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Youngs Modulus Decrease After Cold Forming in HSS 1 Youngs Modulus Decrease After Cold Forming in High Strength Steel (HSS) Supervised By: Eisso atzema.

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Presentation on theme: "Youngs Modulus Decrease After Cold Forming in HSS 1 Youngs Modulus Decrease After Cold Forming in High Strength Steel (HSS) Supervised By: Eisso atzema."— Presentation transcript:

1 Youngs Modulus Decrease After Cold Forming in HSS 1 Youngs Modulus Decrease After Cold Forming in High Strength Steel (HSS) Supervised By: Eisso atzema Pascal Kommelt Presented by: Abdul Haleem

2 Youngs Modulus Decrease After Cold Forming in HSS 2 Contents 1.Introduction to the Problem 2.Theory 3.Experimental Procedure 4.Results 5.Conclusions Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory

3 Youngs Modulus Decrease After Cold Forming in HSS 3 HSS In Automotives Relationship between fuel mileage and automotive weight, Source: Fukizawa(2000) Reduction in car weight and hence fuel economic Solution: Light Weight Design Improvement on Safety Solution: High Strength Design Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory

4 Youngs Modulus Decrease After Cold Forming in HSS 4 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 ProblemConclusionsResultsExperimental ProcedureTheory HSS In Automotives

5 Youngs 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 ProblemConclusionsResultsExperimental ProcedureTheory

6 Youngs Modulus Decrease After Cold Forming in HSS 6 Source: CORUS Deep Drawing (DD) a main forming technique for automotive sheet metal forming. Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Sheet Metal Forming Techniques

7 Youngs 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 Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory

8 Youngs Modulus Decrease After Cold Forming in HSS 8 Bending Animation Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Springback

9 Youngs Modulus Decrease After Cold Forming in HSS 9 Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Springback Bending Animation

10 Youngs Modulus Decrease After Cold Forming in HSS 10 Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Springback Bending Animation

11 Youngs Modulus Decrease After Cold Forming in HSS 11 Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Springback Bending Animation

12 Youngs Modulus Decrease After Cold Forming in HSS 12 Springback Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Springback Bending Animation

13 Youngs Modulus Decrease After Cold Forming in HSS 2nd Feb For bending, springback is [Burchtiz, 2008] M: App. Bending Moment, t: Thickness, E: Youngs Modulus ρ,θ: Circumferential radius and direction Stress and Strain Profile in plane bending strain, Source: Burchitz(2008) Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Springback

14 Youngs Modulus Decrease After Cold Forming in HSS 14 In general, Spring Back 1. with in Yield Strength 2. with in Thickness of the material For HSS, both (1) & (2) are there, so higher SB 3. with in Youngs Modulus e.g. Aluminium Alloy 4.Also depends on the Hardening of the material Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Springback

15 Youngs Modulus Decrease After Cold Forming in HSS 15 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 ProblemConclusionsResultsExperimental ProcedureTheory Prediction of Springback

16 Youngs Modulus Decrease After Cold Forming in HSS 16 Source: Burchitz[2008] Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Prediction of Springback

17 Youngs Modulus Decrease After Cold Forming in HSS 17 Youngs 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 ProblemConclusionsResultsExperimental ProcedureTheory Prediction of Springback

18 Youngs Modulus Decrease After Cold Forming in HSS 18 1.Introduction to the Problem 2.Theory 3.Experimental Procedure 4.Results 5.Conclusions Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Contents

19 Youngs 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, ;where σ=applied stress ε el = Elastic Strain ε dis = Dislocation Strain Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory

20 Youngs Modulus Decrease After Cold Forming in HSS 20 Literature suggests modulus degradation a function of loop length and dislocation density Lems[1963] proposed the model Nowick[1972] suggested the model ρ:Dislocation density; : loop length, G: shear Modulus E: Youngs Modulus Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Theory of Degradation

21 Youngs Modulus Decrease After Cold Forming in HSS Introduction to the ProblemConclusionsResultsExperimental 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

22 Youngs 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 ProblemConclusionsResultsExperimental ProcedureTheory Theory of Recovery

23 Youngs 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 Youngs Modulus do not show decrease after baking treatment. Source: Elsen,Hougardy[1993] Source: The US steel Automotive Group Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Bake Hardenable Steel

24 Youngs Modulus Decrease After Cold Forming in HSS 24 1.Introduction to the Problem 2.Theory 3.Experimental Procedure 4.Results 5.Conclusions Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Contents

