High-Frequency Low-Loss Ultrasonic Modes in Imbedded Bars

Abstract

The dispersion relationships of a system comprising a circular bar imbedded in a solid medium having a lower acoustic impedance than the bar have been predicted. A generic study of such systems has been undertaken, motivated by a particular interest in the case of a circular steel bar imbedded in cement grout which has application to the inspection of tendons in post-tensioned concrete bridges; measurements to confirm the predictions have been carried out for this case. The attenuation dispersion curves show a series of attenuation minima at roughly equal frequency spacing. The attenuation minima occur at the same frequencies as energy velocity maxima and they correspond to points at which the particle displacements and energy of the particular mode are concentrated towards the center of the bar so leakage of energy into the imbedding medium is minimized. The attenuation at the minima decreases with increasing frequency as the energy becomes more concentrated at the middle of the bar, until the material attenuation in the bar becomes a significant factor and the attenuation at the minima rises again. For the particular case of a steel bar in cement grout, the minimum attenuation is reached at a frequency-radius product of about 23 MHz-mm. The frequency-radius product at which the minimum attenuation is reached and the value of the minimum attenuation both increase as the acoustic impedance of the imbedding medium increases.

A review of Pochhammer–Chree dispersion in the Hopkinson bar

“…This approximation is valid for bars in air, however, when the bar is surrounded by a dense fluid or a solid then leakage into the surrounding material becomes significant. Pavlakovic et al (2001) studied the dispersion behaviour of steel bars embedded in grout in order to detect damage in steel tendons in post-tensioned concrete bridges. This also has applications for damage detection in buried pipelines (Leinov et al, 2016).…”

Contribution of leaky modes in the modal analysis of unbounded problems with perfectly matched layers

“…For instance, they can give the leakage attenuation of wave packets in open waveguides, which is of particular interest in the field of nondestructive evaluation (NDE). 3 However, leaky modes correspond to improper modes that spatially grow to infinity in the unbounded direction of the problem (they are not part of the modal basis). Their use in modal analysis hence requires special care.…”

Time Reversal Method for Guided Waves with Multimode and Multipath on Corrosion Defect Detection in Wire

“…Reference-free algorithms were proposed to estimate the cross-section loss based on the differential time-of-flight of propagating guided waves in rod-like waveguides [7,8]. Pavlakovich and Beard extensively studied the attenuation properties of guided waves determined by the material properties, propagation distance, frequency et al in an imbedded bar, and then applied it to the anchor structures detection [9][10][11][12]. Surface and core seeking guided waves were excited and respectively used to monitor 2 of 15 reinforced-concrete beams suffering from damages with different corrosion mechanisms, such as delamination and pitting corrosion on steels [13,14].…”