But if the item in question is chainmail that has hundreds of links and if few links are broken I think it is unreasonable that the whole chainmail is then useless.
Really depends on how magic works in your setting and how do you see that enchantment works.
You can say that magic enchants the whole object, so if the object is broken (even if the damage done to it is very small) then it isn't complete anymore and the enchantment fail. This can mean that the object simply turns non-magical, or that it loose its magical powers untill it's repaired.
Or you can say that every part of the object is enchanted (a longer process of enchanting, I think), so even if the object is broken
each of its parts remains enchanted!
Or that the enchantment is placed on a specific part of the object (maybe the one with the glowing magical symbol
) so, untill that part is intact, the item retains its powers.
Or that the item can sustain a certain amount of damage before loosing its powers: in this case, when the item is damaged I would have it make a RR (level depending on damage). If it fails the RR it bacomes non magical.
Probably in TC you'll find something more helpful, but I don't have the book handy...