Experiment and design methodology of an IODR flange connection under bending load

A new type of inner and outer double-ringed (IODR) flange was introduced in this paper, which can make full use of the inside and outside space of circular tube and reduce the dimensions of connecting bolt and flange plate. Five groups of down-scaled IODR flanges with different flange plate dimensions were designed and tested under bending moment to study the failure mode and force-transferring mechanism. The finite element model was built to further investigate the influence of connection dimensions on the bending performance from different aspects, such as bearing capacity, location of rotation axis and bolt tension ratio. It reveals that the failure of IODR flanges occurred due to the necking of bolt. The bolt tension ratio has an insignificant change when the applied load is smaller than about 80% of bending capacity, then it tends to increase until approaching 1.0. A theoretical method for rotation axis position and bolt force was proposed considering flange geometry parameters and bolt arrangement. It was found that the obtained rotation axis positions are about 0.7r, which is smaller than 0.8r recommended in DL/T 5629–2021, meaning that the existing design method is unsafe. Based on the theoretical analyses, the design methods of flange plate, stiffener and bolt were proposed and a corresponding design process was also established.