Comparisons of second- and third-order ultrasonic nonlinearity parameters measured using through-transmission and pulse-echo methods

It is well known that ultrasonic nonlinearity parameters are effective for assessing material degradation, so many applied studies have been actively conducted. However, a method of measuring the second-order nonlinearity parameter by the through-transmission method (TT) has been used in most cases. In these cases, the second-order harmonic component generated during the transmission of ultrasonic waves in the material is measured. In this study, we demonstrate the utility of the method for measuring the third-order nonlinearity parameter by the pulse-echo method (PE), which is advantageous for field applications because it measures only one side. In addition, the second harmonic component is difficult to measure by PE due to the phase inversion effect, but the third harmonic is not affected by phase inversion. To experimentally verify such differences in different measurement methods, second- and third-order nonlinearity parameters were measured using both TT and PE for the same specimen. For this, two Al6061-T6 alloy specimen groups, high-temperature heat-treated and tensile fatigued, were prepared, of which trends in the second-order nonlinearity parameters measured by TT were well-known. In both specimen groups, second- and third-order nonlinearity parameters measured by TT exhibited the same trend according to heat treatment time or fatigue. On the other hand, in the results measured by PE, only the third-order nonlinearity parameter showed the same tendency as the results of TT, whereas almost no change in the second-order nonlinearity parameter was observed. This result confirms that the third-order nonlinearity parameter measurement by PE can be used equally with the second-order nonlinearity parameter measurement by TT and is expected to remarkably expand the field applicability of nonlinear ultrasonic techniques.