Research On Load Distribution And Progressive Damage Of Multi-bolt Composite Joints

Composite materials have been widely used in various engineering fields due to their many advantages such as light weight,high strength,and designability.The application range of composite materials has changed from non-bearing parts to secondary and primary bearing parts,which will inevitably involve the connection of composite materials.In engineering,multiple bolts are often used to connect components to transfer large loads.In general,due to the asymmetry of parameters such as materials and dimensions between composite bolted structures,the load distribution of each bolt is not uniform on multi-bolt composite joints.At the same time,due to the brittleness and low elongation of the composite material,the load redistribution ability is poor.The non-uniformity of load distribution will affect the damage behavior and characteristics of bolted joints,resulting in damage to the bolt holes with larger loads,even when the bolt holes with smaller loads are far from damaged when they are damaged,thus seriously reducing the connection structure transfer efficiency.Therefore,the bolt load distribution and damage mode of the multi-bolt composite joint structure are studied,the bolt load distribution rule is found,the connection structure is reasonably designed to achieve the maximum load transfer efficiency,and the bearing efficiency of the composite connection structure is improved,and the fatigue life of the connection structure is extended.It is of great significance to give full play to the advantages of composite materials.The main work of the thesis is as follows:Based on the traditional spring method and finite element theory,an improved spring method is proposed,which considers bolt-hole clearance and friction effects.Taking the composite single-lap and three-bolt connection structure as an example,the system stiffness matrix and the system equivalent load vector are derived,and the matrix form of the equivalent equation based on the improved spring method is obtained.Taking the carbon fiber composite material and aluminum alloy double overlap seven-bolt connection structure as an example,the 2D and 3D finite element models of the structure are established.The comparison between the calculation results of the improved spring method and the finite element simulation results shows that The maximum relative error between the calculation result of the improved spring method and the three-dimensional finite element calculation is 8.5%,and the maximum relative error from the test result is 10.7%,which verifies the effectiveness and accuracy of the improved spring method.Based on the improved spring method and finite element method,the influence of structural parameters such as geometric parameters,overlap form,and fit form on the joint load distribution of the carbon fiber composite and aluminum alloy double-lap five-bolt structure are analyzed.Meanwhile,in order to objectively and accurately measure the load distribution unevenness of a particular multi-bolt composite joints structure,this paper proposes a method for calculating the load distribution unevenness based on the concept of the mean squared dispersion coefficient.The research results show that: increasing the thickness and width of the laminate can make the load ratio of each bolt more uniform;reducing the diameter of bolts can promote the uniform bearing of bolts.Then based on the above analysis,a uniform load distribution strategy for bolted structures is obtained.The progressive damage analysis of the multi-bolt composite joint structure is performed.First,the three-dimensional Hashin criterion and the Tserpes criterion are selected as the damage failure criterion and the material stiffness degradation criterion of the composite,respectively,and the final failure judgment criterion of the joint is determined.Progressive damage analysis process and model for multi-bolt composite joint structures is established.Based on this,The effect of facetors,such as geometrical..dimensions.(end..distance/diameter,row..distance/diameter,edge.distance/diameter),pre-tightening force,and layup sequence on the ultimate load and damage failure of the structure is studied.