In my historical renders I many times postwork in the wounds and torn clothes of my knights. Here some easy tips how to do it.
Photoshop	Paintshop
Load the image and make a new layer.
Choose the "clone" or "stamp" brush.
Click the clone or stamp brush.
(above sample in Paintshop Pro)
Choose a fitting cut/crack brush. You can download such brushes for free at Devianart.com for example. (browse resources, application resources, brushes),
(Above Sample in Photoshop)
If you want a torn chainmail, click some skincolor in your image and stamp it on the chainmail (see skin trough the chainmail). If you want a cut in a tunic, click the chainmail and you see the chainmail under the tunic coming trough the torn tunic.
Above Sample of a cut in the chainmail
Above Sample of a cut in the tunic (on the chest).
Another sample.
And another sample.