铝合金5052材料表征(英文版)

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1、Chapter 3 Material model and material characterization of automotive aluminium alloy 5052H343.1 preface The basic property of material decides the use function of the structure.in the research of car crashes and large deformation of key components,the key point of solving the problem is to build cor

2、responding physics-math model in order to acquire the specific material parameters.As for the automotive aluminium alloy in our research ,we need to consider the response characteristic of the material machanism on condition of real impact (such as dynamic loading,high-stress condition,including pri

3、mary defects or damage,etc) .By this way, numerous performance test and full-size componnet crash test need to be conducted ,which would exhaust great labor power and physical resources,but the result is still dubious.With the limited discret data obtain by the experiment,it can hardly cover the spa

4、ce of all solution of the model.For some extreme condition such as shearing stress condtion, high strain rate(12102),it hard to predict its machanism response. Therefore,this model we built has its comparatively large limitations.But combine with numerical simulations,it can expand the space of the

5、model effectively,while obtain complete material parameters. The result shows that the precision of numerical simulation is largely depend on the precision of material parameter to input. In the analysis of large deformation,mater-ial machanism must describe the material change from elastic deformat

6、ion,elastoplastic deformation,damage progession,evolvement,fracture,which is multi-step machanism procedure. Besides,various strain rate,anisotropic caused by processing history,etc need to be take into account to consider its affection with constitutive relations. Previously,this is a extremely com

7、plex project,where many problems is still unsolved. J.G.Blauel and D.Z.Su,etc simply used tensile bar experiment combined with finite element to simulate the characterization of AW6016 automotive aluminium alloy and crash test of the typical components in automobiles.It predicts the static and dynam

8、ic response of the aluminium alloy and key components in macro perspective,and expand G-T-N model in micro perspective,which explain the ductile fracture of the aluminium and its initiation and evlovement by machanism of growth of voids,that could be applied in engneering calculations.Franck Lauro a

9、nd Bruno Bennani built dynamical loading machanism model on finite-element calculation terrace.This model take elastic flow and anisotropic of micro holes damage evolvement into account.Thereby this could be used to predict situation of damage and fracture of materials with obvious texture deformati

10、on such as extruded aluminium alloys in circumstances of dynamic loading with large deformation. The other one contribution of Franks research is to develop iterative algorithm which is a experiment combine with numerical simulation to achieve solving and correct the key parameter in material model.

11、This method uses notch tensile data as the basic input,which can derive parameter of G-T-N damage model, and revise these parameters in consider of anisotropic factors.Eventually,the result of numerical simulation hit off with experiments.In compare with other material characterization method ,this

12、could avoid a great amount of experiment expenses,and improving the efficency of modeling rate. On the other hand,this method does not consider the changes of damage machanism of the materials under lower stress condition.The result proved that when three dimension stress degree is below 1,with the

13、reducing of stress degree,shear slip failure would take the dominent position, and damage of void type will be supressed. So,to adopte single damge model would limit its applicability. This chapter is trying to conduct material characterization,acquire fracture parameter of the material under simple

14、 tension,notch and shear stress,to carry out tensile test in condition of static,quasi-static and dynamic loading,and numerical simulation,furthermore,gain machanical property under different strain rate.Two kinds of damage model were built in this basis,compare the precision of model and sphere of

15、application of these two,to describe the damage and fracture behaviour in large deformation from macro phenomenon an micro machanism.3.2 Research of machanical property test of material under static and dynamic loading.3.2.1 Basic introduction of the experiment1.overall thinking of the experiments.

