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Young’s Modulus Decrease After Cold Forming in HSS
1
Young’s Modulus Decrease After Cold Forming in High Strength Steel (HSS)
Supervised By: Eisso atzema
Pascal Kommelt
Presented by:Abdul Haleem
034157
Young’s Modulus Decrease After Cold Forming in HSS
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Contents
1. Introduction to the Problem2. Theory3. Experimental Procedure4. Results5. Conclusions
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Young’s Modulus Decrease After Cold Forming in HSS
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HSS In Automotives
Relationship between fuel mileage and automotive weight, Source: Fukizawa(2000)
• Reduction in car weight and hence fuel economicSolution: Light Weight Design
• Improvement on Safety Solution: High Strength
Design
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Young’s Modulus Decrease After Cold Forming in HSS
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Source: “Structural Material in Automotive Industries: Application and Challenges” GM R&D Center
• The soln is use of High Strength Steel (HSS), Advanced High Strength Steel(AHSS) and Ultra High Strength Steel (UHSS) with thinner gauges
•Alternative materials like Aluminium Alloy are more expensive
•Mass Market Remains that of Steel
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
HSS In Automotives
Young’s Modulus Decrease After Cold Forming in HSS
Car body parts are made of steel sheet mainly by the following processes
• Bending• Hydro Forming• Deep Drawing• Others
Sheet Metal Forming Techniques
Edge or Wipe Bending
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Young’s Modulus Decrease After Cold Forming in HSS
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Source: CORUS
Deep Drawing (DD) a main forming technique for automotive sheet metal forming.
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Sheet Metal Forming Techniques
Young’s Modulus Decrease After Cold Forming in HSS
Springback
• Weight Saving Achieved• But at the expense of higher springback.• An elastic driven change of shape during load removal• Governed by the stress state obtained at the end of
deformation
2nd Feb 2009 7
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Young’s Modulus Decrease After Cold Forming in HSS
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Bending Animation
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Springback
Young’s Modulus Decrease After Cold Forming in HSS
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Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Springback
Bending Animation
Young’s Modulus Decrease After Cold Forming in HSS
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Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Springback
Bending Animation
Young’s Modulus Decrease After Cold Forming in HSS
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Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Springback
Bending Animation
Young’s Modulus Decrease After Cold Forming in HSS
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Springback
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Springback
Bending Animation
Young’s Modulus Decrease After Cold Forming in HSS
2nd Feb 2009 13
For bending, springback is [Burchtiz, 2008]
M: App. Bending Moment, t: Thickness, E: Young’s Modulusρ,θ: Circumferential radius and direction
3
12M
Et
Stress and Strain Profile in plane bending strain, Source: Burchitz(2008)
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Springback
Young’s Modulus Decrease After Cold Forming in HSS
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In general, Spring Back 1. ↑ with ↑ in Yield Strength2.↑ with ↓ in Thickness of the material
For HSS, both (1) & (2) are there, so higher SB
3. ↑ with ↓ in Young’s Modulus e.g. Aluminium Alloy4.Also depends on the Hardening of the material
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Springback
Young’s Modulus Decrease After Cold Forming in HSS
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• Living with SB is acceptable as long as it can be predicted correctly
• Prediction is possible by the use of CAE and FE• Prediction is important because we can -compensate springback in the tooling design
-save labour of reworking -Reduce design to production time• Implimentation of CAE helps in producing the “first time
right” product.`• Unfortunately, the prediction with FE at the moment is
not very accurate
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Prediction of Springback
Young’s Modulus Decrease After Cold Forming in HSS
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Source: Burchitz[2008]
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Prediction of Springback
Young’s Modulus Decrease After Cold Forming in HSS
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• Young’s modulus reduces before saturation during plastic deformation
• One of the reasons for under prediction of springback is assumption of constant E-modulus in FE Analysis.
For XC38 steel, Source: Morestin, 1996
Source: Corus Internal Report
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Prediction of Springback
Young’s Modulus Decrease After Cold Forming in HSS
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1. Introduction to the Problem2. Theory3. Experimental Procedure4. Results5. Conclusions
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Contents
Young’s Modulus Decrease After Cold Forming in HSS
Theory of Degradation
• In addition to elastic strain, there is a dislocation strain caused by deformation. Effective E modulus is then,
effel dis
E
;where σ=applied stressεel= Elastic Strainεdis = Dislocation Strain
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Young’s Modulus Decrease After Cold Forming in HSS
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•Literature suggests modulus degradation a function of loop length and dislocation density•Lems[1963] proposed the model
•Nowick[1972] suggested the model
2
2
24
1 24
E l
E l
21
6
Gl
G
ρ:Dislocation density;
ℓ: loop length,
G: shear Modulus
E: Young’s Modulus
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Theory of Degradation
Young’s Modulus Decrease After Cold Forming in HSS
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Theory of Recovery
Source: Baumer [2007]
•Degradation of E modulus disappears with time•Effect of prestraining and heat treatment for DP/TRIP is shown in figure
•This offers opportunity to validate the mechanism by experiments
Young’s Modulus Decrease After Cold Forming in HSS
From literature, it has been found that recovery in E modulus is characterized by three stages
• Snoek Relaxation• Cottrell Atmosphere Formation• Carbide Precipitates• Among them Cottrell atmosphere is the most important in
recovery of E modulus• Diffusion of interstitials in Cottrell atmosphere is temp
dependent
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Theory of Recovery
Young’s Modulus Decrease After Cold Forming in HSS
• Selection of Bake Hardenable (BH) steel
• Good Formability and low initial yield strength
• Increased Final yield Strength in the product
• Excellent Dent Resistance• Young’s Modulus do not
show decrease after baking treatment.
