Loosening Mechanism And Experimental Study Of Bolted Structure Under Vibration

Bolt connection is the most commonly used form of structural connection,which has an irreplaceable role in the transfer of force and movement,and ensure the sealing of the sealing structure.The mechanical properties of joints connecting structure restricts the performance of whole structure,but because of its applications,service conditions,installation and maintenance etc.uncertain factors,it is difficult to completely avoid the connection failure,and once the connection structure failure,especially the failure of important connecting positions,will cause very serious consequences.As one of the main failure form of invalidation,in the early period,loosening might not affect the normal operation of the mechanical equipment,but with a further increase in loose,may cause the failure of the whole structure,and thus lead to serious accidents in production.In order to reduce the adverse effects caused by bolt looseness,the study on the loosening mechanism of bolt connections has important theoretical and practical value.In this paper,in order to the study the loosening mechanism of bolt connection,the related theory and experimental research are carried out,mainly include:By using the theoretical solution of the nonlinear free vibration of a cantilever beam with clamped boundary.Ratio between second harmonic amplitude and fundamental frequency amplitude,r,is defined as a characteristic quantity to investigate the loosening laws of clamped structures.The finite element model considering the contact is established,and the nonlinear dynamic response of the clamped structure is solved by using the nonlinear solver.Meanwhile,experiments of structures with different clamping force are carried out.Numerical and experimental results both show that the second harmonic amplitude of the nonlinear response is decreasing when the clamping force is increasing,the ratio,r,is also decrease with the increase clamping force according to the power exponent relationship.The ratio between second harmonic amplitude and fundamental frequency amplitude,r,is an effective characteristic quantity for identifying the state of clamped structures.To study the metal bolts connecting structure,its stress distribution is non-uniform in the connecting part.The finite element model considering preload and contact of the bolt is established,the stress distribution of bolt and the dynamic response of the cantilever at the end of the structure are calculated,and it is proved that the structural looseness characteristic r will decrease with the increase of the bolt preload.Meanwhile,the loose test of metal bolt connection structure in different vibration excitation conditions is carried out,use the displacement response signal of the bolt connection under different tightening torques after the low and high excitation,calculate the structural looseness characteristic r,combine with the change of the axial force of the screw(i.e.the clamping force)which is calculated by the strain values,comprehensive determine the loose state of connection structure.In order to adapt to the harsh working environment,which is high temperature,high pressure and strong vibration,it is difficult to meet the engineering requirements for the special connection parts.So it is necessary to use composite materials with higher temperature performance.One of the key problems to promote the application of composite materials is to ensure the reliability of connection in service stage.In this paper,use the 3D weave C/C composite bolt connection test piece,vibration tests at room temperature and high temperature environment under different excitation are carried out respectively by shaker.Use the change of the looseness characteristic and fundamental frequency,determine the loose status in each stage at the environment of room temperature;in high temperature environment,the test data are analyzed using the short-time Fourier transform,get the time-frequency map in each stage of structure test,determine the loose status in each stage combining the looseness characteristic quantity of structure.The results show that this connection form is reliable for low order vibration excitation in the high temperature environment,and the anti-loosing property under the condition of high order vibration excitation needs to be further improved.It is proved that the looseness characteristic r is also applicable in high temperature environment.