Aluminum structures can develop fatique cracks. These problems have led to fatal accidents, including the loss of 520 lives on a Boeing 747 back in 1985. As a result, there are several nondestructive inspection systems (e.g. Magnaflux) to inspect metal for cracks.

On the other hand, Airbus has insisted that visual inspections are adequate for composite structures. This just doesn't seem to be the case. Delamination of composite structures can seriously weaken them before visible damage appears, so ultrasound inspection is a better way to go. However, ultrasound inspections are more expensive than visual inspections and Airbus wants to keep the operating costs low, perhaps too low for safety. According to this article, Airbus has ordered emergency inspections on A300 & A310 rudders.

Just for the record, B-52s had rudder failure problems back in the 1950s and early 1960s, leading to several fatal crashes. There are several things that can lead to rudder/vertical stabilizer failure, and composite structure problems are only part of the problem.

Composite Primary Structures