I've been fooling around with backpack designs for a couple of decades now. I favor designs where the major parts of the backpack are separate from each other and easily detached/replaced rather than sewn together into one entity. So, for my purposes, I divide the backpack into 5 parts: frame, waist belt, shoulder straps, back bag and front bag. This allows my simple mind to address one part at a time and it is much less daunting (for me) than attacking the entire pack design at once.
Also, when one of the 5 parts fails in testing or post test use I only need to replace that part.....not the whole backpack.
I'm curious how others approach the design and testing process of backpacks.