Continuous monitoring of tightening condition of bolted composite joints using intrinsic mode functions of acoustic emission signals

Intrinsic mode functions (IMFs) of acoustic emission (AE) signals, extracted from signals using empirical mode decomposition (EMD), were used to characterize the contact conditions of asperities (e.g., sliding friction or collision) in the mating parts of bolted composite joints undergone flexural vibration, whereby to evaluate the tightening condition of the joints quantitatively. Specifically, the sliding friction-related IMFs, generated in the mating parts of the two joining composite components (termed as C-C contact) were ascertained from those generated from the contacts between the joining components and metallic fasteners (termed as M-C contact), via a Hilbert-Huang transform (HHT). Subsequently, the C-C contact-related IMFs were linked to the contact behaviours of asperities at the joining interfaces, reflecting quantitatively the degree of the residual torque of the bolted joints. The fatigue performance of the joints was further evaluated according to the changes in the energy ratios of the C-C contact-related IMFs. Experimental Results have revealed that the gross energy of AE signals is capable of evaluating the residual torque of the joints within a limited range. Vibration loosening of composite joints was found to result in an increase in the energy ratios of C-C contact-related highfrequency IMFs, on which basis the detectability of the AE-based structural health monitoring is further improved, making it possible to evaluate the tightening condition of a bolted joint when the joint undergoes vibration fatigue.