【Power amplifier application】 Lamb wave signal analysis based on dry-coupled ultrasonic testing

The dry coupling ultrasonic testing method does not require the application of liquid coupling agents such as water or oil on the surface of the material to be tested. It is easy to operate and flexible to move. It is suitable for solid rocket engine casings, aircraft wings and other parts that have high requirements for structural integrity and need to be used or stored for a long time. When the dry coupling method is used to detect composite flat structures, the Lamb waves excited carry a lot of structural or defect information, such as the type, size, and location of the defects. By using reasonable analysis methods to extract useful information from the signal, the damage in the material can be evaluated.

Dry-coupled Lamb wave signal analysis:

Dry-coupled ultrasonic testing experimental system: signal generator, power amplifier, oscilloscope, dry-coupled ultrasonic probe, where the piezoelectric vibrator of the dry-coupled probe is in the form of a three-layer laminate, with the upper and lower layers being two piezoelectric ceramic discs polarized along the thickness direction, serving as the driving material of the piezoelectric vibrator, and the middle layer being a metal sheet, whose diameter is slightly larger than the piezoelectric ceramic disc, used for support and circuit connection;

A signal generator is used to generate a sine signal with a frequency of 100KHz as an excitation signal. After amplification by a power amplifier, the output voltage is 40Vpp and the output impedance is a low resistance of 50Ω. During the detection, a pair of dry-coupled probes are placed at both ends of the defect with a constant spacing and fixed by pressure through a mass block. Lamb wave detection signals are collected in the 10mm and 30mm defect areas in turn. In traditional ultrasonic testing, the presence of defects is usually judged based on the difference in the amplitude of the time domain signal under different states. However, as can be seen from the figure below, the amplitudes of the two signals are similar, and the size of the defect cannot be distinguished simply based on the amplitude change. The detected Lamb waves contain multiple wave packets with different shapes and quantities, and are mixed together to varying degrees, so its mode needs to be decomposed.

Detection signals in different areas

The EWT method is used to process the defect signals of 10mm and 30mm respectively, and the frequency band boundaries supported by each filter are shown in the figure:

According to the adaptive spectrum segmentation method, the EWT transformation result is shown in the figure: