Investigating the influence of pre-tightening on radial-type flight intersection joints using a spring-mass model

Flight intersection joints (FIJs) connect different airframe sections of a slender launch vehicle, and their effectiveness is assessed by joint rotational stiffness (JRS). The JRS value of the FIJs is influenced by various parameters such as fastener type, joint dimensions, joint component properties, pre-tightening of fasteners, friction, joint clearance, etc. The current work is focused on investigating the influence of pre-torque magnitude on radial FIJs with certain clearance values using the spring-mass-based analytical model. The predictions of the model are verified by conducting appropriate experiments and numerical simulations. The study demonstrates that pre-torque and clearance induce a tri-linear joint behavior in FIJ under an external bending moment. The tightening torque increases both the JRS magnitude as well as the net load-carrying capacity, whereas the clearance drastically reduces the magnitude of these two parameters. Also, the position of the fasteners in the FIJ with respect to loading direction affects the JRS significantly, and this effect gets subdued with an increase in screw numbers for a given airframe diameter. In addition, the absence of pre-load on one more screw and its influence on the JRS magnitude is investigated by employing the aforementioned analytical model simulating a practical situation.