Measuring of Closure Forces with Ultrasonic Diffracted Waves

Two crack parameters determine crack propagation rate when a structure is subjected to cyclic loading; crack size and crack closure. These can be measured with the ultrasonic diffraction method. The application of diffracted ultrasonic waves for measuring the profile of the front of a fatigue crack have been reported in previous works. It has been shown that points of closure on the crack surface act as scattering centers and can be detected with the diffraction method. The present work is the first attempt to measure the magnitude and location of closure forces which act on the surface of a fatigue crack utilizing diffracted ultrasonic waves. It has been shown that the closure forces do not present a continuous spectrum as in the model proposed by ELBER but rather randomly distributed forces which might superimpose on a continuous spectrum of lower level.