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RoadBikeRider
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I am in the market for a replacement frame and during my internet searches I keep reading the term "fatigue" when I am reading about aluminum frames. What is up with that? Do AL frames really only last 3-4 years now? I rode my last aluminum road frame for 8 years plus and never even thought about fatigue. I then sold it and it is probably still out there on the road. Is this a problem only with the super light AL? It is hard to justify an AL frame if I feel the need to replace it every 3-4 years which seems to be the number that keeps coming up. How can you tell if the frame is fatigued? My last frame never looked any different or rode any different after all that time. It would be silly to buy a used AL frame if it were more than a couple of years old if this really is an issue. Somebody please set me straight.

Thanks, T
 

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I worked in aerospace for many years and used a lot of the same aluminum used in bicycle frames like 6061-t6 and 7005 series. They did not seem to worry about fatigue on airplane structures. Although some of the mechanical systems did have hours of life but thats because of metal on metal situations. Some of the planes are 40 plus years old and still flying.
 

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People often confuse aluminum's failure mode with its life span. After millions of stress cycles, aluminum will crack and quickly progress towards complete failure. After millions of stress cycles, steel will crack and slowly progress towards complete failure. But given that millions of stress cycles are good for several decades of riding, aluminum's short time span from crack to complete failure is a non-issue for almost all riders.
 

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When to worry

andulong said:
I am in the market for a replacement frame and during my internet searches I keep reading the term "fatigue" when I am reading about aluminum frames. What is up with that? Do AL frames really only last 3-4 years now?
If you're heavy, ride a lot on rough roads, don't try to dodge potholes, crash occasionally, and buy a super light Al frame, then you should worry and do regular inspections for cracks. For the rest of us, not an issue.
 

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wim said:
People often confuse aluminum's failure mode with its life span. After millions of stress cycles, aluminum will crack and quickly progress towards complete failure. After millions of stress cycles, steel will crack and slowly progress towards complete failure. But given that millions of stress cycles are good for several decades of riding, aluminum's short time span from crack to complete failure is a non-issue for almost all riders.
The underlying fact is that Al. WILL fail eventually, no matter how small the load. Steel and Ti will never fail given a small enough load, below their endurance/fatigue limit. Again, this is not something that most cyclists need to worry about.
 

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SleeveleSS said:
The underlying fact is that Al. WILL fail eventually, no matter how small the load. Steel and Ti will never fail given a small enough load, below their endurance/fatigue limit. Again, this is not something that most cyclists need to worry about.
Yes, but as you say, those 'never fail' steel frames would have to be designed so that peak loads never exceed that fatigue limit. With modern steel frames, it's likely that the limit is exceeded, at least on occasion. All frames fatigue and crack eventually. The only way to make them last longer is to use more material.
 

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Aluminum usually doesn't fair well with fusion welding and in many types of Al the stress allowable drops by a factor almost greater than 2. The only way to get around this is to increase its effective weld area. Therefore, that is why you see Al bikes with larger diameter tubes (epecially the down tube) to account for a longer weld length to strength the joint. Steel doesn't have this problem and its welded joints is just as strong. And yes, the S-N curve (stress vs # of loads) for aluminum is much lower than that of steel and titanium. Then on the other hand, steel bikes I believe are just a bit heavier.

Heat treatment after welding on Al tubes helps but not sure if manufacturers label that as their manufacturing process.
 

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SleeveleSS said:
The underlying fact is that Al. WILL fail eventually, no matter how small the load. Steel and Ti will never fail given a small enough load, below their endurance/fatigue limit. Again, this is not something that most cyclists need to worry about.
Yep. Might be worth noting that one thing often missed in the simplifications of internet postings is that simply loading aluminum will do nothing.

For a movement to qualify as a load cycle, a stress must have both a compression and (most importantly) a tension component. Many of the stresses visited on a bicycle frame have no tension component, and so simply have no effect on the life of the frame. With decent designs, the stresses are arranged and realigned such that the tension components hit portions that are 'overbuilt' such that practically speaking, the fatigue life will never be approached.
 

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SleeveleSS said:
The underlying fact is that Al. WILL fail eventually, no matter how small the load. Steel and Ti will never fail given a small enough load, below their endurance/fatigue limit. Again, this is not something that most cyclists need to worry about.
Bicycles are not designed to keep stresses below the endurance limit, no matter what they are made out of. So, eventually, all bikes will fail. Eventually.
 
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