A f t e r p r e s t r a i n i n g a n d a g e i n g
P r e s t r a i n i n gt
P r e s t r a i n
S t r a i n
St r
es
s
R b L
R t
I n c r e a s e i n l o w e r y i e l d s t r e s s
B H
A s - r e c e i v e dA f t e r p r e s s -
f o r m i n gA f t e r b a k e - h a r d e n i n g
t r e a t m e n t
S o l u t e C D i s l o c a t i o n
Source: Elsen,Hougardy[1993]
Source: The US steel Automotive Group
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Bake Hardenable Steel
Young’s Modulus Decrease After Cold Forming in HSS
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1. Introduction to the Problem2. Theory3. Experimental Procedure4. Results5. Conclusions
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Contents
Young’s Modulus Decrease After Cold Forming in HSS
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• Sample prestrained to different level.• Samples heat treated in silicon based oil bath for required temp
and time.• E Modulus is measured by a static method i.e. Tension and a
dynamic Method (i.e. Impulse Excitation Tech(IET).)
T (ºC)
Room temperature
time (minutes)
R t
prestrain
R bL
re-strain
Experimental Procedure
Time
TemperatureUni-axial Pre Strain by Tensile Machine Baking Time
Room Temp 0,2,4,6,8,10,14,18% -
160°C 0,2,6,10,14,18% 10 and 20 min
180°C 0,2,6,10,14,18% 11 and 20 min
200°C 0,2,6,10,14,18% 12 and 20 min
230°C 0,2,6,10,14,18% 13 and 20 min
Scheme of experiments
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Young’s Modulus Decrease After Cold Forming in HSS
IET Set Up Laser Vibrometer
•Pre Strained Samples were transported to TU Delft for measurement with IET•Measurement velocities from few micron/sec to 1km/sec and vibration frequency from 0.01 Hz to few MHz is possible.
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
IET Setup
Young’s Modulus Decrease After Cold Forming in HSS
• Modified Setup of Support• Norms used ASTM E1876-07
and NEN-EN 843-2
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
IET Setup(2)
Young’s Modulus Decrease After Cold Forming in HSS
• E Modulus is calculated from the static tenisle test from stress-strain curve as shown below
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Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Data Analysis for Tensile Test
Young’s Modulus Decrease After Cold Forming in HSS
• E Modulus is calculated from Dynamic Measurement as[ASTM standard]
2 31
3
0.9465( )fmf L TE
bt
21 [1 6.585( ) ]
tT
L
Wherem=Mass of the sample in gramff=fund. Resonant frequency of the samples measured in flexure;HzL=Length of the samples,mmt=thickness of the samples, mmb= breadth of the samples, mmT1=Correction factor
For (L/t)≥20
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Data Analysis for IET
Young’s Modulus Decrease After Cold Forming in HSS
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1. Introduction to the Problem2. Theory3. Experimental Procedure4. Results5. Conclusions
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Contents
Young’s Modulus Decrease After Cold Forming in HSS
No Heat Treated(HT) Samples
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•No degradation observed (10 to 20% Decrease was expected)
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
• What was wrong? Strange results
Young’s Modulus Decrease After Cold Forming in HSS
No Heat Treated(HT) Samples[2]
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•True stress true strain data revealed existence of strain ageing phenomenon beyond 2% prestrain level.
0
50
100
150
200
250
300
350
400
450
500
0 5 10 15 20 25
True Strain(%)
Tru
e S
tres
s( M
Pa)
As received
2% Pre-Strain
6% Pre Strain
10% Pre Strain
14% Pre Strain
18% Pre Strain
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
•Even with care for not ageing during transportation, strain ageing took place.•Retesting needed for Non Heat treated samples
Young’s Modulus Decrease After Cold Forming in HSS
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•Re testing with only tension test for two conditions prestrained only and prestrained and aged for 24 hours at Room Temperature.
