Does aluminium degrade over time?

[andrewl]

Wahey! Rody to the rescue!

So it DOES degrade in time then

I'm probably coming in a bit late on the debate but, after having this discussion many years ago with an elderly freind of mine who is a Dr of Physical Chemistry and specialised in chemical properties of various metals and alloys and well as contrete dams etc. (FYI did you know that concrete continues to grow throughout its life).

I'd say it really depends on a large number of factors that at best you could say is likely.

Its also interesting to note that in the last 6 months I've seen more steel frame failures than anything else. Application obviously has a lot of influence.

Jez, maybe you should tell what you are thinking of buying so we can give you a more considered answer.

 

[jez-4-bikes-max]

Not a great fan of aluminium frames Im afraid... no Im not buying one of these

 

[Jeroen]

Very much doubt it, but you're not thinking of fatigue life are you? Ie aluminium has a fixed fatigue life - meaning an ally frame will eventually fail if ridden normally where steel might last 'forever'...

'Fixed' as in pre-determind with a big 'x' factor taken into account.

Simple explination; if you take steel or ti for example, theres a fixed max. stress that you can put on the material, at which beyond it the material will no longer go back to its unloaded form (thus permanent deformation). Stay under this load and your construction will last for ever. Meaning; no fatigue.

However; on aluminium alloys this treshold/boundry degrades over time, much like an ever decreasing line to some extend, especially when its subjected to forces. To cope with this, there are studies preformed to determine what will be save for a given lifetime. This still is a not an exact science and thus the margins are much, much bigger so you'll never come close to failure.

Alumium in general is not a material to be used in areas where its subjected to 'moving' stresses, meaning flexing, spring loads and such, because of the absence of a fixed fatique treshold. This, since the fatique limited to which it will not deform, will decrease overtime. There are some exceptions, but limited. By inmensly overengineering the construction you can use it in applications where the stresses are not constant (constant changing loadcycles) or subject to temperature changes (has significant impact on material properties). This way its possible to use it in aeroplanes for instance, where its subjected to great differences in stresses due impact loads and the constant change in temperature.

As said, the margins to be save are much, much greater than with other materials and its the reason why there are so much stricked regulations around aeroplane constructions. The decreasing fatique of aluminium became really known during the '50/'60s due plane crashes.

 

[Jeroen]

On a side-note and perhaps common knowledge to many;

since aluminium cannot withstand big deformations since it will decrease its fatique life; alu frames have this big tubes, so no movement/deformation will occure during its use. Thats the only reason why alumium frames are so stiff. Steel as a material is per given density 'stiffer'; its withstands deformation to bigger forces. But since its a lot stronger than alu, you can use less to obtain a similar strong construction. Using less steel, makes smaller tubes. Smaller tubes 'give' easier under loads than large diameter tubing. Making it more 'forgiving'.

A steel frame with the same tube diameters as a Klein would be roughly 3x as stiff as the alu Klein.

Pegoretti is a nice example of what can be done with oversized steel tubing; incredible sprinting frames...

 

[mikedesign]

aluminium doesnt have a fatigue life

 

[stevet1]

:shock: you can't just say that and leave ...

 

[Ziggy]

Maybe 'fatigue life' isn't the correct technical term? Or maybe he meant that it has no fatigue limit? Maybe he's an expert or maybe he's talking cr@p... either way, methinks a little more explanation's needed!

 

[Jeroen]

Maybe I missuse terms perhaps, Im not English (Dutch).

But read this again, and hopefully you get the picture of what Im trying to 'show':

However; on aluminium alloys this treshold/boundry degrades over time, much like an ever decreasing line to some extend, especially when its subjected to forces. To cope with this, there are studies preformed to determine what will be save for a given lifetime. This still is a not an exact science and thus the margins are much, much bigger so you'll never come close to failure.

... so..in a word, yes Aluminium has no Fatigue, since it decreases in strenght, therefor there is no set boundry to which it will last indefinetly. The boundry decreases over time and one cannot tell where its at at a given point in time. Thus; no fatique is correct. My explination might be a bit tricky to follow since I more or less missused 'fatigue' in this story.

 

[stevek]

I disagree, aluminium can suffer from fatigue. Bigger brains than ours have told us this.

I worked in the aircraft industry for years, airframes are manufactured from aluminium and as such have a "fatigue life" Why would an aluminium bike frame be different?

This describes it well I think:

In materials science, fatigue is the progressive, localised, and permanent structural damage that occurs when a material is subjected to cyclic or fluctuating strains at nominal stresses that have maximum values less than (often much less than) the static yield strength of the material. The resulting stress may be below the ultimate tensile stress, or even the yield stress of the material, yet still cause catastrophic failure.

A practical example of low-cycle fatigue would be the bending of a paperclip. A metal paperclip can be bent past its yield point (i.e., bent so it will stay bent) without breaking, but repeated bending in the same section of wire will cause the material to fail.


Any clearer?

 

[Russell]

Paperclips are steel aren't they? They're magnetic.