RESUMEN
During electromagnetic ultrasonic testing, it is difficult to recognize small-size bottom cracks by time of flight (ToF), and the lift-off fluctuation of the probe affects the accuracy and consistency of the inspection results. In order to overcome the difficulty, a novel composite sensor of an electromagnetic acoustic transducer (EMAT) and pulse eddy current (PEC) is designed. We use the amplitude of a bottom echo recorded by EMAT to identify the tiny bottom crack as well as the amplitude of PEC signals picked up by the integrated symmetric coils to measure the average lift-off of the probe in real time. Firstly, the effects of lift-off and bottom cracks on the amplitude of bottom echo are distinguished by combining the theoretical analysis and finite element method (FEM). And then an amplitude correction method based on the fusion of EMAT and PEC signals is proposed to reduce the impact of lift-off on the defect signal. The experimental results demonstrate that the designed composite sensor can effectively detect a bottom crack as small as 0.1 mm × 0.3 mm. The signal fusion method can accurately correct the amplitude of defect signals and the relative error is less than ±8%.
RESUMEN
Conventional water immersion ultrasonic testing faces limitations due to factors such as environmental conditions, workpiece dimensions, corrosion, and resource wastage. Contact-based coupling methods, which employ coupling media or specific coupling structures, offer a convenient approach to coupling acoustic waves and reduce signal attenuation. However, these methods are time-sensitive and lack adaptability to uneven surfaces, particularly when dealing with workpieces featuring subtle undulations, resulting in significant signal decay. This paper presents an ultrasonic coupling method based on a flexible capillary water column array. By employing a stable and flexible water column array within the micro-channels as the coupling medium, stable contact-based transmission of ultrasonic signals is achieved. The influence of water column array unit dimensions and array structures is explored through theoretical analysis and experimentation, demonstrating lower energy attenuation compared to reductions in water column area. Notably, the tests revealed the method's adaptability at oblique angles below 20°, which surpasses the performance of submerged detection at similar angles. This research presents an innovative and stable approach for contact-based ultrasonic coupling testing, particularly in scenarios involving dynamic contact scanning between ultrasonic waves and workpieces.