An ultrasonic fatigue crack growth monitor and load controller

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Abstract
The design of an ultrasonic crack follower which continuously monitors the position of a fatigue crack in a 36 mm thick C.K.S. type specimen is described. An open-bottomed ‘tank’ containing a mobile immersion probe is mounted on the top surface of the sample. The probe is used to measure the position of the crack tip by normal ultrasonic techniques; as the crack grows the probe is automatically advanced to keep the reflected signal intensity constant. The probe drive mechanism is geared to two potentiometers, one of which produces an output proportional to the probe position, the other is used to attenuate the load control signal so as to keep the stress intensity range constant. The unit was capable of detecting crack growth increments of 450 μm. The automatic nature of the measurement greatly simplified the collection of fatigue crack growth data.