In-Plane Compression and Tensile Testing of AZ31B ...

In-Plane Compression and Tensile Testing of AZ31B Sheet and their Deformation Mechanisms Vineet V. Joshi1, Eric Nyberg1, Ayoub Soulami1, Danny J. Edwards1, Alexandra. Chua2, Curt Lavender1 1

Pacific Northwest National Laboratory, 902, Battelle Blvd, Richland, WA 99354, USA 2 University of California, Davis, One Shields Avenue, Davis, CA 95616, USA Keywords: Compression Testing, AZ31B, Sheet Metals, Digital Image Correlation, ABAQUS Abstract The anisotropic behavior of magnesium alloys or HCP structures is well-known; however the quantification of these properties in sheet product form has been inherently more difficult to determine. The present work describes a novel in-plane compression test fixture to determine the yield stress of these metals/ alloys. The strains in this sheet were mapped along the length of the specimen using digital image correlation (DIC) cameras and software. Using this test it was determined that in a 2.33 mm thick AZ31B sheet, the compressive yield stress along the rolling direction was approximately 80% of that in tension, whereas, in the transverse direction it was approximately 60% of that in tension. In order to validate the test fixture finite elemental analysis was conducted using ABAQUS to simulate the test conditions and the stress state within the sheet. Microstructural characterization and electron backscattered diffraction (EBSD) were conducted on the compression samples to determine the deformation texture, twinning and grain orientation within these samples. Introduction Magnesium and its alloys possess high strength to weight ratio and is being widely investigated as a potential material for the transportation industry as well in consumer electronics [1-4]. The anisotropic tensile and compressive strengths of magnesium/HCP sheet metals along the rolling and transverse direction are well known.[5, 6] However recent developments in the rolling and annealing process (thermo-mechanical) have helped eliminate or reduce the rolling textures (basal texture) responsible for the anisotropic behavior in the magnesium alloys. [7, 8] Recent progress in superplastic forming of magnesium alloys has made its applications all the more attractive. [9, 10] In order to successfully incorporate these new alloys/processes requires systematic investigation of the material properties to simulate the forming operations as well as service conditions of the part. This process thus requires generation of data significant for the designers. Despite the requirements and advancements in the testing technologies it was observed that there was no specific standard relevant for the compression testing of sheet metals. Testing the sheet metals in compression similar to that in tension requires significant reduction in the length to thickness ratio of the test specimens, thus limiting accurate strain measurements. Whereas slight increase in this ratio ceases the uniform load distribution across the entire gage section and results in the buckling of the sample. The ASTM E9[11] standard describes the compressive tests of sheet metals and refers to standards developed in mid-1940. These reports on the compression testing of sheet materials were developed by Kotanchik et.al.,[12] LaTour and Wolford[13] and Paul et.al.[14]. The techniques developed relied on the Presented at Magnesium Alloys and Their Applications, July 8-12, 2012 Vancouver, B.C. Canada

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use of su upporting thee sheet metaals via severral fixtures oor pack methhods. The suupported fixxtures developeed by these authors a successfully meaasured the sttrains up to 00.2%. Further increase iin the strain cau used bucklin ng in the unssupported len ngth. The usse of the pacck method developed byy Paul et.al.[14]] required a fairly large number of specimens tto provide tthe lateral suupport to prevent buckling whilst testin ng. This tech hnique also required r largge load capaacity test fram mes while teesting materialss with high strength. s In n order to ov vercome thee limitations of the aforeementioned ttest methodss it was therrefore imperativ ve to develo op a jig thaat would meeasure strainns beyond th the yield strress and dissplays versatility y to accom mmodate strrain mappin ng technologgies such aas digital im mage correllation software.. The ASTM D695-08 8[15] standaard has beeen used to determine the compreessive propertiees in rigid pllastics. This standard uses a jig as sshown in Figgure 1a to provide the laateral support for f the testin ng material. The supporrting jig usees a saw toooth/ groovedd configuration to hold the specimen to ogether. Usin ng this techn nique one caan successfuully measure strains in exxcess of 0.6% without w any damage to the fixture. In I order to aaccommodatte the DIC aapparatus intto the existing system; s one end of the sample s was machined w whilst takingg care that siignificant suupport was prov vided to prev vent bucklin ng. In the preesent work tthe existing standard jigg was modifiied to accommo odate the maagnesium sh heet materiall and the straains in this ssheet were m mapped alonng the length of o the specimen using digital imaage correlatiion (DIC) cameras annd software. The validation n of the testt was condu ucted by ABA AQUS to siimulate the ttest conditioons and the sstress state witthin the sheet. Microstrructural charracterizationn and electrron backscatttered diffraaction (EBSD) were condu ucted on th he compresssion and teensile testedd samples tto determinee the nd grain orien ntation withiin these sam mples. deformattion texture, twinning an

or compressiv ve Figure 1:: The supporrt jig used fo testing off sheet metalls

Figuure 2: Comprression test specimens aas per the A ASTM D6955 standard.[115]

