Hi,

I'm wondering. Is an aluminum frame bound to fail at some time of its life? If well maintained to prevent rust, will steel outlive an AL frame?

Thanks for enlightening me

I'm not an engineer, but I feel that as far as durability steel is a more durable material. After all I don't see crankshafts nor bridges made out of aluminum.

Aluminum has no fatigue limit, so it will eventually fail even from small stress amplitudes. Steel, OTOH, does have fatigue limits, so repetitive stress cycles with amplitudes below the fatigue limit can be applied an infinite number of times without failure.

Frame designers know this, and design aluminum frames conservatively to compensate for the material's lack of fatigue limit.

Scot Nicol, founder of Ibis Cycles, wrote a great series of articles in the early nineties, Metallurgy for Cyclists, and they're a must-read for anyone interested in the properties of materials used for building bicycle frames.

Here's an excerpt:

Fatigue Strength

Guess what? This is another important property to consider but, once again, not by itself. Fatigue failure occurs by applying cyclic stress of a maximum value less than the static tensile strength of the material ... until your specimen fails. This can be a cool test, because the alternating stress mimics vibrations and impacts that happen when you ride your bicycle down the long and winding road.

The fatigue strength itself is a measure of the stress at which a material fails after a specific number of cycles. What's tough though, is designing the proper test. Again, a bicycle is a complex puzzle to consider. There is no standard test for fatigue. Another kink is that fatigue tests are done by cyclic loading of similar stress, whereas the loads you apply to your bicycle parts are uniform.

Ferrous alloys (a.k.a. steel) and titanium have a threshold below which a repeating load may be applied an infinite number of times without causing failure. This is called the fatigue limit, or endurance limit. Aluminum and magnesium don't exhibit an endurance limit, meaning that even with a miniscule load, they will eventually fail after enough load cycles.

i wonder if pro teams are supplying more frames to the riders knowing that alu frames dont last as long as steel ones. and what about carbon? will carbon outlast steel or alu for a season?

Aluminum frames are designed so it will take many years for them to fatigue.

How many of you worry about the aluminum wheels on your car?

how can you compare an alu frame to alu wheels?

This claim appears again and again, and [I hope I may be forgiven for pointing this out] it is false.

If you had a mosquito [even a heavy one] jumping up and down on a sturdy, thick aluminum frame [a Santa Cruz Bullet, for example], the frame would not reach its fatigue limit in your lifetime, my lifetime, or all of our lifetimes put together, or within any other reasonable time frame -- and not within most wildly unreasonable time frames.

[Perhaps there is a metallurgist out there who could tell us when and how failure might eventually occur -- I suspect that these materials slowly degrade or decay over the course of hundreds of millions of years. Assuming an immortal mosquito, I doubt if its many millennia of activities would be the primary cause of the failure.]

It depends on the way the frames are made, including the exact steel(s) used [there are many different steels, with widely differing properties, ultimate yield strengths, corrosion resistance, and fatigue limits], the tubing diameters [other things being equal, larger diameters will usually be stiffer and more durable], and the wall thicknesses [very thin-walled steel frames are often not particularly durable].

The same sorts of things can be said for aluminums and various types of aluminum tubing.

Some aluminum frames will far outlast most steel frames.

Some aluminum and steel frames will be about equal in longevity.

Some steel frames will far outlast most aluminum frames.

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"Is an aluminum frame bound to fail at some time of its life?"

There are strong, well built aluminum frames that can easily last a lifetime.

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Rider weight, strength, and riding style have a lot to do with it.

What causes most frames to fail eventually is repeated high stresses.

A 120-pound rider who has a gentle riding style, and rides on smooth roads, can make a frame last for centuries.

A muscular 240-pound hard-riding athletic type who rides aggressively on steep and rough off-road surfaces and high-speed downhills on a regular basis will destroy most frames, steel or aluminum, in short order.

Everyone is entitled to his/her own opinions, but not his/her facts.

It is a fact that aluminum has no fatigue limit; it's not just my claim or my opinion. The lack of fatigue limit is a well documented physical property of the material. I made the point in my post that competent designers of aluminum bicycle frames consider this fact when designing frames.

