Application Cases

Experiment Name: Research on Synthetic Aperture Nondestructive Testing Method Based on Lamb Waves

Research Direction:
Building on an understanding of the propagation model of Lamb waves in thin aluminum plates and the excitation principle of Lamb waves via Lorentz force, an EMAT transducer capable of exciting a single A0 mode Lamb wave was designed. Combining the propagation characteristics of Lamb waves with the structural features of the test plate, a novel synthetic aperture focusing imaging algorithm based on the Fourier domain was proposed. This algorithm was used to successfully conduct damage imaging experiments on square through-holes in aluminum plates.

Experiment Objective:
To design a specialized EMAT transducer capable of exciting a single-mode Lamb wave, and to demonstrate that the proposed improved synthetic aperture focusing imaging algorithm significantly enhances the resolution of damage imaging in plate materials. This provides a theoretical foundation for the excitation of single-mode Lamb waves and their application in damage imaging within plates.

Testing Equipment:
RF power amplifier, preamplifier, oscilloscope, transducer

Experimental Process:
After fabricating the EMAT transducer, the experimental setup was prepared. Due to the low transduction efficiency of the EMAT transducer, appropriate analog equipment was required to process the received echoes, including amplification and filtering. The diagram below illustrates the Lamb wave excitation experiment setup:

The test plate was a 1mm-thick aluminum sheet. The excitation signal applied to the EMAT transducer was a 7-period pulse excitation signal generated by a signal generator at a frequency of 132 kHz. The input signal from the signal source was not directly fed to the transmitting EMAT transducer but was instead driven by an ATA-8035 RF power amplifier. The pulse excitation signal was then input to the meander coil, where the Lorentz force generated by the interaction between the induced eddy currents in the metal sample and the magnetic field from the permanent magnet drove particle vibrations in the plate. These vibrations propagated forward as waves. The propagating signals were captured by the receiving transducer, amplified by a preamplifier, and stored in binary format in an oscilloscope. The experimental probe consisted of two EMAT transducers (transmitting and receiving), and the experimental setup employed a pitch-catch configuration.

1. Three excitation frequencies were selected to generate A0 mode waves: 130 kHz (near the optimal excitation frequency), 110 kHz (below the optimal excitation frequency), and 150 kHz (above the optimal excitation frequency). Three sets of comparative experiments were conducted to validate the accuracy of the optimal excitation frequency.

2. To enhance the credibility of the results, a frequency sweep experiment for A0 mode waves was performed. The peak values of the A0 mode Lamb waves were recorded to complete the comparative experiments and ensure accurate analysis of each wave packet.

   
   

Figure: Waveforms of A0 Mode Lamb Waves at Different Excitation Frequencies

From the figure above, it can be observed that the A0 mode Lamb wave with the maximum amplitude was generated at a center frequency of 132 kHz. The A0 mode Lamb wave signals were weaker at frequencies either below or above this optimal excitation frequency, further validating the correctness of the meander coil design and the accuracy of the excitation frequency calculation.

To avoid errors, a frequency sweep experiment was conducted. As the excitation frequency increased from 100 kHz to 150 kHz in steps of 1 Hz, the peak values of the first received A0 mode echoes are shown in the figure below:

Figure: Variation of the First Received A0 Mode Wave Peak with Frequency

As the excitation frequency gradually increased from 100 kHz to 160 kHz, the peak values of the first received A0 mode echoes initially increased and then decreased, reaching a maximum at 132 kHz with an amplitude of 0.0502 mV. This further verified the accuracy of the theoretical optimal excitation frequency analyzed in Section 3.3, providing a theoretical basis for subsequent nondestructive testing experiments using A0 mode Lamb waves.

Experimental Results:
A common method for identifying Lamb wave modes involves using the relationship between group velocity and frequency-thickness product, calculating the propagation velocity of the corresponding mode wave based on waveform propagation time, or determining whether a wave packet is a reflected echo from the edge based on its position. Additionally, frequency domain analysis of the waveform was conducted to calculate the center frequency of the A0 mode wave and compare it with the experimental excitation frequency, thereby identifying the mode of the waveform and each wave packet in the echo signal.

The figure below shows the echo signal received at an excitation center frequency of 132 kHz:

Figure: A0 Mode Lamb Wave Echo Signal Received at 132 kHz Frequency

The figure below shows the frequency spectrum analysis of the echo signal at an excitation center frequency of 132 kHz:

Figure: Frequency Spectrum Analysis of A0 Mode Lamb Wave at 132 kHz

The experimentally obtained signal center frequency was 130 kHz, which closely matches the theoretically calculated optimal excitation frequency of 132 kHz, with an error of only 1.5%. The error between the theoretical group velocity and the experimentally obtained group velocity was 4.2%. Therefore, within acceptable error limits, it can be concluded that the echo signal received in this experiment is a single A0 mode Lamb wave signal.

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