A thermal adhesive wouldn't be messy depending on the type they use. I've never seen the internals of an iPhone, but I'd be curious to see the internals of the Razr. Although, glass is pretty rigid. From what I understand it's one of many reasons the iPhone 4 feels so solid.
If you are interested, you can find take apart videos on Youtube for practically any phone out in the wild. It might interest you to see what the guts actually look like.
I would actually think the Kevlar is more a marketing gimmick than the stainless steel frame...".. get your bulletproof phone!" There's no mention of the Kevlar being anywhere other than that back cover, which, in that case means it's only as strong as it's supporting structure and the resin used to stiffen the fibers.
This, unfortunately, is absolutely incorrect. All these modern big screen phones use a monocoque structure, which requires the external shell to be the load bearing part of the structure--that includes the screen glass. If you were to build an internal frame capable of handling the torsional and bending stresses a large relatively flat phone undergoes, it would be prohibitively large and heavy.
By all accounts, the Razr is "unbelievably light and thin". You can't achieve that with an internal frame or the gorilla glass alone.
I'm not a materials engineer by profession, but I believe the "stronger than steel" property only refers to Kevlar's tensile strength in its pliable fiber form. Obviously there's no compressive strength if it's pliable since it would just fold over. The problem with rigid forms of fiber composites such as carbon and Kevlar is that they're relatively easy to break i comparison to the pliable form. I'm guessing it has to do with the cracked resin edge shearing the fibers like a knife. This is actually a good property for say, race cars, to have a strong rigid body to enhance handling performance, but, if in a collision, allows the pieces to break off, crumble, and dissipate the impact into other forms of energy.
Fortunately, I am an engineer by trade and have a heavy background in materials. Composites do not generally fail that way. While the thought of razor sharp shards of resin matrix sounds like fun, the actual failure mechanism is a lot more mundane. Resin composites like Kevlar fail basically through delamination. As the resin matrix bends and is stressed, microfractues form. As these microfractures expand and intersect the kevlar fibers, they tend to travel along the fiber resulting in localized delamination of the resin from the fiber. As the stresses continue, the delam regions will grow and coalesce, leaving the unsupported fiber to handle all the loads. At some point, there is a stress that exceeds the ability of the fibers to hold, and you end up with some sort of catastrophic failure. The Kevlar fiber does indeed fail, even if the tensile strength is greater than steel. Once the matrix breaks down, the mtutal support is gone and the fibers are rather weak individually. And in this usage, "Kevlar" does not refer to simply the spun fiber, but the composite material. So, making a distinction between the two isn't really useful here.
Again, I'm no materials or structural engineer by profession so I could be completely wrong. But until I see that inevitable tear down someone's likely to perform, I'm more likely to believe that the Kevlar is a thin sheet plastered onto some sheet metal substrate. My OG droid battery cover is very strong.
Two points here. First, the Kevlar is likely molded to the rest of the back shell. That is no way reduces its contribution to the rigidity or overall strength of the monocoque structure. Second, battery covers have to be strong because in these types of phones, they are semi load bearing as part of the monocque structure. I say semi, because they aren't completely load bearing. The shell around the battery door has to be beefed up in two ways--one to transfer stress to the battery door, and two to make up for the incomplete load carried by the battery door.
The Razr's thinness is only possible at this time becuase it does have a solid, load bearing back shell allowing for a more elegant and thin solution.
One final "Oh, by the way": Metals are omni directional in strength, meaning equally strong in all ways--including in directions you don't need strength at all. That results in wasted material and mass. By choosing a composite back, the Motorola designers could conceivably have designed the back to be specificaly strong in torsion and bending, but not particularly strong in tension or compression (though bending is a localized form of both tension and compression).
My question is this: does the Kevlar shell wrap around the whole back shell, or just the visible plate? I suspect the rim "edge" of the back plate is simply a thin layer of protective material to prevent exposed Kevlar corners for getting abraded and ultimately delamming.
