NONLINEAR PROPAGATION IN WATER AND ITS EFFECT ON ULTRASONIC C-SCANNING

In an attempt to reduce the uncertainties in quantitative ultrasonic C -scanning, it has been found that the effects of nonlinear propagation in water cannot be ignored. Nonlinear propagation has been studied fo r many years in the medical ultrasonic field and other areas of acoust ics. However, the subject has not been addressed in relation to non-de structive testing (NDT) because conventional bandwidth-limited NDT equ ipment filters out the characteristic fingerprint - a distorted acoust ic waveform. In many ultrasonic fields, nonlinear propagation is not i mportant because low amplitudes and short propagation distances are us ed. However, it has been found that several popular commercial flaw de tectors can produce large waveform distortions in water at the standar d frequencies and amplitudes used for C-scan ir inspections. The prese nce of this phenomenon explains why increasing the voltage to the tran smitting transducer does not always result in a proportionate increase in the received signal - it is possible to saturate the ultrasonic fi eld. Errors in measured attenuation coefficients of up to -34% (-0.3 d Bmm(-1) at 10 MHz), due to nonlinear propagation, have been recorded u sing conventional equipment through Perspex(TM). This paper-describes the phenomenon, how it affects attenuation measurements, and how to mi nimise these effects.