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• A semi-analytic method to explain the interaction between lamb waves and bolt joint. • The power transmission coefficients change periodically with contact area. • The first S0 wave packet in the received signal is used to monitor bolt torque. • The semi-analytic method is helpful for frequency selection in bolt monitoring. Bolt joints are extensively used in aerospace and civil engineering. Online monitoring of bolt looseness is crucial to ensure the function and safety of these structures. Understanding the interaction between guided waves and the joint interfaces can aid in the design and optimization of such monitoring systems, and hence becomes important. However, the influence of contact area on guided wave energy passing through these jointed plates is still not clear. In this paper, a semi-analytic method, which combines finite element models with a wave superposition method, is proposed to predict the transmission of guided waves in joint interfaces. The wave mode conversions in the contact area are included. The model was validated by comparison with independent finite element models. In addition, the influence of the contact length on transmission coefficients of guided waves passing through the joint interface is investigated. The results were experimentally validated by wire electrical discharge machined specimens with various contact areas. Then, the transmitted guided wave and the contact area in a bolt jointed plate under different torques were measured experimentally and compared with the present method. The results of the two different experiments agree well with those predicted using the semi-analytic method, respectively. In particular, the experimental results of the bolt jointed plate show that the direct S0 wave packet energy decreases with the bolt preload, which can be explained by the proposed model. Thus, the proposed semi-analytic method demonstrates efficiency in the prediction of the power transmission coefficient and has potential applications in the selection of excitation frequencies in the online monitoring of jointed structures. On this basis, a normalized power transmission coefficient can be directly calculated by the energy of the first S0 wave packet in the received signal to monitor bolt torque. [ABSTRACT FROM AUTHOR]