Loosening Behaviour Of Bolted Joints Under Coupled Thermo-mechanical Loading

As an important connection component in engineering applications,bolt connection is prone to looseness and failure in service environments,such as temperature,vibration and shock,thereby leading to failure of the connection structure.Therefore,scholars in China and abroad have continuously explored the loosening mechanism and protection methods of bolted connections.However,few studies have been conducted on the loosening failure of bolted connections under the coupling effect of temperature and vibration.This thesis focuses on the study of the loosening behaviour of bolted connections under the coupled thermo-mechanical loading.The findings serve as reference to the practical engineering application and expansion of bolted connections under alternating temperature and vibration environments.In this thesis,a test fixture and temperature loading device for bolted connection looseness were designed.The loosening behaviour of bolted connection structures under different temperature environments was studied.The loosening mechanism of the bolt connection under the temperature environment was explored through the analysis of the dynamic response of the bolt connection structure and the time-varying curve of the bolt axial force under the temperature environment combined with the microscopic analysis of thread surface morphology and profile morphology.Two types of coating were selected to study the anti-loosening performance of two types of coating bolt connection under temperature environment.The finite element method was used to simulate the loosening process of the bolt connection in room temperature and high-temperature environment(180℃).The contact stress and relative slip amplitude of the thread surface under different temperature environments and friction coefficients were analysed to explore the contact state of the thread contact interface.The main conclusions obtained in this thesis are as follows:(1)The loosening behaviour of bolt connections under room temperature and hightemperature environment can be divided into two periods.In the first period,the axial force of the bolt drops rapidly due to the plastic deformation of the bolt connection structure and the removal of the rough peaks on the thread contact surface.During the second period,the axial force of the bolt slowly decreases due to the fretting wear on the contact thread surface.(2)The bolted connection structure is placed in a high-temperature environment.After cooling to room temperature,the degree of looseness increases with the increase in temperature.Under the same vibration environment,the degree of looseness of the bolt connection increases with the increase in temperature.Fatigue wear,abrasive wear and oxidative wear are the main wear mechanisms that contact the threaded surface.When the working temperature is high,fatigue cracks are induced at the thread root due to stress concentration,and the bolts are loosened.(3)Diamond-like-carbon-coated bolt/nut connection structure has certain anti-loosening performance at room temperature and high-temperature environment.Chromium aluminium nitride-coated bolt/uncoated nut connection structure has certain anti-loosening performance at room temperature but poor antiloosening performance at high-temperature environment.Abrasive wear and fatigue wear are the main wear mechanisms between screw contact surfaces.(4)The contact stress distribution on the thread surface is uneven,and the uneven distribution on the thread surface in a high-temperature environment is greater than that at room temperature.The contact stress and relative slip amplitude of the thread surface decrease with the increase in the number of working threads.The greater the friction coefficient between the thread contact interface,the less likely to occur relative slip,and the contact stress on the thread surface increases.