Application Cases

Experiment Name: Nonlinear Ultrasonic Testing Experiment

Research Direction:
This study focuses on RTM (Resin Transfer Molding)/textile composites as the test subjects. The nonlinear ultrasonic testing method is employed to characterize porosity defects in RTM composites. Through the measurement of nonlinear characteristic parameters, a preliminary quantitative characterization of porosity in RTM/textile composites is achieved.

Experiment Objective:
To validate the basic requirements of a nonlinear ultrasonic system and several nonlinear ultrasonic testing methods. Based on the nonlinear response characteristics of ultrasonic signals to porosity defects in RTM/textile composites, a detection experimental system using the finite-amplitude nonlinear ultrasonic testing method is designed.

Testing Equipment:
The system primarily includes a Tektronix AFG3102 signal generator, an ATA-8202 RF power amplifier, a digital phosphor oscilloscope, transmitting and receiving piezoelectric sensors, a computer, and fixtures.

Experimental Procedure:
The nonlinear ultrasonic experimental system in this study adopts the acoustic wave transmission method with broadband reception to measure the amplitudes of the fundamental wave, second-order, and third-order harmonics. First, a sinusoidal pulse train is generated by the signal generator, amplified by a power amplifier, and used to excite the transmitting piezoelectric sensor to emit a single-frequency ultrasonic wave. This wave is coupled into the test material. The high-amplitude ultrasonic wave interacts with micro-damages in the material, such as lattice distortions, micro-cracks, and lattice anharmonicity, causing significant nonlinear distortion of the transmitted acoustic wave. The receiving transducer on the opposite side of the sample captures this nonlinear ultrasonic signal. The received ultrasonic signal is then processed using MATLAB software for Fast Fourier Transform (FFT) to obtain the amplitudes of the fundamental and second harmonic waves. The higher-order relative nonlinear coefficients of the material are calculated using relevant formulas. The schematic diagram of the system is as follows:

(a) Selection of Excitation Waveforms for the Nonlinear Ultrasonic Experimental System

Figure 3-7 shows three common excitation waveforms. Continuous wave excitation (Figure 3-7(a)) can generate a single-frequency ultrasonic wave (Figure 3-7(b)). Since nonlinear ultrasonic testing primarily analyzes signals in the frequency domain, the frequency components in the excitation signal significantly affect the experimental results. Therefore, ensuring a single-frequency excitation signal is a prerequisite for nonlinear ultrasonic testing. However, generating high-power continuous waves is often challenging, and the axial resolution in continuous wave excitation is low. Using a single-pulse mode (Figure 3-7(c)) as the excitation method, although it provides high waveform amplitude and high axial resolution, the single-pulse signal itself has a wide frequency range (Figure 3-7(d)). This may introduce frequency components other than the fundamental wave at the transmitting end. Different frequency components interact with micro-defects differently, leading to nonlinear harmonic components in the received signal that are not generated by the material itself, thereby reducing measurement accuracy. Additionally, due to the short interaction time between the ultrasonic wave and the material under single-pulse excitation, it is difficult to produce strong nonlinear effects. Therefore, for the reasons mentioned above, this experiment adopts the pulse train mode (Figure 3-7(e)) as the excitation signal. This mode can produce excitation signals with relatively high output power and single-frequency components, making it widely used in nonlinear ultrasonic testing.

In the pulse train mode, the number of waveform cycles is adjustable. The greater the number of cycles, the more single the frequency components. Considering the test specimens and the available transducers in the laboratory, this experiment uses a pulse train with a frequency of 2.25 MHz and 30 cycles. Figure 3-8(a) shows the time-domain waveform of the 20-cycle pulse train excitation signal used in this experiment. Figure 3-8(b) shows the frequency-domain waveform after FFT transformation. From the frequency domain, no second harmonic appears at the 5 MHz position, indicating that the signal has a single-frequency component and will not affect the experimental results.

Experimental Results:

The experiment selected a narrowband PZT material piezoelectric chip with a center frequency of 2.25 MHz as the transmitting transducer. The reason for choosing a bare chip as the transmitting transducer is that, compared to commercial transducers, the vibration of a bare chip is free and unaffected by damping blocks or other components, significantly reducing nonlinear interference signals introduced by the transducer that could affect the detection results. Figure 3-9 shows the physical image of the PZT chip. Figure 3-10 shows the initial vibration waveform of the PZT chip excited by a square wave. Figure 3-11 shows the frequency sweep diagram of the PZT chip, indicating its center frequency of 2.25 MHz and a relatively narrow bandwidth. The ultrasonic wave generated by this chip has a single-frequency component, meeting the conditions for selecting a transmitting transducer for nonlinear ultrasonic experiments.

This experiment adopts broadband reception, as shown in Figure 3-12, where a transducer with a relatively wide frequency band simultaneously receives the fundamental and higher-order harmonic waves. This ensures a good response to both the fundamental and harmonic waves in the received signal, maintaining consistency and synchronization in the experiment. Based on this theory, the experiment selected a commercial probe from the Olympus Panametrics series, model Olympus C109, with a center frequency of 4 MHz. Figure 3-13 shows the physical image of this transducer, and Figure 3-14 shows its frequency sweep diagram. The diagram indicates that the transducer has a wide bandwidth, responding to frequencies ranging from 1.5 MHz to 6.5 MHz, facilitating the observation of second and third harmonics in the experiment.

This experiment aims to validate the basic requirements of a nonlinear ultrasonic system and several nonlinear ultrasonic testing methods. Based on the nonlinear response characteristics of ultrasonic signals to porosity defects in RTM/textile composites, a detection experimental system using the finite-amplitude nonlinear ultrasonic testing method is designed. The potential nonlinear interference sources in the experimental system are analyzed, and appropriate excitation waveforms, transmitting and receiving transducers for nonlinear ultrasonic experiments are selected to ensure the reliability of the experimental system.