Study On Mechanism Of Loosening Of Threaded Connection

Bolted joints have been widely used in mechanical industry, which play crucial rolesin the transmission of force (or torque), ensuring the performance of sealing structure, etc.As one of the major failure reasons for bolted joints, although loosening may not causeserious accidents during its initial stages, in the course of the failure, it decreases the bolttension continually, and may lead to leakage, abnormal noise and other structural failure,which cannot be ignored. Therefore, it is of significant engineering and theoretical value, tostudy the loosening mechanism. Focusing on the loosening of thread connection, thisdissertation was carried out in the following four aspects:1) Using mixed cyclic plastic hardening model based on Ziegler kinematic hardeningrule, this paper studied the initial stage loosening of bolted joints with large preload, due tomaterial’s ratcheting behavior. And then, the effects of some associated factors on theprogress werediscussed.2) Basing onanalysis of the mathematical expression of the bolt thread surfaces, aparametric mesh generation method was proposed. The method couldefficiently providehelical thread models with accurate geometry. Then, the loosening resultedfrom contactstate changes were studied, and the changes of contact state during the process and theinfluence of relevantfactors were investigated.3) Plastic deformation and creep of bolted joints induced by temperature wasexplored.Considering nonlinear material properties change with temperature, and dividingthe bolt into two sections, which are bolt shank and bolt thread, a one dimensional modelwas established, to reflect the variation of bolt tension, strain, and stresswith cyclictemperature changes, whose results were in good agreement with the ones of the finiteelement model.4) The relative motion of thread parts caused by temperature was analyzed. To studythe relative motion caused by thermo expansion in the radial direction of the joint, amathematical model of the problem for rectangle thread was proposed with appropriatehypothesis,which can provide the variations of bolt load, bolt rotation, friction force andfriction torque, while temperature changes cyclically. According to the model, influence ofsome correlative factors was discussed.