presentation on bending processes and springback elimination issue

1. Bending Processes & Springback
Elimination Issue

2. Content
• Bending
• Bending process
• Springback
• Size effect on springback
• Springback prediction & compensation
• Die design methods
• Displacement Adjustment method
• Application on DA-method
• Bending of 304 Stainless Steel Tubes

3. Content
• Die design methods
• Displacement Adjustment method
• Application on DA-method

4. Bending Process

5. Bending
• Bending of sheet metal is a common process in manufacturing
industry. Sheet metal bending is the plastic deformation of the work over
an axis

6. Bending Processes

7. Bending Processes

8. Bending Processes

9. Bending Processes

10. Bending Processes

11. Bending Processes

12. Bending Processes

13. Bending Processes

14. Springback

16. Elastic Recovery
Fig.(1): engineering stress and strain curve

17. Size effect affected springback in micro scale
bending process
(Experiments)

18. 1. Specimen
preparation
• Pure copper sheet
metals with the
thickness
of 0.1, 0.2 and 0.4
mm
• were employed as
the testing material
in this study. The
specimens were
heat treated at
different conditions
to obtain different
grain sizes
Experiments

19. 2. Experimental setup
• The geometric dimensions
of the specimen were
designed as shown in Fig.
1
• The bending experimental
setup is shown in Fig. 2
Three punch angles, i.e.,
30°, 60° and 90°, were
designed to test the
specimens with different
thicknesses and grain
sizes.
Experiments
Fig. 1 Geometric dimensions of the bending specimen.

20. • The micro-bending
experiments were
conducted by
employing the Instron
5966 material testing
machine. A high
resolution camera
was used to record the
deformation behavior
during the bending
process continuously .
Experiments

21. • After the punch
reached the pre-
determined distance,
The punch velocity 3
mm/min returned at
the same speed.
During the
loading/unloading
process, The digital
camera was employed
to take continuous
images
Experiments
3. Experimental procedure

22. • The digital images
before and after
springback were
employed to
obtain the bending
and spring back
angle. The measuring
process
Experiments
Fig. 4 Specimens after the micro-bending test
4. Results and discussion

23. • The specimens with the
thickness of 0.1 mm were
taken as an example
to analyze the
deformation process. The
loading force-
displacement curves
recorded during the tests
Experiments
4.1 The displacement-loading process

24. • It can be observed that springback
angle decreases with the increase
of grain size for each condition This
is because the yield and flow stress
has been revealed to decrease with
the increase of grain size. As the
specimen is subjected to the same
punch angle
Experiments
4.2 The springback angle analysis

25. • As illustrates the springback
results of specimens subjected to
different punch angles
Experiments
4.3 The effect of deformation angle

26. Why springback is an issue
• Assembly difficulties
• Cost revision
• Decrease quality
• Time consuming (delay production)
• US automotive industry alone spends more than $50 million per year
because of rejection of unqualified parts [1].

27. Springback Prediction
1. It was made by empirical hand book rules or simple analysis and it’s applied to :
a) Pure bending
b) Very simple shapes
c) Constant radii of curvature
d) Well studied material (usually mild steel)
2. Prediction using FE-methods
During Last decade Prediction was done using Finite element methods (FE) for arbitrary
shapes
For accurate Prediction we take in consideration:
1. Numerical sensitivity
2. Physical sensitivity
3. Poorly characterized material behavior

28. • While Springback is large; compensation is made to the final part to
approaches the desired product
 Trial & Error during die manufacturing
• Very expensive , time consuming , depend on skills and experience
• Some times luck (For complex aluminum panels, more than 6 months can
be required during die tryout to correct Springback error [2] ).
 Karafillis and Boyce [K&B]
• based on computing the constraint forces to maintain equilibrium
• It’s application suffers from lack of convergence unless the forming
operation is symmetric or has very limited geometric change during
springback
Springback compensation

29. • DA was found to converge incases when K&B does not, and in cases when
both methods converge, DA is many times faster
(i.e. nonsymmetric parts, K&B can return inaccurate results whereas DA does not) .
• It’s Concept based on comparing Target shape & FE Simulation by Moving
the surface nodes defining the die surface in the direction opposite to the
springback error
Displacement Adjustment Method

30. Displacement Adjustment Method
Fig.2: Displacement Adjustment Method

31. 1. First, a flat sheet of metal is
deformed to a trial die shape (for
the first cycle, the trial die shape
is the target shape.
DA-method procedure

