Dunno if it's true, but that's what my cousin's sister's mom's best friend's uncle told me.
well, i can only speak for myself, and i have to admit, that some of it is illogical. i used to be a civil engineer in my past life, and in material science we studied mostly concrete, and then ductile alloys like steel (A572) and aluminum (not the 6061 thats typically used in bike applications). based on that, i guess that i just like the ductile failure mode of alloys where you see plastic deformation before ultimate failure, as compared to the slightly more sudden failure mode of carbon fiber. i understand that carbon fiber has a much higher yield strength, and is probably much stronger (esp w/ repeated loading cycles) than alloys.gticlay said:For you "Aluminum" fans why do you prefer a material that is guaranteed to break in a measureable, finite number of stress cycles vs. a material that is proven to be quite a bit stronger in this specific frame design with a nearly infinate fatigue life? I'm just curious... Maybe it's the classic, "I don't know much about the material so I don't trust it syndrome"? That would be funny 'cause the same thing happened when more and more aluminum frames started to be made.
eleven-yo,eleven-yo said:@dth656 - Metals experience crack growth during low load cycles. Aluminum does not have an endurance limit, so the cracks can continue to grow under low loads (just riding along) until the crack reaches a critical size - and the material will then experience a brittle failure under a low load. The ductile failure you are describing would happen if you drove your bike into your garage door when it was on the roof, but not necessarily when you hit a rut at speed after you've had it it awhile. I think this is a common misconception about metal failure modes versus composites.
It was explained to me in terms of civil engineering with a question posed like this:
A bridge is designed, and a truck drives over it every day for 20 years. One day, the same truck drives over it and the bridge collapses suddenly. Why?
And then I got the dissertation on material toughness, which gets pretty complicated.
When a material has no endurance limit you do not need a defect to cause failure. Defects such as a stress riser make matters worse. An aluminum frame has a finite number of cycles before failure... its predictible in fact, on a frame cycling machine.dth656 said:eleven-yo,
i'm going to have to come clean here, i ended up w/ a B- in my mat sci class (maybe i shouldn't have ditched lecture to go biking, ha). can you explain a bit more about the low load cycles and the cracks they cause? my (probably incorrect) thought was that you only have cracks form at a defect in the alloy microstructure (like at a weld), or maybe at a sharp angle that causes a stress concentration). i understand that repeated loading that causes plastic deformation will cause a failure (kinda like repeatedly bending a paper clip until it breaks), but was curious about this repeated low loading causing failure.
This is completely untrue. A regular frame designed for regular use will not last 20 years. No way, no how. Think of it more like a jar of pennies. Each 'hit' no matter how big or small spends some of those pennies. There isn't 20 years of pennies in any frame and if you ride every day, it's a few years worth of pennies with no crashes, no unusual use... not a decade.@dam said:Of course, if aluminum suffers a fatigue failure in a bike, it was probably underdesigned to start with. It's not like you expect somebody to be hammering on your bike in 200 years, let alone 20.
I guess if you get silly about it and are talking about a comfort bike or something. I'm telling you no MTB that is ridden on trails will last 10 years. It is not under designed either, it is because they are specifically designed to be fairly light (and cheap). I'm sure it varies from company to company but I would guess an aluminum frame is generally expected to last 3 years with regular use, 2 years of that under warranty.@dam said:I'm a mechanical engineer, gticlay; I understand fatigue. Go to your local airport and look at the Cessna's or 737. There's an extremely weight-critical device made of aluminum, and they generally last many MANY decades of regular, hard use before fatigue becomes an issue. Under a low enough load, it'd take billions of years to fatigue a part. If it fails in thousands of cycles, it's probably underdesigned with regards to fatifue. My point is that a typical bike usually only sees maybe 10 years of hard use, and if it can't last that long, it's underdesigned and the typical stress of merely pedaling it had to be way too close to the ultimate stress.