Ex xperimental Setup and Procedure AZ31B A sheetts 2.33 mm in thicknesss were obtaiined from M Magnesium E Elektron, UK K. In order to perform thee compressio on tests, AS STM D695-008 standard test jig wass adopted foor the same (Fiigure 1). Thee samples were w later maachined intoo dog-bone sspecimens uusing a wire saw/ EDM (Fiigure 2) as specified s in the ASTM D695 D standaard. Using thhe current fiixture the saample can be sttrained easily y up to 0.7% % across the gage lengthh. A window w was machinned into the front end of th he fixture to o expose thee gage sectiion (Figure 1), for straiin mapping using the ddigital image co orrelation (D DIC) software. The com mpression teests were peerformed in the MTS 25 kN MTS mo odel 312.21 1 frame equ uipped with h MTS 6611.21A-01 looad cell. Thhe strain inn the compresssion tests weere measureed separately y using standdard extensoometers (25.4 mm, 634.31E24 MTS S extensomeeter) and ussing the DIIC system. The initiall sets of exxperiments were Presented at a Magnesium Alloys and Th heir Application ns, July 8-12, 22012 Vancouvver, B.C. Canadda

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performeed by attachiing an exten nsometer to the t test sampple (Figure 33). The straiin rate durinng the tests wass 5×10-4 s-1. Later L the exteensometer was w removed and the com mpression teests were connducted usinng the DIC setu up. Test speecimen weree first spray--painted whhite and specckled with bblack spray--paint drops/speeckles to creeate contrastt. DIC uses a camera that allows foor full two dimensional sstrain measurem ments. The system opticcally tracks the specklee pattern moovement durring the testt. Vic Snap, 20 010 (Correllated Solutions Inc., SC, S USA) w was used tto capture the imagess and simultaneeous load daata during th he test and Vic-2D 20009 (Correlateed Solutionss Inc., SC, U USA) was used d to analyze the t data.

Figure 3: 3 Setup used to meassure the stress strain datta using an extensometer e r.

Figuure 4: Partts of the model andd the asseembly used to validatee the tests uusing ABA AQUS

An A Abaqus-sttandard finitte element model m represeenting the coompressing sspecimen annd the supportin ng blades was w develop ped to simu ulate the deeformation process (Fiigure 4). S Sheet specimen n was mesh hed using 3D D 8-node lin near brick eelements, w whereas the supporting jjig is meshed with 3D 4-node 4 lineaar tetrahedro on elementss. Displaccement conttrolled bounndary condition ns were app plied to simu ulate the compression ttest. Materiials behavioor parameterrs are summarizzed in table1 1: ble 1: Materrials parametters Tab

Materiial

ng Youn (MPaa)

AZ31B B Mild Steel S

Mod dulus

Poissson Ratio

Yield Streess (MPa)

45000

0.35

183

180000

0.3

380

Microstructur M ral characterrization and texture anaalysis were pperformed on a JEOL 77600F field emiission gun SEM S equipp ped with a Nordlys cam mera and thhe HKL Chaannel 5 soft ftware package. The EBSD D maps were taken at 20 kv, 150x maagnification at 1 µm stepp spacing ussing a 4x4 binned patterns. Using thesee parameterss the regionss inside and ooutside of deeformation bbands were cap ptured over an a area rough hly 0.8 mm x 0.6 mm, w with roughly 8000 grains per map.

