Prediction And Design Method Of Mechanical Performance Of Bolted Joints Based On Damage Constitutive Model Of Aluminum Alloy

Aluminum alloy has been widely used in large-span spatial structures,owing to its advantages of lightness,high strength,corrosion resistance,formability,elegant appearance and so on.The aluminum alloy gusset joint,as one of the most regularly utilized bolted joint systems in single-layer latticed shells,has gained extensive study focus on its failure modes and ultimate resistance.Fracture,such as block tensile–shear and net section fracture of aluminum plates or bars,is a prevalent failure mode in aluminum alloy gusset joints.Aluminum alloy features poor ductility,inevident plastic deformation before fracture,as well as the abrupt fracture process.Therefore,the collapse of aluminum alloy structures cased by component fracture may lead to serious economic loss and casualties.The potential fracture regions are in three-dimensional complex stress state.At present the research of mechanics behavior of aluminum alloy material are mostly confined to the theory under uniaxial stress state,which cannot reflect the real characteristics of materials under the three dimensional stress state,leding to the huge deviation between predicted ultimate bearing capacity and test results and great safe hidden trouble.In this paper,the experimental research,theoretical analysis and numerical simulation are carried out from the three levels of aluminum alloy materials,connections and joints,in order to study the plastic evolution law and ductile fracture characteristics of aluminum alloy materials,and establish the bearing performance design method of aluminum alloy bolted systems based on refined finite element models.Firstly,plastic and ductile fracture properties of 6061-T6 extruded aluminum alloy is investigated.Notched bars,grooved plates and shear plates with varying detailed configurations are designed to obtain the full range force-deformation curves via monotonic static tensile tests.By comparison with test results and simulation rerults based on Mises yield+Swift hardening,the MBW yield model+piecewise power hardening is proposed.This plastic constitutive model is used to simulate these material specimens under different stress states,so as to obtain the evolution of stress state variables as well as equivalent plastic strain of crack initiation element.The MMC fracture criterion considering triaxiality and Lode angle dependence is used to describe its softening surface,which is calibrated based on the extracted average triaxiality and Lode angle and equivalent plastic strain at softening initiation of the critical element.The damage-induced softening model is introduced to consider the stress reduction caused by the development of plastic damage,where the rate of damage evolution is related to the softening strain.This softening model is calibrated based on testing specimens under different stress states.Based on the comparison of loaddeformation curves and fracture modes between experimental and finite element simulation results,the MBW yield criterion and piecewise power hardening rule as well as MMC fracture model considering damage softening are validated in characterization of the plastic evolution and ductile fracture of 6061-T6 extruded aluminum alloy under complex stress states.Secondly,anisotropic plastic propertis of 6061-T6 aluminum alloy rolled sheet is studied.24 types of specimens are designed considering different geometric configurations,material orientatons and loading directions.The plastic strain ratio is measured by uniaxial tensile test of smooth square bars extracted along the direction of0°,45°,90° to the rolling direction.It is proved that the plastic volume of this aluminum alloy changes with the development of plastic strain,which overturns the traditional ideal assumption of plastic incompressibility of metal.Based on the experimental results,a new orthotropic yield criterion is proposed,which can fully characterize the plastic properties of aluminum alloy materials such as transverse isotropy(in-plane isotropy while out-of-plane anisotropy),tension /compression anisotropy,and hydrostatic pressure independence.The anisotropic ductile fracture behavior of 6061-T6 aluminum alloy sheet is further investigated after the anisotropic plastic properties is fully characterized.From the load-deformation curves and element softening strain under different stress states,it can be seen that its ductile fracture exhibits significant anisotropy both in plane and outside plane.To describe these features,a new anisotropic fracture model based on the direction of maximum principal stress is proposed.The orientation of principal stress can be specifically determined by the geometrical configuration,material orientation and loading direction of these material specimens.Hence,the parameters in this fracture model can be calibrated directly and simply.By comparing with the test results of specimens with different geometric structures and different material orientations,it is validated that the finite element model based on this model can accuratly predict deformation,damage indicator at softening initiation and final failure mode.Moreover,single bolt double shear connection is investigated.Within the design range of edge and end distance stipulated by European regulations,9 kinds of representative specimens with different edge and end distance are designed and tested.Based on the newly proposed anisotropic plastic evolution and ductile fracture model,a refined finite element model is established for these bolt connections.Compared with the experimental results,the accuracy of the refined finite element model in the prediction of full range load-deformation curves and failure mode is validated.Based on the validated finite element method,47 finite element models is established.Then,the influence of crucial parameters on ultimate bearing capacity is analyzed.The concept of failure mode mape is introduced.The 47 finite element simulation results are classified according to the failure modes.Accordingly,the design formula of bearing capacity are proposed for bearing failure and net cross section failure.Compared with the European code,the formula proposed in this paper is more accurate and reasonable in the consideration of ductility reserve of bearing failure and suddenness of net section failure.Afterward,the resistance of aluminum alloy gusset joints under monitonic tensile loading are studied by means of experimental,numerial and theoretical analysis.Two groups of four joint specimens are designed and tested,considering different bolt arrangement and joint plate diameter.The main failure modes is the fracture of I-shaped Aluminum bars,accompanied by local buckling of the web in the connection domain.The fracture modes of the flanges are different: block tensile–shear fracture in the small plate(SP)group,whereas net-section fracture in the big plate(BP)group.The traditional simulation method without considering fracture cannot predict the final fracture path and ultimate resistance.In order to accurately simulate the full range loadelongation response and the crack initiation and propagation,this paper introduces the Modified Mohr-Columnb(MMC)ductile fracture criterion with damage-induced softening,which is implemented by VUHARD and VUSDFLD user subroutines embedded in ABAQUS/Explicit.Compared with the experimental results,the accuracy of the refined finite element model in prediction of web buckling,full range loadelongation curve and final fracture path is validated.This research framework,particularly the fracture model and its detailed calibration procedures,have a broad prospect of applications for evaluating the resistance of aluminum components that are governed by various ductile fracture modes under monotonic loadings,such as the rupture of bolt connections and block tearing of AAG joint plates.Finally,this paper summarizes the research results and puts forward the future research direction.