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INTERNATIONAL Mechanical Engineering and Technology (IJMET), ISSN 0976 –
 International Journal of JOURNAL OF MECHANICAL ENGINEERING
 6340(Print), ISSN 0976 – AND TECHNOLOGY (IJMET) © IAEME
                          6359(Online) Volume 3, Issue 3, Sep- Dec (2012)



ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
                                                                            IJMET
Volume 3, Issue 3, September - December (2012), pp. 517-530
© IAEME: www.iaeme.com/ijmet.asp                                        ©IAEME
Journal Impact Factor (2012): 3.8071 (Calculated by GISI)
www.jifactor.com



            EFFECT OF PUNCH PROFILE RADIUS AND LOCALISED
           COMPRESSION ON SPRINGBACK IN V-BENDING OF HIGH
                STRENGTH STEEL AND ITS FEA SIMULATION
                 Vijay Gautam1, Parveen Kumar2, Aadityeshwar Singh Deo3
     1
         (Department of Mechanical Engineering, Delhi Technological University, Road, Delhi-
                               110042, India, vijay.dce@gmail.com)
    2
        (Department of Mechanical Engineering, Delhi Technological University, Road, Delhi-
                            110042, India, dahiya.sonu1@gmail.com)
     3
         (Department of Mechanical Engineering, Delhi Technological University, Road, Delhi-
                             110042, India, dce.aaditya@gmail.com)

  ABSTRACT

  Spring-back is a very common and critical phenomenon in sheet metal forming operations,
  which is caused by the elastic redistribution of the internal stresses after the removal of
  deforming forces. Spring-back compensation is absolutely essential for the accurate geometry
  of sheet metal components. In this study an experimental investigation was carried out to
  determine the effect of punch corner radii on springback in free V- bending operation. The
  springback compensation was done by localised compressive stresses on bend curvature by
  the application of compressive load between punch and the die. This experimental springback
  phenomenon was analysed and validated by an Explicit finite element program using
  ABAQUS 6.10. In order to determine spring-back in V-bending operation, six numbers of
  ‘‘V’’ shaped dies and punches with required clearances were designed and fabricated with
  included angle of 90° for bending of high strength sheet metal with thicknesses: 0.85, 1.15
  and 1.55mm. Keeping other parameters same increase in punch corner radius increases the
  springback and increase in sheet thickness reduces the springback. Springback compensation
  by localised compressive stress showed negligible springback and the same results were
  supported by FEA simulations. This model is very useful to control springback on a press
  brake equipped with controlled computer integrated data acquisition system.

  Keywords: Bending dies, Explicit solution, Mn-High Strength steel, Springback, V-bending.




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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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1. INTRODUCTION

Bending is one of the most important sheet metal forming operations by which a straight
length of metal strip is transformed into a curved one with the help of suitably designed die
and punch. It is very common process of forming steel sheets and plates into channels, drums,
automotive and aircraft components.

   Especially V-Bending process has been thoroughly studied and there is plenty of literature
available, among which the most important contribution is Hill's basic theory on pure
bending of sheet metals[l]. Hill has derived the complete solution for pure bending of a non-
hardening sheet and showed the shift of the neutral surface during bending. Lubahn and
Sachs [2] studied the bending of rigid perfectly plastic materials in cases of both plane stress
and plane strain, and they predicted no change in material thickness by assuming that the
surfaces, including the neutral surface, Crafoord [3]considered the Bauschinger effect by
assuming the constant yield surface on reverse straining by fibres overtaken by the neutral
surface. And he predicted obvious thickness thinning of rigid-strain-hardening metal sheets.

   Pure bending is rarely achieved in actual bending process, except that, it is the desired
profile of a bend than the temporal stress and strain distribution that is important. The
assumptions made in the study of pure bending are generally different from real conditions in
v-die bending. Unlike pure bending, V-die bending is not a steady process. A sheet metal is
laid over a die and bent as the punch inserts into the die, the bending moment and curvature
vary continuously along the sheet and during the deformation, the sheet is stressed in tension
on one surface and compression on the other, it is shift of the neutral surface during bending
that complicates the analysis[4].

