New Characterization Methods for Mechanical Properties. S. Hoffmann; Department of Ferrous Metallurgy (IEHK), RWTH Aachen University

Transcription

1 New Characterization Methods for Mechanical Properties S. Hoffmann; Department of Ferrous Metallurgy (IEHK), RWTH Aachen University

2 Introduction Characterisation methods Tensile tests Local strain measurement Infrared thermography Confocal microscopy Compression tests Determination of the compression flowcurve Bulgetest Determination of the biaxial flowcurve Results of mechanical testing 2 Outline

3 Typical Approach: Characterization of a standardized sample (eg. DIN EN 10002) Measurement of force and elongation Assumption 1: homogenous stress and strain in the sample Assumption 2: temperature constant and/or influence neglible Observations on high manganese steel: Inhomogenous strain distribution Locally increased temperature 3 Tensile test on high manganese steels

4 Standard evaluation methods in the typical approach: Measurement of force Measurement of elongation and necking in the parallel length with a videoextensometer Calculation of stress and strain (resp. true stress and true strain) temperature measurement on three sample spots (only for T RT) Problem: Tensile test delivers only global information! 4 Tensile test on high manganese steels

5 PLC-Effect (Shear Bands) Also leads to problems with computer aided flowcurve determination True strain will often be underestimated Not centered fracture of the sample due to shear band interaction High notch sensitivity of the material Low yield strength and exceptional hardening Leads to high deformation in the sample head Conclusion: Measurement of localized phenomena is necessary 5 Challenges in testing high manganese steels

6 Additional characterisation methods: Localized strain measurement by photogrammetic methods Localized temperature measurement by infrared thermography Local observation of the microstructural determined surface effects by confocal microscopy 6 Tensile Test on high manganese steel

7 Introduction Characterisation methods Tensile tests Local strain measurement Infrared thermography Confocal microscopy Compression tests Determination of the compression flowcurve Bulgetest Determination of the biaxial flowcurve Results of mechanical testing 7 Outline

8 Localized strain measurement by photogrammetic methods Vialux Autogrid strain analysis 4 Cameras with 1366*1024 pixels resolution Video and/or single image analysis Sample or structural component analysis 1 or 2 mm gridsize 8 Vialux Autogrid Strain Analysis

9 Photogrammetric strain evaluation shows severe difference in plastic behaviour of standard austenitic materials and high manganese austenites. The PLC-Effect leads to deformation bands running through the material. 9 Localized Strain analysis

10 Localized deformation leads to inhomogeneous cross section reduction Visible differences in the width of the parallel length at uniform elongation (measured 1.8% width reduction difference, A 80 sample) Where to determine the stress at uniform elongation? 10 Effects of inhomogenous strain distribution

11 The strain analysis shows that each volume element sees a different straining history. Localized plastic deformation implies localized friction by dislocation glide, TRIP or TWIP-Effect and thus localized heating of the material. Additionally heat transfer is 11 Strain analysis

12 Introduction Characterisation methods Tensile tests Local strain measurement Infrared thermography Confocal microscopy Compression tests Determination of the compression flowcurve Bulgetest Determination of the biaxial flowcurve Results of mechanical testing 12 Outline

13 Localized temperature measurement by infrared thermography High speed, stirling cooled infrared thermal camera Resolution at 10kHz : 192*1 pixels (line mode) Resolution at 200Hz: 320*200 pixels (full frame mode) -10 to 600 C in different overlaped calibration ranges 13 Jade CEDIP infrared thermography

14 Parallel to the strain measurement the determination of sample temperature distribution shows higher temperatures in the deformation bands. 14 Localized temperature measurement

15 15 Temperature elevation during deformation Temperature, C metastable, 1/s, 12.7% Martensite metastable, 100/s, 11% Martensite stable, 1/s, 0.5% Martensite stable, 100/s, 0.6% Martensite ,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 true strain, -

16 Standard tests do not take the adiabatic heating into account: "isothermal" tensile test standard tensile test High manganese TRIP-steel, hot rolled condition 16 Quasistatic Tensile Test of HMS (standard and isothermal)

17 Introduction Characterisation methods Tensile tests Local strain measurement Infrared thermography Confocal microscopy Compression tests Determination of the compression flowcurve Bulgetest Determination of the biaxial flowcurve Results of mechanical testing 17 Outline

18 Local observation of the microoutline evolution by confocal microscopy Confocal Microscope µsurf Explorer custom 18 Confocal Microscope

19 α µm Characterisation of micrographs and identification of different phases due to different etching behaviour α pixelcount α height α Etched micrograph of a dual phase microstructure 100x, 160µm edge length 19 Microstructure identification by confocal microscopy

