Measurement of the radial-bolt-hole crack size using the Doppler-shift technique

A technique based on the Doppler effect has been developed to measure the size of radial-bolt-hole cracks in mechanically fastened aircraft structures. Aluminum samples having radial cracks were mounted on a rotating platform in a water bath and interrogated using a train of tone bursts from a stationary, focused transducer. The Doppler-shift spectrum of the backscattered ultrasonic beam from the rotating crack is recorded, and the maximum Doppler shift is then related to the radial size of the crack. The technique does not depend on the amplitude or time delay of the reflected signal from the crack and is more accurate than those previously used.