   The stress state is complex in bending. Around the neutral plane, the stress must be elastic
because complete tensile and compressive stress-strain curves of the material are traversed on
both bend side. When the forming tool is removed from the metal, the elastic components of
stress cause spring back which changes both the angle and radius of the bent part as shown in
Fig.1. The part tends to recover elastically after bending, and its bend radius becomes larger.
This elastically-driven change in shape of a part upon unloading after forming is referred to
as spring back.




                       Figure 1: terminology for springback in bending [5]
  Spring-back causes following problems in sheet-metal forming:


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1) The assembly of the sheet metal components becomes problematic thereby increasing the
   assembly time and reducing the productivity.
2) In automobile industry different punch corner radius are used for different bending
   operations which in turn affects the spring-back in components.
3) A wide range of thickness is used in sheet-metal components which again affects the
   spring-back.
4) High strength sheets are preferred for automotive body as to reduce the thickness which
   results in reduction of the overall weight of the vehicle. Lighter vehicles are in demand for
   higher fuel efficiency.

    However, spring-back characteristic of IFHS has not been investigated widely and very
little information is available about its behaviour during V-bending operations.

   Both material parameters and process parameters affect springback, parameters such as
elastic modulus, yield strength, strain hardening ability and thickness of the sheet metal as
well as die opening, punch radius and so on interfere the springback in a very complicated
way.




           Figure 2: Methods of reducing springback in V-bending operations [5].

   We can calculate springback approximately, in terms of the radii Ri and Rf i.e. initial and
final radius of bend curvature (Fig.1) as[5] :

                           ோ௜       ோ௜        ோ௜
                                = 4(ா௧)ଷ − 3 ቀா௧ቁ + 1             (1)
                           ோ௙


  Note that springback increases as the R/t ratio and yield stress Y of the material increases
and the elastic modulus E decreases [5].

2. COMPENSATION FOR SPRINGBACK
In general practice there are different ways for springback compensation as shown in the
Fig.2 :over-bending (In Fig.2 (a) & (b)), coining or bottoming the punch (shown in Fig.2 (c)
and (d)), stretch bending and warm bending[5].

   Over-bending is an effective way to compensate for the springback, this can be done in air
bending by adjusting the punch/ die angle or punch stroke. Several trials may be necessary to
obtain the desired results. Stelson and co-workers have introduced an adaptive control model
[6-8] and this model estimates the material characteristics of a sheet being bent from the
punch force-displacement data taken early in the bending process, and the in-process

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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measured parameters are then used in the calculation of the current final punch position so
that the elastic springback can be compensated by over-bending and desired unloaded angle
of a bend can be obtained. By this model, disturbances in operation due to variations in
material characteristics of a sheet will not affect the modelling results. To predict the loaded
shape and the springback of a sheet being bent, however, strenuous measurement and
calculation must be performed. In analysis of springback in v-die bending, the curvature of a
sheet metal subjected to bending needs to be known. In most analyses, the inner radius of a
bend is commonly assumed to be the same as that of the punch. In fact, the radius of
curvature is a function of both material and process parameters. If the radius of a punch is of
the same order of the sheet thickness, the radius of curvature underneath the punch will be
larger than that of the punch, while a sufficiently large punch will cause a smaller bending
curvature [9].

   Another method is stretch bending, in which the part is subjected to tension while being
bent, the springback is reduced as the neutral surface is shifted out of the sheet metal [10].
Since the springback decreases as yield stress decreases, all other parameters being the same,
bending may also be carried out at elevated temperatures to reduce springback known as
warm bending [11].

  Little data is available for springback compensation by bottoming the punch or coining
and hence localised compression was the main objective of the study.

3. MATERIAL SELECTION & METHODOLOGY
Materials and techniques for cutting weight from vehicles and thereby improving fuel
efficiency, are a part of routine automotive engineering practice. Large reductions in weight
while maintaining size and enhancing vehicle utility, safety, performance, ride and handling
are often thought of as the driving force for future vehicles [12].The body of a car, including
the interior, accounts for nearly 40% of the car’s total weight and offers a high potential for
lightweight construction [13]. Materials for car body panels require certain specific
characteristics to meet the industry’s challenges: rationalisation of specifications for leaner
inventory, improved formability for reduced rejection rate and better quality. Higher Strength
Low Alloy (HSLA) steels of thinner gauges are getting preference for weight reduction and
the resulting better fuel economy. Other quality characteristics under demand are higher yield
stress (strength), toughness, fatigue strength, improved dent resistance as well as corrosion
resistance in materials used for body panels for improved durability and reliability.
   Keeping in view of the above factors low carbon high strength steel, was chosen for the
springback study and the sheet metal was procured from leading automobile manufacturer
with thickness 0.85, 1.15 and 1.55mm. Chemical analysis of the material as per the ASTM-
E415-08 reveals that the material is high Manganese and low Carbon and the composition of
the steel is given in TABLE 1.

