The iPhone 6 and iPhone 6 Plus suffered from a weak frame in some cases that led to reports of bending. With the iPhone 6s and iPhone 6s Plus Apple uses a new stronger aluminum that Apple calls, "the strongest alloy we've ever used in an iPhone."
The stronger aluminum alloy is the same level used in aerospace applications and it should help prevent bending and stand up to other damage better. This is a good sign for users who prefer to use the iPhone without a case.
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