Fatigue corrosion or stress corrosion in car chassis

[SEBASTIANOFA]

Hello to all, thinking about car chassis, when it is used we should not always have corrosion due to fatigue at the welding points since they are points of high stress that often do not have a fatigue limit? shouldn't they show micro cracks that will corrode in a short time? I think the coating will not protect from this type of corrosion. what's the difference with stress corrosion? Will stress corrosion also occur for static loads at room temperature?
if anyone could explain these things to me I would be grateful

if instead a car is kept still but it already has some micro fractures in the welding points that are however protected by the paint, these will corrode even with only a static weight

 

Those are many questions about a wide array of potential fracture mechanisms, and impossible to answer.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[SEBASTIANOFA]

I try to simplify my questions then, talking about a properly painted and protected car frame, what can happen to the frame itself if it is kept at room temperature without being used? that is, the most critical points I imagine are the weldings since they have to hold the whole frame together, I imagine that if used it could generate fatigue corrosion, if instead it is left standing?

 

[EdStainless]

In general the stresses involved when not in use are so low that the life would be infinite.
There is no cyclic loading in induce fatigue.
And as long as it is dry it will not suffer any corrosion.
Why would you think that sitting unused would lead to any derogation?

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P.E. Metallurgy, Plymouth Tube

 

[SEBASTIANOFA]

I read that is not easy to establish a fatigue limit on welds and it may happen that even small strain make them to work close to the yield strength, this would mean having immediately micro fractures and therefore corrosion i imagine. On the other hand I see frames with more than 400000km with still perfect welds and no traces of corrosion, so I guess I lose some passage.

 

[cap1a79]

Generally static fatigue is not an accepted concept. Refer to the below link.
"The page describes static fatigue definition is limited to ceramics literature which involves a time dependant reduction of material properties whereas the more widely accepted definition of fatigue needs cyclic loading to occur (see Fatigue (material) and Fatigue of Materials by Suresh 2004)"

https://en.m.wikipedia.org/wiki/Static_fatigue

I would give some guideline to underatand better for you.

1. Check if the weldmenet is exposed under out of elastic area by stress. If yes, you have to find a solution from stress and strain curve , not from corrosion
2. Based on chemical corrosion model, generally metal shall contact with corrosive "fluid". If the metal contacts with dry air, corrosion is not a main concern as dry air is non-corrosive.



Lee SiHyoung,

 

[SEBASTIANOFA]

the stress of weldings in a car should be largely in the elastic range. Is the concept of fatigue limit also applicable to welds? I read that it can happen that certain weldings due to their shape can always work close to the yield point, this should not lead to microfractures that can rust already in the first work cycles?

I mean, i don’t understand how they can design a car with infinite life fatigue, but having welds which cannot be set at a fatigue limit

 

[EdStainless]

There are steels that will suffer delayed cracking after welding, but that is not fatigue it is welding the wrong steel.
In cubic structure metals (Fe, Ni based mostly) you have fatigue curves that actually flatten out with increasing cycles.
That is, if you are below a critical stress level they will have infinite fatigue life.
This is why stainless steel aircraft hydraulic tubing will last longer than the airframe.


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P.E. Metallurgy, Plymouth Tube