                      Table 1: Chemical composition of HS-steel (wt %)
                     C       Si   Mn     P     S       Ti    Nb      Al
                   0.077 0.013 1.4 0.05 0.011 0.04 0.001 0.032



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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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   The microstructure of high strength steel was revealed and carefully studied under
magnification of 200X. The microstructure shown in the Fig. 3 depicts the fine grains of
ferrite and complex Manganese Carbides uniformly distributed in ferrite matrix responsible
for high strength of the steel.




Figure 3: microstructure of high strength steel at 200x showing complex carbides in the
matrix of ferrite
3.1. Tensile Properties of High Strength Steel
The tension tests were carried out as per ASTM standard E 8M-04 (2004) on INSTRON
4482, 100KN machine, in strength of materials laboratory at DTU Delhi. The HS sheets of
0.85, 1.15 and 1.55mm thickness were tested for the mechanical properties. The tension test
specimens were cut from the sheet metal at 0° i.e. parallel to, inclined at 45° and
perpendicular at 90° with respect to the rolling direction of the sheet metal. The tensile testing
of material was carried out with standard size specimen as shown in Fig.4.




Figure 4: tensile specimens cut as per ASTM-E8M in direction parallel to, perpendicular to
and inclined at 45° to the rolling direction
   The typical stress strain curve obtained from the tests is shown in Fig.5 to Fig. 7. Since the
departure from the linear elastic region cannot be easily identified, the yield stress was
obtained using the 0.2 % offset method. UTS was determined for the maximum load and
original cross section area of specimen. The tensile properties of the specimens show that the
sheet metal is slightly anisotropic. Hence it can be regarded as isotropic material.




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                 Table 2: Mechanical properties of High Strength steel sheet

                              Young’s                   Direction                      Strength       Strain
     Sheet     thickness                                                σy yield
                              Modulus                     wrt.                        Coefficient   hardening
     metal       (mm)                                                    (MPa)
                               (MPa)                     rolling                          (k)     coefficient(n)

                                                           0˚             355           791.39        0.188
      High
                  0.85             210000                  45˚            367            762.0        0.179
    strength
                                                           90˚            376            785.4        0.179

                                                           0˚             312            744.7        0.199
      High
                  1.15             210000                  45˚            320           730.18        0.190
    strength
                                                           90˚            314            741.7        0.188

                                                           0˚             315            735.9        0.196
      High
                  1.55             210000                  45˚            319            732.0        0.192
    strength
                                                           90˚            324           716.51        0.184



                                           Stress- Strain curve for 0.85mm sheet
                                            600
                                                                                engg stress
                           stress in MPa




                                            400                                 strain curve
                                                                                X-0-1
                                            200

                                               0                                 stress
                                                                                strain
                                                   0      0.2     0.4           curve:x-90-
                                            -200
                                                       strain in mm/mm          1


Figure 5: engineering stress strain curve of 0.85mm thick sheet as obtained from tensile test
on UTM, depicting that the metal is almost isotropic.




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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME


                                           stress -strain curve for 1.15mm sheet
                                           500
                                           400                            stress




                           stress in MPa
                                                                          strain
                                           300
                                                                          curve Y-0-1
                                           200
                                           100                            stress
                                              0                           strain
                                           -100 0         0.2     0.4     curve: y-
                                                                          45-1
                                                      strain in mm/mm


Figure 6: engineering stress strain curve of 1.15mm thick sheet as obtained from tensile test
on UTM, depicting that the metal is almost isotropic.


                                           stress strain curve for 1.55mm sheet

                                           600
                                                                          engg stress
                                                                          strain
                           Stress in MPa




                                           400
                                                                          curve z-0-1
                                           200
                                                                          engg stress
                                              0
                                                                          strain
                                                  0        0.2    0.4     curve z-45-
                                           -200
                                                      strain in mm/mm     1


Figure 7: engineering stress strain curve of 1.55mm thick sheet as obtained from tensile test
on UTM, depicting that the metal is almost isotropic.

