Mechanisms in Incremental Mechanisms in Incremental Sheet FormingSheet Forming
W. C. Emmens, CORUS RD&TA.H. van den Boogaard, University of Twente
The Netherlands
© W. C. Emmens 2008 - [email protected]
Seminar NSF08, Helsinki, Finland, 29-30 oct 2008
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What this is all about
In incremental forming large strains can be obtained, well beyond the classic FLC.
WHY?Figure: Shim, Park; 2001
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Enhanced formability
Questions to be asked:
• Observing plane-strain deformation in ISF, why is the process not limited by the instabilities that normally limit plane-strain stamping?
• What mechanism does finally limit the process?
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Process limits
AA 5154
(Hosford 1999)
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What is ISF?Incremental forming: A variety of forming processes all characterized by the fact that at any time only a small part of the product is actually deforming. This zone of local deformation is moving over the entire part by some means, mostly mechanical.
Or in brief:A progression of localized deformation. (K.J.)
(Mason 1984)
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What is ISF?
Source: DTU, Kopenhagen, Denmark
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What is ISF?
Characteristics of incremental forming:
• The deformation is localized into a small zone
• In that zone special conditions exist that raise the formability
⇒ How and Why?
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Next:
Forming/stabilizing mechanisms in ISF:
Continuous mechanisms:• 1 Shear• 2 Contact stress
Intermittent mechanisms:• 3 Bending-Under-Tension• 4 Cyclic effects
Other mechanisms:• 5 Hydrostatic stress
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1. Shear - principle
• Which type of shear? Not all are easy detectable!
• Distinguish between intermediate strain state and final strain state
A B C
in-plane shear through-thickness shear out-of-plane shear
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1. Shear - principle
-1.6 1.6-1.6
1.6
A shear stress reduces the yield stress in tension and raises the necking limit.
τ = 0 τ <> 0
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1. Shear - principle
A shear stress reduces the yield stress in tension and raises the necking limit.
pure tension pure shear
0.0 0.1 0.2 0.3 0.4 0.5 0.6
τ / σ f
0
1
2
3
4
5
ε z,neck / n
σ y / σ f
(data from Tekkaya 2006)
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1. Shear - historyDemonstrated by Hoogenboom (TUE), 2002Observed as occurring in ISF:- Sawada (Tokyo Un.) 2001, FEM- Bambach (RWTH) 2003, FEM- Eyckens (KUL) 2007+, FEM, exp.
Proposed as mechanism in ISF:- Allwood/Jackson (Cambridge), 2007+, FEM, exp.
Analysis: - Tekkaya (IUL) 2006, analytical - Eyckens (KUL) 2008, M-K
Investigated by Allwood c.s. “paddle forming”- Obtained high level of uniform elongation
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1. Shear - effects
effect on FLC, M-K analysis
(Eyckens 2008)Direction of shear
A B
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2. Contact stress - principle
A surface stress in compression reduces the yield stress in tension and raises the necking limit.
-1.6 1.6-1.6
1.6
σ3 = 0 σ3 < 0
contact stress
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Observed as occurring in ISF: - Bambach (RWTH) 2003, FEM ( hydrostatic stress)- Eyckens (Leuven) 2008, FEM
Proposed as mechanism in ISF:- Huang (US) 2008
Analysis:- Gotoh (Japan) 1995, anal. - Smith, Matin (US) 2003, 2005, anal. - Banabic (Cluj) 2008, M-K
Experimental investigation:- Taraldsen (Oslo) 1956, 1964, 1972, 600% (Cu-rod) - Rijken (HO) 1965, 100% (steel strip)- Olejnik (Warsaw) 1987, 590% (steel bar)
2. Contact stress - history
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2. Contact stress - effects
(Smith 2003, weakened in 2005)
γ = σ3 / σ1
Smith model
Gotoh model
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2. Contact stress - effects
0.0 0.1 0.2 0.3 0.4 0.5 0.6− σ 3 / σ 1
1
2
3
45678normalized necking limit
Smithn = 0.2
Smithn = 1.0
GotohBanabicAA3104
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specimen
set of tworotating rolls
2. Contact stress - testing
σn = 1-10% σy
forming is concentrated in the
contact zone: incremental forming
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2. Contact stress - results
(Taraldsen 1964, OFHC copper)
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2. Contact stress - results
measured stress-strain curves for steel (Taraldsen 1964)a, b = different steel grades
true stress engineering stress
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1/2. Shear & Contact stress
Both affect the stress state.
