indeed, you can not be in both the plastic and elastic regions of the stress/strain curve! Once somthing has been stretched (convert this into strain) above the elastic limit of the material it will never return to its original shpe and size.

Worth noting that the wing is probably still capable of carrying the design criteria. I would think that this event has shortened the fatigue life of the aircraft by some amount.

If there was no structural failure, or adequate repairs could be made to the structure, there would be no need to take the aircraft off the line.