| Multi-bolt composite joints are common in aerospace industry.Because the location of bolt-holes in mechanical joints is also prone to failure,the strength of composite mechanical joints directly affects the safety and life cycle of the overall structure of aircraft composite joints.Based on this,in order to give full play to the excellent material properties of composite materials,this paper studies the reasonable design and strength prediction method of pin-load distribution ratio of multi-bolt composite joints from the application of aerospace field.The main contents and achievements of this paper are as follows:1)Considering the bending caused by eccentric load and the friction caused by tightening moment,a strain measurement method is proposed,which can measure the pin-load distribution ratio of each bolt in multi-bolt single-lap joint.The load distribution ratio of three-bolt,single-lap joints was studied by using this method and the spring mass method considering clearance and tightening moment.The results show that the measurement method has high accuracy,simple operation and is easy to be popularized in engineering.2)Through the comparative study of the measurement method and spring mass method,the influencing factors of load distribution of three-bolt single-lap composite joints are discussed.The results show that the clearance of bolt-holes has the greatest influence on the initial pin-load distribution ratio;the tightening moment also has a more obvious effect on the pin-load distribution;increasing the clamping area(by changing the outside diameter of the gasket)can delay the strengthening of the equivalent contact stiffness caused by high pre-tightening force,and has a certain effect on the adjustment of the pin-load distribution,but the effect is not particularly obvious.3)A linear programming optimization model considering the effects of screw-hole clearance and bolt tightening moment is proposed for the uniform distribution of screw loads in multi-bolt single-lap composite joints,and the corresponding solution method is given.After optimization,each bolt-hole of the three-bolt,single-lap composite joint almost uniform load.The failure mode changes from tensile failure to extrusion failure,which improves the comprehensive mechanical properties of multi-bolt composite joints.This method is easy to understand and fast to calculate.It has important engineering application value for the design and optimization of multi-bolt composite joints.4)The application of OMLED O failure criterion and VALENZA stiffness degradation model is realized by ABAQUS user subroutine,and the three-dimensional finite element progressive damage analysis model of three-bolt single-lap joint is established.The model has good accuracy,high coincidence with test results,and accurate prediction of failure location and failure form.The stress distribution and progressive damage process of homogenized and initial parameter joints of three-bolt single-lap composite joints were analyzed by using the model.The results show that the pin-load distribution homogenization joint reduces the coupling stress of bypass load and extrusion load at the hole edge,greatly improves the stress condition of the joint,helps to improve the bearing capacity of the joint,is not easy to cause failure,and the comprehensive mechanical properties are better.5)The progressive damage model and characteristic curve method are used to predict the strength of three-bolt single-lap joint,and the effects of bolt-hole clearance and tightening moment on the strength of multi-bolt joint are studied through experiments.The results show that:(1)bolt hole clearance will reduce the strength of multi-bolt joint;(2)tightening moment will increase the strength of multi-bolt joint;(3)each bolt will be improved separately.The strength of multi-bolt joints is improved by both methods,in which the tightening moment increases the strength more obviously,and the influence of the clearance of bolt-holes on the distribution of pin-load is more obvious. |
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