16、As for the automotive alminium alloy 5052H3.4,it is mainly used in weld production in bearing weight component of car-body,in actual collision design,these structure would mostly have large elastic deformation before failure.Therefore ,its not enough for material characterization only considering to

17、 satisfy strength demand,the rheological future of material in large elastic deformation,such as work hardening,anisotropic and facture features of primary damage and evolvement,etc.While these machnical properties are tied up with inside factors such as microstruture,distribution,and outside factor

18、s of working temperature,loading rate and stress condition.As for specific aluminium alloy,its organization structure is fixed,but the work temperature and loading condition varies with the actual impact process,so we need to conduct a series of sysmatical experiments on characterization of this kin

19、d of aluminium alloy.Take research in machanical response under static and dynamic condition.On this sense,this is a project,which needs particular category and unify requirements.2.Basic information of specimens In accordance with the actual impact condition,we need to take static and dynamic loadi

20、ng machanical property test in aluminium alloy 5052H43,the experiment was divided into five groups,which is smooth tensile test,notch tensile test,double notch shear test,Iosipescu shear test,dynamic tensile loading test.Serial number of the test specimens and experiment condition are show in Table.

21、3.1,sizes of the specimens are show in Fig.3.2,the specimens are cut from original plate by using (电火花技术),the distribution of all the specimens on plate can be seen on Fig 3.1. As for the serial number of the specimens ,taking PT1-UT-S1Q as an example for explanation:3.2.2 Static uniaxial tesnile te

22、st3.2.2.1 experiment equipment Uniaxial smooth tensile test is one of the basic machanical property test,which is mainly used to measure flow curve of material and strength index.The primary aim of this test is to measure the actual stress-strain curve and fracture parameter,in order to provide with

23、 material property for following numerical simulation.So as to expand the scale of the test data,obtain practical data as much as possible,the experiment adopted two sets of equipments.one is the strength-displacement detect system,which uses extend-measurement displacement,also loading sensor to me

24、asure load,which is owned by Instron 1886 tensile test machine. Another set of machine is optical deformation measure system,including static camera,digital transition card and following image processing software and diagram calculation softeware.The system is equiped with two sets of digital camera

25、, the shooting angle is seperatively perpendicular to front side and lateral side of the specimens, distributted by 90 degrees.Therefore it could detect changes in width and thickness direction immediately when the specimen is in the tensile process,with dynamic image detecting to acquire changes in

26、 specimen section,so we could calculate the minium section size Ai ,according to the formulation in figure3.2 to calculate the actual stress ,strain in tensile process,which could get stress-strain relation of the tensile specimens after necking. Fig3.3 is the schematic chart of the experiment equip

27、ments.These two sets of equipments works collaboratively.The common stress-displcement is mainly responsible for recording of whole course loading detection and elastic deformation and partial ductile deformation(before maxmium loading),while the optical detecting system aim at recording of detectio

28、n of elastic and ductile(before fracture) deformation. We can see from Chaper 2.3.3 that the true strain of aluminium alloy 5052H34 is 9.6% correspond to static maxmium tensile load at room temperature,and fracture strain f is 73% at fracturing,at this time,these data segement is undetectable by usi

29、ng ordinary tensile experiment,and there will be severe error in the coming up numerical simulation. By contrast,segment data after necking can be repaired if optical detecting system is used,and the maxmium true strain can be measured up to 56.1%.The measure problem of stress-strain curve can be fi

30、xed by linear interpolation.Comparison of Curves of specific materials and precision of numerical simulation will be discussed in this chapter in related diagrams.3.2.2.2 Discussion1 Relationships of stress-strain The true curves by two sets of different kinds of detecting system can be show in Fig3

31、.4,we can see that the data scale of optial deformation system covered wider,while the complexity and costs of this detecting system is relatively higher.2 Material characterization of relativity of anisotropic and stress condition Specimen of smooth tensile test is cut from the direction of vertica

32、l and parallel to the rolling direction,the engineering stress-strian curves can be seen in Fig.3.5.From the result,the experiment displayed excellent repitiveness and stablity of data recording.Curves react that vertical yeild stress is 22Mpa higher than parallel yield stress,tensile strength is 26

33、Mpa higher,which demostrates particular anisotropic.The size and distribution of grains is regular from further research in microstructure.First,grian sizes of the suface is larger than center grains (Fig.3.6). sec-ondly,there are certain grain orientation in the center of the material(Fig.3.7).Part

34、 of the grain in the axis along the rolling direction is obviously larger than the other two axises,which proved that grain refinement along the thickness direction varies in rolling process.In the following aneal and artificial aging,there are certain preferred orientation in the refined grains aft

