youngs modulus decrease after cold forming in hss 1 youngs modulus decrease after cold forming in...

Young’s Modulus Decrease After Cold Forming in HSS

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Young’s Modulus Decrease After Cold Forming in High Strength Steel (HSS)

Supervised By: Eisso atzema

Pascal Kommelt

Presented by:Abdul Haleem

034157


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Contents

1. Introduction to the Problem2. Theory3. Experimental Procedure4. Results5. Conclusions

Introduction to the Problem ConclusionsResultsExperimental ProcedureTheory


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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



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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


HSS In Automotives


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



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Source: CORUS

Deep Drawing (DD) a main forming technique for automotive sheet metal forming.


Sheet Metal Forming Techniques


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

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Bending Animation


Springback


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Springback

Bending Animation


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Springback

Bending Animation


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Springback

Bending Animation


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Springback


Springback

Bending Animation


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)


Springback


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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


Springback


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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


Prediction of Springback


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Source: Burchitz[2008]




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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




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Contents


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



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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


Theory of Degradation



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


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


Theory of Recovery


• 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


Bake Hardenable Steel


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Contents


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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



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.


IET Setup


• Modified Setup of Support• Norms used ASTM E1876-07

and NEN-EN 843-2


IET Setup(2)


• E Modulus is calculated from the static tenisle test from stress-strain curve as shown below

2nd Feb 2009 28


Data Analysis for Tensile Test


• 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


Data Analysis for IET


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Contents


No Heat Treated(HT) Samples

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•No degradation observed (10 to 20% Decrease was expected)


• What was wrong? Strange results


No Heat Treated(HT) Samples[2]

2nd Feb 2009 32

•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


•Even with care for not ageing during transportation, strain ageing took place.•Retesting needed for Non Heat treated samples


2nd Feb 2009 33

•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)


No HT Samples (Retested)


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•Results for 20 and 10 minutes baking times•Average E modulus results for a specific temperature and time


HT Samples (IET)


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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


HT Samples (TT)


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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


Discussion


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•Loop Length calculated from Lems model[ 1963]


Discussion


2nd Feb 2009 38

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.


Comparison of measurement methods


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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.


Comparison of measurement methods


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Contents


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



• 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


Conclusions


• 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.

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Recommendations


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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



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Thank You for Your Attention

Questions/Comments?


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