Comparison of Attenuation and Phase Velocity Measurements in Composites Made using Unipolar and Bipolar Pulses
We have made a comparison of the ultrasonic attenuation coefficient and phase velocity obtained from two different experimental systems. The first set-up employed unipolar (either one relative maximum or minimum) ultrasonic pulses to interogate a specimen of porous woven graphite/epoxy composite. In the second, bipolar (one relative maximum and one relative minimum) ultrasonic pulses were used to interogate the same specimen. The replacement of bipolar pulses by unipolar pulses lead to an increase in fractional bandwidth of at least 200% for measurements of both phase velocity and attenuation coefficient. Most of this increase arose from improvements in sensitivity at lower frequencies. Gains of this nature will significantly improve the stability of flaw inversion algorithms in either thick composites or porous composites where attenuation losses prevent the acquisition of data at high frequencies. Use of unipolar pulses also doubles the temporal (spatial) resolution that may be obtained using ultrasound generated by any particular transducer; this is a consequence of the fact that the unipolar pulse will be one half as long as the bipolar pulse produced by the same transducer.
KeywordsPhase Velocity Ultrasonic Pulse Magnitude Curve Bipolar Pulse Fractional Bandwidth
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- 1.D. O. Thompson and D. K. Hsu, “Technique for Generation of Unipolar Ultrasonic Pulses,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 35, No.4, pp. 450–455Google Scholar