I’m wondering if this is ever a concern for manufacturers, particularly aluminium frames, or if a frame that’s designed stiff enough will have a very long fatigue life anyway. Anyone know?

well?

IME Nothing is designed with a fatigue lif ein mind.

Most things are designed to withstand a “design” load, and then a series of tests are carried out to show that the structure will last a significant period of time at general loads which are significantly less than the design loads.

Dom

this video is an extreme case of what I mean (btw I don’t think the A380 has undergone such testing – no proof only hearsay though, happy to be proven wrong.)

http://www.boeing.com/Features/2010/09/bca_fatigue_testing_09_13_10.html

Having been on the inside and outside of the A380 program, the answer, as you may expect is ‘…it depends…’ I don’t think fatigue testing was done on the full size wing (1.5 x proof load only IIRC), but material/coupon/scale testing that was done in the pyramid of data leading up to the full scale structures was impressive/thorough.

bristolbiker – would be interested in your experiences if you’re allowed to talk about it in detail!!!!

I’d be very surprised if there isn’t some thought put into the fatigue life of a bike frame as there is obviously no point in designing a beautifully light frame which rides wondering yet fails after a short period of time due to fatigue.

I’d imagine that it’s just one of the large number of variables that needs to be juggled in order to create the frame, and not a specific design aim for a designer though…

Some of it is public domain, some isn’t…. alot of the interesting stuff probably isn’t – they are very good at harvesting/protecting any IP from any work done for them. If you want to know anything specific email me and I’ll see if I can talk about it

EDIT: gshort AT eatec DOT co DOT uk

WRT bike frames – is the EN test not a half-way-house between proof/fatigue loading (i.e. cyclical loads applied to the frame, but the loads are relatively high and relatively constant – IIRC the DIN test is like this as well)? Brant/Cy etc will obv be in a better position to answer this…

You can’t not consider fatigue life if building an aluminium alloy frame. You probably won’t mention it with a MTB though because you’ll build it so far beyond the minimum to survive bigger hits (and you won’t do enough big hits to exceed go beyond the design spec for smaller hits). On earlier Bromptons all the aluminium alloy parts (hinge clamps, chainset, handlebars) were designed to be replaced after 5,000 miles.

Well the CEN testing includes fatigue testing (or cyclic loading as BB correctly states) so yes they are built to a minimum no of cycles under the tests in CEN, how this translates to real world riding is possibly tricky. Interestingly we had a discussion a few months ago (I’ll find it in a bit) and Brant made some comments that were not massively informative, but suffice to say, he said if steel frames were built under the endurance limit (Brant also alluded to the fact he didn’t believe in the endurance limit or at least wasn’t sure of it) then they would be massively heavy. I dunno if this means anything in reality…

you’ll build it so far beyond the minimum to survive bigger hits

This is potentially a non issue due to crack closure or overload effects on fatigue life which can be positive.

I think the issue here is that there are some metallurgists engineers on here (like me and BB) who know about fatigue, but eff all about bike design as its all kept a bit secret, so it really is hard to tell.. (although BB might know lots about bike design, but I don’t know much despite observing it for years..)

For example for many years I assumed that the whole steel vs aluminium thing was about the endurance limit, until Brant said what I quoted above, although the steel vs aluminium thing is probably really about the effect of better fatigue response in steel and the ability to make more flexible sections from steel due to the interplay of stiffness and section properties. Or just marketing bullsquat…

Brant/Cy/Sam etc to the forum please!

We just need someone to tell us when our bikes might break.
I imagine most will be obsolete before they reach that point.

From my recollection, and I think I read it on Sheldon Brown, both alu and steel are built to a certain stiffness, which means the alu tubes are generally much larger diameter (the metal itself being softer).

But you don’t expect an aluminium frame to last forever, as at some stage it will fail from fatigue. I can’t remember about steel but I think it’s fatigue life is better.

“If you drop a feather on an airoplane wing enough times it will eventually break.” According to my old materials professor.

Thats what I meant by the endurance limit – steels have a level below which if you load them then they never fail. Although some recent researchers doubt this, but there is a least a marked change in slope, whereas in most Aluminium Alloys (2000 series are more “damage tolerant” ) it hits zero much quicker…

The fatigue life of an aluminium frame is dependant on many factors.
A part can fail more quickly with just lower load cycles than if a higher load is occasionally seen. Plastic deformation at the crack tip due to a higher load leads to a residual compresive stress at the tip which must be overcome for the crack to advance.
Stress concentrations due to changes of section, joints, corrosion and concentrated loading will have an impact of the fatigue life.
Other considerations; is the load fully reversible (R=-1)?, the relationship between mean and alternating stress (Sa and Sm)

From what I’ve sussed out, ably assisted by Cotic, the issue is thus:

Steel has fatigue limit = X
Aluminimum has fatigue limit = 0

In normal riding (just riding along) 0 < load < X therefore aluminium frames constantly fatigue. When the interesting stuff happens 0 < X < load therefore steel frames stuffer fatigue at these moments (as do aluminium ones but they’re always fatiguing).

Over the lifespan of a MTB there will not be enough interesting moments to break a steel frame, nor will there be enough general fatigue to break an aluminium frame.

However, the CEN test involves a high number of cycles of a high load. I was working this out earlier and its perfectly reasonably for an aluminium frame to suffer 10,000,000 crank turns (and thus torsional BB loading) over a lifespan so its built to ensure these numerous small loads don’t cause failure. Apply the 50,000 large loads for the CEN test and it isn’t bothered because the fatigue limit = 0 means it’s been ‘overbuilt’ to handle lots of small loads. Take a steel frame and those 10,000,000 crank turns won’t load the frame above the fatigue limit, so you don’t need to add material to compensate for this fatigue. Apply 50,000 large loads for the CEN test and bang, crash, wallop, crack. But in the real world you won’t be applying 50,000 large loads, you’ll be applying many more small loads and many fewer large loads (some of which may be larger still). Hence the steel frame redesigns to meet CEN.