20 Effect of deformation on surface topography Experimental Procedure Topography determination by in situ-confocal microscopy during tensile test Stepwise deformation and measurements of surface topography in each step Time required: approx. 60s for 25600µm² (160*160µm)

21 Evaluation of the microstructural changes of TRIP- and TWIP-HMS during deformation. 0% 12.5% Inspection of the polished surface with confocal microscopy and description of the surface evolution depending on the deformation. 27.5% TE Problems encountered: Unevenness at higher strain covers surface characteristics of twinning and martensite formation 21 Microstructure evolution

22 Introduction Characterisation methods Tensile tests Local strain measurement Infrared thermography Confocal microscopy Compression tests Determination of the compression flowcurve Bulgetest Determination of the biaxial flowcurve Results of mechanical testing 22 Outline

23 compression tests determination of flow curve Rastegaev-geometry Lubrication: C: Boron-nitride C: Glass Processing: Isothermal compression tests Rastegaev cylinders Lubrication Temperatures (700 to 1200 C) Constant strain rates (0.1, 1, 10 and 100 1/s) Samples of I: Mn23C0.3 (V16), II: Mn23C0.6 (V15) Calculation of stress (k f ) and strain 23 before test after test Compression tests (Partproject B2)

24 Introduction Characterisation methods Tensile tests Infrared thermography Local strain measurement Confocal microscopy Compression tests Determination of the compression flowcurve Bulgetest Determination of the biaxial flowcurve Results of mechanical testing 24 Outline

25 Erichsen Universal-Sheet-testing-machine 145/60: Punch-Force: up to 600 kn Sheetholder-Force: up to 400 kn Punch-Speed: mm/min Shape Bulge Analyzer Hydraulic Bulgetest

26 Schematic Setup of the Bulge Analyser Side view 26 Flowcurve Evaluation

27 Laserline and cameraimage during forming: Continuous evaluation of pointgrid and laserline with determination of 3D coordinates (x,y: camera, z: laser) Determination of strain and stress Output Data

28 Introduction Characterisation methods Tensile tests Local strain measurement Infrared thermography Confocal microscopy Compression tests Determination of the compression flowcurve Bulgetest Determination of the biaxial flowcurve Results of mechanical testing 28

29 Alloy I (TRIP), flowcurve from tensile test true stress, MPa ,00 0,10 0,20 0,30 0,40 0,50 true strain, - 29 Tensile Tests

30 30 true stress / strain hardening rate, MPa Alloy I (TRIP), flowcurve and hardening curve d Considère Criterion: dε 0 0,1 0,2 0,3 0,4 0,5 true strain, - Considère-Criterion as point of instability not present or measurement artifact? Tensile Tests (Strain Hardening) σ w = w σ w

31 true stress / strain hardening rate, MPa Alloy I, Hardening and true stress- true strain curve up to fracture ,1 0,2 0,3 0,4 0,5 true strain, - 31 Tensile Test (Strain Hardening 2)

32 true stress / strain hardening rate, MPa Alloy I, strain hardening and true stress- true strain curve up to fracture SFB_D1_Zug SFB_D1_Ver SFB_L3_Zug SFB_L3_Ver ,25 0,27 0,29 0,31 0,33 0,35 0,37 0,39 true strain, - Tensile test (Strain Hardening 2), magnification

33 2000 Alloy I, flowcurves from compression test, strain rates 0,1/s to 100/s true stress, MPa ,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 true strain, - Compression Tests V16_0,1_K1_Druck V16_0,1_K2_Druck V16_1_K1_Druck V16_1_k2_Druck V16_10_K1_Druck V16_10_K2_Druck V16_100_K1_Druck

34 hardening rate strain hardening of Alloy I (TRIP) in compression test V16_0,1_K1 V16_0,1_K2 V16_1_K1 V16_1_K2 V16_10_K1 V16_10_K2 V16_100_K1 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 true strain 34 Compression Tests (Hardening)

35 1.600 Industrial Fe22Mn0,6C-Alloy, flowcurves from bulge test FeMnC (tensile test) true tress, MPa (bulge test) ,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 true strain, - 35 Bulgetest

36 Is the Considère-Criterion valid for high manganese steels? While the total elongation of high manganese steels exceeds the elongation of conventional steels in tensile test the biaxial elongation is not as high as expected. Is this a measurement artefact or the characteristic material behaviour? How are the mechanical properties influenced by strain rate and stress state of the test? 36 Questions induced by our results

37 Thank you for your attention "Steel - ab initio; quantum mechanics guided design of new Fe based materials" The authors gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Center (SFB) 761 "Steel ab initio".