Failure Behaviour Of Coaxial And Eccentric Clamping Bolted Joints Under Transverse Load

Bolted joint structures are widely used in industrial manufacturing,and the reliability of their connection affects the safe operation of equipment.Scholars from various countries regard the loosening mechanism and anti-loosening design of bolted joints as important research directions due to the particularity and importance of threaded fasteners.The loosening behaviour of bolted joints under single loading conditions(axial,transverse and torsion)has been extensively studied under the condition of coaxial clamping.However,few studies have been conducted on the loosening behaviour of bolted joints under eccentric installation conditions,especially the eccentric clamping conditions under transverse load.In this thesis,the damage morphology and mechanical properties of bolts with different operating mileage were analysed.On the basis of self-designed eccentric clamping bolted joint looseness test device and fatigue test fixture,the failure behaviour of eccentric/coaxial clamping bolted joint under transverse load was systematically investigated.The evolution law of fretting wear between threaded contact interfaces and the axial force of bolted joint structure was comprehensively analysed.The findings enrich the theoretical system of bolted joint structure failure,and has positive importance for the application of fretting tribology in engineering practice.The main conclusions of this thesis are as follows:The damage area of the serviced thread surface is discontinuous,the number of thread turns increases,and the thread damage is alleviated(the first thread bears the greatest load).The damage characteristics are mainly manifested as spalling,plastic deformation and fatigue cracks initiating due to the stress concentration of spalling pit.With the increase in service mileage,the thread surface damage increases,the tensile strength of the bolt slightly decreases,the thread surface hardening layer thickness increases,and the surface friction coefficient decreases.The wear mechanism of the coaxial and eccentric clamping bolted joint is similar to that of the serviced bolt.Fatigue cracks are initiated from the root,and the degree of looseness increases with the increase in the vibration amplitude.Compared with coaxial clamping,bolt damage and structural looseness of eccentric clamping are increased.The increase in eccentric distance promotes the stage I of bolted joint loosening.The rotation angle has no obvious effect on it.The stage II of loosening is affected by the joint action of rotation angle and eccentric distance.The fatigue life of bolt conforms to the normal distribution.The bolt axial force attenuation is divided into three stages: rapid,slow and rapid stage.The first two stages constitute the bolt crack initiation life period,and the last stage is the bolt crack propagation life period.During the crack initiation period,small difference is found in the stiffness of the bolted joint structure.During the crack propagation period,the stiffness of the structure decreases with the increase in the number of cycles.The fatigue source of bolt fracture is multisource,and the morphology near the fatigue source is river-like.