Steel molecules - do they get tired?

General cycling advice ( NOT technical ! )
Brucey
Posts: 39836
Joined: 4 Jan 2012, 6:25pm

Re: Steel molecules - do they get tired?

Postby Brucey » 2 Aug 2020, 9:59am

asides from a semantic side debate the original question could be said to boil down to "does steel creep or change at room temperature, when stresses are lower than yield?".

The answer is 'yes' but it is not normally a significant effect. But occasionally it is; so cheap clock springs can lose their springiness, and sometimes it is found that much older castings are dimensionally more stable in service, that kind of thing.

What is happening? Well it varies but dislocations can have an energy barrier to further (sometimes permanent) movement which is comparable to thermal energies, so it becomes a statistical process, strongly related to temperature. In other cases there are species (such as monatomic hydrogen) which can dissolve in the interstices between larger atoms and can affect the mechanical properties of the material.

cheers
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seph
Posts: 155
Joined: 3 Sep 2010, 8:22pm

Re: Steel molecules - do they get tired?

Postby seph » 2 Aug 2020, 10:27am

I agree that the terms used are not important here.
But....
...At a general sort of a level...
Metal molecules is incorrect because of the type of bonding present.

Non metals bond covalently, by sharing electrons, in materials like carbon dioxide and water. These small entities are molecules and have weak attractions for other molecules around them. (Giant covalent structures exist too, like graphite).


Metals and non metals form ionic bonds, with the metal donating electrons to the non metal. The bond consists of the electrostatic attraction between the oppositely charged ions. This can form large crystal structures, like salt (ie sodium chloride) crystals. They can be of any size, and so are not molecules

Metals have metallic bonds. The metal atoms outer electrons are lost, leaving lots of positive metal ions, and lots of delocalised electrons. The metallic bond is the attraction of these ions for the electrons in the sea of delocalised electrons. It may be a few metal ions, or billions, this is why they are not termed molecules. This type of bond gives metals their properties of flexibility, conductivity etc. (Metal structure is more complex than this, but in essence I think it’s true)

So, generally, molecules refers to small, covalently bonded (ie non metal) entities.

As I say, of no importance to the topic discussed, but

JakobW
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Joined: 9 Jun 2014, 1:26pm
Location: The glorious West Midlands

Re: Steel molecules - do they get tired?

Postby JakobW » 2 Aug 2020, 1:42pm

Casting my mind back to my materials lectures I recall that we distinguished between creep (= material extension/strain at constant stress) and relaxation (= reduction in stress at constant strain). As mentioned above, neither are common in usual engineering materials unless operating in a high-temperature regime. You sometimes see advice against storing springs under tension, but I'd always assumed that was so a knock or w.h.y. didn't take it beyond the elastic limit rather than to do with relaxation.