The Dispersion of Ultrasonic Pulse Velocity in Cylindrical Rods

Abstract

Velocity dispersion of compressional stress waves in cylindrical rods of 61S-T6 aluminum alloy has been investigated experimentally. Electronically generated pulses of the desired shape and carrier frequency were used to drive the barium titanate and quartz transducers. Both the pulse echo and pulse transmission methods were employed. It was found that the group velocity depends strongly on the ratio of a, the radius of the specimen, to λ, the wavelength of the stress disturbance, when the value of a/λ is less than 2.5. For instance, in the neighborhood of a/λ equal to unity, the group velocity approaches that of Rayleigh surface waves which is 0.95 times that of the shear wave velocity when Poisson's ratio equals 0.29. As a/λ is increased, the group velocity also increases and approaches the bulk velocity as a limit. When a/λ is greater than 2.5, the group velocity does not deviate appreciably from the bulk velocity. This dispersion effect disappears when pulsed shear waves are used. The experimental error is estimated to be less than 2 percent.