Study On The Loosening Phenomenon Of Bolt Connection Structure Under Thermal And Vibration Loads

Bolts are one of the most common connection methods and are widely used in aerospace,automotive and other engineering fields.Under normal working conditions,the bolt connection can reliably complete the load transfer between the connected structures.However,under the action of vibration and high-temperature load,the decrease in preload caused by the loosening of the bolts seriously compromises the safety and reliability of the overall structure.In response to this problem,the loosening rule of the bolt connection structure under thermal and vibration loads is studied from the two aspects of finite element simulation and experiment.The content mainly includes the following parts:Firstly,a refined finite element model of the bolt connection structure considering the thread angle is established,and a modeling method for bolt loosening simulation is given.For this model,the axial force distribution of the bolt is compared with the results in the existing literature to verify the effectiveness of the modeling method of the bolt connection structure.Based on the refined model,the bolt loosening research under normal temperature vibration is carried out.The reason why the bolt connection structure loosens under the vibration load is that the lateral load destroys the friction self-locking condition at the thread.The cause of the loosening of the bolt connection structure under vibration load is analyzed,and it is considered that the lateral load destroys the friction self-locking condition at the thread and causes the bolt to loosen.The characteristics of the contact state of the thread contact surface and the screw head pressure surface during loosening are analyzed,the stress distribution law of the thread root bottom is studied,and the factors such as load amplitude,load frequency and initial preload force on the loosening characteristics of the bolt connection are discussed.Long-term load calculations are carried out on the simplified disc model,and the several preload tightening modes of bolt connection structures are summarized.Then,considering the effect of temperature load,a simulation study is carried out on the bolt connection structure under thermal and vibration loads.The influence of the thermal environment on the structure is discussed.The contribution of elastic,plastic and thermal expansion deformation methods to bolt loosening under the thermal environment is analyzed.On this basis,the effects of thermal load parameters and initial preload on bolt loosening are further studied.Vibration loads are introduced at different temperature points to study the loosening of bolts under the combined action of the thermal and vibration loads.Finally,for the single-bolt connection structure,an exciter is used to apply a sinusoidal vibration load to the bolt connection plate,and a certain thermal load is applied through the high-temperature environment box.The bolt connection looseness test under vibration and temperature load is studied.The test directly measures the looseness of the preload force through the gasket piezoelectric sensor to obtain the time history curve of the preload force.The test analyzes the effects of load amplitude,load frequency,bolt preload and temperature on the loosening of bolted structure,and summarizes the loosening of bolted structure.The results show that the magnitude of the load amplitude is the main factor affecting loosening for vibration loads and thermal expansion deformation is the main factor affecting bolt loosening for temperature loads.For a certain bolt connection structure,there is a critical load for loosening under a certain tightening torque.The critical load increases with the increase of the tightening torque.The temperature load will reduce the critical load of the bolt connection structure for loosening.When the load is sufficient to loosen the bolt,the smaller the tightening torque,the larger the amplitude of the load and the higher the temperature,the greater the preload loss of the bolt connection structure will be.