Finite Element Study on the Stiffness Variation Mechanisms of Radially Bolted Cylindrical–Cylindrical Shell Joints Under Transient Thermo-Mechanical Loading

Radially bolted cylindrical–cylindrical shell joints are critical load-bearing components in aerospace vehicles. These joints experience complex thermo–mechanical environments during flight, where aerodynamic heating and mechanical loads jointly induce nonlinear deformation and stiffness variation through evolving interfacial contact states. To elucidate these mechanisms, this study develops a sequentially coupled thermo–mechanical finite-element framework to analyze the stiffness evolution of RBCCSJs under transient heating and combined mechanical loads (tension, compression, and bending). The results show that the global stiffness evolves through distinct contact-controlled stages (sticking → microslip → macroslip → mechanical bearing), producing pronounced nonlinear stiffness troughs spanning over two orders of magnitude. Under tension and bending, stiffness peaks during full sticking and decreases with slip, whereas under compression, it recovers earlier due to its end-face-bearing formation. Transient heating introduces two competing effects, thermal-expansion-induced frictional stiffening during short-term heating and temperature-dependent material softening during sustained exposure, leading to a 19.2–34% reduction in stiffness under steady thermal conditions. These findings clarify the dominant role of contact-state evolution and thermo–mechanical coupling in joint behavior and provide a quantitative analytical basis for enhancing the stiffness reliability and design optimization of aerospace bolted assemblies operating in transient thermal environments.