Hello,
What you have described is normal design practice for aircraft, and is called buckling or post buckling.
You will see this on most semi-monocoque aluminium aircraft especially on the top skin, my most notable recollection of seeing this effect was in a PA38 tomahawk in a steep turn.
I was fortunately involved some years back with designing composite parts using semi-monocoque design techniques for Boeing, ribs, stringers etc, and still had post buckling in the composite panels, we were able to photograph the buckling using special techniques.
Its nothing to be concerned about, the buckling is designed for, and is used by the engineers as a means of achieving a minimum weight for the component.