   The strain hardening exponent (n) and the strength coefficient (K) values are calculated
from the stress strain data in uniform elongation region of the stress strain curve. The plot of
log (True stress) versus log (True strain) which is a straight line is plotted. The power law of
strain hardening is given as:

                                           σ = K. εn                        (2)

  Where, σ and ε are the true stress and true strain.

  Taking log on both sides:

                                           log (σ) = log (K) + n. log (ε) (3)

   This is an equation of straight line the slope of which gives the value of ‘n’ and ‘K’ can be
calculated taking inverse natural log of the y-intercept of the line (i.e. ln (K)) as shown in
Fig.8.


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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME


                                                      log stress strain curve
                                                     n=0.188, K=791.39MPa
                                                       6.8




                     Ln(true stress)
                                       y = 0.188x + 6.673
                                                       6.6
                                           R² = 0.999 6.4                       log stress strain
                                                                                curve
                                                      6.2
                                                        6
                                                      5.8                       Linear (log
                                                                                stress strain
                               -4               -2           0         2        curve)
                                              Ln (true strain)


                 Figure 8: calculation of n and k value of high strength steel

3.2. Fabrication of Bending Tools
As discussed earlier two sets of dies and punches with the punch corner radius 7.5 mm and 10
mm were required for the experimental setup of V-bending in addition to other accessories.
The included angle for dies and punches were kept as 90°.

   The D-2 tool steel for cold working was selected for the bending dies. The drawings of
tooling were made in CATIA-V5 as shown in the Fig.9 and Fig.10. The DXF file was used in
EDM-wire cut to fabricate the dies and punches. A total of six dies and two punches were
designed with two different punch corner radii. The clearance between dies and punch was
made equal to sheet thickness to avoid the localized compressive stresses during bending
operation. The dies and punches were designed with a holding shank of 25mm length and
12.5mm thickness for easy holding and proper alignment in UTM. The setup was designed
for UTM for capturing the data for load and deflection.

   After fabrication the dies and punches were hardened and tempered in Metallurgy
laboratory. D-2 steel dies and punches were heated to 910°C in a muffle furnace for 4hrs.
And hardened in air and then tempered at 250°C. After air hardening the hardness was 65Rc
and after tempering the hardness was 62HRc.




             Figure 9: a CAD drawing of the v-die showing various dimensions



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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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           Figure 10: a CAD drawing of the v-punch showing various dimensions

4. FEA SIMULATIONS: RESULTS AND DISCUSSIONS
The FEA simulations for the above experimental procedure were carried out using ABAQUS
6.10 in CAD lab. The material model for 2D deformable blank was prepared as isotropic
hardening following power law of hardening, with young’s Modulus of Elasticity as
210000MPa and Poisson ratio of 0.3 with structure insensitive elastic constants for steel.

   The plastic data of the steel was directly taken from data acquisition of UTM were true
stress and strain. The plastic strain at stress level 350MPa was 0.0, and at 445MPa strain was
0.058mm/mm. The dies and punches were modelled as 2D analytical rigid requiring no
meshing properties. The friction condition between the punch and blank was frictionless and
a friction value of 0.1 was used between blank and the die.

                  Table 3: Different part attributes used in the FEA Model

                           PARTS                       2D

                             DIE                ANALYTICAL RIGID

                           PUNCH                ANALYTICAL RIGID

                          BLANK                   DEFORMABLE

  The simulation results are listed as below:

Case-1.Bending of HS steel 0.85mm thick with punch corner radii 7.5mm.

Case-2.Bending of HS steel 1.15mm thick with punch corner radii 7.5mm.

Case-3.Bending of HS steel 1.55mm thick with punch corner radii 7.5mm.

Case-4.Bending of HS steel 0.85mm thick with punch corner radii 10mm.

Case-5.Bending of HS steel 1.15mm thick with punch corner radii 10mm.

Case-6.Bending of HS steel 1.55mm thick with punch corner radii 10mm.


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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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      CASE 1: High Strength steel 0.85mm thick with punch corner radii 7.5mm.




