Modelling acoustic emission generated by microslip friction between bolted joint interfaces

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.