redditor explains why the aluminum iPhone 6 bends and the plastic Note 3 doesn’t

We have all had our fun with the BendGate jokes. But sometimes it’s important to set the jokes aside and ask why is it that it’s happening and how come some of the other devices don’t share the same problem.

Most people have blamed the aluminum shell on the iPhone 6 for being the culprit here, because aluminum is a soft metal. However, one redditor has explained that it doesn’t really have anything to do with the shell but how the internal geometry of the phone is.

What follows below is his comment on reddit:

This isn’t right. iPhone’s bending problem has nothing to do with aluminum. It has everything to do with geometry.

((Edit: Material choice matters, always. But material has to work together with the geometry it’s been shaped into. The choice of aluminum here isn’t the limiting factor. The geometric design of the aluminum chassis is.))

Nobody uses pure aluminum for general purpose manufacturing. They use aluminum alloys instead. And the alloys themselves are incredibly diverse.
You can get extremely rigid alloys that will be incredibly difficult to bend even in thin structures prone to bending (such as 7075), but the disadvantage of that is difficulty of machining (and often welding too). This type is dominantly used in transportation industry (automotive, marine, aircraft, etc). Manufacturers take the additional manufacturing costs in order to reap the great strength-to-density ratio.

And then there are buttery soft alloys that are extremely easy to machine (3031 for instance), even manually, but they’re also way too deformable to be useful for any load bearing purposes. I don’t have too much experience with these, but they’re cheap, and generally a good choice for decorative uses.
Apple uses anodized 6000 series aluminum (most likely 6061, possibly a tempered variant like -T4 edit: apparently it’s 6003, which is similar to 6061 in properties). This is a good compromise between the extremes, and is the most ubiquitous aluminum alloy out there. It’s got good mechanical properties, easy to machine, easy to weld. Their choice of material was correct in this case.

The problem with the iPhone 6 chassis comes from something we call “stress concentration” in engineering and this phenomenon is related to the geometry of an object. More specifically, it has to do with the cross section profile that is being bent.

If you watch the bending test video, you’ll notice that iPhone 6 bent exactly at the root of the volume buttons. And if you look even more closely, you’ll notice that the bending is actually on just one side — the side of the volume buttons. The opposite side is actually mostly unscathed.

This is because the cross section area of the bending profile decreases dramatically right at that point. They have cut out a hole to accommodate the volume buttons, and when under loading, the internal stresses of the structure are being concentrated at the base of this cutout. So when the structure fails, it fails at that point. The lower cross section area decreases the resistance to bending, and makes it possible to bend the chassis at a lower applied force than what it would take otherwise, had the volume buttons not been there (but of course they have to be there).

The result here wouldn’t have changed if Apple had used plastic in place of aluminum. In fact, it probably would have been worse. Typically phone manufacturers use brittle plastic in their devices (ductile plastic is the kind that feels really cheap and terrible), so the chassis would have broken entirely at the same point. They could have avoided the issue, maybe, if they opted for steel or a tougher aluminum alloy but then you run into other problems and have to retool essentially your entire product line.

The reason why Galaxy Note 3 passes the bending test doesn’t have anything to do with the material it’s made out of. It has everything to do with the internal geometry of the chassis. The internal magnesium alloy chassis (which isn’t any better than aluminum as a material) has an I-beam cross section that is great against bending, and it’s further sandwiched between two shells, which are in this case plastic. It’s reinforced very thoroughly, to the point where human-applied forces cannot bend the device beyond its “elastic range” (this is the deformation range within which the device can recover to its original state when loads are removed).

Apple could have designed the aluminum chassis in a way that would accomplish the exact same thing, and if they had, people wouldn’t be mistakenly criticizing the aluminum here. They would just be talking about how nice the material feels to the touch (because it does, and yes, it is “premium” much more so than plastic). Unfortunately, they fucked it up. Again, it’s all in the geometry.

So to be clear, Apple is at fault here, and the company’s obsession with making its products thinner has once again resulted in design compromises being made elsewhere. It’s not the aluminum to blame, however, (the HTC One M8 doesn’t bend) but rather the internal geometry.

Should this be a cause for concern, though? Considering how few actual people have reported the problem, I would say no. Not unless you put your phone in your pocket and do yoga. It takes a lot of force for the phone to bend like that and merely having it in your pocket won’t bend it like that. But it’s something to keep in mind and consider as a trade-off for having such a thin metal phone.

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