32. 2. Springback occurs
3. Comparing springback with target
and defining the error as (Delta y)
DA-method procedure

33. 4. (Delta Y) is added to the current
die shape nodal positions, obtaining
new tooling shape (1 cycle).
DA-method procedure

34. New flat sheet, deforming by last die,
springback occurs, if not within
specific tolerance ; another iteration
is done.
DA-method procedure

35.  DA is a numerical method, its usefulness relies on the accuracy of
springback prediction (it can be done by FE simulation)With
proper care taken in the material laws and contact conditions
DA-method procedure

36. Arc bending

37. Arc bending (simple example)
Fig.3: Arc bending test
• ABAQUS STANDARD 5.8 [3]
• The punch and die are treated as rigid
bodies.
• There are 100 beam elements along the
sheet length (Element B21).
• W=0.4572 mm , t=0.1778 mm in thickness
• 51 integration pt. thickness [4]
• Young’s Modulus = 207 Gpa
• Poisson’s ratio = 0:3
• hardening equation:
sigma (Mpa)=307 + 4460 ^0.76
• µ=0.2

38. Arc bending (simple example)
Fig.3: Arc bending Results
Results:
• 1 cycle of DA iteration
• springback error becomes
negligible.
• identicality of springback
shape to the desired
target

39. Residual stress and springback analysis for
304 stainless steel tubes in flexible-
bending process

40. 1. medical engineering
2. petrochemical industry
3. urban/residential construction
4. automotive manufacturing
5. aerospace industry
Aspects of use

41. What is flexible bending process ?
Flexible-bending is an innovative
processing technique for profiles and
tubes, which adjusts the curved shape
of the forming part through numerical
control.
Figure 1 illustrates the principle of
flexible-bending.

42. Challenges
the effects of residual stress and springback on the forming quality and
geometric accuracy of 304 stainless steel bent tubes

43. Analysis
The springback and residual stress of tubes were analyzed under
different bending conditions through both simulations and experiments

44. Numerical model
In order to study the flexible-bending process of 304 stainless steel
tubes, we applied ABAQUS/explicit to numerical simulations

45. The mechanical properties of 304 stainless steel tube were as
follows:
density ρ = 7.93 g/cm 3
elastic modulus E = 193 G pa
yield strength σs = 242.31MPa,
Poisson’s ratio ν=0.3,
tensile strength σb =694.72MPa
elongationδ=40%
initial size = 500 × Φ 12 × t 2 mm 3
The bending die had thicknessof20mm and radius of 20mm
The gap between the guide, the bending die, and the outer contour of the tube was 0.05mm.
The gap between the sleeve and the bending die was 0.02 mm

46. Results and analysis
1) Analysis of residual stresses:
During the bending process, the forming
quality was represented by some special
points

47. Effects of wall thickness on residual stresses

48. T = 2 mm ( a, a′), 1.5 mm (b, b′), 1 mm (c, c′), and 0.75 mm (d, d′)

49. 2) Optimization of the springback:
H is the Y-direction rise of the bending die when the pusher is fixed; L is the
horizontal distance between the right side of the guide device and the gravity
center of the bending die; a is the bending angle; and r is the bending radius. At
this time, springback did not occur

50. After unloading, springback occurred and the figures below illustrates the
effect of changing the outer diameter(D) and the thickness(T) on springback

51. By adjusting the process parameters L and H, they were matched according to the following
equation

52. Comparison of forming radius and target
radius
Springback angle of different L values

53. [1] USCAR (Big 3 Automaker Consortium), estimate provided by Dr. M.L. Wenner,
General Motors R&D Center,
Warren, Michigan.
[2] Xia ZC. Springback compensation technology for aluminum stamping. NADDRG Spring
Meeting. Columbus,
Ohio, May 9, 2001
[3] ABAQUS user Manual, version 5.8.
[4] Li KP, Geng LM, Wagoner RH. In: Meech JA, Veiga MM, Smith MH, LeClair SR, editors.
Simulation of
springback with the draw/bend test, IPMM ’99. Vancouver, BC, Canada: IEEE; ISBN 0-7803-
5489-3, 1999. p. 1.
References

54.  Elsevier: ScienceDirect
 Elsevier: Engineering Village
 Cambridge University Press
 Oxford University Press
 Springer
 Research Gate
 Scientific American
All of this cannot be done except by :
• Google Scholar
• Egyptian Knowledge Bank (EKB)
Research Basis

55. A Great Thanks for making up this
opportunity to make this Research & to
stand here in front of you
Dr. Mohamed Daha