Figure 11: overlay plot showing springback of HS 0.85mm thick sheet with punch corner
radius of 7.5mm.

      CASE 2: High Strength steel 1.15mm thick with punch corner radii 7.5mm.




Figure 12: overlay plot showing springback of HS 1.15mm thick sheet with punch corner
radius of 7.5mm

      CASE 3: High Strength steel 1.55mm thick with punch corner radii 7.5mm.




Figure 13: overlay plot showing springback of HS 1.55mm thick sheet with punch corner
radius of 7.5mm



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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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      CASE 4: High Strength steel 0.85mm thick with punch corner radii 10mm.




Figure 14: overlay plot showing springback of HS 0.85mm thick sheet with punch corner
radius of 10mm

       CASE 5: High Strength steel 0.9mm thick with punch corner radii 10mm.




Figure 15: overlay plot showing springback of HS 1.15mm thick sheet with punch corner
radius of 10mm

       CASE 6 High Strength steel 1.55mm thick with punch corner radii 10mm.




Figure 16: overlay plot showing springback of HS 1.55mm thick sheet with punch corner
radius of 10mm




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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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Table 4: Comparison of springback values in experimental and simulation results by bending
           with punch corner radius of 7.5mm when no compensation was done.

  SHEET             CONFORMING
           ROLLING             EXPERIMENTAL BY SIMULATION
THICKNESS             BENDING
          DIRECTION             (IN DEGREE)  (IN DEGREE)
   (mm)              LOAD (N)

    0.85             0˚              200                4.61˚                4.12˚

    0.85            45˚              200                5.66˚                4.46˚

    0.85            90˚              200                4.81˚                4.33˚

    1.15             0˚              400                3.21˚                2.41˚

    1.15            45˚              400                3.90˚                2.21˚

    1.15            90˚              400                3.64˚                2.10˚

    1.55             0˚              800                1.92˚                1.62˚

    1.55            45˚              800                2.78˚                2.13˚

    1.55            90˚              800                2.12˚                1.92˚

Table 5: Comparison of springback values in experimental and simulation results by bending
           with punch corner radius of 10mm when no compensation was done.

       SHEET                       CONFORMING                               BY
                      ROLLING                 EXPERIMENTAL
     THICKNESS                       BENDIG                            SIMULATION
                     DIRECTION                 (IN DEGREE)
        (mm)                         LOAD(N)                           (IN DEGREE)

           0.85           0˚               200            5.76˚             5.45˚

           0.85           45˚              200            5.99˚             5.15°

           0.85           90˚              200            5.76˚             5.54˚

           1.15           0˚               400            4.14˚             3.48˚

           1.15           45˚              400            4.49˚             3.18˚

           1.15           90˚              400            4.91˚             3.79˚

           1.55           0˚               800            2.18˚             2.56˚

           1.55           45˚              800            2.94˚             2.44˚

           1.55           90˚              800            2.98˚             2.17˚


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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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    Table 6: Table depicts final spring-back values after spring-back compensation due to
                             localisation of compressive stresses.

                                     INITIAL         FINAL
  SHEET                              ANGLE           ANGLE       SPRING-BACK         BY
                ROLLING       Load
THICKNESS                                                       EXPERIMENTAL    SIMULATION
               DIRECTION       (N)    ( ɵi ) (in     (ɵf) (in
   (mm)                                                           (IN DEGREE)   (IN DEGREE)
                                      degree)        degree)

    0.85            0˚        1200    44.95˚         44.57˚        0.3805˚          Nil

    0.85           45˚        1200    44.95˚         44.25˚        0.7009˚          Nil

    0.85           90˚        1200    44.95˚         43.43˚        0.5212˚          Nil

    1.15            0˚        3000     44.92         44.71         0.2101˚          Nil

    1.15           45˚        3000    44.92˚         44.48˚        0.4416˚          Nil

    1.15           90˚        3000    44.92˚         44.33˚        0.5868˚          Nil

    1.55            0˚        4000    44.96˚          45.6˚        -0.6351˚         Nil

    1.55           45˚        4000    44.96˚         45.46˚        -0.4966˚         Nil

    1.55           90˚        4000    44.96˚         44.72˚        -0.2416˚         Nil



Discussions: when a sheet metal is bent as discussed above, it will be in compression on
punch side and tension on die side, and at the mid surface it has elastic stress component
which primarily depends on bend angle. When localised compression is applied then the
neutral surface vanishes forcing the sheet section in compression only and springback is
compensated by localised compression.