Presented at a Magnesium Alloys and Th heir Application ns, July 8-12, 22012 Vancouvver, B.C. Canadda

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Results and Discusssion 1. Mechanical M Testing: T Durin ng the comp pression testts the samples made a ddistinct poppping sound upon yieldinng. It was obseerved that thee samples haad no signifiicant strain hhardening annd thus the teests were stoopped when thee samples weere strained to t 0.5%. There was no eevidence of bbuckling whhatsoever in these samples. Figures 5a and 5b show w the stress strain curvees in tensionn and comprression alonng the rolling diirection (lon ngitudinal) and a in transv verse directioon respectivvely. It was oobserved thaat the along thee rolling dirrection the yield y strengtth (YS) wass approximaately 80% oof that in tensile, whereas in the transsverse direction the YS S was approoximately 600% of that in tensile. T These values co orrelated well with thosee mentioned d in the ASM M handbook[[16], however the source and the metho od of the tessting the sam me has not beeen describedd in the handdbook. The load and sttrain data co ollected via the convenntional extennsometer rooute and thee one collected d via the DIC C software overlapped o perfectly p (noot shown), tthus validatiing the said tests. The DIC C was more helpful in mapping m thee variation inn localized strain as a function of load. Prior to yielding/ y yieeld point thee strain distrribution acrooss the entire cross sectiion was unifform. Upon yieelding it waas observed that as the test progresssed the straain propagatted from thee end towards the center of the gagee. Distinctiv ve strain baands rangingg from 0.2% % to 1.9% were observed d (Figure 6). The largest strains weree located at th the ends of thhe gage secttion, the lenggth of which in ncreased as the tests continued. At A 0.5% oveerall strain (just beforee the end oof the experimeent), the regiion of maxim mum strain (1.9%) ( had aan approxim mate length oof 4 mm, whhereas the centeer of the gagee had a strain n of 0.2% with w a length that spannedd approximaately 8 mm.

(a) (b) Figure 5: Stress-straiin curve for the sampless tested in ccompressionn and in tenssion (a) alonng the rolling diirection, (b) transverse direction. d 2. Test T Validatio on: Finite element sim mulations of the uncon nstrained coompression ttest (no suppporting jig)) and constrain ned compresssion test were w conduccted and reesults show w that there is no bucckling happenin ng for both cases up to 0.3% strain. Unconstrain U ned test beginns to buckle for strains aabove 0.3% and constraineed sample shows s smalll contact strresses whichh do not afffect the uniiaxial behavior (Figure 7-b).


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Figure 6:: The strain distribution as a function of load viaa DIC. (a) U Uniform straiin at proof sstress, (b)-(d) deevelopment of localized strains emerrging from tthe ends of thhe gage secttions

(a)

((b)

niaxial comp pression straain contours for the connstrained sam mple (b) Coontact Figurre 7: (a) Un (samp ple/blades) stresses s distrribution, Lig ght grey ~ 0 M MPa and blaack ~ 0.01 M MPa 3. Microstructur M ral Characterrization & Texture T Anallysis: Figurre 8 shows the t compressed specimeen and form mation of defformation baands, which were perpendicular to the loading l direction. Thesee bands weree observed accross the enttire cross secction. An EBSD D (Figure 9)) analysis off the as receiived materiaal revealed a strong basaal texture paarallel to the sheeet specimen n. The longiitudinal com mpression sam mples had a slightly smaaller grain siize of 5 µm thaan the transv verse samplees (7 µm). Itt was observved that in bboth sampless the deform mation occurred via twinnin ng and was activated by y c-axis exttension in thhe > syystem. Further analysis a indiicated that th he density of o twins wass significanttly higher inn the deform mation bands thaan in the no on-deformed d regions and d that after twinning thhe basal polees of the twiinned volume were w closely y aligned witth the comprression axis . The finer ggrain size off the longituudinal samples yielded a hiigher twin density d of tw wins compareed to the traansverse sam mples. Qualittative analysis of the locall misorientattion maps in ndicated greeater strains in the longgitudinal sam mples than the transverse samples. This T may bee a directly related to the finer ggrain size, w which promotess greater twin nning and po ossibly dislo ocation slip ccompared too the transverrse samples. Presented at a Magnesium Alloys and Th heir Application ns, July 8-12, 22012 Vancouvver, B.C. Canadda

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(a) (b) Figure 8:: The formattion of deforrmation map ps, and (b) a magnified iimage showing the defoormed region

(a) (b) Figure 9: 9 (a) Pole figures f is sho own for the grains insidde a deformaation band inn the transveerse sample, while (b) sh hows the tex xture inside a deformatiion band in tthe longituddinal sample. In both casses the basaal poles in tw winned volum me are closeely aligned w with the com mpression axxis, however, there is a split texture present in th he longitudinnal sample.


