Feasibility of pulse-echo thickness measurements in air with a laterally displaced receiver

G. Waag, Petter Norli, L. Hoff
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引用次数: 1

Abstract

Air-coupled ultrasound (ACU) is an attractive option in non-destructive testing when the target of inspection is affected by the coupling liquid, or when the target is too big to be immersed in the coupling liquid. The challenge with ACU is the huge impedance mismatch between the air and the target. In pulse-echo measurements this mismatch causes a huge difference in level between the first reflection, from the air-target interface, and the tail, from multiple reflections inside the target. This can cause the first reflection to mask the tail signal. An analytical model for pulse-echo and through-transmission measurements has been developed by using the angular spectrum method. This model was verified against measurements in a water tank in a through-transmission setup. The model was then used to model echoes from a steel plate in air. These studies show that moving the receiving transducer laterally away from the acoustic axis of the transmitter will reduce the level of the first reflection more than the level of the tail of the signal. Hence, to avoid masking the signals from the interior of the plate by the strong reflection from the air-steel interface, the receiving transducer should be placed off the acoustic axis of the transmitter. Displacing the receiver from the transmitter acoustical axis represents a new challenge, as dispersion in the plate could potentially make the measured spectra difficult to interpret. Our studies show that the dispersion does not interefere with the resonance frequencies, as the spectral peaks are insensitive to the start of the time window the spectra are calculated from. This makes the computation of the thickness robust and simple, as the half wave resonance frequencies can be used.
用横向位移接收机测量空气中脉冲回波厚度的可行性
当被测目标受到耦合液的影响,或者被测目标体积过大而无法浸入耦合液中时,空气耦合超声(ACU)是一种有吸引力的无损检测方法。ACU面临的挑战是空气和目标之间的巨大阻抗不匹配。在脉冲回波测量中,这种不匹配导致第一次反射(来自空中-目标界面)和尾反射(来自目标内部的多次反射)之间的电平差异很大。这可能导致第一次反射掩盖尾部信号。利用角谱法建立了脉冲回波和透透射测量的解析模型。该模型通过在一个直通传动装置的水箱中的测量进行了验证。该模型随后被用于模拟空气中钢板的回波。这些研究表明,将接收换能器横向移动,远离发射机的声轴,将比降低信号尾部的水平更能降低第一次反射的水平。因此,为了避免由空气-钢界面的强反射掩盖来自板内部的信号,接收换能器应放置在发射机的声轴之外。将接收器从发射机声轴上移开是一个新的挑战,因为板中的色散可能会使测量的光谱难以解释。我们的研究表明,色散不影响共振频率,因为谱峰对计算光谱的时间窗的开始不敏感。由于可以使用半波共振频率,使得厚度的计算变得简单可靠。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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