5. CONCLUSIONS
With reference to the above studies and results following conclusions are drawn:

1. When the bending load was low and enough to conform to the shape of the die then
   considerable value of springback was seen experimentally and by numerical simulations.

2. It was confirmed by numerical simulation that as the sheet thickness increases the spring-
   back decreases.

3. It was determined that as the punch corner radii increases the spring-back effect increases
   significantly keeping other factors same. The result is in agreement with some of the
   literatures [14].



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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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4. Experiments show that the spring-back can be effectively compensated by localization of
   compressive stresses by the increase in the load in the final phase of bending when the
   sheet is conforming to the shape of the die fully. Experiments show negligible springback
   value but these values should be carefully used while designing the toolings for v-bending
   operation. Thicker sheets showed negative springback but the same was not reflected in
   the numerical simulation results.

6. ACKNOWLEDGEMENTS

The Authors thank the Management of Delhi Technological University for their continuous
support and constant encouragement to carry out this research work.

REFERENCES
[1]. R Hill, the mathematical theory of plasticity Oxford, London, 1950.
[2]. J. Lubahn et al, Bending of an ideal plastic metal, Transactions of the ASME, Volume
72, 1950, 201-208.
[3]. R. Crafoord, Steel sheet surface topography and its influence on friction in a bending
under tension friction test, International Journal of Machine Tools and Manufacture, Vol. 41,
issues 13-14, 2001, 1953-1959.
[4]. Z. Tan et al, An empiric model for controlling springback in V-die bending of sheet
metals, Journal of Materials Processing Technology, 34, 1992, 449-455.
[5]. S. Kalpakjian and R. Schmid, manufacturing processes for engineering materials 4th
edition( Pearson Education Inc., 2003).
[6]. K. A. Stelson and D.C. Gossard, An Adaptive Pressbrake control using an Elastic-Plastic
Material Model, ASME, Journal of Engineering for Industry, 104, 1982, 389-393.
[7]. S. Kim and K. A. Stelson, Real time identification of workpiece material characteristics
from measurements during brakeforming, ASME Journal of Engineering for Industry, 105,
1983, 45-53.
[8]. A. K. Stelson, An Adaptive Pressbrake Control for Strain Hardening Materials, ASME
Journal of Engineering for industry, 108, 1986, 127-132.
[9]. K. J. Weinmann and R.J. Shippell, Bending of HSLA steel Plate, 6th North America
Metal Working Research, Conf. Proc., April-1978, Florida, USA.
[10]. ZaferTekiner, An experimental study on the examination of springback of sheet metals
with several thicknesses and properties in bending dies, Journal of Material Processing
Technology, 145, 2004, 109-117.
[11]. J. Yanagimoto and K. Oyamada, Springback of High-Strength Steel after Hot and
Warm Sheet Formings, Annals of CIRP, Vol. 54/1, 2005, 213-216.
[12]. De Cicco J.M., Projected fuel savings and emmissons reductions from light vehicle fuel
economy standards, Transportation Research, Part-A: Policy and Practice, Vol. 29, no.3,
1995, 205-228.
[13]. Auto Steel Partnership, Tailor welded blank design and manufacturing manual,
Southfield, MI, Report, 2001.
[14]. W.D. Carden, R. H. Wagoner et al, Measurement of springback, International journal of
Mechanical sciences, 44, 2002, 79-101.




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Effect of punch profile radius and localised compression

