urbanknight

That's what I was thinking. I just dismissed it since it doesn't make sense. I think he was trying to say that steel can take inifinite loads of a certain weight while aluminum has a finite number of stresses in any porportion. Not sure, though.

well biked

Here's an explanation: https://en.wikipedia.org/wiki/Fatigue_Limit

urbanknight

Yeah, it's poorly worded there too. Limit is supposed to mean maximum, while in this case they are saying that even the slightest load is over the limit.

well biked

Think of the fatigue limit as a threshold that, as long as it's not crossed, the material (steel or titanium) doesn't get any closer to failure. With most metals, including aluminum, there is no threshold (fatigue limit), which means any and all stresses put it closer to failure-

DannoXYZ 

The word "limit" means something like "approaching but not exceeding". Such as "speed limit" on the highway, as long as you don't exceed it, you'll be OK. It says nothing about the maximum at which you can go or how high you can stress a part. As long as you don't load steel above a certain amount, it will never fail. There is so such "load limit" on aluminium where it will never fail. Stresses no matter how low, will accumulate on aluminium until it fails. Here's an SN diagram:



Fatigue-limit is also called "endurance limit" as well. Note that both the blue and red lines refer to "fatigue-strength", just that "fatigue-limit" refers to a specific spot on this curve. With standard-sized samples, note that with steel (blue), if you keep stresses below a certain level, it can take an infinite amount of cycles. With aluminium (red), it will take fewer cycles at the same load as steel to fail. Additionally, there is no minimum load below which aluminium will last forever... it will always fail given enough cycles. That's why aluminium parts always have an extra safety-margin designed into them, to last X-number of years at typical load-levels so that you'll end up gettig bored with the part and buy a new one before it even approaches failure.

So for the various "limits" of materials, it basically means this:

Fatigue limit = stress levels below which the part will never fail
Yield limit = stress level below which a part will spring back to original shape, above which will cause a permanent bend
Ultimate limit = stress level above which will cause a break in the part (aka tensile limit)

urbanknight

I completely understand that, but having NO LIMIT means it can handle any stress. It would be more appropriate to say that it has a limit of 0. Some places in Montana have no speed limit. That doesn't mean you can't drive at all. You see what I'm getting at? I understand what they're saying, but it's worded like an AP physics student flunked grade school grammar.

urbanknight

You basically proved what I'm saying. You can't exceed a limit if there is none. Yet you are saying aluminum has exceeded it no matter what the load. Therefore, saying "no limit" means you can't exceed it and it therefore could handle any load. Now I KNOW THAT'S NOT WHAT THEY"RE TRYING TO SAY, but that's how they worded it. Once again, it's poor grammer. If there was "no limit", then you can't exceed it. The better way to put it, if anyone cares about being grammatically correct, is that it has a limit of 0, Zero.

well biked

I think the point is that aluminum has no fatigue limit as it's defined in this context. So even though aluminum certainly has its limits , it has no fatigue limit...............When I got my first aluminum-framed bike in the '90's, I was talking to a friend who's an aircraft engineer about it, and he explained to me, in a very rudimentary way, the fact that aluminum has no fatigue limit and what this means. It was kind of depressing to realize that every time I rode the bike, the frame was a little closer to failure. I realize that aluminum frames are typically overbuilt enough that it's not something to obsess about, but it's still a little troubling, especially since my aluminum-frame experience has been almost entirely on mountain bikes ridden hard, off-road. I've seen several aluminum frames where tubes have cracked, and I've seen steel frames develop cracks at brazed joints, but I've never seen a steel frame's tubing crack along the length of a tube (not saying it doesn't happen, I've just never seen it). What I would find interesting in terms of real world biking is to know more about the specific fatigue limits that come into play on a chromoly steel frame. In the case of a road bike, for example, not used for racing and not ridden particularly agressively, is the fatigue limit reached often, or at all, anywhere on the frame during normal riding?

urbanknight

{grumble} That's the problem, "NO" limit is not the same as saying it can fatigue at any point. Am I the only person on this board educated in anything above 3rd grade grammar? Anyway, I get the point and I'll stop complaining that people don't know what it means to have no limits.

