Application Cases

Experiment Name: Study on the Influence of Excitation Conditions on Nonlinear Ultrasonic Testing

Research Direction:
Analysis of interference mechanisms of detection system nonlinearity, comparison of detection suitability of different types of excitation signal waveforms, investigation of the impact of excitation signal cycle count (energy) on nonlinear parameters

Experimental Objective:
By systematically exploring the influence of variables related to excitation conditions on nonlinear ultrasonic testing results, interference factors are eliminated, and the optimal excitation conditions that accurately characterize damage in carbon fiber composite materials are identified. This lays the foundation for subsequent research on nonlinear ultrasonic diagnostic imaging.

Testing Equipment:
Function generator, ATA-2031 high-voltage amplifier, air-coupled ultrasonic transmitting transducer, air-coupled receiving transducer, broadband preamplifier, oscilloscope, MATLAB spectral analysis, DC power supply, carbon fiber plates.

Experimental Procedure:
First, the excitation signal is amplified by the power amplifier and fed to the transmitting transducer. The transducer converts the electrical signal into mechanical vibrations through the piezoelectric effect, generating ultrasonic waves. The ultrasonic waves pass through a coupling layer and propagate into the test specimen. During propagation in the specimen, distortion occurs due to the material's elastic nonlinearity, generating a nonlinear response. The ultrasonic waves then pass through another coupling layer and are received by the receiving transducer, which converts them back into electrical signals. The received signals undergo pre-amplification and analog-to-digital conversion, and spectral analysis is performed to obtain the fundamental wave amplitude and second harmonic amplitude, from which the nonlinear ultrasonic parameters are calculated.

In this detection system, a pulse signal with a frequency of 200 kHz, peak-to-peak voltage of 10 V, and transmission interval of 200 ms is generated by a function generator (Rigol DG4062). This signal is amplified 14 times by a power amplifier (Aigtek ATA-2031) to increase its energy and fed to the transmitting ultrasonic transducer. The transmitting transducer sends ultrasonic waves at a fixed angle, which are received by the receiving transducer. The received signal is then amplified by a low-noise, broadband preamplifier (PXPA6) with a gain of +60 dB and stored in an oscilloscope (Rigol MSO5074). Finally, the data is imported into a PC, and spectral analysis of the signal is performed using MATLAB. The entire measurement process is synchronized and controlled by the trigger signal from the function generator.

Figure 1: Factors Influencing Nonlinear Ultrasonic Testing

Figure 2: Oblique-Incidence, Same-Side Air-Coupled Ultrasonic Testing System

Figure 3: Schematic Diagram of Components of the Oblique-Incidence, Same-Side Air-Coupled Ultrasonic Testing System

Experimental Results:

1. In unidirectional carbon fiber composite plates, whether cracks are present or not, the received signals under both healthy and damaged conditions contain higher harmonic components. However, the harmonic components in the damaged signals are more pronounced, and the relative nonlinear parameters calculated from the fundamental and higher harmonic amplitudes show a significant increase compared to the healthy state.

2. The nonlinear responses in two perpendicular directions of unidirectional carbon fiber composite plates differ. Signals obtained from cracks parallel to the carbon fiber arrangement (90°) exhibit larger harmonic amplitudes, and the changes in nonlinear parameters before and after damage are more distinct.

3. Excitation signals with too low energy cannot induce significant nonlinear responses, and increasing excitation energy does not guarantee a continuous increase in nonlinear parameters after damage.

   
   

Figure 4: Time-Frequency Analysis of Three Different Pulse Signals

Recommended Voltage Amplifier: ATA-2031

Figure: ATA-2031 High-Voltage Amplifier Specifications

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