Both reduce the yield stress in tension. ⇒ localize the deformation
Both increase the formability to a certain level.⇒ additional stabilizing effect
Both depend on yield- and hardening-behaviour of the material.
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3. BUT - principle
R
t
w
X
YZ
F, TT
ee b
.tσ
e = t/2Rb
relation between tension force per unit width T and stretching strain e in BUT, assuming constant bending radius R
reduces tension force, creates stable deformation
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3. BUT - principle
0 50 100 1500
2
4
6
8
10
12
14maximum strain (%)
water jet ISFspinning
material thickness (µ m)
Results from can shaping experiments (1999-2002) using incremental techniques (Emmens 2004)
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Bending-under-tension: textbooksObserved as occurring in ISF:- Sawada (Tokyo Univ.) 2001, FEM
Proposed as mechanism in ISF:- Emmens (Corus) 2004, exp.
Investigations (using repetitive bending):- Benedyk (IIT) 1971, 2002, exp. 26 - 140% (var. mat) (mat. characterization)- Jongenburger/Mols (TUD) 1972, exp. 30 - 110% (var. mat.) (tension-levelling)- Hadoush (UT) 2007, FEM 200% (steel)- Emmens (UT) 2007+, exp. 430% (steel) (incremental forming)
3. BUT - history
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3. BUT - testing
specimen
set of threerotating rolls
only material actually being bent will deform ⇒ forming is
concentrated in the 6 bending zones: incremental forming
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3. BUT - results
0 50 100 150 200 250
elongation (mm)
0.0
0.5
1.0
1.5
2.0true length strain
normal tensile test
series 1series 2series 3
100
eng. strain (%)
50
150 200
300
500 400
600
dashed line= theory
(Emmens 2008)
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3. BUT - results
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4true strain
0
100
200
300
400
500
600true stress (MPa)
Ludwik-Nadai curve
(Emmens 2008)
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3. Bending-Under-tension
Does NOT affect the stress state: part of the sheet is in compression.
Reduces the yield force in tension and creates stable elongation to a certain strain increment.⇒ localizes the deformation⇒ additional stabilizing effect
Independent of yield- and hardening-behaviour or level of straining. (except Bauschinger effect)
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Summary so far
Shear and contact stress:- reduction of yield stress in tension due to change of the strain state;- capable of creating higher levels of deformation to an absolute level;
Bending-under-tension:- reduces yield force in tension due to simultaneous bending;- capable of creating higher levels of deformation to a relative level;
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Situation in ISF
PUNCHPUNCH
zt
R
position at last contour
zone of localized deformation
stretchedwall
SB
B
S = shearB = BUT = contact stress
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4. Cyclic effects - principle
Strong cyclic effects in ISF.
Repeated bending / unbending.
(Eyckens 2007)
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4. Cyclic effects - history
Cyclic effects in ISF:
- Emphasized by Bambach (RWTH) 2003
- Analysed using M-K theory by Eyckens (KUL) 2007, Van Bael (KUL) 2007
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4. Cyclic effects - results
(Van Bael 2007)
limit for straight strain path
limits in serrated straining
M-K analysis
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4. Cyclic effects - summary
Non-straight strain path (serrated yielding) can create enhanced formability.
Effects caused by non-isotropic hardening.
