Structural fatigue

If possible, I would like some insight or find out about other people's experiences with metal fatigue on high mileage unit - body cars.

I have owned many vehicles of all kinds. Two of them ('88 Pontiac Bonneville and '87 Mustang 4 cyl) showed substantial signs of fatigue; excess body flexing, shaking and twisting, not only over bumps but also when braking or cornering (both have about 250,000 km). These problems manifested themselves in many obvious ways and were not caused by worn suspension parts, etc. They were not present when new, but got progressively worse over time.

Both also showed a small amount of fatigued metal cracking in several places.

Both cars were in the family since new and were rust free and were not in an accident or overloaded.

Other unit body cars and light trucks I have owned with similar mileage are still rock solid.

This got me to wonder if ALL cars have a structure "lifespan" and, if driven enough, will suffer a fatigue related collapse.

Or, if the above two cars simply had a faulty structure, either in design or assembly. If yes, 250 k km (about 158k miles) does not seem like particularly high mileage to cause this kind of damage. Any thoughts on this?

Finally, does anyone else have similar experiences with their own cars?

I think each car design may have built in long term weaknesses, type of driving conditions of course enter the picture, rust naturally, my race car, before installing a full cage back then, showed fatigue in the torque boxes just behind the k member, even got a kink over the drivers door. Granted with the Hemi in it for 100 runs I was asking for trouble, that of course is the extreme example. Most unibodies seem to have pretty good floor bracing, a convertible is asking for trouble if not designed properly.

One of my '97 Ventures showed signs of fatigue to the point where it would not pass my state's safety part of the yearly inspection.It also was flexing and my friend who inspected it told me it had gotten to the point if I had driven over some rough railroad tracks, severe damage could result. The vehicle had around 115,000 miles on it and I was the second owner purchasing it in June 1998 at 16,0000 miles. It appeared that salt usage on our roads was a major contributor to the damage. I had noticed a little "flexing" and odd sounds if I hit a pothole or went over railroad tracks leading up to this.There were signs of visible cracks and other areas that showed fatigue.

road salt will destroy any car in amazingly short amount of time. (to me anyway)

In WA salt is not allowed on the street, so I see salt-free cars 95% of the time, and in that situation, I almost never see what I would call structural failure without something abnormal (such as an accident) very often at all. I regularly work on cars that have 200K-300K miles.

Basically, if the material stress is low enough, then fatigue isn't going to be an issue. It all comes down to how well the part has been designed and tested.

Fatigue cracks start in areas of high stress, if a manufacturer doesn't do enough testing to find those areas, it may take a few design cycles to sort them out later.

Reinforcement isn't always easy as a poorly designed reinforcement can create other high stress areas and cause cracking there.

Think of it this way. Body parts were designed to specs to resist a certain amount of torque and tension. Over time, the body probably experiences alot of forces that warp the metal very slightly and make it easier to flex the metal in the direction it was not intended to flex in. Other than that, the constant heating and cooling of the metal (due to engine bay temperatures and environmental temperatures), the metal changes structure and becomes more/less brittle. Add in salt, chemicals in the air, and all that wonderful stuff, and you end up with slightly weaker body parts.

maybe I'm wrong, but that seems to make sense to me.

All materials put under stress and strain will eventually, over time, fail from fatigue. It just depends on the materials used and how well whatever it is was designed.

Other than that, the constant heating and cooling of the metal (due to engine bay temperatures and environmental temperatures), the metal changes structure and becomes more/less brittle.

It's a myth that metal gets brittle over time.
Fatigue is simply accumulated damage, crack initiation then crack growth.

If the stress is low enough to prevent the original damage, then cracks can't start or grow.
Think about how many cycles your engine crankshaft or suspension springs have taken.

i remember hearing somewhere (i think it was in maximum Boost) that the average load on an average crankshaft is well over 4000 psi.

Add to the salt usage in our area of the country, we also have a large number of potholes that form in the late winter and early spring from the freeze/thaw cycles. Some of these potholes are so bad that people have destroyed tires, screwed up alignments , and busted front end components.That definitely puts unneeded stress on the undercarriage.
Something UncleBob said,"I regularly work on cars that have 200K-300K miles." made me remember a conversation I had with an associate many years ago. He told me he took a flatbed with a bunch of low mileage engines from cars and trucks whose bodies rusted out before their time and sold or traded them down South for cars and trucks whose engines had failed but the bodies were still in excellent condition.

Something UncleBob said,"I regularly work on cars that have 200K-300K miles." made me remember a conversation I had with an associate many years ago. He told me he took a flatbed with a bunch of low mileage engines from cars and trucks whose bodies rusted out before their time and sold or traded them down South for cars and trucks whose engines had failed but the bodies were still in excellent condition.

this is actually a very common problem here. Less savery people will buy rusted out cars that don't show it externally for little money, then ship them to a place like here and sell them at auctions. Since salted cars are so uncommon here, people don't look for it, and pay full price for them.

As always, having a car inspected before buying it can be quite advantageous

Reminds me of a few years back when I bought my 1973 Ford Gran Torino. It had spent most of it's life in Florida. After only a couple of winters the car had completely rusted out to the point where I couldn't keep up with trying to slow it down and ended up scrapping it:frown: . It got so bad the roof under the vinyl top had developed holes and weak areas.
This past Fall I bought a 2006 Uplander and had it Ziebarted ( rust proofed and under coated with paint protection) in hopes it will help the vehicle last longer than the one it replaced.