Microslip friction often occurs at the contact interface of bolted joint structures under long-term oscillating loads. Frequent microslip may cause the bolt preload to decrease or even cause the bolt to loosen. Acoustic emission (AE) is a phenomenon that occurs during microslip friction and contains abundant physical information. Developing a microslip friction AE model can link AE signal characteristics with tribological conditions and further guide the application of AE technique to detect bolt looseness. Therefore, this paper establishes a theoretical microslip AE model for bolted joint structures. The main steps of establishing the theoretical AE model are as follows: First, the elastic energy stored in a single asperity during stick‒slip friction is studied. Second, the overall elastic strain energy is obtained via the statistical analysis method, and the characteristics of microslip are considered on the basis of the Mindlin solution. Finally, the strain energy release rate and the AE root mean square (RMS) are calculated considering the additional sliding velocity and the AE signal conversion ratio. Reciprocating microslip experiments of a bolted joint structure are carried out to verify the accuracy of the proposed AE model. The tangential and normal behavior of the bolted joint structure are changed. The results show that the proposed theoretical model can accurately describe the variation trend of AE RMS with respect to the operating parameters. The theoretical AE RMS values calculated with the proposed AE model are more coincident with the experimental results than those calculated with the gross-slip AE model are. In summary, this paper proposes a novel microslip AE model based on the Mindlin contact solution and considers the relationship between surface separation and bolt preload, which lays the foundation for applying AE technique in the early stage of bolt looseness monitoring.

Stiffened panels have been widely utilized in fuselages and wings as critical load-bearing components. These structures are prone to be damaged under long-term and extreme loads, and their health monitoring has been a common concern. The guided wave-based monitoring method is regarded as an efficient approach to detect the damage in stiffened plates because of its wide monitoring range and high sensitivity to micro-damage. Efficient simulation of wave propagation can theoretically demonstrate the detection mechanism of the method. In this study, a Time-Domain Spectral Finite Element Method (TD-SFEM) is adopted to study the wavefield in stiffened plates, where continuous Absorbing Layers with Increasing Damping (ALID) strategy is proposed to circumvent the disturbance of reflected waves on boundaries. After the convergence analysis, the developed TD-SFEM with ALID is validated by the finite element method first. Then, wave scattering and the influence of the stiffener are investigated in detail by comparing the results with the non-stiffened structure. Finally, the effects of the parameters of the stiffener, such as the height and width, on wave propagation are studied, respectively. The results illustrate that the proposed TD-SFEM with ALID is an efficient approach to study the wave propagation in the stiffened plate and can reveal the mechanism of influence of the stiffener. It is found that the height of the stiffener changes the interference of wavefield in the plate, while the effects of the width are mainly in wave scattering and mode conversion.