But yeah, have you seen the size of the tubing on the early Cannondale frames? They really compensated for it. They still had some break, but most of them are still riding around today.

urbanknight

come to think of it, I kind of like you guys. If politicians in office were told that they had no spending limit, I wish they would interpret that to say that they can't spend any money at all

well biked

[QUOTE=urbanknight Am I the only person on this board educated in anything above 3rd grade grammar? [/QUOTE]

It has nothing to do with grammar, it's a case of simply using an established, defined term in a sentence. At first, I was trying to be nice, because I took it from your post that you didn't understand the concept of "fatigue limit." Then, when I realized you understood but you were for some reason hung up on the wording, I tried to politely change the subject and move on. Now you come back with this high and mighty BS about 3rd grade grammar. Whatever...........One thing to remember is that, in regard to bicycles, there would be no reason to mention "fatigue limit" if it weren't for steel and titanium. In the case of aluminum, the best term in regard to fatigue limit would probably be "does not apply." But I certainly don't have a problem getting a handle on what fatigue limit means, or the wording, when it comes to steel and titanium, and I don't see why anyone else would either-

wroomwroomoops

Actually, that's wrong, too. Alluminum and steel differ in their crystal structure in such way that, while a steel object can be bent to a certain extent, without causing plastic (only elastic) deformations and hence causing fatigue, alluminum can't - which means, ANY amount of bending WILL cause fatigue.

In other words, if you don't exceed a certain amount (angular or linear) in bending a steel object, it will endure an infinite number of such deformations without fatigue - which is clearly demonstrated by springs.
With alluminum there isn't such an amount, alluminum will accrue fatigue no matter how little it is bent.

wroomwroomoops

DannoXYZ is, actually, quite correct in what and how he wrote it. I know this may sound a bit confusing at first. Maybe my previous post has cleared this, hopefully.


Now, said all this: does anyone know of steel MTB handlebars?

urbanknight

Sorry, I didn't mean to cause such a disturbance. It has been more clearly defined for everybody and that's what matters.

rmfnla

You're responding out of the context of what I wrote; re-read the thread so I don't have to repeat it.

I also stated that aluminum does not like to be bent; I just didn't feel like showing off my rusty engineering jargon (or Googling skills).

As to the "no-load" load, does that mean my cranks are weakening every second of every day & night, whether I am riding on them or not?

wroomwroomoops

The reason I wanted to elucidate the issue to you, was the way you replied to DannoXYZ. If you feel that I have insulted you by explaining something you understood aboundantly clearly already, I apologize. My impression is that you still don't quite get it, but if I am wrong in that assumption, I appeal to your indulgence.

I am really not sure whether your last question was serious, or not.

rmfnla

This is what DannoXYZ said: However, steel DOES have a fatigue-limit whereas aluminium DOES NOT. There's a load-level below which steel will NEVER fail. There is no such limit with aluminium, no matter how low the load, eventually it will fail.

I replied the way I did because the statements were badly worded to the point of making no sense.

My feelings are fine, but thanks for your concern.

wroomwroomoops

DannoXYZ used the most common, most precise, terminology when expressing him/herself.
I found a very good definition for fatigue limit: "Maximum stress that a material will endure without failure for an infinite number of load cycles." What DannoXYZ was trying to tell you is, that this maximum stress for alluminum is 0; alluminum can't endure any stress an infinite number of times without failing.

Sorry about the comment on feelings; I tried to censor myself by way of editing my post but, apparently, not quickly enough.

DannoXYZ 

I think the part that's confusing for urbanknight and rmfnla is that mention of "fagitue limit" refers to a specific feature on the SN diagram. It does not refer to any actual force that you apply to a real part in your hands nor does mean any type of failure mode. The line on the diagram is an asymptote that is never reached, similar to dividing by zero to approach but never reach infinity. Or the two asymptotes present when graphing x^3 functions.

