As far as I know, a shell inpacting on a metal plate would damage anyway its structure -no matter if shell's AP are lesser than box AP- by means of deformation and micro-creaks.
These factors would affect the armour level by decreasing it, and they should be cumulative with an exponential trend. In other words, the more stress a metal structure had previously suffered, the bigger the effect of further stresses, until the critical point is reached and a mechanical overload (sudden collapse of the structure) is occurring.
I suppose as well that any improvised damage repair could restore the original armour only in minimal part.
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