[Closed] Metal Fatigue

In theory yes. In practice it does not seem to matter.

Wot TJ said. I've never heard of anything snapping just due to age.

(You watch the massive rush of people that have seen it posting now...!)

[i]I've never heard of anything snapping just due to age.[/i]

I snap regularly now I am old

Arguably any crack is due to the fatigue life of a part being exceeded Shirley?

All metals fatigue but at different rates. Aluminium fatigues way faster than steel or Titanium. Steel and Titanium fatigue slow enough to never worry about it. Its all about stress cycles. i.e. if you give your aluminum bars grief a few times a week I would suggest you replace them every 3 years. If you use them less you would probably get away with it for longer. Its not to say your bars will snap after 3 years just that they will loose some strength.

Carbon does not fatigue in the same way though 😉

but you need something to start a fatigue crack - so poor design/finishing/construction are (always?)to blame

with handlebars if you exceed the design limits (big biffa lots of miles) [i]and[/i]overcrank the stem i might be a bit worried

The reason I ask is that my bike snapped on Saturday with no real reason on the last little step at the bottom of Telegraph Road, it completely let go at one of the welds. I had a spare swingarm which I've fitted but I'm now concerned that after 10 years of use the frame and replacement swingarm is likely to fail again.

It might. It is complex and difficult to answer... There are some snippets of accurate information above and some misunderstandings...

Aluminium has more of a fatigue problem than steel in that it does not have an "endurance limit" . In steels the endurance limit is a stress which it can experience below which it will never fail, or has never failed with current measurement techniques. In Aluminiums the limit is not really there, so no matter how low the applied load it will still fail after a certain no of cycles. Although some aluminiums are more "damage tolerant" than others like 2000 series alloys.

In reality it would be impossible to predict when your frame would fail without a detailed knowledge of the stress distribution under load, the load history, the manufacturing process, the materials, any residual stress, any existing defects and an observation routine that would trace cracks as they grow. This does exist in industry, for example the aircraft industry, power generation industry, ski lift/cable car industry.

I assume it is an aluminium full suspension bike? I had a similar problem with a 2nd hand bullit I bought, I stripped it fro a repaint and discovered that the swing arm was full of cracks. Frame was fine. So I replaced the swing arm and its been fine ever since.

I think the suspension shields the main triangle from some of the load by reducing the instant hit - the summed impulse (force seconds)is the same but it is distributed over time. I have done a few sums about this in the past to see what the difference is, not conclusively mind you, and it is very much dependant on frame geometry and all the other things listed two paragraphs up. I might start thinking about it again now you have raised it..

One more point - the fatigue limit for steel may not even apply here. For example Brant has said in this [url= http://www.singletrackworld.com/forum/topic/any-mechanical-bike-engineers-in-the-house-opinions-needed/page/2 ]thread[/url] and others that

I can assure you that all high performance mountainbike parts are always designed OUTSIDE of the "fatigue limit" zone - as they would be ludicrously heavy and stiff if they were.

So this would imply that even if it were steel it would have a finite fatigue life, and I reckon 10 years is a long time in the world of full suspension mountain bikes.

Also lots of testing has shown that under certain load conditions some aluminium frames lasting longer than steels ones. Not that this proves anything as the designs are different etc etc. ([url= http://www.efbe.de/ ]see here[/url])

What I am trying to say is that you can learn lots about this and still know **** all about your bike's lifetime...

What I am trying to say is that you can learn lots about this and still know **** all about your bike's lifetime...

Amen to that - the only thing I can add is that fatigue life is generally much worse around the welds due to the poorer post-weld material properties and the stress concentrations due to the weld material itself. In this area - even for steel - for design purposes, the "safety net" of the endurance limit doesn't apply.

This is a can of worms that we could kick around all day 😉

its impossible to get bored of metal!

IGMC

Know any doctors? Ask to get it X-rayed or MRI scan but take the ferrous bits off 😉 ,failing that buy a new bike wouldn't like to be stuck somewhere because my frame had snapped, just a thought.

It is indeed a FS bike. I've ridden 2-3 times a week since 2001 ish.

I suppose because the nature of both the breakages I've had were not from a single incident, like a big drop to flat for example, I have to conclude that it was fatigue due to repeated load that made it fail.

Allowing for no design issues (like the ones Commencal were having with a batch of Metas a few years back) it seems that an aluminium bike will have a fatigue life span.

Shame as I really like the way my bike rides and would have expected more from it in terms of life.

You're right though I think a new frame beckons however I'd still expect a frame to last more than the absolute max of 10,000 miles that I've covered, allowing for 30 miles a week (with a good few weeks off) over 10 years.

Leads me back to my tread yesterday choosing between a Five, Heckler , Nicolai or Morewood.

9 years is a long time, even 'lifetime' warrenties tend to have a cut off at about 10 years. Then again lifetime warenties are a bit of a con as the manufacturer is banking on the buyer getting itchy feet after a few years and selling it on, so in reality it costs most manufacturers no more to offer a 4 year over a 3 year warrenty, or a lifetiem over a 4 year warrenty.

I usualy bin bars after 2 years, and relegate stems to commuter bikes as failiures in that area dont end well, aluminium frames I'd make an individual decision on, as usualy theres a lot of other damage by the time I'd wory about fatigue, my last aluminium frame I wore through the chainstays with my heal! Now I just buy cheep and/or steel so theres no need to worry.

Well I believe our chums in Halifax have my way of thinking when it comes to frames. I've not heard of any failures of fives under normal conditions.

Now to find a grand down the back of the sofa!

joolsburger - Member

"I've not heard of any failures of fives under normal conditions."

It does happen... Anything can break, saw a Five that had split its BB shell on glentress blue earlier this year, and heard of a couple that pulled their headtubes off. But they're tough enough so don't worry too much.

Nicely timed thread.

My FSR snapped on Sunday, driveside chainstay as I hopped over a pothole on some doubletrack. Ended up walking half the loop of the Newnham 60 with it (if you were one of the dozens who asked how I was - cheers!).

I bought the frame new 5 years ago and it's been used a lot since then, all year riding and trips abroad. The stays have a 2 year guarantee and the mainframe 'lifetime'. I've ordered a new chainstay assembly from Spesh as I need the bike working for another trip in 2 weeks.

I was wondering if I should be worried about the mainframe? It was cleaned on Sunday before the ride and there were no obvious cracks but to be fair I wasn't looking for minor ones, hadn't occurred to me that it would fail that way.

Bagpuss just check it over.

I usualy bin bars after 2 years, and relegate stems to commuter bikes as failiures in that area dont end well

Done that Al, black frame, black cracks.

Paranoid?

Me? 😳