Research On Loosening Mechanism Of Threaded Connection Under Axial Impact Loading

The threaded connection structure is the most common industrial fastening method,which mainly provides the clamping force through the cooperation of bolts and nuts to fasten the parts to be connected.It is directly related to the reliability and safety of the structure and system.With the increasing complexity of industrial structures and performance requirements,the number of bolted connections is raising.More importantly,the load environment of screw connection structure is becoming more and more complex and severe,so the issues of bolt loosening have aroused widespread concern.The connection structures on typical equipment such as high-speed railways and fuze-projectiles often withstand complex,high-frequency,high-amplitude impacting loads.It is a challenging and basic problem to fully understand the loosening evolution and failure history process of bolted structures under impacting loads.And it is difficult to accurately predict the loosening behavior of bolted joints under impact loading due to their high nonlinearities of thread interface and the broadness and complexity of the shock wave.The literature survey showed that the loosening of the threaded fastener under impact loading has not been systematically studied,especially by using the experimental approach.In this work,a typical structural device of threaded connection assembly similar to the fuze-projectile structure was proposed,and a modified SHPB apparatus was carried out to provide axial impacting loads.Meanwhile,based on the transient photoelectric technology,a loosening phase diagram was established based on the measurement of axial stress variation of threaded fastener and amplitude of the stress wave during the impact loading.The results showed that the loosening phase diagram was divided into three parts: completely loosening,partially loosening,and non-loosening and the boundary between adjacent areas was linear.At the same time,the loosening rotation angle evolution of the threaded fastener under typical impact loading conditions was recorded with a high-speed camera system and analyzed using digital image correlation technique.The loosening of threaded connection under impact loading is usually the result of multiple shock waves with dispersion effects.However,the essence of loosening is formed by the accumulation of microscopic movements,so the experimental study is limited.In order to study the loosening mechanism of threaded fasteners under impacting load,this paper used ABAQUS software to establish the impact dynamic model of threaded connection structure.The transient response behavior of the threaded fastener under a single shock wave was obtained,and the alternating process of tightening-loosening of the internal and external threads at 8 typical moments was analyzed in detail.And the evolution of the parameters such as the interface clearance and contact force of thread engagement were extracted.Besides,the effects of different parameters on bolt loosening were studied,including the applied pre-tightening torque and friction coefficient of the thread surface.The S shape relationship between the maximum rotational angle during the loosening process and stress wave amplitude was established.Finally,this paper explained and analyzed the rationality of the above curve from the perspective of stress wave energy.This study provides an understanding of the self-loosening process of threaded fastener under impact loading and is helpful for optimizing structural design under extreme conditions and provide basic support and analysis tools for related engineering design and applications.55 pictures,3 tables,99 references.