35、er grain growth or recrytalization,and these factors lead material with certain anisotropic. Fig.3.8 compares smooth tensile test with notch tensile test,the notch radius is 4mm,the test showed that the deformation resistance increased at notch condition.With the same deformtion,load of smooth test

36、is lower.Tri-axiality is bigger at the foot of notch under geometry restraint.In the simple tensile process,larger additional stress derived from the thickness and width direction,and then tri-axis tensile stress forned.Effective shear stress or equivalent Mises stress that ductile deformation neede

37、d decreased,which would lead to ductile difficulty.From the mcro aspect,minish the notch radius,stress tri-axiality of component increased,stress concertration increased at the foot of the notch,ductilty decreased,and fracture strain decreased too.The engineering strain of the notch specimen is 4.19

38、%,which is only 35% of the engineering strain of smooth tensile specimen. From the experiment analysis,its clear that stress condition is not related to yielding obviously,but it matters badly to losing effectiveness of fracture.3.2.3 Dynamic tensile test3.2.3.1 Experiment equipment and measurement

39、principle Car accidents mostly happens at the velocity of 50Km per hour, the impact kinetic energy of mid-weight car can be up to 150KJ.And the strain rate of these similar impact accidents is about 100s-1 ,but the limitation of local strain rate is within 1*10-3.Mostly,the strain of dynamic test is

40、 in the scale of 102103s-1 on the basis of safe impact design. Currently,the applied test equipment are as follows:1 Falling weight device Different heights of falling hammer acquire various kinetic energy, experiment data are collected and processed by using signal processing device such as oscillo

41、graph.Advantages of such devices are simple structure,easy operation and low costs.It fits impact tests of the level of 10m/s. Previously, there have been such devices such as Tinghua university;State Key Laboratory of Auotmotive Safety and Energy and Haribin institute of technology throughout the n

42、ation.2.Ejection experiment devices There are vertical type and horizontal type,it gains great speed by exploded high energy ballon, the specimens are fastened on data acquisition board with piezoelectric sensor,specimens are impacted with hammer loaded with different masses,while record the impact

43、process with high-speed camera.Advanced digital shooting technology can acquire video image of 10000 frames per second,and the frequency of impact data reach up to 200kHz.At present,all the advance insitute are capable of this test,such as Cambridge,Kaiserslautern university and Fraunhofer IWM macha

44、nical institute ,who use this device to test impact responses of components.3.Improved Hopkinson draw(press) poles test devices This method is first proposed in 1914,its the earliest method of testing transient state pulse stress.lately,many researchers changed and developed this method,in which the

45、 improved discret Hopkinson draw poles by Koisky is the most typical one,that has become major testing tool of dynamic property of material.This kind of experiment is taken in metallic large defomation dynamic machinsm respose of strain rate within 102104s-1. The key point of Hopkinsons testing tech

46、nology is to assure that separate the stress wave effect with dynamic buckling effect within the impact load.Therefore,the incidence pole and test pole of the test device are designed to be longer,in order to sepperate the infunce from reflex waves of press pole,so the equipment would take much spac

47、e. On the other hand,due to the strict restriction of specimen size,which could make sure ductile stress wave in the component take small proportion in structure buckling response time,compress stress is well distributed in components. Key physical qualities need to be determined in this experiment

48、are as follows.(1) input response of impact load(2) response,value and distribution of structural load(3) response,value and distribution of structural displacement(4) strain rate of material,the key formula are as follows:process of impacting load (3.1)Course of input and output loading response of

49、 components (3.2) (3.3) Course of input and output displacement response of components (3.4) (3.5)A1 and A0 represent section surface of input pole and output pole seperatively,E represents elastic mudulus; I(t) is course of input strain andR(t) is the course of response strain. Since the disadvanta

50、ges of complexity and bulky of Hopkinsons test device, Nicholas, Ramesh, Franz, Ellwood,many researcher have improved it in recent years,such as repalce incident devices with concussive bullet shoot by high-pressure gas gun,which reduced the volume fo device greatly,increased the flexibility,which m

