChooMotherFudger 08/14/2015 at 14:00

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Don’t get me wrong - I love the idea of aluminum being used in cars. I just worry that eventually a car will come with a “best before” sort of date, meaning that it has been engineered to only theoretically last so long.

BaconSandwich is tasty. > Azrek 08/14/2015 at 14:02

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The thing with professional racing is that parts don’t have to last all that long. E.g.: I don’t think anyone is going to be driving a Formula 1 car for 300,000+ miles without entirely rebuilding it.

That does bring up an interesting point, though: How long will we be using aluminum before we make the jump to carbon fiber body panels in consumer-level cars? I know the BMW i3 is largely carbon fiber, although I’d still consider it to be a bit of a niche car.

ChooChooMotherFudger > BaconSandwich is tasty. 08/14/2015 at 14:12

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I agree, but as cars get more advanced there will have to be compromises.

I don’t know what the standard number of cycles something in the production car industry is for something like suspension. My experience with the FSAE uprights I have designed is that if it is stiff enough then it is strong enough to last a long time.

I don’t like the idea of parts fatiguing early, but then again aluminum doesn’t rust. Maybe they would rust away normally before the would ever fatigue.

Azrek > BaconSandwich is tasty. 08/14/2015 at 16:04

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Absolutely correct, but they use it at higher tolerances than we do in daily driving. I have a fast car, but I am not going to keep my aluminum framed car with an aluminum V8 at 130+mph for an hour or more. So they are stress testing at higher speeds and tolerances. If their car can handle those speeds/heat for a few races then it should be easily modified for daily use for a long period of time.

Carbon fiber and/or magnesium is the next logical choice for major car parts in terms of lightness with strength.

BaconSandwich is tasty. > ChooChooMotherFudger 08/14/2015 at 17:02

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I love aluminum for that reason (no rust).

samssun > BaconSandwich is tasty. 08/14/2015 at 18:19

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It may be true, but it won’t be a date so much as a use amount, which may asymptote to infinity. You go from 10-12-15k miles a year to 5k/year on your 70s muscle car, to 1k a year on your 50s classic, to 50/year on your 20s roadster.

Being halfway worn out after 20 years doesn’t mean it falls apart at 40, it may mean a 75+ year life, or forever.

gogmorgo - rowing gears in a Grand Cherokee > BaconSandwich is tasty. 08/14/2015 at 20:11

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Most steels are reasonably elastic, which is why they last longer. IIRC the rule from materials class was that if you didn’t exceed 40% of yeild stress in a cycle, then it wouldn’t fatigue.

Aluminum, on the other hand, is fairly brittle. It’s why it bends so easily... and then breaks. However, if the piece itself isn’t loaded enough to really deflect all that much, it won’t break as quickly. But there’s still a finite fatigue life.

Under normal driving conditions (i.e. paved roads with the occasional pothole and not severe washboard gravel), you wouldn’t really be cycling much load. If you’re twisting the chassis, you’re either going over some seriously unlevel terrain (i.e. ditch twist) or going around a corner HARD. Otherwise, it’s a fairly constant load. Accel and deccel loads don’t stress the chassis all that much unless you’ve got a lot of horsepower. Hitting a pothole is more of a shock load and is more likely to cause an instant fracture than small incremental stress.

You can tell a Finn but you can't tell him much > BaconSandwich is tasty. 08/17/2015 at 15:36

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The curves you have pictured are likely for fully reversed bending. Fully reversed bending is the type of loading that a solid axle on a four wheeler sees. If you look at this Suzuki image courtesy of theatvchannel.com you can see that all of the weight of the machine and rider is transferred to the ground through the rear axle. As the axle rotates the load gets fully reversed, that is the axle is completely loaded in one direction and then completely loaded in the opposite direction.

This is the harshest kind of fatigue loading and will result in the shortest time to failure. Fortunately two things help us out, first fully reversed bending is pretty rare in the case of a car and second, steel has a fatigue limit above which it won’t fail.

In the case of the aluminum bodied F-150 and other aluminum bodied vehicles there shouldn’t be a reduction in service life of the body panels. Body panels on a truck are essentially unstressed as the body sits on a steel frame that takes all the loading. Possibly in some body locations there will be local areas that see high vibration and may be likely to develop cracking, but that is pretty unlikely. Ford has been using aluminum hoods on their trucks since about 2004-05 or somewhere in there and I’ve never heard of hoods failing due to fatigue loading.

In cases where the vehicle has an aluminum load bearing structure or members, they are designed to a certain number of cycles. What the cycle number is or how it is developed is proprietary to the particular manufacturer, but think in terms of millions or tens or hundreds of millions of cycles. Take the upper strut mounts on your Lotus. Because of the application they are not subject to fully reversed bending. The load on the strut mounts will be the weight of the car plus or minus whatever loads are put on it due to bumps or cornering. Unless the car goes airborne there will not be negative loading on the strut mounts. Even if it does go airborne it still won’t be fully reversed bending since the weight of the suspension bits being supported by the strut mount is less than the weight of the car.

For entertainment purposes google “ Subaru sti lower control arm failure ”. STi’s have aluminum front lower control arms. The only results I get are for regular Imprezas and Foresters that have steel lower control arms. The steel models were recalled because they were corroding and failing. This ignores all the results for bushing failure which are obviously a completely different issue.

As for your original question, cars are already designed for a particular service life. I’m sure switching to aluminum for various parts doesn’t change the design life of the vehicle. They just account for the different properties of the aluminum by adjusting part size and geometry accordingly.

BaconSandwich is tasty. > You can tell a Finn but you can't tell him much 08/17/2015 at 16:01

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Well said!