Relaxation stability and fatigue strength of thread elements made of VT6 alloy under cyclic load

Abstract

1. A regularity of stress relaxation has been established for the investigated alloy under cyclic load, and a link was shown between the relaxation process intensity and cyclic as well as static stress components. An analytical description is given of the relaxation process at the steady stage, and a method has been put forward for assessing the relaxation stability of the material — the arbitrary relaxation stress limit — based on the change of the relaxation process intensity as a function of stress amplitude. 2. An evaluation of the contribution to the relaxation process of various structural elements of a threaded joint was carried out. It is shown that the highest specific contribution to the relaxation stress is that supplied by the working thread. Such a joint will work at tightening stresses σm = (0.5–0.7)σ0.2 and a cyclic load at normal temperature under the conditions of stress relaxation. 3. An evaluation of the service life of a material working under conditions of cyclic stress relaxation was carried out which takes into account its fatigue and relaxation characteristics; this evaluation is based on the linear hypothesis of the summation of fatigue damage.