Fundamental Research On Deformation Damage And Unerring Fracture Of Plastic Hinge Of 7050 Aluminum Alloy

With the rapid development of aerospace industry,the requirements of design reliability become higher and higher.It is crucial to use hinge for the design of fairing in aerospace structure,the hinge is simple though,it needs to have large carrying capacity and high operational reliability.The fewer and simpler of action components in aerospace structure are,the higher of the using reliability is.On the condition of rational design,using plastic hinge can increase the safety margin of aerospace structure,save materials,and increase service reliability.Therefore,it is important to research the unerring rotation and switch-type fracture for plastic hinge in aerospace structure.In this thesis,based on plastic deformation theory,damage mechanics,and fracture mechanics theory,the issues of damage and fracture in deformation process of materials were fundamentally studied by using theoretical derivation,experiments,and finite element method(FEM).Firstly,the alternative materials of plastic hinge in aerospace structure were studied using Hollomon,Swift,and Voce constitutive equaitons,and then the characterization parameter and the different descriptions in different coordinate systems were proposed to predict the working hardening behavior in different stages of tension deformation.Secondly,based on continuous damage mechanics and fracture mechanics,the competitive ductile-brittle fracture was proposed to study the effect of stress concentration factor on fracture mechanism in tension process and fracture behavior of deformation by adopting uniaxial tension experiments and FEM of notched specimens.Thirdly,regarding the complex multiaxial stress state,the modified Mohr-Coulomb elliptical fracture criterion was proposed to clarify the effect of?on fracture loci and to predict the fracture forming limiting diagram of 7050 high-strength aluminum alloy.Then,the Hill criterion and modified elliptical fracture criterion were incorporated to characterize fracture behavior of anisotropic plate and to study the effect of plasticity anisotropy on the fracture loci and fracture froming limiting diagram of 7050 aluminum alloy.At last,the plastic hinge of aeroscpace structure was modeled to reveal the influence law of notch radius and notch type and to analyze the realiability by using the three-point bending experiments and simulations of notch bending and groov bending specimens.The working hardening behavior,deformation damage ability,and fracture characteristics of alternative materials(carbon steel,stainless steel,aluminum alloy,titanium alloy,and nickel alloy)of plastic hinge were studied,the characterion parameter?_P and different manifestations of predicting working hardening behavior in the different stages of tension deformation are proposed.It is found that Hollomon and Swift equations have unsaturated characteristics,and Voce equation has saturated characteristic;the S-V equation can be used to describe the tensile stress-strain of nonuniform deformation,such as necking,by linearly combining Swift equation and Voce equation;the predictive accuracy of yield strength is better to use piecewise fitting than overall fitting for Hollomon,while it is opposite for Swift equation;the predictive accuracy of ultimate tensile strength by using Voce equation is higher whether it is overall fitting or piecewise fitting,the predictive error is in the range of±5%.For the deformation damage process of materials,the ductile-brittle competitive fracture criteria were proposed based on R-T model and elastic strain energy density.Then,the experiments and simulations of 7050 aluminum alloy specimens with different stress concentration factors were carried out to analyze deformation damage and fracture process and quantitatively describe the the relations between average stress triaxiality,Lode parameter and elastic strain energy density,deformation damage.It is found that the intrinsic characteristics of materials(fracture toughness,elastic modulus,and poisson's ratio etc),deformation damage and eometric features of structure comprehensively affect the fracture process.the tensile fracture mechanism of smooth specimen is that voids and micro defects occur first in the center of specimen and then shear fracture occurs at the side of specimen finally;the higher of stress concentration factor is,the easier it is to reach critical elastic strain energy density and then brittle fracture;it is easier to failure during-0.4 and-0.2 of average Lode parameter,and the fracture is competitively affected by ductile damage and brittle fracture.Regarding the complex multiaxial stress state,a modified Mohr-Coulomb elliptical fracture criterion was proposed to predict the fracture loci of 7050high-strength aluminum alloy under different stress and strain spaces and to get fracture forming limiting diagram.The relation between the square of intrinsic parameter?and the ratio of shear modulus to bulk modulus is derived based on the effect of volumetric and distortional strain energy density on deformation damage and fracture behavior,meanwhile,the relation between intrinsic parameter?and Poisson's ratio is obtained.Then,the critical Poisson's ratio is given as 2/7 to distinguish the toughness and brittleness of bearing deformation.It is found that the fracture locus predicted by modified elliptical fracture criterion is closer to experimental data;the asymmetry of fracture loci with the variation of intrinsic parameter?is related to the different deformation damage and fracture mechanism under tensile and compressive stress states.Regarding the anisotropy of materials,Hill48 criterion and modified elliptical fracture criterion are incorporated to characterize the anisotropic coefficents on deformation path and fracture loci of 7050-T7451 high-strenght aluminum alloy plate.It is found that the effect of anisotropic coefficients on fracture loci essentially is the effect of anisotropy on Huh parameter T;the fracture stress increases with the increase of F and G at L_e=1;the fracture stress is constant with the variation of H and N at L_e=-1;the fracture loci at 45odirection are not affected by effect of H,and fracture loci at0oand 90 directions are not affected by the effect of N;the effect of anisotropic coeffients on fracture loci is large at L_e=-1 and L_e=0,is none at L_e=1;the fracture stress at 90odirection is lowest at L_e=-1 due to the effect of G and H,and the strength at 90odirection is large though,the forming ability is very low.For the engineering application of plastic hinge,the experiments and simulation of three-point bending for the notch and groove specimens were carried out.The influence law of notch or groove radius and gap type on the failure of plastic hinge is revealed,and the realiability analysis of the bending failure of plastic hinge in three-point bending is carried out by using response function and Monte-Carlo method.It is found that the valid combination of notch and groove can completely cover all the designing range of bending failure for plastic hinge;small groove or notch radius can obtain good switch-type fracture characteristics,while large groove or notch raidus can get good rotation characteristics;the designability of rotation for notch bending specimen is bad,while the characteristics of switch-tye are obvious,it is suitable for the rotation of large angle;the designability of rotation for groove bending specimen is good,while the characteristics of switch-type is not good,it is suitable for the rotation of small angle.