Anyway, that’s how this mechanical engineer is supposing it all works! So to recap, I believe aluminium alloy frames are built to withstand constant fatigue, steel frames were built to handle big hits and but cumulative fatigue from them was too low to worry about but with the new CEN regulations they’re now built to handle repeated large hit fatigue.

Of course I could be wrong…

I am not a metallurgist, but the fatigue limit or endurance limit apeears to be there for composites, which may be why some manufacturers offer lifetime warranty on composite bits but not alu bits.

The Aluminium Federation ‘big book of aluminium’ quotes a fatigue endurance limit for various aluminiums. This is the stress at which failure occurs after 1*10^8 cycles of rotary bending.
190MPa for 6061-T6

“I’m wondering if this is ever a concern for manufacturers, particularly aluminium frames, or if a frame that’s designed stiff enough will have a very long fatigue life anyway. Anyone know? “

That is the aim of trying to make an aluminium bike frame; a stiffer frame will flex less, the flexing of aluminium is where the problems start with aluminium fatigue life. Although a stiffer frame may be heavier to avoid flexing; an aluminium frame that does flex (a lot) will have a shorter fatigue life than one that does not flex as much.

A steel (or titanium frame) can flex with less of a problem for the fatigue life of the frame.
Car springs are made of steel instead of aluminium for the same reasons.

Don Ferris of Anvil has some thoughts on the subject:
http://www.sportandme.com/docs/sports/cycling/frame_weight_wars.html
“When discussing aluminium, someone always brings up airplanes.
Airplane design showcases what aluminium does best: acceptable strength and a low relative weight. But, aluminium’s lack of a fatigue limit is one very good reason why there is stringent monitoring of dynamically or cyclically stressed aluminium structures. Airplanes are also designed to allow sub-assembly replacement as they approach the end of their life cycle, which is an option you really don’t have on a bicycle frame. The inverse example along these lines may be why springs and paperclips aren’t made of aluminium. “

http://www.frameforum.org/forum2/index.php?showtopic=2317
“Even the largest and most efficient cargo aircraft have a payload ratio of less than 4 to 1, with most being less than 3 to 1. In other words, for one pound of aircraft weight, they can carry 3 pounds of payload. A 20 pound road bike with only a 150 pound rider has a load carrying ratio of 8.5 to 1. Put a 200-pounder on that bike and it grows to 11 to 1. Put that 200 pounder on a 40-pound DH rig designed for 9-g loads, and you’re still at 6 to 1. No the best way to compare stresses obviously, but it does put into perspective how bicycle frames are exceptionally stressed for their mass.”

Similar story applies to CFRP, if used on aeroplanes; there is a system of classification of parts on planes eg critical parts, sensitive part. Sensitive parts, if they fail, will cause a drop in aircraft performance in flight eg speed, fuel consumption etc, Critical parts, if they fail, will lead to the plane falling out of the sky. CFRP is not first choice for critical parts.

Wow some great info there 💡

An alloy frame is made stiffer than a steel one, to avoid lots of flex, and premature fatigue. Yes.

One other thing to bear in mind is that for welded structures (that’s bike frames – and let’s stay with steel for the moment), whilst the plain material of the tubes might have an endurance limit, the welds themselves do not (for design purposes) due to a) the stress concentraions caused by the geometry fo the weld and b) the local metalurgy (for S-N curves of various classes of welds, choose your weapon – BS7608 or the current EC3,Part 1-8). Hence, unless something has gone very wrong in the design (other stress raisers are present – holes, fillets etc), fatigue cracks will inititate at the weld and then either propogate through the crack or the parent material, depeding on the loading on the joint and how the deveopment of the crack affects that load path.

Fatigue of composite material is one of my pet subjects – whilst it is true that, until recently, fatigue was not considered too much of an issue if there was sufficient static strength built into the design, there is now alot of work going into understanding the fatigue mechanisms in composites and coming up with design tools to truly optimise composite structres for fatigue life.

Recommended reading Structures – or why things don’t fall down by J E Gordon
(and New Science of Strong Materials for info of Fracture Toughness, Griffiths crack theory, surface tension etc)

http://www.aoe.vt.edu/~gurdal/Public/COURSES/2104-Docs/LECTURES/Lect-04-00.pdf
“It is impossible in practice to plan for a “safe”
life of exactly so many hours or years. We can
only consider the problem in statistical terms
and in the light of accumulated data and
experience. We build in whatever margin of
safety seems reasonable. All the time we are
working on a basis of probability and
estimates.”

Plus: Light Alloys by Polmear , for Supersaturated Solid Solution, aging/ precipitation hardening /aka heat-treatment, stress corrosion cracking etc
http://130.236.35.10/kmt/education/course-material/tmkm06/Summary%20of%20POLMEAR02.pdf

“1.4 Fatigue
For most of non ferrous alloys usually to an increment in tensile strength, it does not correspond to a proportionate increment in fatigue proprieties. For age-hardened aluminium alloys, usually the so called fatigue ratio is the lowest. The more an alloy is dependent upon
precipitation-hardening for tensile strength, the lower its fatigue ratio becomes.
Normally the initiation of cracks, occurs at the surface; it is here that strain becomes localized due to the presence of pre-existing stress concentrations (like mechanical notches or corrosion
pits).
The bad fatigue proprieties of age-hardened aluminium alloys can be attributed to the metastable nature of the metallurgical structure under conditions of cycling stressing.”