PSI - Issue 37

958 Reza Soleimanpour et al. / Procedia Structural Integrity 37 (2022) 956–963 Reza Soleimanpour, Sayed Mohamad Soleimani and Naser Khaled Mohammad/ Structural Integrity Procedia 00 (2019) 000 – 000 3 1.2 Article objectives This study is presented in two stages; it is initially focused on using CAN and SHG for detecting bolt loosening and then proposes an efficient method for bolt loosening localisation using signal processing along with acquired data from damage detection stage. This study provides numerical insights into the complex contact nonlinearity caused by bolt loosening. The study outcomes could further advance the nondestructive techniques for health monitoring of the bolted joints using nonlinear guided waves that can benefit many industries such as oil and gas, aviation, car manufacturing and the construction industry. 2 Damage detection and localisation methodology 2.1 Damage detection As discussed, most of linear guided waves techniques rely on base line data. However, this study proposes to use nonlinear features of guided waves such as SHG to detect and locate the bolt loosening, and hence, the assessment process does not rely on the baseline data. Since the fundamental anti-symmetric mode (A0) guided wave possesses a smaller wavelength compared to the fundamental symmetric mode (S0) guided wave at the same frequency and it has larger out-of-plane displacement magnitude than in-plane direction, it provides a better correlation in terms of higher harmonic guided wave generation at the bolted joint. Therefore, A0 guided wave is used as the incident wave in this study. 2.2 Transducers arrangement A transducer network of two transducers is used to detect and locate the imperfect bolted joint in this study. In general there are two conditions, pulse-echo and pitch-catch, depending on the location of the defect. In this study a general approach, which considers pulse-echo condition where the defect is detected and located using reflected waves from defect, is proposed to detect and locate the loose bolted joint using SGH. The reason for selecting pulse echo is that previous studies (Soleimanpour et al. 2017) show that pulse echo condition provides more information regarding the location of defects. However pith catch method still can detect the defects. 2.3 Damage localisation In the case of pulse-echo condition, the defect is located at one side of both actuator and receiver. The incident wave generated by the actuator passes through the receiver and then arrives at loose bolted joint. There will be reflections from loose bolted joint that may mix with boundary reflections. However the interaction of incident wave with surfaces of plates will generate wave packs with double frequency of incident wave which may be mixed with linear waves and reflected from the defect. The receiver captures the reflected waves that contain linear and nonlinear waves. The linear wave reflection is at the same frequency as the incident wave (fc) while the second harmonic frequency is at frequency 2fc. Soleimanpour et. al (2017) proposed the following equation for determining the location of defect using first and second harmonics of guided waves. − = ∆ . (2 ). ( ) (2 )+ ( ) where ∆ = 2 − (1) where − is the distance between the actuator and receiver, ( ) is the group velocity of the incident A0 guided wave at the excitation frequency , (2 ) is the group velocity of the second harmonic guided wave and − is the distance between the breathing defect (e.g loose bolted joint) and the receiver. ∆ is the time difference between the arrival time of the incident wave ( ) and the second harmonic guided wave ( 2 ). The loose bolted joint location − can be determined once ∆ is calculated from the measured guided wave data. In the case of using linear scattered wave information to detect and locate the defect, it usually requires the baseline data to extract the linear scattered wave information. However, if the defect is close to the receiver or the inspection area is small or geometrically complex, the linear scattered wave overlaps with the incident wave or wave reflected from boundaries which makes the damage detection or localisation process complicated. In contrast the determination of the defect location using the higher harmonic guided waves only relies on the information of the arrival time of the