•At 2% and higher, 11.5% reduction in E modulus from 192 GPa to 170 Gpa
•Gradual restoring in E modulus in aged samples (Quicker than expected)
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
No HT Samples (Retested)
Young’s Modulus Decrease After Cold Forming in HSS
2nd Feb 2009 34
•Results for 20 and 10 minutes baking times•Average E modulus results for a specific temperature and time
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
HT Samples (IET)
Young’s Modulus Decrease After Cold Forming in HSS
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•Static Tensile Test results for 20 and 10 minutes baking times•Average E modulus results for a specific temperature and time
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
HT Samples (TT)
Young’s Modulus Decrease After Cold Forming in HSS
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•Calculated dislocation density for different prestrain levels through following relation
whereσf=Flow stress; σ0= back stress; b=2.5x10-10 m(burger’s vector)G= 7.8x104 MPa (shear Mod)
•Minimum Dislocation Density required for effective bake hardening
Pre Strain ρ [1012 m-2]
0% 1[ Cottrell 1949]
2% 10.98
4% 24.96
6% 36.41
8% 45.18
10% 51.26
14% 58.68
18% 61.49
0f Gb
Condition T [°C] Time[sec]
Diffusion (micron)
ρ [1012 m-
2]
24 hours at RT 20 86400 0.02 3400
160°C+10 min 160 600 0.29 12
160°C+20 min 160 1200 0.41 6
180°C+10 min 180 600 0.47 4.5
180°C+20 min 180 1200 0.67 2.3
200°C+10 min 200 600 0.74 1.8
200°C+20 min 200 1200 1.04 0.92
230°C+10 min 230 600 1.35 0.55
230°C+20 min 230 1200 1.91 0.27
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Discussion
Young’s Modulus Decrease After Cold Forming in HSS
2nd Feb 2009 37
•Loop Length calculated from Lems model[ 1963]
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Discussion
Young’s Modulus Decrease After Cold Forming in HSS
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Dynamic IET Method
1) Lower Standard Deviation 2) Dimensions and Mass Dependent3) Non destructive method.4) Non contact Laser vibrometer with high accuracy5) Our experiment’s measurement resolution was 0.06Hz. Higher
resolution is possible easily6) Shearing not a good option.
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Comparison of measurement methods
Young’s Modulus Decrease After Cold Forming in HSS
2nd Feb 2009 39
Static Tensile Test Method
1) For lower standard deviation in E modulus, 3 samples are not sufficient.
2) Destructive Method3) More information per one set of test.4) More Accuracy emphasized.5) Lack of standardization( Only ASTM standard, No
European Standard exists)6) Some factors responsible for inaccuracy in E modulus are
conditions of clamps, extensometers, test conditions e.g. pre load, temperature, stress rate, way of finding linear regression, material condition etc.
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Comparison of measurement methods
Young’s Modulus Decrease After Cold Forming in HSS
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1. Introduction to the Problem2. Theory3. Experimental Procedure4. Results5. Conclusions
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Contents
Young’s Modulus Decrease After Cold Forming in HSS
Conclusions
• 10% to 12 %Reduction in E modulus on prestraining• Heat treatment restores the original E modulus of the material
after prestrain• Recovery in E modulus is more sensitive to ageing than the yield
strength increment• Cottrell atmosphere formation by the carbon diffusion is the main
mechanism of recovery• The effect of prestrain on recovery is visible before restoration of
E modulus as for aged samples.• No baking time dependence is found.• E modulus is a function of dislocation density and av.loop length
between pinning points.• .2nd Feb 2009 41
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Young’s Modulus Decrease After Cold Forming in HSS
• Increase/Recovery of E modulus after prestraining by heat treatment is bound by physical constraint
• Dynamic IET is more reproducible and adoption of higher resolution is easier.
• With one test of T.T, more info is possible unlike IET.
2nd Feb 2009 42
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Conclusions
Young’s Modulus Decrease After Cold Forming in HSS
• Loop Length as a function of prestrain should be found for other grades of steel.
• The need of more accurate E mod measurement is still there and can be done by high resolution in (a) IET of LDV and (b) of extensometer in T.T.
• For BH material, a non heat producing tech. should be adopted for cutting/shearing and tension samples.
• Measurement time of IET can be reduced if all samples are of same size and dimension.
• For distorted sheet metal samples, band supports are more convenient than the rigid knife edged supports.
• To cater for anisotropic nature of sheet metal, it should be measured along other directions than the RD.
2nd Feb 2009 43
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Recommendations
Young’s Modulus Decrease After Cold Forming in HSS
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Ultimate Goals of the Research
• Better Springback Prediction• First Time Accurate Production of stamping dies and
tooling• Labour of Reworking reduced• Design to Production Time Reduced• All of above, results in reduction of production costs
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory
Young’s Modulus Decrease After Cold Forming in HSS
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Thank You for Your Attention
Questions/Comments?
Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory