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Significance of "fatigue life" engineers are welcome(19 posts)

Significance of "fatigue life" engineers are welcomestinkfoot247
May 4, 2003 5:54 PM
Aluminum frames are compensated for fatigue life by over sized tubing. other than crashing your bike what significance does fatigue life have.
Engineer replies!!!Rusty McNasty
May 4, 2003 5:59 PM
Fatigue life means essentially this: If your frame is made of aluminum, eventually, despite anything you do, it will fail due to metal fatigue. Steel and titanium both have ENDURANCE LIMITS, which mean, if the stress is kept below a certain level, the frame COULD last forever. Aluminum does not. Period. Ask any metallurgist.
I don't understandstinkfoot247
May 4, 2003 6:23 PM
Still not understanding you. how about what causes fatigue life. any stess on the frame right. well if I apply that stress for 1 second and then stop, If I didn't max out on the frames ability to handle it then what difference is it going to make if I do that a bunch of times. Ok, the way I am percieving this whole fatigue life is this, aluminum and carbon do not "stetch" when they undergo an impact, they take it like a man, if it is too much they break. that threshold is called fatigue life. Please help me understand, and maybe you know some resources that would help me to do so. BTW what kind of engineer are you, only ask cuz I stive to do so one day(mechanical). ok, thank you
I don't understandBruno S
May 4, 2003 6:48 PM
Fatigue generally refers to failure by crack initiation and
growth due to cyclic loading. It follows these steps:

1. Crack initiation: A small crack forms at some point of high stress concentration (sharp corners usually)

2. Crack propagation: The crack advances incrementally with
each stress cycle (bump in the road, pedal stroke)

3. Final failure: Occurs very rapidly once
the advancing crack has reached a critical size (frame fails suddenly without warning)
I don't understandEvilDeer
May 4, 2003 6:51 PM
To oversimplify things...

A metal under stress can fail in one of two ways:
1) a very large force applied to the metal will exceed the yield strength of the metal and will permanently deform the metal. This is what happens when you bend a piece of metal far enough that it doesn't return to its original shape.
2) Fatigue. Stress on metal, although lower than the yield limit, but repeated many times (10^3 -> 10^8), will cause microscopic cracks and imperfections in the metal to gradually grow until the part fails

Ian Dromey P.Eng.
re: I don't understand -- long rant, sorrydtufts
May 4, 2003 7:03 PM
Fatigue life is measured by taking a sample of the metal being tested and putting it in a machine that applies a load briefly, then repeats, cycling as may times as it takes for the specimen to fail. Just like you described. The load applied is not enough to cause a failure with one application, but the repeated application of the load over and over again will cause failure eventually. Different metals have a different fatigue life because steel and titanium can take more cycles than aluminum. Eventually, however, every material will wear out and fail (even CF).

You can learn all about this stuff by earning a degree in Civil Engineering, like I did, or Mechanical Engineering, or many of the other engineering diciplines. Books are ok, but its much easier to learn by putting samples of steel, alum, and titanium in the stress machine and watching them go until failure.

Also, people have tested every material under the sun and cataloged all of the data in reference books. You can look up the modulus of failure for alum. and compare it to titanium if you want to. You don't have to test it yourself.

Anyway, this can be very interesting but it really has nothing to do with how a rider should evaluate the durability of any particular frame. Sure a CAAD7 is made of aluminum, and aluminum has lower "fatigue life" than steel (i.e., a steel sample takes more cycles to fail than a similarly sized aluminum), but there is so much more to be considered to be able to judge the life of a FRAME than the type of material. In addition to the material's fatigue life, you would also want to consider: size of tubes, craftmanship of joints, imprefections in materials, geometry, etc. In fact, the best way (the only reliable way) to predict how a frame will hold up over time is to observe it over time while someone rides it hard. Not even the best engineers could say that one well-made frame is more durable or "better" than another just be doing an engineering analysis.
Fatigue life = how long it takes to get tired of the bike!dzrider
May 5, 2003 4:27 AM
Thus a bike that is a joy to ride and is beautiful in the eyes of its owner has a much greater fatigue life than an ugly bike that busts your butt with every bump or makes the rider feel terrorized by every high speed descent. This definition of fatigue life cannot be quantifed, but is far more real than a fatigue life formula that leaves bike lust out of the equation.
...for the second owner, the fatigue counter starts over! LOL!nmSpunout
May 5, 2003 6:11 AM
heh, metallurgy.....(nm)longhorn
May 4, 2003 10:02 PM
re: Significance of "fatigue life" engineers are welcomeBruno S
May 4, 2003 6:15 PM
AL tubes are not as strong as steel or titanium so they need to be larger. The tube size doesn't have anything to do with avoiding fatigue failure. All materials fatigue if they are worked past certain limits. In some AL frames these limits will be reached causing fatigue that will eventually break the frame. In fact, steel frames can also fail because of high cycle fatigue if the frame design stresses the material too much.

