Strength improvement of bolted joints by post-yield festening.

There have been many kinds of troubles of bolted joints, such as loosening and fatigue failure of bolts. The main reason of these troubles is a lack of initial fastened stress. As a prevention of these troubles the post-yield fastening method of boltedjoints has attracted the attention recently. This post-yield fastening method has many advantages such as higher load-capacity of bolted joints and stability of initial fastening stress. This method is being used in variable industrial fields and contributing to the improvement of reliability and to the reduction of the size of the bolt. In this paper the fatigue strength and loosening behevior of elastic and post yield fastened joints under off-axial loading condition are studied analytically and experimentally. The deformation behavior of bolted joints under off-axial loading condition is analysed with following procedures. Below the load at which the side edges open, the relation between incremental is linear. This relation is analysed already in VDI 2230. On the other hand, over this load the stress of bolt and applied load relation between incremental bolt stress and applied load become nonlinear, which case is not analysed in VDI 2230 yet. In this paper this relation is derived using the equivalent fastened body, which is definend taking account of the opend contact surface. These analytical relations, linear and nonlinear, were confirmed experimentally. Using these relations between incremental bolt stress and applied load the fatigue strength of bolted joints were estimated analytically and confirmed by the fatigue test. Moreover, the loosening behavior of bolted joints were obtained by measuring the bolt stresses during the fatigue test.<br>From these studies following conclusions are obtained.<br>(1) The fatgue strength of post-yield fastened joints is about 50% higher than that of elastically fastened joint.<br>(2) The drop of fastening stress by loosening of post-yield fastened joint during the fatigue tests is almost the same as that of elastically fastened joint.