  • 1. INTERNATIONAL Mechanical Engineering and Technology (IJMET), ISSN 0976 – International Journal of JOURNAL OF MECHANICAL ENGINEERING 6340(Print), ISSN 0976 – AND TECHNOLOGY (IJMET) © IAEME 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) IJMET Volume 3, Issue 3, September - December (2012), pp. 517-530 © IAEME: www.iaeme.com/ijmet.asp ©IAEME Journal Impact Factor (2012): 3.8071 (Calculated by GISI) www.jifactor.com EFFECT OF PUNCH PROFILE RADIUS AND LOCALISED COMPRESSION ON SPRINGBACK IN V-BENDING OF HIGH STRENGTH STEEL AND ITS FEA SIMULATION Vijay Gautam1, Parveen Kumar2, Aadityeshwar Singh Deo3 1 (Department of Mechanical Engineering, Delhi Technological University, Road, Delhi- 110042, India, vijay.dce@gmail.com) 2 (Department of Mechanical Engineering, Delhi Technological University, Road, Delhi- 110042, India, dahiya.sonu1@gmail.com) 3 (Department of Mechanical Engineering, Delhi Technological University, Road, Delhi- 110042, India, dce.aaditya@gmail.com) ABSTRACT Spring-back is a very common and critical phenomenon in sheet metal forming operations, which is caused by the elastic redistribution of the internal stresses after the removal of deforming forces. Spring-back compensation is absolutely essential for the accurate geometry of sheet metal components. In this study an experimental investigation was carried out to determine the effect of punch corner radii on springback in free V- bending operation. The springback compensation was done by localised compressive stresses on bend curvature by the application of compressive load between punch and the die. This experimental springback phenomenon was analysed and validated by an Explicit finite element program using ABAQUS 6.10. In order to determine spring-back in V-bending operation, six numbers of ‘‘V’’ shaped dies and punches with required clearances were designed and fabricated with included angle of 90° for bending of high strength sheet metal with thicknesses: 0.85, 1.15 and 1.55mm. Keeping other parameters same increase in punch corner radius increases the springback and increase in sheet thickness reduces the springback. Springback compensation by localised compressive stress showed negligible springback and the same results were supported by FEA simulations. This model is very useful to control springback on a press brake equipped with controlled computer integrated data acquisition system. Keywords: Bending dies, Explicit solution, Mn-High Strength steel, Springback, V-bending. 517
  • 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME 1. INTRODUCTION Bending is one of the most important sheet metal forming operations by which a straight length of metal strip is transformed into a curved one with the help of suitably designed die and punch. It is very common process of forming steel sheets and plates into channels, drums, automotive and aircraft components. Especially V-Bending process has been thoroughly studied and there is plenty of literature available, among which the most important contribution is Hill's basic theory on pure bending of sheet metals[l]. Hill has derived the complete solution for pure bending of a non- hardening sheet and showed the shift of the neutral surface during bending. Lubahn and Sachs [2] studied the bending of rigid perfectly plastic materials in cases of both plane stress and plane strain, and they predicted no change in material thickness by assuming that the surfaces, including the neutral surface, Crafoord [3]considered the Bauschinger effect by assuming the constant yield surface on reverse straining by fibres overtaken by the neutral surface. And he predicted obvious thickness thinning of rigid-strain-hardening metal sheets. Pure bending is rarely achieved in actual bending process, except that, it is the desired profile of a bend than the temporal stress and strain distribution that is important. The assumptions made in the study of pure bending are generally different from real conditions in v-die bending. Unlike pure bending, V-die bending is not a steady process. A sheet metal is laid over a die and bent as the punch inserts into the die, the bending moment and curvature vary continuously along the sheet and during the deformation, the sheet is stressed in tension on one surface and compression on the other, it is shift of the neutral surface during bending that complicates the analysis[4]. The stress state is complex in bending. Around the neutral plane, the stress must be elastic because complete tensile and compressive stress-strain curves of the material are traversed on both bend side. When the forming tool is removed from the metal, the elastic components of stress cause spring back which changes both the angle and radius of the bent part as shown in Fig.1. The part tends to recover elastically after bending, and its bend radius becomes larger. This elastically-driven change in shape of a part upon unloading after forming is referred to as spring back. Figure 1: terminology for springback in bending [5] Spring-back causes following problems in sheet-metal forming: 518