So when I say "steel DOES have a fatigue-limit", it refers to the existence of a horizontal asymptote line on the SN diagram for steel where it has infinite life-span. When I say "aluminium DOES NOT" that refers to a lack of such a horizontal line for alloy on the SN diagram. It's a simple matter of looking on the diagram and seeing YES or NO a certain material has a "fatigue limit" line.

Similar comparison to when the cop pulls you over in Montana, "Did you or did you not see the speed-limit sign?". You can argue perceptions and semantics all you want, but the concrete fact in reality remains that there is or isn't a sign posted. The issue isn't over what the actual speed-limit is or what MPH it is, but rather on the existence of such a sign.


So I'll rephrase it again specifically in a way that when talking to the guy designing your bars, they'll get what you're saying:

"When referring to the S-N diagram of stress-levels versus number of load-cycles, steel DOES have a horizontal fatigue-limit line whereas aluminium DOES NOT. There's a stress/load-level below which steel will NEVER fail. That's the horizontal line on the right that goes out to infinite number of cycles. There is no such horizontal fatigue-limit line with aluminium when looking at the fatigue-strength curve. No matter how low the load, eventually it will fail with sufficient numbers of load-cycles."

DannoXYZ 

Another thing about fatigue-failures, they are cumulative stresses and are not "forced" failures from overloading. For example, let's take the specific case of the alloy handlebar:

ultimate strength break would require say.. a single 10-feet drop landing loading the bar-ends at.... 400 lbs on each end and the bar snaps in two.

yield strength bend takes less force... so a kerb-hop with full-weight landing and loading the bar-ends at... 300 lbs and the bar takes a permanent 20-degree bend per side

fatigue-strength break takes even less force... like 75 lbs load multiplied by 500,000 cycles. Fatigue failure is due to microscopic surface cracks that develop with each load cycle. Each time the part is loaded, a tiny crack develops. Over time, more and more of these cracks accumulate all over the stressed regions. Slowly they propapate, grow and eventually join, then >SNAP< the part breaks.

The main difference between fatigue-failures and the forced breaks is that the part is not deformed in any way. You can line up the broken part and it'll fit and match perfectly. Whereas a forced failure beyond yield and ultimate strength of the part will always show some bending and deformity and the part can't be re-assembled and have the crack line up precisely.

In most failure cases, it's usually a combination of all three. The fatigue-failure starts the crack, which overloads the remaining parts of the bar that's still attached, which then bends at past its yield-strength and eventually breaks past its ultimate strength. The top-half of the bar will have a matching crack line that lines up the two pieces and the bottom half will most likely be bent and twisted.

urbanknight

I think that was my own poorly worded interpretation. "Any load" instead of "no load", meaning your cranks are getting weaker any time you exert any force on them, but not when they are resting.

Once again, sorry for being so picky about the wording. Now people have gone out and oversimplyfied, reworded, and techno-defined it for us all. Thanks for being so specific, as I'm sure if someone can't understand it with the disertation above, they'll never get it.

rmfnla

Both you and wroomwroomoops are gentlemen and I appreciate that.

However, the bolded part of your statement above is just plain wrong, and that is the part to which I took exception.

oilman_15106

Your suggesting steel handlebars? I should replace the bars on my 1984 Centurion I guess.

urbanknight

How is that wrong? I thought the comments above about aluminum would mean that any force would fatigue it, and most cranks are aluminum, right?

Oh, and don't tell my wife about the gentleman thing. She might expect me to act civilized.

DannoXYZ 

I think he takes offence at the word "weakened" that you used. That's subject to a lot of interpretation on what "weakened" means. A lot of people get stiffness, rigidity and strength/weakness mixed up, when they really are different properties. But yes, you can arm-wrestle an alloy crank and it WILL eventually fail. Might take 1000 years, but it will fail... Quantifying the discussion always makes things clearer for people.