Research On Load Introduction Factor Of Bolted Joints

Bolted connection play the role of connecting,fastening,locating and sealing in the mechanical components.It has merits such as good load-carrying properties and anti-vibration performance,high connection stiffness,anti-fatigue,simple construction and convenient assembly,etc.It will be affected by pretension load in the assembly states,and loads on the jointed bolts are complicated and changeable in the working states.It is the key to accurately analyze the distribution of external load in the bolt connection system under the working states.By the stress analysis of the simplified bolt connection structure at different loading positions,it is obtained that the ratio of external load on the bolt is determined by the relative stiffness of the clamping parts and bolts and the load introduction factor of the bolt when the external load is applied to the clamping part.The load introduction factor can be explained as the ratio between the axial elastic deformation of the clamping part in the unloading area due to the external applied load and the axial elastic deformation of the clamping part.Based on the finite element simulation,a large number of finite element models are created by controlling the different variables that affecting the load introduction factor in this thesis.The load introduction factor of different variables is calculated from the relative stiffness of bolt connection structure and the load extracting from the cross section of the bolt.Then,all the parameters influence are analyzed fully.First,considering the calculation of the axial compression deformation of the clamping parts,the correction coefficient is put forward for the initial diffusion diameter of the compression stress region of the clamping part,and modified formula for calculating the stiffness of the bolts is obtained,which the compressive stress of axial cross section is uniform distribution and four times non-uniform distribution.At the same time,the axial compression stress of the clamping part perpendicular to the arbitrary section of the bolt shaft is extracted.Fit the axial compressive stress accurately and the average strain then is calculated.The true axial compression deformation of the finite element is obtained by the integral calculation of the average strain,and the accuracy of the modified formula is verified by comparison.Secondly,through the comprehensive comparison analysis,the axial average displacement of the outer nodes of the contact area of the bolt head and the nut is obtained as the equivalent displacement to calculate the stiffness of the clamping part more accurately.By controlling the different influence variables of load introduction factor,the finite element simulation analysis is used to get the curve diagram of the load introduction factor of different variables,and fit data to get the relation formula of the dimensionless influence variables,which provides guidance for the engineering application.The analysis shows that when the distance between the loading point of the external load and the edge of the preloaded region is larger,it is considered that the loading region mainly acts outside the compression stress region or the small compression stress region.With the continuous increase of the distance,the additional tensile load of the bolt cannot be produced.With the increase of loading height,the additional load of bolt increases approximately linearly at first,and decreases slowly after a critical value.The decrease is due to the additional bending moment produced by the external load,and the reduction is greater with the farther loading position,the smaller plate thickness and the higher loading height.As the relative distance between the loading position or the side edge of the clamping part to the edge of the preload area is farther,the smaller the additional load of the bolt and load introduction factor are,the relative distance between the outer diameter of the bolt head and the height of the clamping part does not affect the load introduction factor basically.As the external load increases or the loading location is closer to the joint surface,the local contact state of the joint surface will be detached and the additional load of the bolt will be increased.