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.)]