No offense was intended.

I just wanted to point out that it is somewhat overstated.

In (theoretical) cases of very light loads, I don't think that the statement applies. Certainly if you bounced a helium atom against a piece of aluminum tubing (at low velocities), there would be no real limit to the number of stress cycles.

I also have to wonder whether a frame built of hefty, thick aluminum tubing would ever really fail when used by a light and gentle rider. The frame would probably last for centuries if not millennia -- almost certainly so if the rider were gentle enough with it.

I recall some 20 years or so back about 20 people died over that theory, applied to the real world.

It was a large power catamaran that ran a taxi service around the Bahamas. The hulls were connected by a series of large steel I beams, well maintained with paint etc, and deemed such serious overkill at to give the boat the highest commercial rating. But the stress cycles built up over time (many years), and there was a series of popping noises that every beam broke off before they figured out what the sound was ( maybe three minutes ) and they suddenly were two boats sinking where there had been one before.

Only a couple people survived to tell the story and it was quite a lot of comment at the time, but I came away with a lot of respect for repeated stress, and the need to spread it out and not allow it to focus. Also sudden shocks to a rigid system produce stresses thousands of times greater than if there is a shock absorber (a main reason for inflated rubber tires).

To that end I am investigating linear fiberglass rod or tubing for the long bits with attached stainless tips to allow for flexing and take a bit of harshness out of the ride on a tadpole trike. If anyone has tried this, I would like to hear about the results.

A well built aluminum frame will last longer than most people will care to keep it. It's true about the lack of a fatigue limit for Al, that is one reason aluminum frames typically have such large tubes - to reduce the flex thus, to extend the frames life. I'd be leery of about the life expectancy of a small diameter tubed aluminum frame designed for comfort, ie, flex. Alas, the bicycle industry doesn't seem to make any of these...for good reason.

"This claim appears again and again, and [I hope I may be forgiven for pointing this out] it is false.

If you had a mosquito [even a heavy one] jumping up and down on a sturdy, thick aluminum frame [a Santa Cruz Bullet, for example], the frame would not reach its fatigue limit in your lifetime, my lifetime, or all of our lifetimes put together, or within any other reasonable time frame -- and not within most wildly unreasonable time frames."

I'm no engineer, but I know in cycles to failure tests they will do cycles of 10% of the tensile strength, and that these apparently low levels cause material failures. So it is somewhere above the level of a helium atom. The 10 percent test is taken as important because, for instance, a boat just gently tipping back and forth in it's slip may experience a demasting when the spar clumples, even though nothing in it's service life could explain such a failure. So I guess the issue for Al might be something like carrying it around on the back of your car for eons just in case you want a ride, a lot of low impact cycles on the bike rack might prove damaging.

Hi Pluc,

On a practical level, no -- a good aluminum frame is not bound to fail. Assuming you are not doing something unusual with it (if you are a more or less average recreational rider), it can easily last for a lifetime or more.

Most well made, well maintained steel frames will also last a very long time.

Both are fine.

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Aluminum got a bad reputation a long time ago, in the early experiments with its use for bike frames (and for other applications). Since that time, though, changes have been made and reliability is much higher. Properly made modern aluminum frames just don't have the same sorts of problems. As long as you stay away from the very lightest aluminum frames (in which durability is sometimes sacrificed to save on weight), quality aluminum frames are plenty durable.

There are other factors, though, in addition to fatigue -- gouges and scratches, dents, and other sorts of damage, apart from fatigue, are not uncommon in the long run. Steels tend to be harder and more resistant to these sorts of damage. Some steels are not only harder but much harder. If certain surface treatments are added, they are harder still.

Some of the steel frames have superior fatigue properties as well and, if well made and well maintained, they will outlast most aluminum frames in the *very* long run; but the well made aluminum frames will -- in practical terms, for many people, under real-world conditions and within real-world time frames -- have more than enough ability to withstand the stresses they will actually receive.

