Last year, some iPhone 6 and iPhone 6 Plus owners experienced a phenomenon known as "bendgate". Although Apple downplayed the issue, it looks like they might be improving this year"s upcoming iPhone by using better materials.
Lewis from "Unbox Therapy" has taken testing to a new level by finding out the metal composition of an alleged "iPhone 6S" shell by using a XRF Analyzer. By analyzing the upcoming shell, he found that the new shell is not just made from aluminum, but it also contains about 7% zinc. By adding this material, it looks as though Apple intends to strengthen the new shell. The added zinc does have its drawbacks, with a higher chance of corrosion. But, in order to combat this, the new shell has a higher grade/layer of anodization.
Naturally, Lewis also tested the strength of the upcoming shell by measuring how much force can be applied before bending. While the older model shell was able to take about 30 pounds of force, the new shell was able to easily double that amount and withstand a force of 80 pounds. If the tested shell is from the next iPhone, it should give users a better experience by lowering the chance of a "bendgate" incident .
Source: Unbox Therapy via 9to5Mac