Even as smartphones continue to grow in length and width to accommodate larger screens, they've continued to shrink in thickness. The iPhone 6's thin, attractive body is no exception, with Apple shaving off 0.7mm (0.5mm for the iPhone 6 Plus) compared to the iPhone 5s. Reducing thickness partially offsets the weight penalty of an overall larger phone, improving in-hand feel, and giving the marketing department something to brag about.
But can a phone be too thin? Amid reports of the iPhone 6 Plus' bending in pockets and Unbox Therapy’s video showing one being bent by hand, it's tempting to think Apple went too far, compromising durability for vanity. While the Internet is rife with panic and ridicule, it's important to remember that any phone, particularly larger phones, can bend or break if forced to assume the curvature of one's posterior, and no sane person is going to spend hundreds of dollars on a new phone just to see if they're strong enough to bend it with their hands. The new iPhone, or any smartphone, regardless of thickness, doesn't need to be indestructible, just strong enough.
As exciting as seeing a product tested to destruction is, we have neither the budget nor the equipment to start bending phones. I know, bummer. Instead, we'll defer to the test conducted by Consumer Reports, which pitted the thin aluminum bodies of both new iPhones against several competing devices. A three-point flexural test was performed to determine the force required to permanently bend and then break each device. In this limited test, both the LG G3 and Samsung Galaxy Note 3 prove more durable than either of the new iPhones. However, both iPhones outperformed the HTC One (M8). Interestingly, the iPhone 6 Plus was slightly more durable than its smaller sibling. This could be an aberration due to the limited sample size (only one of each phone was tested), or perhaps the sides of the aluminum frame on the iPhone 6 are thinner in cross section.
One of the criticisms leveled against the Consumer Reports test is its failure to replicate the loading condition shown in the Unbox Therapy video, which is essentially a four-point bending condition. However, plotting the bending moment versus distance from the top edge of the phone shows that the three-point flexural test produces a greater bending moment at the critical section of the volume controls for the same applied load. In other words, the three-point flexural test used by Consumer Reports requires less force to permanently bend an iPhone 6 then the force required to bend it by hand as shown in the video.
While the iPhone 6 Plus is undeserved of its overly thin, fragile reputation, all of the physical testing has identified a weak point in its design by the left-side volume controls. The bending induced by these tests creates a tensile force above the volume buttons and a compressive force below. Since axial stress is equal to force divided by cross-sectional area, it follows that stress is higher in this location due to the cutouts to accommodate the controls.
Inside the phone, behind the volume buttons, is a fairly thick bar, attached by a screw on each end, that looks like it could reinforce this area. However, the only way to transfer force from the frame to the bar is through friction between the mating faces underneath the screw. With such a small screw being threaded into a relatively soft aluminum, it’s unlikely that enough preload can be created to keep the joint from sliding with a bending load applied to the frame, nullifying any load transfer into the bar and isolating the weak point in the frame.
A better solution would be to machine a long slot into the aluminum frame for the bar to fit into. Then, when the frame bends, the bar would contact the upper and lower portions of the slot. The bar would then carry a significant portion of the bending load, relieving the weak point around the volume controls.
Another way to fix the problem is to use a material other than aluminum. Titanium is both stiffer and stronger than aluminum. However, it’s heavier, more expensive and more difficult to machine. Steel is also stiffer and stronger, but considerably heavier. The only reasonable material that’s stiffer, stronger and lighter than aluminum is carbon fiber composites. It would be more expensive to use than aluminum, but not by much. If Apple is going to continue reducing weight and thickness, it needs to embrace composites. I predict we’ll see a carbon fiber iPhone 7.