Car structure weaking rate over time/mileage

[SlavaYkraine]

Hello,

I'm extremely curious as to your professional thoughts on how the car structure's strength and stiffness weaken over time/mileage. If we take a very basic and simplified example of, say, 2015 Honda Accord. Let's assume that initial stiffness was at 30,000 lb.ft and that the car is driven moderately - not too gently, not too harshly on average roads that are neither smooth, nor perfect. Assume standard wheels that have average 215/45 R17 wheels. Assume rust plays no role (California car)

1. What is the rate -approximately/vaguely - at which the structure would lose its strength and stiffness?

2. Would the rate of stiffness and weakness loss be faster in the beginning (first 30,000 miles) and slower thereafter? Or vise versa?

3. Is it possible that two different cars - for example 2015 Honda Accord and 2015 Mercedes E class - assuming both had the same initial structure stiffness would age differently under the same conditions?

P.S. is it possible for an automaker to build a car structure that would not groan and moan after 300,000 average miles and 20 years on the road?


Thank You very much for your insights/thoughts/suppositions on this matter

 

[BrianPetersen]

I don't think you're going to find much on this subject which is not proprietary (secret).

It's also going to vary enormously. I can tell you that where I live, the number one factor is rust. My friend's Pontiac Montana minivan needed the doors kept closed if you had to jack it up to change a wheel to stop the whole thing from simply twisting, mostly because the rocker panels weren't there any more. I doubt if the crusher in the junkyard had much trouble flattening it.

Certainly this is going to vary between models ... because some have much better corrosion protection than others.

 

[GregLocock]

Yes we measure it. No I'm not going to risk my job by telling you the answer. Yes it will vary between models and makes.

More usefully, in your PS, yes, a rough calculation suggests that a car that weighed twice as much would maintain its squeaks and rattles performance for 300000 miles. So if you can persuade enough people that they want to pay twice as much new for a car so they can drive around in an expensive, slow, heavy quiet 15 year old car, perhaps you'll find a market.

Bear in mind the luxury end seems to have gone the opposite way - a 10 year old 7 series is just an electronic timebomb, by all means buy one and enjoy the ride, but if anything goes wrong electrically you have scrap metal on your hands.



Cheers

Greg Locock


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[JayMaechtlen]

Aside from rust, I'd expect the car to keep most of its strength and stiffness.
Of course, hinges and latches still wear, and weatherstrip and seals age.
Cars that start out heavier and stiffer might wearn the hinges and latches less. Or those pieces might be better to start.


Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[GregLocock]

Yes, /most/ of the increase in S&R is due to bolted joints, and other contact points rather than the actual bodyshell. But some spotwelds will break over a normal durability test so BIW integrity is a contributor.

Cheers

Greg Locock


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[dicer]

If spot welds are breaking during testing then are those areas being reinforced better?
If spot welds are being stressed to breaking points during normal and adverse driving conditions then the structure is not designed correctly.
Breaking in crash tests is another subject.
If the body structure is correctly designed then it should never lose its stiffness, only suspension and shock absorbing rubbers etc. would wear and degrade.
So out of all the cars manufactured which ones are the best for maintaining body structural stiffness?

 

[GregLocock]

"If spot welds are being stressed to breaking points during normal and adverse driving conditions then the structure is not designed correctly. "

Ever heard of statistics? The spotwelds strength will fall on a distribution, inevitably on some events a weaker weld will be on a high stress point and will break.


Cheers

Greg Locock


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[JayMaechtlen]

A car's service life is pretty unpredictable, except through statistics.
"Durability test" - I bet many (most?) cars in service never get used as hard as in a mfg's durability test?

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[GregLocock]

More crashes,fewer square edge potholes!


Cheers

Greg Locock


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[dicer]

Just like if aircraft are popping rivets during testing, its not going to be FAA rated till the problem is fixed.
Or the same with cylinders falling off engines. So then auto manufacture is not into correct design of their products?
If there is a problem fix it, before it becomes the big trillion dollar recall that is a statistic to think about.

 

[GregLocock]

So then auto manufacture is not into correct design of their products?

There is no such thing as correct design, but anyway, how many recalls involve /structural/ issues in the bodyshell?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[mbt22]

As a non-auto engineer, I would have thought that if a manufacturer's durability test always stopped before something failed, then it's a pretty poor test. Make it arduous enough to understand where it fails, hten decide if that point is acceptable or not.

Just like if aircraft are popping rivets during testing, its not going to be FAA rated till the problem is fixed.

Actually no, there are some wonderful videos of Boeing 777/787 wings being loaded until they snap.

Matt

 

[dicer]

I was not talking destructive testing. That is a different test format than a durability test. Now if those wings had parts falling off while doing stalls and high G maneuvers the test is a fail.