Saltfever,
I agree that a piece of material that has been strained past its yeild strength is typically weaker but that is mostly do to the reduced material cross section and not the actual strength of the material itself. Most materials that we work with, steel, aluminum etc actually exibit improved physical properties once they have been stretched. This is a type of cold working that refines the metal grain sizes and increases strenght, but what happens, as Willie's test have shown, the material thins and the percent of thinning vs the percent of increase in material properities can make the part weaker.
Strain hardening is an important component of most of the "heat treatable" aluminums. To get 6061 to the T651 the material has to be stretched a certain percentage to get the desired properties.
I do agree that once you have a structure that has failed in "plastic deformation" it is junk and not repairable.
Rex