Effect of Multimode Guided-Wave Propagation on Ultrasonic Phase Velocity Measurements: Problem and Remedy

Multimode guided-wave patterns in bounded ultrasonic specimens have been shown to interfere with phase delay measurements made by the pulse superposition method and the pulse-echo-overlap method. The phase shifts associated with the nodes in the multimode guided wave patterns invalidate the McSkimin ΔT criterion for the choice of the proper cyclic matching between echoes when the nodes are near the echoes used. The adverse effect of nodes upon the McSkimin criterion has been documented by extensive measurements employing shear waves in square barsand thick plates. In free-wave propagation without nodes, the ΔT criterion remains valid. A method employing video pulses to establish the proper cyclic match for rf measurements has been developed to supplement or replace the McSkimin ΔT criterion in all cases including those of adverse specimen geometry. Advantages and limitations of the video overlap criterion are discussed.