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1、A Study of Early Stage Self-Loosening of Bolted JointsYanyao Jiange-mail:yjianghttp:/ Zhang Mechanical Engineering,University of Nevada,Reno,NV89557Chu-Hwa LeeFord Motor Company,Advanced Engineering Center,20000Rotunda Drive,Dearborn,MI48121A Study of Early StageSelf-Loosening of Bolted JointsBoth e
2、xperimental investigation and?nite element analysis were conducted to explore the mechanisms for the early stage self-loosening of bolted joints under transverse cyclic loading.The nuts were glued to the bolts using a strong thread locker in the self-loosening experiments to ensure that no backing-o
3、ff of the nut occurred.Depending on the loading magnitude,the clamping force reduction ranged from10%to more than40%of the initial preload after200loading cycles.Three-dimensional elastic-plastic?nite element analysis was conducted with the implementation of an advanced cyclic plasticity model.The?n
4、ite element results revealed that the local cyclic plasticity occurring near the roots of the engaged threads resulted in cyclic strain ratcheting.The localized cyclic plastic deforma-tion caused the stresses to redistribute in the bolt,and the result was the gradual loss of clamping force with load
5、ing cycles.The?nite element results agreed with the experimen-tal observations quantitatively.When the two clamped plates started to slip and the slip displacement was controlled,both experiments and?nite element simulations suggested that the friction between the clamped plates has an insigni?cant
6、in?uence on the early stage self-loosening.?DOI:10.1115/1.1586936?IntroductionBolted joints are the most commonly used components in ma-chines and structures.The fasteners generally represent the largest single cause of the warranty claims faced by the automobile manufacturers?1?.Often,potential dur
7、ability problems in a ma-chine or structure are proportional to the number of the boltedjoints used.Two failure modes are found in bolted joints subjectedto cyclic loading:fatigue and self-loosening.Fatigue is a majorfailure form for a bolted joint subjected to tensile load and self-loosening is oft
8、en found in a bolted joint subjected to transverseor shear load.Self-loosening is the gradual loss of the clampingforce in the bolted connections under cyclic external loading,es-pecially transverse loading.It can result in a decrease in structuralstiffness or the separation of clamped members.Exper
9、imental observations reveal that self-loosening starts witha gradual relaxation of initial preload.Only when the clampingforce has fallen below a certain critical value does the nut actuallystart to back off?1?.Loosening is rapidly beyond this point.Fig-ure1shows a typical self-loosening sequence du
10、e to transversecyclic loading from an experiment?2?.The symbol P representsthe clamping force,P0denotes the initial clamping force or pre-load,and?is the rotation angle of the nut against the bolt.Two distinct stages can be identi?ed from Fig.1.During the?rst ?early?stage,there is no relative motion
11、 between the nut and the bolt.Loosening is caused by material deformation.For the caseswith large slip between the two clamped plates,the?rst stage canbe very short?2?.The other stage is characterized by the obviousbacking-off of the nut and rapid loosening of the clamping force.There is a possible
12、transition in between the two stages whereboth material deformation and the backing-off of the nut contrib-ute to the loosening.The current investigation attempted to under-stand the mechanisms governing the early stage self-loosening.Early investigations concentrated on the behavior of boltedjoints
13、 under axial dynamic loading?3?.Junker?4?was the?rst tonotice the importance of the transverse dynamic load on the oc-currence of self-loosening.It is generally asserted that the clamp-ing force serves as the source of internal off-torque force whichresults in the rotational loosening of the bolt or
14、 the nut.Such ahypothesis has been the most convincing explanation for self-loosening.However,since the relative rotation between the bolt and the nut is a necessary condition,the theory cannot explain the experimental observation that signi?cant loss of preload can occur before the nut starts to ba
15、ck off.Most investigations tried to understand how the nut slides on the bolt thread due to internal clamping force and the external loading on the bolted joints.Yamamoto and Kasei?5?developed quantitative models based on a two-stage theory for the nut to slide along the thread of the bolt.However,n
16、o quantitative results were provided for the veri?cation of the models.Following Junk-ers basic assumptions,Sakai?6,7?proceeded to conduct a theo-retical analysis,trying to derive the conditions necessary for a bolt to loosen by self-rotation.It was concluded that unless the friction coef?cient was less than0.03,the bolt would not rotate loosely.It