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Conclusion 1. The present work successfully demonstrated the use of the ASTM D-695 test and fixture as a technique for compression testing of sheet metals. 2. Repeatability in the results and no buckling or deformity were observed in the compressed specimens (up to yield) 3. DIC analysis was helpful in determining the localized strain and correlated well with the stress strain curves generated using an extensometer. 4. The anisotropic behavior of AZ31 was successfully determined using this technique, (different than reported values). 5. The setup was validated using FEM and found to be in truly uniaxial compression. 6. EBSD measurements confirmed a strong basal texture in the sheet plane, and that 1012 1011> type twinning reactions occurred predominately inside the deformation bands. DIC indicated localized strain, which was confirmed by using EBSD, revealing a high density of twins and misorientation boundaries inside the deformation bands. . Acknowledgements The author’s would like to thank Mr. Dave Klingensmith for his help in material testing, Mr. Clyde Chamberlin for his assistance in the preparation of the metallographic samples used in the study and Elizabeth Steven for DIC equipment training and Michael Dahl and Tyler Kafentzis of PNNL for mechanical testing. Support for this work was provided in part by the U.S. Department of Energy – Office of Science (Science Undergraduate Laboratory Internship & Academies Creating Teacher Scientists), and the DOE Office of Energy Efficiency and Renewable Energy. The Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the United States Department of Energy (U.S. DOE) under Contract DE-AC0676RLO 1830. References [1] R. Gradinger, P. Stolfig, in: Magnesium Technology 2003 Symposium, 2-6 March 2003, TMS, Warrendale, PA, USA, 2003, pp. 231-236. [2] K. Johnson, Advanced Materials and Processes, 160 (2002) 62-65. [3] A.A. Luo, A.K. Sachdev, in: Magnesium Technology 2006. Proceedings of Symposium by the Magnesium Committee of the Light Metals Division (LMD) of TMS (The Minerals, Metals & Materials Society) held during the TMS 2006 Annual Meeting, 12-16 March 2006, TMS (The Minerals, Metals & Materials Society), Warrendale, PA, USA, 2006, pp. 333-339. [4] F.-K. Chen, T.-B. Huang, S.-G. Chen, The International Journal of Advanced Manufacturing Technology, 32 (2007) 272-279. [5] S. Graff, W. Brocks, D. Steglich, International Journal of Plasticity, 23 (2007) 1957-1978. [6] Y. Chino, K. Kimura, M. Hakamada, M. Mabuchi, Materials Science & Engineering: A (Structural Materials: Properties, Microstructure and Processing), 485 (2008) 311-317. [7] M. Masoumi, F. Zarandi, M.O. Pekguleryuz, Scripta Materialia, 62 (2010) 823-826. [8] M. Masoumi, F. Zarandi, M. Pekguleryuz, Materials Science and Engineering: A, 528 (2011) 1268-1279. [9] O. Duygulu, S.R. Agnew, in: Magnesium Technology 2005. Proceedings of the Symposium Sponsored by the Magnesium Committee of the Light Metals Division (LMD) of TMS with the International Magnesium Association, 13-17 Feb. 2005, Minerals, Metals & Materials Society, Warrendale, PA, USA, 2005, pp. 517. Presented at Magnesium Alloys and Their Applications, July 8-12, 2012 Vancouver, B.C. Canada

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[10] R. Lapovok, P.F. Thomson, R. Cottam, Y. Estrin, Materials Science & Engineering A (Structural Materials: Properties, Microstructure and Processing), 410-411 (2005) 390-393. [11] ASTM E09-09 Standard Test Methods of Compression Testing of Metallic Materials at Room Temperature, ASTM International West Conshohocken, PA, 2009. [12] J.N. Kotanchik, W. Woods, R.A. Weinberger, in, National Advisory Committee for Aeronautics, Washington, DC, United States, 1945, pp. 10. [13] H. Latour, D.S. Wolford, in: American Society for Testing Materials -- Meeting, Jun 27 1945, American Society for Testing Materials (ASTM), Philadelphia, PA, United States, 1945, pp. 18. [14] D.A. Paul, F. M.Howell, H.E. Grieshaber, in: Comparison of Stress-Strain Curves Obtained by Single Thickness and Pack Methods, Technical Note No. 819, National Advisory Committee for Aeronautics, 1941. [15] ASTM D695-08 Standard Test Method for Compressive Properties of Rigid Plastics, ASTM International, West Conshohocken, PA, 2008. [16] Magnesium and Magnesium Alloys, ASM International, Materials Park, OH., 1999.

Presented at Magnesium Alloys and Their Applications, July 8-12, 2012 Vancouver, B.C. Canada

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