Multilayer adhesive bonded structures/materials (MABS) are widely used as structural components, especially in the field of aerospace. However, for MABS workpieces, the facts that the weak echo of the deep interfacial debonding defects (DB) caused by the large acoustic attenuation coefficient of each layer and this echo, which generally aliases with the excitation wave and the backwall echo of the surface layer, pose a great challenge for the conventional longitudinal wave ultrasonic nondestructive testing methods. In this work, an ultrasonic resonance evaluation method for deep interfacial DBs of MABS is proposed based on the ultrasonic resonance theory and the aliasing effect of ultrasonic waves in MABS. Theoretical and simulation analysis show that the optimal inspection frequency for II-interfacial DBs is 500 kHz when the shell thickness is 1.5 mm and the ethylene propylene diene monomer (EPDM) thickness is 1.5 mm, and the optimal inspection frequency is 250 kHz when the shell thickness is 1.5 or 2.0 mm and the EPDM thickness is 2.0 mm. Verification experiments show that the presence of a DB in the II-interface causes a resonance effect, and in the same inspection configuration, the larger the defect size, the more pronounced this effect is. This resonance effect manifests itself as an increase in the amplitude and an increase in the vibration time of the A-scan signal as well as a pronounced change in the frequency of the received ultrasonic wave. In addition, the increase in the excitation voltage further highlights the ultrasonic resonance effect. Four imaging methods – the integrations of the signal and the signal envelope curve, the maximum amplitude of the fast Fourier transform (FFT) of the signal, and the signal energy – were used for C-scan imaging of ultrasonic resonance evaluation of MABS’s deep interfacial DBs and all these methods can clearly show the sizes and locations of the artificial defects and internal natural defect. The normalized C-scan imaging method proposed in this study can further highlight the weak changes in the signals in the C-scan image. The research results of this study have laid a solid theoretical and practical foundation for the ultrasonic resonance evaluation of MABS.

中文翻译：

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多层胶粘材料深界面脱粘缺陷的超声共振评价方法

多层粘合结构/材料（MABS）广泛用作结构部件，特别是在航空航天领域。然而，对于MABS工件，由于各层的声衰减系数较大而引起深部界面脱粘缺陷（DB）的微弱回波，并且该回波通常与激励波和表层的后壁回波混叠。 ，对传统的纵波超声无损检测方法提出了巨大的挑战。本文基于超声共振理论和MABS中超声波的混叠效应，提出了一种MABS深界面DB的超声共振评估方法。理论和仿真分析表明，当壳体厚度为1.5 mm、三元乙丙橡胶(EPDM)厚度为1.5 mm时，II界面DB的最佳检测频率为500 kHz；当壳体厚度为1.5 mm时，最佳检测频率为250 kHz。厚度为1.5或2.0毫米，EPDM厚度为2.0毫米。验证实验表明，II界面中DB的存在会引起共振效应，并且在相同的检测配置下，缺陷尺寸越大，这种效应越明显。这种共振效应表现为A扫描信号的振幅增加和振动时间增加以及接收到的超声波频率的显着变化。此外，激励电压的增加进一步凸显了超声谐振效应。四种成像方法——信号和信号包络曲线的积分、信号的快速傅里叶变换（FFT）的最大幅值以及信号能量——用于MABS深层界面超声共振评估的C扫描成像DB 和所有这些方法都可以清楚地显示人为缺陷和内部自然缺陷的大小和位置。本研究提出的归一化C扫描成像方法可以进一步突出C扫描图像中信号的微弱变化。本研究的研究成果为MABS的超声共振评价奠定了坚实的理论和实践基础。