ARM for server

By: Ungo (a.delete@this.b.c.d.e), September 22, 2009 4:25 pm
Room: Moderated Discussions
Anon (no@email.com) on 9/19/09 wrote:
---------------------------

>Ah, I see you dont actually know what you are talking about.
>
>Aluminium has inherent problems due to its tendency to fatigue, it also has issues
>with thermal cycling and vibration. Aluminium used in aircraft tends to be very
>specific (and expensive) alloys, and inspected very regularly, AND not used in locations
>where structural fatigue is likely to happen.

You've just proven you have zero clue. Negative clue, even.

First, almost all commercial jetliners use aluminum as the primary structural material. The only aircraft which don't are the very recent designs which have been using increasing amounts of carbon composite (with the not-yet-in-service 787 being the first aircraft design to flip from majority aluminum construction to majority composite).

Aluminum is routinely used where structural fatigue is likely to happen. Care must be taken, but that's why they pay talented mechanical engineers lots of money to design aircraft. And of course the industry has built up a very extensive body of knowledge about aluminum structure design techniques, since it has been used as a main material in aircraft for at least 75 years.

Now, some specific examples: Consider the aircraft's skin. Not only is it a key structural element (most aircraft use monocoque or semi-monocoque construction), it's exposed to a lot of fatigue cycling due to pressurization cycles.

It's also used for internal framework. Go read this archived article about the 747:

http://www.flightglobal.com/pdfarchive/view/1968/1968-1%20-%200117.html

The money quote: "Wing-box materials are as used on the 727, namely: 2024 aluminium lower skin and stringers; 7075 aluminium upper skin, stringers, spars and ribs."

You can't find a more critical structure in an aircraft than the wing box. It was standard practice to make them with aluminum alloys for a very long time, and the material which is supplementing and/or replacing aluminum is not another metal.

For that matter, one of the most notable _spacecraft_, the Space Shuttle, uses aluminum structure. It can't use monocoque construction, because its skin is largely materials which cannot tolerate significant mechanical loading (thermal protection ceramic tiles are very brittle), so instead it has a strong internal frame supporting a non-structural skin. That frame is more or less all-aluminum.

>The wonder metial you are looking for is magnesium and titanium alloys (not together,
>both separately), and guess what the carriers is most high end laptops are made of?

"Carriers"? Do you mean "frames", perhaps?

Are you even aware that Apple's notebooks don't really have a frame? The current models seem to have one internal stiffening rib, judging by the teardowns at ifixit.com. Otherwise they are monocoque designs where the skin is the structure.

Apple's design target is thin, light, and long battery life, with few performance compromises. The combination of these four requirements means that internal volume eaten by framing is extremely undesirable.

Could they use magnesium instead of aluminum? Maybe, but aesthetics do matter, and aluminum offers some advantages there. Aluminum surface treatments (anodizing) are very attractive and can be made highly durable. Magnesium, on the other hand, tends to require paint to look attractive. Apple has had some bad experiences with the durability of paint-on-metal finishes in the past. The G4 "Titanium" models used painted titanium sheet for the skin, and users frequently complained that the paint was easily damaged (both by palmrest wear and scratches to the other surfaces).

Speaking of which... those G4 models were Apple's original foray into using metal construction for laptops. They had titanium skin on a carbon composite frame, in other words two of the materials you've been worshipping mindlessly in this thread. The original G4 Ti models were notoriously unreliable because they were very flexy, leading to lots of failed PCBs. Worse, they often suffered gross mechanical failures (the carbon composite frame was prone to cracking in certain spots).

Apple iterated the G4 Ti design a couple times and improved its stiffness (and therefore reliability), but after less than 3 years switched PowerBook G4 to aluminum semi-monocoque construction. They've been using aluminum ever since. The aluminum models were a clear upgrade in stiffness and reliability over the titanium.

Maybe this should tell you something?

(FYI, what it _should_ tell you is that it's not just about the materials, _and_ that aluminum can be used in this application.)
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