Study On The Failure Mechanism And The Effect On Mechanical Behavior Of The Whole Structure Of Bolt Loosening

As one of the most standardized parts,the bolted joint structures are widely used in aerospace,chemical machinery and other industrial equipments.Its main function is to connect the different parts together reliably.At the same time,it also needs to be used in combination with sealing washer to improve the sealing effect.Therefore,the preload usually exist in the bolt connection.However,under the complex working environment,when the bolt connection structure is subjected to shock,vibration,thermal cycling and other loads,the preload will gradually decline and result in bolt self-loosing.As the service time extending,when the preload decreases to a certain extent,leakage,abnormal sound,sloshing and other phenomena will occur in the structure.In extreme cases,they may lead to a catastrophic consequence such as structural disintegration.Therefore,it is of great scientific significance to study the bolt self-loosening failure mechanism.In this paper,the loosening mechanism of bolt connection structure under transverse vibration is systematically studied by means of numerical simulation and principle test.And the mechanical mechanism of some main anti-loosening measures is analyzed,which can provide a reference for the design of new lock nut.At last,the influence law of pre-tightening force change on the overall response of the structure is discussed.Main works of this dissertation are:1.The method for generating hexahedron mesh of helical structure and the forming mechanism of pretension force are studied.Through the analysis of the mathematical expression of the thread surface,the parameter modeling idea of modifying the node coordinates and scaling is proposed,and a thread modeling plug-in is made with Abaqus.The accuracy of the model is verified by the comparison between the analytical results on torque-tension relationship.The differences between the tightening process and some other simplified methods are discussed.2.Based on the established hexahedron model of bolt joint,the self-loosening mechanism of bolted connection structure under transverse vibration is analyzed,and the influence of different pretightening modes on the loosening process of the structure is compared.The state change on contact surface is described through the change of friction state and the rotation angle of the node.This paper expounds the alternating state of "adhesive-partial slip-complete slip" on the contact surface and the creep slip phenomenon during the self-loosening process.Then,using the model,.the influence of friction factor,vibration frequency and vibration amplitude on the loosening of bolts are discussesed.3.The self-loosening mechanism of bolts is studied based on the transverse vibration testing machine.According to its principle,a simulation model is established and its accuracy is verified by comparison with the experimental results.Using the machine,the anti-loosening performances of different kinds of locknuts are tested.It is pointed out that the differences between the common nut and the anti-looseing nut on failure mode.In addition,the influences of assembly number,vibration amplitude and initial pre-tightening force on self-loosening process are discussed in combination with the adjustable parameters of the machine.4.By using the modeling method in Chapter 2,a fine model of Hard-Lock nut is established to simulate its pre-tightening and loosening process,which reveal the anti-loosening design mechanism.It was found that the concave nut presents lever locking phenomenon under eccentric connection,which make the friction torque on the thread much higher than that between nuts.So it is difficult for the concave nut to loose during vibration.If the firiction coefficient of the contact surface between nuts increases to a certaion extent,the concave nut will loose along with the convex nut,which make the Hard-Lock nut no longer have the anti-loosening effect.The subsequent anti-loosening design should take the nut rotation as the premise,and try to increase the resistance torque during the process of nut rotation.It can't realize the absolute anti-loosening by increasing of the nut initial rotation condition unilaterally.In addition,using the Spiralock nut as an example,the influence of thread shape and plastic deformation on the load distribution uniformity is illustrated combining theoretical and numerical analysis.5.The influence of bolt preload on axial static and dynamic characteristics of bolt flange connection structure is studied by numerical simulation and test method.And a method of monitoring local bolt looseness by response amplitude ratio is proposed.The change in bolt pretension only impact on the initial stiffness of joint face during the statical analysis.When local bolt pre-tightening force changes,the coupled axial and bending response phenomenon exist on the joint surface under axial load.On the dynamics,the phenomenon shows as the axial and bending coupling vibration under axial impact.And the amplitude ratio in the spectrum is linearly related to the bolt pre-tightening force difference,which can be used to monitor local bolt loosening.then through experiment study,the accuracy of the conclusion above is verified.