M.S. Thesis Presentation by Daniel A. Cook
Thursday, April 13, 2000

(Dr. Yves Berthelot, advisor)

"Monitoring Small Fatigue Cracks Using Ultrasonic Surface Waves"

Abstract

 The detection and measurement of small fatigue cracks is of particular interest because of its applications in the areas of remaining life prediction and condition-based maintenance (CBM).  There is a growing demand for integrated diagnostic systems that can be used to continuously monitor the integrity of key components and provide near real-time estimates of the remaining time to failure.  Fulfilling this need would translate directly into saving time and money associated with current methods of routine maintenance and inspection.  In addition, such systems could enhance safety by giving advance notice of imminent catastrophic failures.

A relatively new method for measuring small surface-breaking cracks consists of monitoring the Rayleigh (or long-wavelength) scatter of pulsed surface waves off the crack.  In this work, the method is applied using 5MHz ultrasonic surface waves propagating on steel specimens under tensile cyclic loading.  The objective is to use changes in the characteristics of the backscattered signal to obtain information about the damaged region.  The backscattered signals are used to detect the presence of cracks as small as 80?m. It is also possible to determine whether a single crack or a distribution of cracks is present.  For small single cracks, optical measurements are combined with acoustical measurements to show that the amplitude of the backscattered signal is proportional to the square of the measured crack radius.  [Work supported by ONR-MURI Center for Integrated Diagnostics]