Do car chassi (?!) suffer from metal fatigue? I read somewhere that spotwelds can even fail as cars age.
Can a chassis literally 'fail' after x years or will it last forever if washed and kept rust free etc?
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depends on the vehicle and the flexing of the shell (chassis)
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Friend of mine used to work as a technical trainer for Volvo. Apparently some years ago they had a problem with chassis cracking just ahead of the rear wheels on one of their models. It wasn't widely publicised, but the dealers knew about it....
In the past I have worked on the odd car with failed spotwelds etc. I once worked on an Alfa where the spotwelds at each end of the bulkhead seemed to have failed and you could get the end of a finger into the gap.
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Even brand new production cars are thought too floppy for racing, hence the special seam welding and comprehensive roll cages which help stiffen the body structure especially to the front spring turrets... After a few years on the road they are worse, with spot welds giving way, rust, metal fatigue and so on. Goes with the worn suspension joints, shot dampers and fully run-in chipped turbo unit, sweet as a nut guv :o)
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Do car chassi (?!) suffer from metal fatigue? I read somewhere that spotwelds can even fail as cars age. Can a chassis literally 'fail' after x years or will it last forever if washed and kept rust free etc?
If you look at a virgin piece of steel, there is a stress range below which you do not get fatigue failures - this is called the endurance limit. For standard steels, this is about 30 MPa.
However, there are some things that interfere with this nice state of affairs;
1) In a corrosive atmosphere, the endurance limit is no longer valid - fluctuating stress will cause failure at some point
2) In a weld, there are a number of factors which conspire to invalidate the endurance limit. These are;
- the presence of inclusions which can nucleate cracks
- the presence of a geometrical discontinuity which concentrates the stress by the weld
- the presence of a zone in the weld where the thermal residual stress is of yield magnitude, and is tensile
In most cases, the chassis is over-tested, and over-designed, and cracks in car chassis are unusual. There are exceptions - mkIII Escort Track Control Arm brackets spring to mind. Finding cracks is much more common in commercial vehicle chassis. Some of my father's old trailers needed some welding every year for their MOT!
I won't go into the problems of cracking and structural integrity on our rail vehicles or nuclear reactor structures!!!
Number_Cruncher
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you forgot to mention on a chassis under stress (or is that the geometrical discontinuity area?) where more than one piece of metal is welded to the other the thinner weaker of the two is most liable to show a metalurgical proposition to subside and open up.............transit mk3 rear chassis hangers come to mind
I always found the best thing to do was mig weld the split and then double plate the piece of chassis with butt welds and then see where it decides to tear next :(
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Full load desert racing Transits, om, or just ordinary ones cruising about with a bed and a coffee table?
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>>or is that the geometrical discontinuity area?
Yes, that's the sort of thing I was on about - perhaps I should have phrased it better - whenever you have a sudden change of shape in a part, you get a stress concentration. If you are applying a one off load, this isn't such a big deal, but if the load varies with time (like just about everything on a car does!), then the stress concentration becomes very important.
The most important work done on this subject was by Inglis (quite some time ago - I'll have to look up the date) - he was the first to demonstrate how stress concentration works - he showed, for example that you get a stress concentration factor of three close to a hole in a plate.
Griffith, then went further, and generalised the results of Inglis from a circle into an ellipse. So what! Well, by making the ellipse very long, and very thin, he was able to understand the stress field around a crack, and by some further maths involving energy, he determined the conditions under which the crack would grow to failure - this work forms the basis of fracture mechanics, which is used extensively in aerospace, rail, and nuclear engineering. But, because there isn't the skilled labour (because motorists are too tight to pay!), the mere presence of a crack means that an automotive structure has failed. Any large structure (plane, train, nuclear reactor, bridge, etc) contains cracks - even from new - we must live with it, and monitor, and manage the cracks by appropriate detection and repair.
Number_Cruncher
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No-one yet mentioned Fords and the famous failures where the tops of the mcpherson struts were mounted, or the bulkhead where the clutch cable went through
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TourVanMan TM < Ex RF >
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