Detection of Radial Bolt-Hole Cracks Using Sampled CW Ultrasonic Doppler-Shift Techniques

Recently there has been considerable interest in detecting radial cracks under fasteners in the wings of C-5A aircraft. Generally, detection is accomplished using the pulse-echo method, reflections from cracks being detected in real time. In the present study, cracks are detected by observing the ·Doppler-shifted frequency. A sample having a radial crack is mounted on a rotating platform in a water bath. A focused transducer transmits a tone burst such that only the shear mode propagates tangential to the hole in the metal. This transducer receives a Doppler-shifted reflected signal whenever a moving crack is in the field of view of the incident beam. The received signal is heterodyned, filtered, and displayed on a low-frequency spectrum analyzer. Merits and limitations of the technique are discussed