Experimental And Numerical Study On Static And Dynamic Mechanical Properties Of Composite Joint

Fibre-reinforced composites have specific high strength,high stiffness and excellent anti-fatigue properties,which provide convincing mechanical performances for the lightweight structures.The parts of composite materials are usually assembled by mechanical joint and adhesive joint.Due to the anisotropy of composite materials and the stress concentration at the joint area,the composite joint becomes the weakening part of the composite structure.Therefore,the design of the composite joint is the critical foundation for the large-scale application of composite structure as the main load-transferring structures.Up to now,most of the researches were mainly focused on the in-plane static and dynamic tensile properties of composite joints experimentally and theoretically.However,the investigations about the response of composite joint under out-of-plane static and dynamic loads are limited.Out-of-plane static and dynamic loading,coupled with in-plane and out-of-plane properties,make the damage mechanism of composite joints more complicated.The main contents of this paper are as follows:(1)A coupled nonlinear damage constitutive model for composite materials was developed.The nonlinear characteristics of woven composites found in the experiment were studied.A three-dimensional progressive damage model was established for the whole process,including damage initiation,damage evolution and element deletion.The parameters of composite materials involved in this paper were obtained by combining macro experiment and microsimulation.The accuracy of the constitutive model was verified by comparing the mechanical response and damage morphology of the tensile specimens.The constitutive model provides a basis for the subsequent research.(2)The key factors affecting the bearing capacity of the unidirectional composite joint were studied.The numerical model of the composite joint was verified,and the accuracy of the unidirectional lamination method and the quasi-isotropic macro homogenization method were compared.The sensitivity analysis of the critical interface parameters of the composite joint was carried out based on the numerical model of the more accurate unidire ctional lamination method.After the global sensitivity analysis,the six analysis of the tolerance fit parameters of the composite hole was carried out.It was found that the interference fit between the hole and the bolt could significantly reduce the pe ak stress of the composite material,thus improving the bearing capacity of the connection.(3)The damage characteristics and failure mechanism of woven composite joints under out-of-plane static load were studied.The static response of thermoplastic composite rivet joint and adhesive/riveting hybrid joints was experimentally studied by three-point bending test.The deformation mode and force-displacement response of the structure under static three-point bending are analyzed in detail.Combined with the experimental and numerical analysis,the damage characteristics and mechanism of composite joints are revealed.The influence of geometric parameters on the mechanical properties was numerically analyzed.(4)The damage characteristics and failure mechanis m of woven composite joints under out-of-plane dynamic load were studied.The dynamic responses of thermoplastic composite rivet joint and adhesive/riveting hybrid joints were experimentally studied by drop hammer impact test.The dynamic damage characteri stics and failure mechanism of thermoplastic composite joints were revealed experimentally and numerically.Besides,the factors affecting the impact resistance performance were analyzed based on the dynamic impact simulation,which guides for improving th e energy absorption efficiency of composite joints in engineering applications.(5)The rate sensitivity of woven composite joints under different loading rates was studied.The response of the joint structure under out-of-plane load at different speeds was investigated experimentally.Then,the influence of loading rate on the dynamic response of the joint structure was discussed.Response difference of the thermoplastic woven composite structures between quasi-static and dynamic compression loading was analyzed.The sensitivity analysis of stiffness and strength shows that the strength is more sensitive to the loading rate than the stiffness.Thus,only the strain rate effect of strength needs to be considered in the numerical analysis.The ultimate bearing capacity of the joint under out-of-plane dynamic load is three to four times more than the static load.