Shield Crushing

From the Rulebook:

A shield that has received any Shield Crushing hits is damaged. A shield that has been damaged any amount may be repaired. Three hits to a shield from a weapon, Magic Ball, etc. with the Shield Crushing Special Effect destroys the shield. Hits do not all have to be from the same source; hits from different sources are cumulative. Denoted by declaring “Shield Crushing”.

Rulebook 8.6.3