I'm sure a materials engineer can explain this better.
Ok, all AL will fail. My poor Specialized is doomed nmBruno S
May 4, 2003 6:18 PM
It depends on tube thicknessMR_GRUMPY
May 4, 2003 7:06 PM
A three pound Aluminum frame might last for 10 years. A 1 kilo frame might last for 1 year if raced on by a pro.
Sometimes an ultra light steel frame may fail before a midweight Aluminum one. Stick a 220 pound rider on a 1 kilo Aluminum frame and you have a good chance of having trouble.
PUH-LEEZE don't mix metric and English in the same sentencebicyclerepairman
May 5, 2003 11:58 AM
especially in a technical discussion...
from cervelo....blanchaa
May 4, 2003 8:50 PM
Taken from Cervelo's website.....

"....In your question seems to lie one of the misconceptions of bike frames. Frames don't go "soft" over time, they don't change, unless it breaks. But the material stays the same, its stiffness stays the same. "

Are they full of it?
They mean that your frame will break without warning...Spunout
May 5, 2003 3:40 AM
in that the frame won't die a gradual death. Thats what all the microfractures are in an Al frame.

OTOH, a lightweight on a beefy Al frame on my club ride on Sunday tore his BB out. Craftsmanship has a very big part of this.
re: Significance of "fatigue life" engineers are welcomeJBergland
May 5, 2003 5:56 AM
If in fact Al. frames do fatigue and fail at a certain point, then there should be a very clear timeline when failures start to happen. Based off of this, no bike manufacture would even consider using Al. because of liability issues. The bike/frame is going to fail... and when it does, there is going to be a lawsuit.

I believe the whole 'fatigue issue' is very similar to an urban myth... sounds good in 'theory'... but the actual application isn't as clear. Ask yourself this: how many frames have you ever heard of 'failing'? For most people, it's a fairly small number. Of those failures, I'd be willing to bet that the frame materials are evening distributed based off of materials being used. There are more frames being made out of Al., therefore there should be more failures with that material. However, I'd be willing to bet that there are just as many failures with steel and Ti based on the corresponding numbers sold.
I personally know of more Ti and steel frames that have failedspeedisgood
May 5, 2003 6:41 AM
than aluminum. Three guys I race with had Ti frames fail for various reasons. A GT Edge (bad weld), Litespeed Ultimate & Colnago BiTitan (both cracked the seatstay near the dropout). Another friend from years ago cracked the steel downtube on his Specialized Stumpjumper. My Al GT of 2 1/2 years of racing is just as stiff (harsh?) as it was when new. It even has a big stress-rising dent in the top tube! The point is that any frame can fail (or last a long time) if it's not (or is) designed or manufactured right, among other things.

I'm no engineer but I vaguely remember from biomechanics class that the modulus of elasticity (ME) plays a role in the fatigue cycle. A higher ME means the metal can bend more without deformation (eg, fatigue) than a metal with a low ME. Since Al has a lower ME (eg, is stiffer) than Ti or steel, Al frames need to be overdesigned so they don't flex beyond their ME (or deformation) limits. Thus the overly stiff nature of Al frames protects them from flexing beyond the deformation point and thus from early fatigue. Am I in the ballpark here?
theory vs. realityDougSloan
May 5, 2003 6:43 AM
While it may be engineering dogma all aluminum will fatigue and fail, what if it does, but 100 years from now. So what?

Fatigue is caused by bending. If the bike isn't bending as much, then it won't fatigue as fast. Large tubes keep the bike frame from bending as much, therefore delaying fatigue failure.

If the frame is over built with enough extra material in the right places, the right tube shapes, or with the right alloys, fatigue failure might well occur well after the otherwise useful life of the bike frame, anyway.

Doug
Yup...JBergland
May 5, 2003 12:40 PM
... I would guess 99.9% of people have replaced thier bikes 5-10 TIMES over before true 'fatigue' would become an issue!! Also... most of the big advocates (that I have experience with) for this 'fatiguing frame' idea fall in one of two categories. Either they never pay for thier own bikes (sponsored races) or they are trying to justify a new purchase to a spouse!!:) Ask yourself which one you fall into??:)

JBergland