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Here are some more or less theoretical points (including some that are germane to the points about infinite cycles below the fatigue limit, and the myth about aluminums' having no fatigue limit):

One important structural limitation of an aluminium alloy is its fatigue properties. While steel has a high fatigue limit (the structure can *theoretically* [this is an important qualifier -- see below] withstand an infinite number of cyclical loadings at this stress), aluminium's fatigue limit is *near* zero, meaning that it will *eventually* fail [so will steel and ti -- see below] under even very small [not to be confused with even smaller] cyclic loadings, but for small stresses this can take an *exceedingly* long time.

[asterisks added]

Also,

Fatigue is a stochastic process, often showing considerable scatter even in controlled environments.
The greater the applied stress, the shorter the life.
Fatigue life scatter tends to increase for longer fatigue lives.
Damage is cumulative. Materials do not recover when rested.
Fatigue life is influenced by a variety of factors, such as temperature, surface finish, presence of oxidizing or inert chemicals, residual stresses, contact (fretting), etc.
Some materials (e.g., some steel and titanium alloys) exhibit a *theoretical* fatigue limit below which continued loading does not lead to failure.
In recent years, researchers (see for example the work of Bathias, Murakami, and Stanzl-Tschegg) have found that *failures occur below the theoretical fatigue limit* at very high fatigue lives (109 to 1010 cycles). An ultrasonic resonance technique is used in these experiments with frequencies around 10-20 kHz.

[asterisks added, and '109' and '1010' indicate ten to the ninth and tenth powers respectively]

and,

Fatigue limit is a property of ferrous alloys and titanium alloys[1]. It is the constant amplitude (or range) of cyclic stress that can be applied to a material without causing fatigue failure. Other structural metals such as aluminium, do not have *a distinct* [an important qualifier, not to be equated with 'any'] fatigue limit and will eventually fail even from *small* [i.e. how small?] stress amplitudes. In these cases, a number of cycles (usually 107) is chosen to represent the fatigue life of the material. The corresponding stress amplitude is then referred to as the "Endurance Limit". Typical values of the endurance limit (Se) for steels are 1/2 the ultimate tensile strength, to a maximum of 100 ksi. *For irons, aluminums, and copper alloys, Se is typically .4 times the ultimate tensile strength*. Maximum typical values for irons are 24 ksi, aluminums 19 ksi, and coppers 14 ksi.


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[Fatigue life may be a useful concept for you -- the fatigue life of a well made aluminum frame (for most riders) is very, very long. If the stresses do not often approach .1 or .2 times the UTS (and, with the sturdier, modern, well engineered aluminum tubings and frames, they should relatively rarely approach or surpass this level, for many riders), then the fatigue life is likely to be *exceedingly* long.

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This is another thing that is often ignored by (and is misleading about) statements that claim 'no' fatigue limit for aluminum: the substantial change or increase in fatigue life as one goes lower and lower in stress amplitude, and particularly when the stresses are substantially below the 'Se,' or edurance limit. (Even there, at the relatively rare, in actual practice, higher-level stresses of the Se, ten million of the higher stress cycles can typically be endured without failure; and at the real-world-typical and much lower stress amplitudes, the fatigue life genuinely does deserve and qualify for -- in the real world -- the words exceedingly long.)]

Thanks a lot! Very enlightening!

What could have happened was that stresses above steel's fatigue limit happened frequently or at least enough times so that the material finally failed (catastrophically at that). Maybe they thought that the infinitely repetitive stresses were under the fatigue limit and/or didn't consider some stresses that were slightly (or even much) higher.

That would be my take also.

The structure connecting the hulls of a catamaran (or other multi-hull oceangoing vessels) is subjected to extreme stresses as wave action will simultaneously lift the bow of one hull and the stern of the other causing a twisting moment. It's probable these were higher than the fatigue limit of the steel I beams, and over time resulted in the failure.

Gee whiz guys, I'm no expert on metallurgy but I do know that all those airplanes we fly on daily have aluminum wing spars and watch 'em flex while you ride. That is unless you fly an oldie with a spruce spar. ( my preference
Any airplanes ever built with steel wing spars? Fuselages, etc. yes (gas welded but that's another subject) but I don't think any wing spars that were successful.

