Intrinsic mode functions of acoustic emission signals, extracted from signals using empirical mode decomposition, 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 intrinsic mode functions, 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. Subsequently, the C-C contact-related intrinsic mode functions were linked to the contact behaviors 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 intrinsic mode functions. Experimental results have revealed that the gross energy of acoustic emission 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 high-frequency intrinsic mode functions, on which basis the detectability of the acoustic emission-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. This proof-of-concept study provides a promising solution to evaluate the contact conditions of bolted composite joints during assembly and to continuously monitor the tightening condition throughout the service life of the joints using the acoustic emission technique.