51、ade this effective dynamic loading test method popular.4.The improved ejection test device in this research Considering the effectiveness and convenience of dynamic tensile experiment,making full use of previous equipment,a new particular type of dynamic tensile device-Instron VHS dynamic tensile te

52、st machine in Fig.3.10 was used in this research.Components of such experiment system can be show in Fig.3.9,including three significant part.(1) Executive part of loading.Pneumatic chuck is the key part,Fig.3.11 shows the prepare condition of the part before testing,the upper is tied on the fixture

53、,well the below is connected with following system.When the below fixture of testing machine reaches certain speed downside driven by high-energy gas,follow up system would fasten fixed part of the specimen below,which could make the down fixture move along with specimen below synchronously. (2) Zim

54、mer laser-transient dispalcement high speed photography and force testing part. When lower clip tied up with specimen to move downward synchronously,which would trigger the laser sensor at the same time,and then turn on the high-speed camera,which would record deformation and dispalcement of the spe

55、cimen with the scale distance.Sampling frequency of shooting is 2500fps.Analyze relationship between dispalcement and time of scale boundery by means of later digital image processing.Comparison of calculation error and data of local foil gauge is smaller than 5%.Measurement of tensile load via acce

56、leration sensor to mutiply instantaneous acceleration by quality to get changes of system dynamic load.Formula 3.6 shows the calculation in details (3.6)Where P(t) is instantaneous tensile load,M represents movable chuck component,g is acceleration ,a(t) is instantaneous acceleration.Accordingly,who

57、le deformation energy and impact energy can be derived by intergal calculus of compounded load-displacement.(3) data collection system.It includes digital oscillograph, later treatment part of computer.Digitalize the amplified signal,with filtering waves,lastly,digital documents will be stored in co

58、mputer.The filtering process adopte high-frequency filtering circuit and spline fit smoothing technique of ultimate data.The advantages is that sorting of wave characteristics can be arificially controlled, in accordance with various experiment condition to choose filter function,which can avoid dat

59、a distortion inn filtering by means of adjusting smoothness. Common spline function is demostrated in formula 3.7 (3.7)fi-1, fi, fi+1,represent function value of i-1,i and i+1. ft represent the function value after t times filtering,this fuction can be iterated endlessly till the result is satisfied

60、.5 Size of specimen and initial conditions Specimen size is as same as the static tensile one,orientation of specimens is perpendicular to rolling directions. Install foil gauges symmetrically on two sides of the specimen within the measure scale before experiment,so as to adjust twist error in dyna

61、mic tensile test.Meanwhile print glisten glue of light colour in the center of test zone,which is used to high-speed photography.Finally fix the whole specimen in the tensile fixure,and adjust laser trigger.3.2.3.2 Experiment result Relationship among engineering strain,initial tensile velocity and

62、scale distance in dynamic tensile test can be demonstrated as, (3.7) In this experiment,its certain that initial velocity of fixure v0 is 1.7m/s,scale distance is 10.08mm.Average strain rate is 169s-1,according to formula 3.7 at tensile condition.Fig3.12 shows the curve of load-dispacement at dynami

63、c tensile condition.Fig3.13 demonstrate the relationship among engineer-stress,strain and true stress-strain.Fig.3.14 shows the comparision between dynamic load and static true stress. Fig.3.15 is the comparison of engineer stress-strain measured by static notch tensile ,smooth tensileand dynamic lo

64、ad smooth tensile test.Fig.3.16 shows that static and dynamic strength of material and plasticity index vary with strain rate.The data imply the slight improvement of the aluminium alloy strength with the strain rate increase from dynamic to static tensile,by contrast, the fracture strain increase b

65、y 60%,shrinkage of section increase by 18%,which means the ductility is enhanced.High-speed deformation is done in short time in dynamic loading test,a lot of heat is generated and unable to consume,so it could be treat as a non-exchange heat process with the outside.So temperature of local material would rise dramatically,machanism response of material has become a heat-stress coupling problem.On the basis of document82,83,as for Al-Mg-Si alloys,work hardening and dynamic softening are concurrence under conditi