25 Youngs Modulus Decrease After Cold Forming in HSS 25 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).) Experimental Procedure Time Temperature Uni-axial Pre Strain by Tensile MachineBaking Time Room Temp0,2,4,6,8,10,14,18%- 160°C0,2,6,10,14,18%10 and 20 min 180°C0,2,6,10,14,18%11 and 20 min 200°C0,2,6,10,14,18%12 and 20 min 230°C0,2,6,10,14,18%13 and 20 min Scheme of experiments Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory

26 Youngs 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 ProblemConclusionsResultsExperimental ProcedureTheory IET Setup

27 Youngs Modulus Decrease After Cold Forming in HSS Modified Setup of Support Norms used ASTM E and NEN-EN Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory IET Setup(2)

28 Youngs Modulus Decrease After Cold Forming in HSS E Modulus is calculated from the static tenisle test from stress- strain curve as shown below 2nd Feb Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Data Analysis for Tensile Test

29 Youngs Modulus Decrease After Cold Forming in HSS E Modulus is calculated from Dynamic Measurement as[ASTM standard] Where m=Mass of the sample in gram f f =fund. Resonant frequency of the samples measured in flexure;Hz L=Length of the samples,mm t=thickness of the samples, mm b= breadth of the samples, mm T 1 =Correction factor For (L/t)20 Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Data Analysis for IET

30 Youngs Modulus Decrease After Cold Forming in HSS 30 1.Introduction to the Problem 2.Theory 3.Experimental Procedure 4.Results 5.Conclusions Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Contents

31 Youngs Modulus Decrease After Cold Forming in HSS No Heat Treated(HT) Samples 2nd Feb No degradation observed (10 to 20% Decrease was expected) Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory What was wrong? Strange results

32 Youngs Modulus Decrease After Cold Forming in HSS No Heat Treated(HT) Samples[2] 2nd Feb True stress true strain data revealed existence of strain ageing phenomenon beyond 2% prestrain level. Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Even with care for not ageing during transportation, strain ageing took place. Retesting needed for Non Heat treated samples

33 Youngs Modulus Decrease After Cold Forming in HSS 2nd Feb 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 ProblemConclusionsResultsExperimental ProcedureTheory No HT Samples (Retested)

34 Youngs Modulus Decrease After Cold Forming in HSS 2nd Feb Results for 20 and 10 minutes baking times Average E modulus results for a specific temperature and time Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory HT Samples (IET)

35 Youngs Modulus Decrease After Cold Forming in HSS 2nd Feb Static Tensile Test results for 20 and 10 minutes baking times Average E modulus results for a specific temperature and time Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory HT Samples (TT)

36 Youngs Modulus Decrease After Cold Forming in HSS 2nd Feb Calculated dislocation density for different prestrain levels through following relation where σf=Flow stress; σ 0 = back stress; b=2.5x m(burgers vector) G= 7.8x10 4 MPa (shear Mod) Minimum Dislocation Density required for effective bake hardening Pre Strainρ [10 12 m -2 ] 0%1[ Cottrell 1949] 2% % % % % % %61.49 ConditionT [°C]Time[sec] Diffusion (micron) ρ [10 12 m - 2 ] 24 hours at RT °C+10 min °C+20 min °C+10 min °C+20 min °C+10 min °C+20 min °C+10 min °C+20 min Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Discussion

37 Youngs Modulus Decrease After Cold Forming in HSS 2nd Feb Loop Length calculated from Lems model[ 1963 ] Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Discussion

38 Youngs Modulus Decrease After Cold Forming in HSS 2nd Feb Dynamic IET Method 1)Lower Standard Deviation 2)Dimensions and Mass Dependent 3)Non destructive method. 4)Non contact Laser vibrometer with high accuracy 5)Our experiments measurement resolution was 0.06Hz. Higher resolution is possible easily 6)Shearing not a good option. Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Comparison of measurement methods

39 Youngs Modulus Decrease After Cold Forming in HSS 2nd Feb Static Tensile Test Method 1)For lower standard deviation in E modulus, 3 samples are not sufficient. 2)Destructive Method 3)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 ProblemConclusionsResultsExperimental ProcedureTheory Comparison of measurement methods

40 Youngs Modulus Decrease After Cold Forming in HSS 40 1.Introduction to the Problem 2.Theory 3.Experimental Procedure 4.Results 5.Conclusions Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Contents

41 Youngs 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 Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory

42 Youngs 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 Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Conclusions

43 Youngs 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 Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory Recommendations

44 Youngs Modulus Decrease After Cold Forming in HSS 44 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 ProblemConclusionsResultsExperimental ProcedureTheory

45 Youngs Modulus Decrease After Cold Forming in HSS 45 Thank You for Your Attention Questions/Comments? Introduction to the ProblemConclusionsResultsExperimental ProcedureTheory


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