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Comment Re:you know not what you speak of (Score 1) 262

The reason that the S-N graph is slightly deceptive is this: I can give you a pre-cracked steel part where the material is perfectly sound except that there is a crack, and I can size the crack so that the part will fail at an arbitrarily low load, in just one half-cycle of loading (you only load up, it fails before you cycle load back to zero). It looks as if I gave you "weaker" steel, but the steel is fine, it's the geometry of the part that's wrong. It may appear to the naked eye that the geometry is fine, because I can make the crack in such a way that you won't see it.

When aluminum fails due to fatigue, the tensile yield stress appears to be lowered, and thus we talk of fatigue "weakening", but only because you ignore the presence of cracks. Say you have a 1 inch square aluminum rod under a 20,000lb normal load, so you think the longitudinal tensile stress in the rod is 20ksi. But in fact it's not, it's very high, at the level of a yield stress, since there are cracks in the material, and the crack surface is a stress-free boundary. So the load has to find elsewhere to go, figuratively speaking. So it looks as if you had a weak rod. But then you can apply a compressive load just under the nominal yield (not any weakened yield), and guess what, if the rod doesn't buckle, nothing else will happen. So the material is not weaker. The part is. That's a subtle difference.

So, there's a very easy way to tell if a material is weaker, or just the part is precracked and thus weaker on average: just apply compressive load instead of a tensile one. In metals, the compressive and tensile strength should be similar. When it isn't, you have a precracked part. When it is, and both are low, you have true material weakening at the microscopic level.

Comment Re:you know not what you speak of (Score 1) 262

This shows the bulk stress needed to fail the part. It will fail by a fatigue crack. The effect is that of a "weakening" but only if you view it in bulk. The effect is as if the material was weaker, but it's really a material that's not set up the same anymore. It has new internal surfaces that didn't exist before.

Comment Re:Steel is stronger than carbon in many instances (Score 1) 262

You're correct, but even then I find the video rather weird. They're supposed to be braking from 160mph down to 0mph. They don't freaking need to apply the brakes at 5000RPM or anything like that, even though they seem to do just that in the video. To me, that's silly, or the superimposed numbers are someone's fantasy. The testing regime for their brakes is spinup to 10kRPM, the dyno braking down to 1600RPM to simulate air drag, then actual braking down to 0RPM at ~1MW braking power, if my assumption of 15s braking is correct, and the assumption that 1000mph = 10kRPM. The highest I've ever recorded my SUV braking at was 1.2MW IIRC, during some emergency braking tests. Of course the brakes probably wouldn't last if I could keep at it for 15 seconds, but for 3 second they were just fine (that was 100mph to 0mph).

Comment Re:you know not what you speak of (Score 1) 262

I hope that you do realize that the braking job described here is something done routinely by disc brakes in large trucks, and occasionally done by disc brakes in run-of-the-mill SUVs. The stuff that happens at the braking speeds is inconsequential. You got taken by a very inaccurate and misleading article. What they worry about is what the disc does when it's not braking at all and is merely spun fast without any braking action. An emergency braking on my SUV dissipates as much power as a 15 second braking would on their vehicle. You can't look at those things without running some numbers. The video is also rather misleading since it overlays thermal imagery on visible image. Nothing is really glowing in visible light. The brake testing that they do is also slightly over-the-top: the brakes will not be used at 5kRPM at all.

Comment Re:you know not what you speak of (Score 1) 262

"when aluminum flexes, it weakens." Nope. The finite fatigue life of aluminum has nothing to do with weakening, unless you simply use the wrong term to really mean fatigue. Fatigue has little to do with strength of the material itslef. A typical fatigue failure mode is fatigue cracking, and it most definitely doesn't make the material weaker. The part gets weaker, but that's because it changes shape. Cracking produces new surfaces and thus changes the shape of the part. A part with a crack in it is not the same part as one without a crack, even if the material is no weaker.

Comment Re:you know not what you speak of (Score 1) 262

There are specific terms for it. Aluminum generally has finite fatigue life no matter what the cyclic stress amplitude is. That means that in presence of cyclic stresses, it will always eventually fail. If you really overdesign things, it may take a very, very large number of cycles - so large that they won't occur in a human lifetime, but still, if you keep cycling the stress, you'll get failure. Many kinds of steel, though, have infinite fatigue life at sufficiently small cyclic stress amplitudes. If you design things properly out of steel, they'll literally "last forever" - or at least they won't fail due to fatigue.

Comment Re:Stronger? (Score 1) 262

It wouldn't solve anything, because you have no clue about mechanics of such composite assemblies. Just forget it. There is no rotor issue. Remember that the article doesn't mention any issues at all. They managed to shatter a carbon disk that wasn't designed for the job. Big deal. There are no other problems at all. It's just sensationalism and innuendo. Get over it.

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