Study On Self-loosening Failure Mechanism Of Bolted Joint Structure

Bolted joint structure is widely used in various engineering structures for its simple,high reliability and convenient disassembly,such as docking between spacecraft and wind turbine shaft.However,when the bolted joint structure is subjected to many kinds of loads,it will cause the loosening failure of the tension decline,even the fracture failure,which endangers the safety of the structure,and reduces the reliability of the structure.But,most bolted connection structures have been loosened before fracture.Therefore,this paper makes a systematic study on the loosening failure and anti-loosening problems.The main contents are as follows:1.The two-dimensional axisymmetric finite element model is used to study the load distribution of screw threads,and the results are in good agreement with Sopwith method,Yamamoto method and discretization method.In addition,various factors affecting the load distribution of screw threads,such as boundary conditions,number of thread engagement,ratio of elastic modulus between bolt and nut,net thickness of nut,friction coefficient and pitch of screw,were studied.The results show that the force and the boundary on the opposite side can improve the load distribution uniformity of the thread significantly.It can also improve the load distribution uniformity of the thread by reducing the elastic modulus of the nut,reducing the pitch and adopting the modification technology,so as to improve the service life of the thread.2.The loosening failure form of bolt connection under transverse load is studied by experiment method.The results show that the nut with repeated disassembly has similar antiloosening property to the anti-loosening nut.That is,the reduction of tensioning force can be divided into three stages,but the bolt and nut with multiple disassembly has less vibration resistance than the locknut.Among all kinds of anti-loosening nuts,Hard-lock nuts and the three compressed type Spiralock nuts have better anti-loosening performance.In addition,the three-dimensional fine finite element model of bolt connection is established,and the factors affecting the loosening of bolt connection under transverse load are studied by finite element method and experimental method.The results show that the amplitude,pretightening force and friction coefficient have great influence,and their influence is dependent on each other.Inertia force can slow down the loosening process,and vibration frequency has little effect on loosening.3.The anti-loosening mechanism of the double nuts was studied based on the method of and test,and it was found that the anti-loosening effect could only be achieved when the pretightening of the double nuts reached the stage of top locking.In this stage,the contact state between the lower surface of the main nut thread and the upper surface of the bolt thread produce a negative force,that is,the effect of top locking.When the top locking state is reached,the pre-tightening torque relation between the auxiliary nut and the main nut is related to the thread fit tolerance,the nominal diameter of the bolt and the friction coefficient.4.The self-loosening mechanism of bolt connection under temperature load mainly includes the loosening caused by plastic deformation and the loosening caused by the relative movement of bolt and nut.A two-dimensional axisymmetric finite element model is used to study the loosening caused by plastic deformation.The results show that higher temperature cycle amplitude,higher pretension force and higher thermal expansion coefficient difference between the connector and the bolt will cause more plastic deformation,and the tensioning force will decrease more.Three dimensional fine finite element model was used to study the relative motion of loosening.The results showed that higher temperature cycle amplitude,lower pretension and higher difference of thermal expansion coefficient between nut and bolt would result in more relative motion and faster decrease of tension force.In addition,a minimum of 3 teeth is recommended for thread fit.5.The self-loosening mechanism of bolt head under fixed and unfixed conditions for rotational load is studied.Based on the loosening theory of nut fixed and free bolt head proposed by Sakai,it is proved that the theory is also applicable to the case of bolt head fixed and free nut.At the same time,this paper put forward the theoretical criterion of loosening when the bolt head and nut are free,and it is pointed out that there are two kinds of loosening failure mechanism in this case,which the first is the loosening failure caused by the bolt head rotation,the second is the loosening failure caused by the nut rotation.The correctness of the theory is verified by finite element analysis,and the error is within 5%.In addition,it is pointed out that the reason of loosening under the action of rotational load is the complete sliding of the contact surface,and local sliding will not cause loosening failure.