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Gorilla Glass cracking
Can confirm, this is the reason right here. Cracked glass means that you no longer have a boxed structure - you're effectively bending a flat sheet of aluminum now.
In automotive and aerospace engineering, we call this a "body in white", compared to a fully assembled vehicle. In a car, the front and rear windshield alone increase the stiffness of the frame by a factor of 2x. This is why crash testing is done with a fully assembled car, rather than just the frame and restraint system.
for example:
https://www.roush.com/portals/1/downloads/articles/application_modal_compliance.pdfQuote
Adding fixed glass to the BIP can be expected to reduce the compliance by a factor of ~ ½, and addition of closures (doors, hatchback, etc.) will also reduce the compliance.
Source: PhD in mechanical engineering, I write finite element software
edit: note that stiffness and strength are two different things. The guy in that first video was scratching something but calling it "toughness". Drove me nuts. Anyway, since incorrect terminology can easily derail a discussion, here are two quick images:
http://imgur.com/Ly2cqVehttp://imgur.com/akKZhoodeformation is the X-axis
force is the Y-axis
stiffness is the slope of the line
strength is how high you can go before you break
toughness is the area under the curve when you break
hardness is something separate, the resistance to damage at the surface.