Application Cases

Experiment Title: Application of High-Voltage Amplifier in Nonlinear Ultrasonic Testing of Duct Grouting

Research Field: Non-destructive Testing

Purpose of the Test:

Ultrasonic waves, with frequencies above 20kHz, are widely used in non-destructive testing (NDT) of various structures. Ultrasonic NDT, known for its unique advantages in detecting concrete defects and damage, includes linear and nonlinear ultrasonic methods. Linear ultrasonic methods identify defects based on changes in linear parameters such as attenuation coefficients, amplitudes, and wave velocities. In contrast, nonlinear ultrasonic methods detect defects based on nonlinear ultrasonic phenomena like higher - harmonic generation, acoustic resonance frequency shifts, and sidebands under intermodulation. Many researchers have confirmed that nonlinear ultrasonic methods are more sensitive than traditional linear methods for detecting minor material defects and are easier to identify.

Test Equipment: ATA-2042 high-voltage amplifier, signal generator, ultrasonic transducer, oscilloscope.

Nonlinear Ultrasonic Testing System

Figure: Nonlinear Ultrasonic Testing System

Experiment Process:

A nonlinear ultrasonic testing system was set up with a signal generator producing electrical signals. However, these signals have low energy and need to be converted into acoustic signals by a transmitting transducer before being incident on concrete specimens. Upon incidence, the acoustic signals carry internal damage information of the concrete specimen and are received by a receiving transducer, which converts them back into electrical signals. These signals are then collected by an oscilloscope for data processing and waveform display.

The test parameters, such as incident frequency and gain voltage, were determined. The ultrasonic test in this study selected the optimal transmission frequency based on the prominence of the second - harmonic amplitude. Drawing on previous tests, transmission frequencies of 40kHz, 45kHz, 50kHz, 55kHz, and 60kHz were considered to suit different specimen sizes. First - harmonic amplitudes were observable under all five frequencies but not comparable. The focus was on whether the second - harmonic amplitude was clearly observable and readable. At 60kHz, the second - harmonic amplitude was barely detectable. At 55kHz, it was readable but required coordinate processing for accurate values, complicating data processing. At 50kHz, the second - harmonic amplitude was observable but not prominent. At 45kHz, it was clearly visible. Further reducing the frequency to 40kHz revealed not only the second - but also the third - harmonic amplitude, but the second - harmonic frequency was unstable, and research on third - harmonic amplitudes remains inconclusive. Thus, 40kHz was excluded to ensure a stable second - harmonic amplitude.

The power amplifier's gain voltage range decreases with increasing ultrasonic transmission frequency. The gain voltage can amplify the electrical signal from the signal generator, and increasing it can boost the first - and second - harmonic amplitudes used to calculate the relative nonlinearity coefficient. For cube specimens of size 200200200mm³ with the same mix proportion, the measured data were analyzed. Spectrum graphs were plotted for gain voltages of 7V, 8V, 9V, 10V, and 11V. To enhance clarity, the second - harmonic amplitude values were magnified. The results showed that regardless of the gain voltage increase, the first - harmonic amplitude appeared at 45kHz and the second - harmonic amplitude at 90kHz, with no frequency changes. Both amplitudes increased with gain voltage. Therefore, varying the gain voltage can yield different first - and second - harmonic amplitudes for nonlinearity coefficient fitting. To verify this, the first - and second - harmonic amplitudes at 45kHz under different gain voltages were fitted.

Figure: Harmonic Amplitude Variation under Different Gain Voltages

As shown, with the gain voltage rising from 7V to 11V, the fundamental amplitude at 45kHz and the second - harmonic amplitude at 90kHz both increased. The frequency remained constant. This indicates that increasing the gain voltage can adjust the amplitudes of the fundamental and second - harmonic components. By collecting data on these amplitudes at different gain voltages, we can better understand the nonlinear characteristics of the concrete specimens and provide a basis for calculating the nonlinear coefficient.

Figure: Fitting of Fundamental and Second-Harmonic Amplitudes at 45kHz

The above figure shows the fitting result of the square of the fundamental amplitude versus the second - harmonic amplitude at an input frequency of 45kHz under different gain voltages. The fitting curve has a good shape, with a linear correlation coefficient of R²=0.923. Therefore, this method can yield relatively accurate values of the relative nonlinear coefficient. In the nonlinear ultrasonic testing of concrete specimens, the relative nonlinear coefficient is a crucial parameter for characterizing the degree of nonlinear interaction in concrete. By analyzing this coefficient, the presence and severity of internal defects and damage in concrete can be effectively assessed.

Experimental Results:

(1) For cube specimens of size 200200200mm³ made with the same mix proportion, an ultrasonic incident frequency of 45kHz and a sine wave - shaped signal are suitable for nonlinear ultrasonic testing of prestressed concrete beam specimens.

(2) The method of obtaining the fundamental and second - harmonic amplitudes by increasing the gain voltage and then fitting them using the least squares method to calculate the relative nonlinear coefficient is feasible. The linear correlation coefficient can reach above 0.9, showing a high degree of linear correlation between the measured values and the fitting results. This indicates that the method can accurately and reliably determine the relative nonlinear coefficient, providing a reference for the nonlinear ultrasonic testing of concrete specimens.

Figure: ATA-2042