Studies on Behavior of Conventional Back-to-Back and Stepped Flange Bolted Joints

Abstract

Structures in aerospace are often connected by different types of bolted flange joints. A basic function of bolted joint is to provide adequate joint strength, stiffness and sealing to minimize leakages. The capability of the joints is determined by analysis, augmented by testing. Therefore accurate evaluation of bolt force in a bolted joint under external loads is a fundamental requirement in bolted joint design. Different types of bolted joints such as conventional back to back flange, stepped flange (Dogleg flange), triple stack flange are generally used to connect casings, bearing housings, rotors of gas turbine aero engine. Bolted joints are complex to design and difference in behavior is expected for different design configuration of bolted joints. While hand calculations are used for simple bolted joint design, finite element analysis found to be effective for complex geometry and loading conditions. The objective of present study is to assess the flange separation, contact pressure distribution between flanges, flange stress and load sharing of bolt for back-to-back and stepped flange bolted joints. The nonlinear contact analyses have been carried out for these two bolted joints using FEA software ANSYSver14.0. Analyses were performed for external axial load with assembly preloading. Finite element analysis results indicate different flange separation, contact and load sharing behavior between back-to-back and stepped flange bolted joint. The flange separation starts at the inner radius of flange and grows towards outer diameter of flange, as the axial load is increased in the back-to-back flange joint. The bolt hole zone is critical compare to between bolts for leakage point of view. In the case of stepped flange, no significant flange separation is observed at the interface of flanges as the entire stack pivots about bolt axis with flange corners taking the bending load. The contact pressure between the two flanges is much higher at the bolt position than between bolts. The flange stress induced in the stepped flange is higher compare to convention flange joint. The major portion of the external load is shared by the flanges in case of stepped flange joint, while the bolt shares major portion of the load in back-to-back flange joint. These critical observations provide insight for selecting appropriate joint to ensure structural integrity or to design bolts for failure for worst load case like fan blade off in case of aero engine to reduce load transfer to Airframe.