Investigation of the natural frequency behavior of fiber-reinforced composite thin-walled conical-cylindrical shells subjected to partial bolt loosening

Under actual operating conditions, structures with bolted connections can develop localized bolt loosening due to external excitations, which may result in structural disintegration or other hazards. To tackle this issue, the current study presents the theoretical model for the analysis of natural characteristics of a fiber‐reinforced composite thin‐walled cone–cylinder joint shell (FTCCJS) under conditions of partial bolt loosening. Firstly, the classical laminated plate theory and Love’s thin-shell assumption are utilized to establish a theoretical model of the FTCCJS under partial bolt loosening, and the virtual artificial spring technique (primary and secondary springs) is introduced to equivalently represent arbitrary bolt connection states. Then, an orthogonal polynomial method is employed to construct the displacement field function, and in combination with the Rayleigh–Ritz method, the natural characteristics of the FTCCJS with partially loosened bolts are solved. Subsequently, using a TC300/epoxy resin-based FTCCJS as the test specimen, a modal experiment with single-point hammer excitation and multi-point response measurement is carried out. Through comparison of experimental results and theoretical calculations, it is found that the error in the first four natural frequencies of the FTCCJS with partial bolt loosening ranges from 0.3% to 5.5%. Furthermore, the mode shapes derived from both methodologies show a high degree of congruence, confirming the validity of the theoretical model. Finally, by varying the number, degree, and pattern of bolt loosening, the impact on the natural frequencies of the FTCCJS is evaluated. This research offers valuable reference data for forecasting the natural characteristics of bolt-connected shell structures.