Originally Posted by
Kontact
Considering that the primary stresses are to the outer skin of a tube, I'm not sure I agree that an outer repair will fail to arrest any cracking.
In general, yes, however with a *thin wall* tube, the limiting factor can sometimes be (resistance to) localized buckling, when that local portion is under compression. In slender columns, the resistance to buckling is a function of the material (elastic modulus) and geometric (section modulus) *stiffness*, not strength. Similarly, in thin wall tubes under bending and torsion, the resistance to localized buckling can be a skin stiffness issue.
If you do a search for <B-52 fuselage wrinkling>, you will see plenty of photographs showing much local buckling in the forward section in front of the wings, the direction of buckling for "longering" (square skin element trying to turn parallelogram), under high positive wing loads (wing pulled hard up, thus fuselage pulled down by gravity and inertia), limited by the semi-monocoque ribs and stringers; The stresses are taken by both the skin and reinforcements that prevent localized collapse of the skin.
The lower the elastic modulus of the material, the more susceptible to localized buckling. This can be mitigated with ribs and stringers, but also, a "sandwich" material, with thin outer and inner skins separated by structural foam, or a "honeycomb" core of the same material as the skins; Fiberglass skin with foam core is common on small and large sailboats. Honeycomb aluminum and composite panels are common on aircraft. The XB-70 (Mach 3 cruise) required stainless steel honeycomb panels with the skins brazed to the cores, to stand up to the heat at max cruise. ($$$$$$)