Clearly, "theory" says that an aluminum tube subjected to the sorts of bending and flexing stesses that bike frames and forks are under should fail rather rapidly. But, in "real life", a failure of an aluminum frame or fork from routine riding stresses is so rare that many experienced bike techs don't remember ever seeing it occur.

The folks who design and build aluminum frames and forks OVER-BUILD them to a very large degree to prevent such failures. Of course, the result of that is that many aluminum frames weigh almost as much as the best steel frames...beefy up the aluminum frame cancels out the supposed weight advantage of aluminum over steel.

Where steel has its BIG advantage over aluminum is under traumatic stress. Run into a curb at high speed with a Reynolds 520 or generic 4130 steel fork and frame, and a skilled tech can realign them as good as new. An aluminum fork or frame that is bent or twisted out of alignment can not be safely realigned (and Reynolds says that heat treated steel such as Reynolds 853 should not be realigned either).

If someone wanted a frame and fork that will last a lifetime, there was never anything better than Reynolds 531 or generic 4130 frame a good supplier...but who really wants to ride the same bike for the rest of their life?

"Any airplanes ever built with steel wing spars? Fuselages, etc. yes (gas welded but that's another subject) but I don't think any wing spars that were successful."

Recomended book: No HWY by Nevil Shute.

I'm pretty sure tons were, welded tubing as you say. I don't actually know what is in the spar of a Boeing, I am ebarassed to say. Al is certainly in the skin etc... don't know about the caps on the spar. Presumably today - carbon.

Aluminum wing spars, like other aluminum structures, have no fatigue limit. However, they are designed using finite element analysis to determine the probability of failure over time so that some minimum spar lifetime is pretty much guaranteed.

Likewise, aluminum fuselages of pressurized airplanes are designed to last a certain number of pressurization/depressurization (ground-air-ground) cycles after which they are usually retired from service.

In fact, the FAA initially "life limits" many new aircraft designs until the design has been in service long enough and examples of the design approaching the initial life limit have been thoroughly inspected for fatigue to justify extending the life limit. A good example is the new Eclipse 500 Very Light Jet, designed for a fatigue life of 20,000 airframe hours and has an initial life limit of 10,000 hours (see Eclipse Press Release announcing completion of static testing).

The wing center section of the U.S. Navy E-2C aiframe has a fatigue life limit of 11,450 flight hours.

The USAF F-15 Eagle fleet was grounded for two months after an accident caused by fatigue failure of a critical upper longeron structure, and during inspections "Time compliance technical order inspections have discovered nine other aircraft with longeron fatigue-cracks. Additionally, approximately 40 percent of inspected aircraft have at least one longeron that does not meet blueprint specifications."

Aircraft Structural Design

From the comments made at the time was the thinking that if you put enough overkill into the design "it couldn't possibly move" and therefore would not be stressed. Silly thinking of course, particularly given the proof of hindsight.

I also think that a clean weld with a half inch radius is better than a quarter inch radius but nowhere near as good as a six inch radius, and that that point alone would be more important in the end than the specific metals used, and that annealing stresses adds as much to long term strength as changes to polymorphism or whatever the rules are in metals. I am more familiar with silicates, but but the relationships of stress and shape must be similar.

All this fuss about bike frames not being solid enough...



Problem solved.

What about mountain bikes? It's pretty difficult to use it for its intended purpose without applying some rather significant stresses, and low-level loads multiple times a second (particularly for a hardtail). This is one of the many reasons I wonder why so few brands still build high-end steel hardtails (hopefully ordering a Jamis Dragon later this week).

Mountain bike manufacturers switched from steel to aluminum for frame material because overbuilt (or, more accurately, appropriately robust) aluminum frames carry a smaller weight penalty than overbuilt steel frames. Any manufacturer who builds a steel mountain bike frame that is meant to be close to an aluminum frame in weight is asking for trouble. As a Trek sales rep told me in the early '90s, Trek found that reducing production of steel frames and increasing production of aluminum frames resulted in a greatly reduced number of frame warranty claims.