Application Cases

Experiment Name: Defect Detection Capability Test of Variable-Size Butterfly Coil EMAT

Research Direction: With the continuous development of China's industrial level, online ultrasonic flaw detection equipment plays an important role in detecting the internal quality of steel products while balancing production efficiency in plate enterprises. It offers advantages such as fast detection speed, high accuracy, and wide plate coverage. Especially for products like pipeline steel and container plates that require high welding standards, the capability for online flaw detection is a requirement for many plate customers. Therefore, ultrasonic flaw detection equipment has become crucial online equipment for China's steel enterprises. Ultrasonic flaw detection equipment mainly includes piezoelectric ultrasonic flaw detection equipment and electromagnetic ultrasonic flaw detection equipment. Piezoelectric ultrasonic flaw detection equipment has relatively mature technology but also has obvious disadvantages.

During traditional piezoelectric ultrasonic (PE) testing, the voltage from a pulse oscillator is applied to a probe made of piezoelectric ceramics or quartz crystal. The ultrasonic pulse emitted by the probe enters the material through an acoustic coupling medium and propagates within it. When it encounters a defect, part of the reflected energy returns along the original path to the probe, which converts it back into an electrical pulse. This signal is then amplified and displayed by the instrument. This method usually requires a couplant to achieve good coupling with the test piece and has high requirements for the surface quality of the test piece, making it difficult to apply in high-temperature, high-speed, and rough surface detection environments. Electromagnetic Acoustic Transducer (EMAT) technology involves a powerful pulsed voltage generating a pulsed current in a coil within a permanent magnetic field, creating a strong electromagnetic field around it. The electromagnetic field radiated onto the surface of the test piece induces eddy currents. These eddy currents experience the Lorentz force, whose direction is perpendicular to the eddy current and points towards its center, generating electromagnetic ultrasonic waves on the surface. If the test piece is ferromagnetic, magnetostrictive forces are also generated. Which of these two effects, Lorentz force or magnetostriction, plays the dominant role depends on the strength of the external magnetic field and the frequency of the excitation current. The receiving effect is the reverse process of the excitation effect. The ultrasonic echo is received by the detection probe, enabling the inspection of the test piece. Since the above process is reversible, the ultrasonic waves reflected from the corners of the specimen or defect areas form eddy currents under the external magnetic field. The eddy current magnetic field causes a change in the voltage across the coil, and this signal, received by the instrument, enables defect detection.

Experiment Purpose: By changing the width and spacing of the conductors at different positions in the coil, study the relationship between the Lorentz force on the conductors at different positions of the butterfly coil EMAT and the type of ultrasonic wave generated, laying the groundwork for subsequent experiments.

Test Equipment: High-voltage power amplifier, Signal generator, Oscilloscope, Pre-amplifier for weak signals.

Experimental Process: The experiment used the ATA-4315 high-voltage power amplifier. An ultrasonic excitation signal generated by a signal generator was amplified by the ATA-4315 high-voltage power amplifier and input into the butterfly coil. Under the action of the Lorentz force, particles in the skin layer of the aluminum block undergo high-frequency vibration, exciting ultrasonic waves on the aluminum plate surface. The ultrasonic waves propagate vertically from the workpiece surface into the specimen interior, reflect upon encountering defects and the bottom surface, and are received by the transducer as a voltage signal. This signal is amplified by the ATA-5620 pre-amplifier for weak signals, realizing the excitation and reception of ultrasonic waves. The echoes are collected and displayed by an oscilloscope. The experimental schematic is shown in Figure 1-1.

Figure 1-1: Schematic diagram of the electromagnetic ultrasonic flaw detection experimental setup

Experimental Results: Figure 1-2 shows the actual echo signals measured by the transducer at a defect-free location before and after optimization. The experiment measured the shear wave velocity in the aluminum alloy test block as 2944 m/s and the longitudinal wave velocity as 5778 m/s. Based on the thickness of the test block, the arrival time of the voltage signal at the transducer can be calculated: the longitudinal wave appears near t=24 μs, and the shear wave appears near t=47 μs, consistent with the echo arrival times in Figure 1-2. This proves that the butterfly coil EMAT generated both shear and longitudinal waves in the aluminum block. Comparing the echoes before and after optimization, it can be found that the amplitude of the longitudinal wave excited by the optimized EMAT decreased from 5.6 mV to 1.9 mV, a reduction of 66.1%. The shear wave amplitude increased from 32.2 mV to 43.9 mV, an enhancement of 36.3%. The purity of the shear wave excited by the EMAT increased from 5.8 to 23.1, indicating a high shear-to-longitudinal wave amplitude ratio. Furthermore, the longitudinal wave component is submerged in noise, and the echo consists almost entirely of a single shear wave.

Figure 1-2: Detection signals from the EMAT on the defect-free aluminum block before and after optimization

Power Amplifier Recommendation: ATA-4315

Figure: ATA-4315 High-Voltage Power Amplifier Specifications

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