Note: bending-under-tension also implies cyclic effects
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5. Hydrostatic stress
Contact stress: σ3<0postpones necking
Compressive hydrostatic stress: σ1+σ2+σ3<0suppresses damage development
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5. Hydrostatic stress
Observed in ISF: - Hirt and Bambach (RWTH), 2002, 2003, FEM- Silva, 2008, analytically
sheared elements !(Bambach 2003)
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5. Hydrostatic stress
Effect of compressive hydrostatic stress:
Slows down damage development but does not suppress instabilities.
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Summary of mechanismsLocalize deformation due to local reduction of stretching force by:- Shear- Bending-under-tension- Contact stress
Stabilizing effects by:- Shear and Contact Stress (postpones necking)- Bending-under-tension (stable deformation)- Cyclic straining (postpones necking)
} experimental evidence it can create large uniform straining
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But …
ISF can be done with a high pressure water jet(Emmens, Iseki, Jurisevic)
- no tool, no tool contact- no friction- no contact pressure- no shear ?
ISF can be done on plastics (Frantzen 2008)
- fundamentally different plastic behaviour(yield stress depends on hydrostatic pressure)- material independent mechanisms?
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And …
2D operations (adapted tensile tests) differ from 3D operations:- no constraints from the surrounding material (back stresses)- max. strain much larger than in 3D ISF
Open questions: - are stabilizing mechanisms really necessary?- what is the effect of the actual moving of the zone of localized deformation?- what is the effect of the constraints caused by the surroundng material?
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Conclusions• Several mechanisms are known that can localize the forming and lift the formability, and are related to some aspect of Incremental Sheet Forming.
• At any moment more mechanisms may be at work simultaneously.
• It is not fully clear yet if and if so to what extent these mechanisms operate in a specific ISF operation.
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CopyrightThis presentation contains material copied from other publications, that is subjected to copyright by the respective authors or publishers. The original sources are:
•Sheet 2: M.S. Shim, J.J. Park; The formability of aluminium sheet in incremental forming; Journal of Materials Processing Technology 113 (2001) 654-658 •Sheet 4: Hosford, W.F., Duncan, J.L., 1999. Sheet Metal forming: A Review. JOM, 51 (11), pp. 39-44. •Sheet 5: B Mason, E. Appleton; Sheet metal forming for small batches using sacificial tooling; Proc. 3rd Int. Conf. on Rotary Metalworking, Kyoto, Japan, 1984, pp 495-51 •Sheet 6: Copyright DTU, Denmark•Sheet 13: P. Eyckens, A. Van Bael, P. Van Houtte; An extended Marciniak-Kuczinski forming limit model to assess the influence of through-thickness shear on formability; Proc. Numisheet 2008, Sept 1-5 2008, Interlaken, Switzerland, pp 193-198 •Sheet 16: Smith, L.M., Averill, R.C., Lucas, J.P., Stoughton, T.B., Matin, P.H., 2005. Influence of transverse normal stress on sheet metal formability. Int. J. of Plasticity 10 (2005) pp. 1567-1583•Sheet 19,20: Taraldsen, A., 1964. Stabilized tensile testing (i.e. without local necking). Materialprűfung, vol 6 (1964), nr. 6, pp 189-195.•Sheet 31: P.Eyckens, S. He, A. Van Bael, P. Van Houtte, J. Duflou; Forming Limit Predictions for the Serrated Strain Paths in Single Point Incremental Forming; Proceedings Numiform 07, june 17-21 2007, Portugal, AIP CP908, pp 141-146 •Sheet 33: A. Van Bael, P. Eyckens, S. He, C. Bouffioux, C. Henrard, A.M. Habraken, J. Duflou. P. Van Houtte; Forming Limit Prediction for Single-Point Incremental Sheet Metal Forming; Proceedings 10th ESAFORM, Zaragoza, Spain, April 18-20 2007, AIP CP907, pp 309-31 •Sheet 36: M. Bambach, G. Hirt, S. Junk; Modelling and Experimental Evaluation of the Incremental CNC Sheet Metal Forming Process; Proceedings 7th COMPLAS, Barcelona, Spain, April 7-10 2003
All other material is Copyright W.C. Emmens