Structures Under Lateral Load Research On Looseness Behavior And Detection Of Bolted Connection

The bolted connection structure has a high proportion in aviation engine connections due to its advantages of convenient assembly and disassembly,reliable performance,and strong interchangeability.The segmented casing connection of the aviation engine is bolted,and the casing is inevitably subjected to external lateral load during the engine’s long-term service.After experiencing a large number of working cycles,the bolted connection structure of the casing is highly likely to suffer from bolt loosening,which leads to a decrease in pre-tension force.The stable pre-tension force of each bolt is key to ensuring the high reliability of the bolted connection structure.The phenomenon of bolt loosening presents a significant safety hazard for stable engine operation and even safe flight of the aircraft.Therefore,conducting research on the tightening process,loosening behavior,and preload detection of the bolted connection structure of the casing under lateral load is of certain significance to reveal the mechanism of bolt loosening,optimize anti-loosening design,and maintain the stable operation of the aviation engine.The main research contents of this paper are as follows:(1)The tightening principles of torque method and torque-angle method were theoretically analyzed,and the torque coefficient was theoretically derived.The tightening quality of torque method,elastic zone torque-angle method,and plastic zone torque-angle method were compared using the stability and self-decay law of bolt pre tightening force as evaluation indicators.The influence of different tightening process parameters,such as the number of repeated tightening,surface roughness,tightening steps,tightening speed,lubricant position,and lubricant type,on the size and stability of bolt pre-tension force was analyzed based on the single bolt tightening experiment.The self-decay law of bolt pre-tension force was determined.The elastic interaction process and distribution law of each bolt were explored through the numerical simulation of bolt flange connection structure tightening.The influence law of factors such as tightening sequence,the number of bolts,the center distance of bolt holes,the wall thickness of the flange,and the wall thickness ratio of upper and lower flanges on elastic interaction were analyzed.The most suitable tightening strategy was selected to consider actual engineering factors and the influencing parameters of internal structure and external process that are coupled and interacted with each other to achieve both tightening effects and tightening costs,in order to obtain the best tightening quality.(2)The mathematical expression of the threaded surface was analyzed,and a finite element model parameterization method for threaded structures with a helix angle was developed.The finite element models of bolted connection structure for phased loosening were stablished,including plastic bolt connection structures without a helix angle,elastic and plastic bolt connection structures with a helix angle,and the ratchet behavior of the cyclic plastic constitutive model was analyzed.The initial pre-tension force was applied using the applied torque method,and the validity of the finite element model was verified using the Yamamoto method and the Spowith method.The loosening behavior of various model bolts under lateral load was simulated and compared.The loosening behavior of single bolt connection structure was analyzed during the non-rotating loose period,the rotating loose period,and the whole loose period under different conditions such as the initial pre-tension force,lateral load amplitude,friction coefficient of the threaded meshing surface,and eccentricity distance,and the critical values of the initial preload and the first and second critical values of the lateral load amplitude during the loosening process were defined.(3)A bolt loosening behavior analysis method based on the adhesion-slip contact state change of the bolt head and threaded contact surface was proposed.The adhesion-slip contact state change process of the bolt head and threaded contact surface during one cycle of load was described in detail.Based on this method,the bolt loosening behavior of a single bolt connection structure under different lateral load amplitude,friction coefficient of bolt head contact surface,friction coefficient of threaded contact surface,and eccentricity distance during the rotating loose period was analyzed.Finally,the accuracy of this bolt loosening behavior analysis method was verified based on the changes in the initial pre-tension force and bolt head rotation angle.(4)The elastic fine finite element model of bolt flange connection structures with different parameters and a helix angle were established.Firstly,the optimal torque sequence and tightening steps were determined based on the size and stability of bolt preload force and modal frequency of the connection structure.Then,the theory of loose condition was analyzed to theoretically reveal the required conditions for bolt loosening in the connection structure.Finally,the numerical simulation of the bolt flange connection structures loosening behavior under lateral load during the rotation loosening period was conducted.The loosening behavior of the connection structure was systematically analyzed based on changes in preload force,adhesion-slip contact surfaces of bolt heads and threads,under different lateral load amplitude,bolt position,bolt quantity,bolt diameter,contact surface friction coefficient,stopper tightness,and load frequency.(5)Based on the piezoelectric equation and piezoelectric coupling system model,the resistance and conductance equations of piezoelectric material in the piezoelectric coupling system were established.The one-dimensional component wave theory analysis was conducted for axial loading,and the relationship between axial stress and natural frequency of the structure was derived.The finite element model of single-bolt connection structure coupling mechanical field and electrostatic field was established.An experimental platform for bolt preload force detection based on piezoelectric impedance was built.Simulation and experimental studies of bolt preload force detection were conducted under different initial preload force.A comprehensive analysis was conducted on the relationship between the peak frequency of electrical admittance in piezoelectric sheets,the stress on the bolt head,and the bolt preload force in theoretical,simulation,and experimental analysis.Finally,a bolt pre tightening force detection based on piezoelectric impedance and characterized by changes in peak frequency of electrical admittance was achieved.