Both amalgam and composite fillings can crack, either soon after they are placed or after the fillings have been in place for some time.
Cracks can occur soon after a filling is placed if the filling is higher than the rest of the tooth surface, and must bear most of the force of biting. Cracks also can occur over time, as the forces from chewing and biting affect the filling.
Small cracks also can occur at the edges of a filling. These usually are caused by wear over time. These cracks often can be repaired.