Experimental Investigation On Self-loosening Mechanism For Bolted Joints Under The Transverse Loading

Bolted joints are widely used in engineering structures because of advantages for transmitting force and torque,and guaranteeing connection characteristics for sealing structures,etc.As one of failures for bolted joints under vibration,dynamic and stochastic loading,self-loosening not only can cause mechanical breakdowns,but also leads to industrial catastrophes.In consideration of its importance,self-loosening mechanism is investigated all over world by academics but owning to its complexity,self-loosening has not been integrated to a same view.Thus,it is of significance for investigating the self-loosening mechanism with the theoretical value and engineering guiding meanings for bolted joints.Using the test devices and designed joints,influence factors and self-loosening mechanism for bolted joints are investigated by a series of tests in the paper.The conclusions can be drawn as following:The self-loosening mechanism for single-bolted joints is investigated and a series of tests are conducted on INSTRON test machine.The results indicate that the clamped force decreases with the increasing cycle number and the bigger the loading amplitude is,the easier the self-loosening occurs.The direct reason for self-loosening is the drop-off of the clamped force,which can't make connection effective.With cycles increasing,the enveloped area of the hysteresis loop is narrowing correspondingly,which means that the applied force to overcome the friction force is decreasing.The sliding types and contact phases can be influenced through the external loading for self-loosening.Aiming at investigating effects of assembling position for single-bolted joints under transverse loading,the experimental set-up is made and a series of tests are also conducted on INSTRON test machine.The test results indicate that if the bolt and slotted hole with good alignment,the bolt is sheared difficultly through the wall of the slotted hole,which makes the bolt connection with good performances.If bolted joints with the poor alignment,the shearing phenomenon occurs easily between the bolt and slotted hole,which make connection with low service life and poor connection performance.Thus,during the bolt assembling,controlling alignment for bolts and slotted holes can improve the connection performance and enhance service life effectively.The results verify effects of the assembling position for bolted joints and are significant for bolt using and designing for anti-loosening structures.In order to investigate effects of self-loosening behaviors for group-bolted joints under cyclic loading,tests are conducted on INSTRON test machine.The test results indicate that self-loosening for group-bolted joints occurs easily under transverse loading and the service life is lower because of shearing.However,the service life and connection reliability are better when joints subjected to the combined loading.The reason is that there is only the clamped force along the bolt axis under the transverse loading and the normal force on the contact faces is low,which means the connection performance is poor.But when joints subjected to the combined loading,there is the redundant loading or loading component existing along the bolt axis,which makes the normal force on contact faces increase.The increasing for the normal force leads to the friction increasing and the relative slip occurs difficultly between contact faces and the ability for anti-slipping for self-loosening increases correspondingly,which could assure connection characteristics for bolted joints.Conclusions verify that changing loading directions could enhance the service life and reliability for bolted joints.Based on above theoretical analyses and experimental investigations,effects for different loading amplitudes are clarified;sliding types and contact phases during the loading process are verified;effects of different assembling positions and variation of the loading direction on bolt self-loosening are investigated.Thus,investigations for bolted joints are of significance with the reliable guidance for bolt using and anti-loosening in industry.