A car chassis will fail aging without use?

[SEBASTIANOFA]

Speaking of cars, in particular of the steel chassis, if we keep a car stopped for years, even a new car, will the weight of the engine and all the components form cracks in the crystalline microstructure of the steel?

 if we think about a spring, if we leave it compressed for so long it will lose its ability to flex, it will not come back as before (i guess), isn't it the same for a frame that has to hold up an engine for years? perhaps there is a load limit below which the piece will return as before also being compressed for many years? What happens to the metal microstructure in these cases? Correct me if I'm wrong. 

if the weight of the motor has been designed to keep the deformation of the underlying frame always in the "elastic" and not "plastic" range, how can it damage the metal in the absence of external environmental attacks?

thanks



 

[IRstuff]

Seems like a dubious assertion to me. If the frame is that overloaded, it would fail under normal usage as well, since any rood shock could result in even further overloading.

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[EdStainless]

As long as loads are within the design range, even springs will take load for decades with no change in size or properties.
This is why we like using metals for construction.
Non-metals may have a different story since many of them will creep even at room temp.

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[SEBASTIANOFA]

Since automotive components are designed to work below the fatigue limit I would say that we are always in the field of elastic deformations and that the weight of the engine and other components (even more if at rest) are well below the limit of fatigue. assuming these conditions, would the frame remain unchanged over the years or should it be damaged even when stationary at room temperature?

the crystalline microstructure of the metal would remain unchanged over time even having an elastic deformation? that is, if weight were to be removed from the frame, would it return to its initial condition without microstructural damage?

 

[IRstuff]

Microstructure changes would require heat, which would not occur for a static load. Cargo trucks are loaded with static load and dynamic loads, and you don't see broken springs, frames, axles, etc., unless there are gross overloads.

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[EdStainless]

We have building structures out of metals for centuries, under continuous load with no changes.
We use metals in stable microstructural conditions, that is the whole idea.
I have taken the valve springs out of an engine that is over 80 years old and they all measured fine.

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[BrianE22]

You might be thinking of "Stress Relaxation". I think it's just a yielding in small areas of the material that are close to plastic. I'm sure others know more about it.

 

[SEBASTIANOFA]

Can someone explain this strees relaxation?

 

[SEBASTIANOFA]

I read something about it, if I understood correctly, having a certain level of strain, we have a certain level of relaxation, but I also read about the proportional limit, being within this limit, can the frame withstand the strain without ever registering damage?
Is something similar to fatigue limit?

 

[IRstuff]

There are extant vehicles from well before the turn of the last century and beyond NOT falling apart; this vehicle looks perfectly fine:

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[btrueblood]

I'd think corrosion, not fatigue or creep, would limit the life expectancy of a modern car. Kept in a temperature controlled, low humidity (and possibly low oxygen) environment it would keep for centuries or perhaps millenia. There are people who purposely do this - buy new cars and roll them into long term storage "bubbles" to (hopefully) appreciate in value over the years as other examples of the vehicle are driven/worn/destroyed and disappear.

 

[SEBASTIANOFA]

you are probably right, also because if the iron deforms due to its own weight, practically everything should be destroyed by itself.

 

[BrianE22]

I found the following that relates to creep and stress relaxation:

https://materion.com/-/media/files/...ors-affecting-stress-relaxation-and-creep.pdf

 

[SEBASTIANOFA]

From what the article say practically all stell objects will have to crumble sooner or later for their own weight. I thought that for static loads below the fatigue limit there were no problems even in long periods like many years.

to generate heat in the steel microstructure, is enough to apply a weight on the piece or must there be some loading / unloading cycles?

 

[IRstuff]

It's a logarithmic scale, as stated in the article. Moreover, reduction in stress does not necessarily equate to collapse or failure

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[SEBASTIANOFA]

I am probably wrong because these things are not my field of work, but of the test scales I seem to understand that even with little strain and low temperatures it will lead to damage even if in a much longer time. Correct me if i’m wrong.
What would it mean for a frame to lose stress? would it mean having a reduced elastic capacity in the future?

I just can not understand if even a low weight will lead to damage to the frame sooner or later or if, of course, without being used, it can remain for an indefinite time

 

[MagBen]

interesting topic. I happened to have a study on ordered (L10) Fe-Ni from disordered (fcc). The idea was from mineral phase change (from Tetrataenite to taenite) found in meteoric iron. The phase change occurs at low temperature (<300C) and takes MILLION years! What I did was to make a nanostructured Fe-Ni material to increase diffusion rate, it still took a week to have the phase transfered.
The point i wanted to make is "Time is a killer"

 

[SwinnyGG]

From what the article say practically all stell objects will have to crumble sooner or later for their own weight.

The article does not say that.

but of the test scales I seem to understand that even with little strain and low temperatures it will lead to damage even if in a much longer time.

That's not the case. What did you read that made you think this was true?

 

[IRstuff]

"Correct me if i’m wrong"

You are wrong; as MagBen alluded, these effects, if they occur at all, will occur over many millennia. The article's chart is in logarithm of time, so complete relaxation will take millennia, and even then it merely a distortion of the crystalline structure, not disintegration.

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Thought I would finally weigh in here with my 2¢.

You are conflating a number of concerns, like new materials science students do, and that's OK.

A coil spring can potentially be overcompressed to the point of yielding, but that is far beyond what the static weight of the vehicle will do.
Springs are subject to sag, a very ordinary word that describes some rather complex submicroscopic processes. This happens after many many load cycles. Metallurgists have figured out how to mitigate that with alloy design and you rarely see that anymore on modern vehicles.

Each of the components you describe has a different set of requirements, but each is prone to the one factor you omitted - corrosion.

The typical life cycle of an automotive (and many other) coil springs is:
Coating is penetrated > corrosion pitting happens > pits act as stress raisers for fatigue > fracture
I recently replaced four springs on my Camry. Being a Toyota you know the steel was good. It was many years and 185,000 miles in a winter environment that killed off one spring. There was no sag to warn me.

This failure sequence can easily occur without yielding of the material (until of course just before final catastrophic failure). That is the pernicious thing about high-cycle fatigue: there is no visible deformation. The important thing to understand with vehicles on roadways is that simple statements about fatigue are impossible to make. This is because loads vary widely and randomly, so we need to talk about it in terms of a load spectrum. Difficult to analyze and to simulate experimentally than the ideal case of constant amplitude loading.

Creep and stress relaxation are very high-temperature processes, from which automotive components are exempt.


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