Fatigue cracks have been found in many thick annular components in aging helicopters such as the pitch shaft, rotor hub, and connecting link in H-46 helicoptes. Fatigue cracks of ten occur on the outer and inner surface of the annular components and may lead to a critical failure. To keep aging helicopters in operation, these cracks must be identified and monitored. At the present time, visual inspection is still the major menas to detect fatigue cracks in these components. Obviously, there is a need to develop an effective and efficient technique for detecting cracks in thick annular components.
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