Finite Element Analysis of Load Distribution of Threaded Connections Made of Dissimilar Materials in High Pressure Facilities

Threaded connections are often used for pressure vessels in high pressure services, and they must be designed to resist the shear forces due to initial tightening and internal pressure. Many investigations of the load distribution in threaded connections have been conducted. One of the most basic investigations was the work of Sopwith, and a design formula was developed. The High Pressure Gas Safety Institute of Japan (KHK) design guide provides design methods based on a modification of Sopwith’s formula for threaded connections. These methods are limited to threads made of similar materials. However, in order to prevent such problems as seizing or corrosion, threaded connections sometimes use dissimilar materials for the female and male threads. In this case, it is necessary to determine whether the modified Sopwith formula can be applied or not. In this paper, linear finite element analyses were performed to calculate the load distribution in threaded connections of dissimilar materials, and the results were compared with those of the modified Sopwith formula. The contact pressure and the friction forces on the surfaces of the threads were considered in the analyses. Two load conditions, the initial tightening and the internal pressure, were considered. From these analyses, the following conclusions were obtained: (1) In practical material combinations with an elastic modulus ratio of 0.5∼2.0, the load concentration factor for threaded connections of dissimilar materials was almost the same as that for threaded connections of similar materials. In other words, it is not necessary to consider the effects of the material combinations in threaded connection design. (2) The load concentration factors were dependent on the load type. The load concentration factor under internal pressure was smaller than that under initial tightening.