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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.

Comment Re:Stronger? (Score 1) 262

Let's get some perspective.

A well-loaded SUV doing pedal-to-the-metal ABS emergency braking from 100mph on dry pavement with summer tires can easily hit 1MW total braking power.
A passenger car doing some only mildly distracted late braking in city traffic easily pulls off 50kW braking power.
An old grandma's scooter can easily exceed 5kW of braking power in city traffic.

The 4.6kW figure is a typo, and anyone who takes it on face value is silly. I don't know where the heck it came from. A 6 ton vehicle doing 15 second braking from 160mph to stop needs to dissipate 1MW.

All figures are average power per vehicle, not per brake. Uneven braking will produce higher peak power.

Comment Re:Stronger? (Score 1) 262

So, let me think. A simple chunk of metal that heats itself by friction, vs. a generator and a bunch of resistor coils. Yes, I thought so. Proposing regen braking for this project is insane. BTW, who the heck told you that heating is a problem? Disc brakes work just fine. On my wife's car, on dry pavement, I can hit 1MW of braking power for a second or two, no biggie. Braking a 6 ton vehicle from 160mph is no problem for disc brakes. Let me repeat: braking is not a problem at all. It's the survivability of a brake disc that wasn't necessarily designed for operation in enterprise hard drive spindle type of a job. A major problem with off-the-shelf brake discs in this application is fatigue cracking. They have various holes and radii that are not designed for the hard drive spindle operation called for here. The whole article is IMHO a big fat decoy, most likely an inadvertent one - due to ignorance, not malice.

Comment Re:Killowatts are power, not energy (Score 1) 262

The hard part is going to be making the brakes survive spinning at 10,000rpm, not dissipating the energy from slowing down.

Finally someone who gets it :) But that's not even the hard part. The hard part is making a much larger diameter wheel that will survive this. The brake disc is, comparably speaking, peanuts.

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