  • 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME 1) The assembly of the sheet metal components becomes problematic thereby increasing the assembly time and reducing the productivity. 2) In automobile industry different punch corner radius are used for different bending operations which in turn affects the spring-back in components. 3) A wide range of thickness is used in sheet-metal components which again affects the spring-back. 4) High strength sheets are preferred for automotive body as to reduce the thickness which results in reduction of the overall weight of the vehicle. Lighter vehicles are in demand for higher fuel efficiency. However, spring-back characteristic of IFHS has not been investigated widely and very little information is available about its behaviour during V-bending operations. Both material parameters and process parameters affect springback, parameters such as elastic modulus, yield strength, strain hardening ability and thickness of the sheet metal as well as die opening, punch radius and so on interfere the springback in a very complicated way. Figure 2: Methods of reducing springback in V-bending operations [5]. We can calculate springback approximately, in terms of the radii Ri and Rf i.e. initial and final radius of bend curvature (Fig.1) as[5] : ோ௜ ோ௜ ோ௜ = 4(ா௧)ଷ − 3 ቀா௧ቁ + 1 (1) ோ௙ Note that springback increases as the R/t ratio and yield stress Y of the material increases and the elastic modulus E decreases [5]. 2. COMPENSATION FOR SPRINGBACK In general practice there are different ways for springback compensation as shown in the Fig.2 :over-bending (In Fig.2 (a) & (b)), coining or bottoming the punch (shown in Fig.2 (c) and (d)), stretch bending and warm bending[5]. Over-bending is an effective way to compensate for the springback, this can be done in air bending by adjusting the punch/ die angle or punch stroke. Several trials may be necessary to obtain the desired results. Stelson and co-workers have introduced an adaptive control model [6-8] and this model estimates the material characteristics of a sheet being bent from the punch force-displacement data taken early in the bending process, and the in-process 519
  • 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME measured parameters are then used in the calculation of the current final punch position so that the elastic springback can be compensated by over-bending and desired unloaded angle of a bend can be obtained. By this model, disturbances in operation due to variations in material characteristics of a sheet will not affect the modelling results. To predict the loaded shape and the springback of a sheet being bent, however, strenuous measurement and calculation must be performed. In analysis of springback in v-die bending, the curvature of a sheet metal subjected to bending needs to be known. In most analyses, the inner radius of a bend is commonly assumed to be the same as that of the punch. In fact, the radius of curvature is a function of both material and process parameters. If the radius of a punch is of the same order of the sheet thickness, the radius of curvature underneath the punch will be larger than that of the punch, while a sufficiently large punch will cause a smaller bending curvature [9]. Another method is stretch bending, in which the part is subjected to tension while being bent, the springback is reduced as the neutral surface is shifted out of the sheet metal [10]. Since the springback decreases as yield stress decreases, all other parameters being the same, bending may also be carried out at elevated temperatures to reduce springback known as warm bending [11]. Little data is available for springback compensation by bottoming the punch or coining and hence localised compression was the main objective of the study. 3. MATERIAL SELECTION & METHODOLOGY Materials and techniques for cutting weight from vehicles and thereby improving fuel efficiency, are a part of routine automotive engineering practice. Large reductions in weight while maintaining size and enhancing vehicle utility, safety, performance, ride and handling are often thought of as the driving force for future vehicles [12].The body of a car, including the interior, accounts for nearly 40% of the car’s total weight and offers a high potential for lightweight construction [13]. Materials for car body panels require certain specific characteristics to meet the industry’s challenges: rationalisation of specifications for leaner inventory, improved formability for reduced rejection rate and better quality. Higher Strength Low Alloy (HSLA) steels of thinner gauges are getting preference for weight reduction and the resulting better fuel economy. Other quality characteristics under demand are higher yield stress (strength), toughness, fatigue strength, improved dent resistance as well as corrosion resistance in materials used for body panels for improved durability and reliability. Keeping in view of the above factors low carbon high strength steel, was chosen for the springback study and the sheet metal was procured from leading automobile manufacturer with thickness 0.85, 1.15 and 1.55mm. Chemical analysis of the material as per the ASTM- E415-08 reveals that the material is high Manganese and low Carbon and the composition of the steel is given in TABLE 1. Table 1: Chemical composition of HS-steel (wt %) C Si Mn P S Ti Nb Al 0.077 0.013 1.4 0.05 0.011 0.04 0.001 0.032 520
  • 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME The microstructure of high strength steel was revealed and carefully studied under magnification of 200X. The microstructure shown in the Fig. 3 depicts the fine grains of ferrite and complex Manganese Carbides uniformly distributed in ferrite matrix responsible for high strength of the steel. Figure 3: microstructure of high strength steel at 200x showing complex carbides in the matrix of ferrite 3.1. Tensile Properties of High Strength Steel The tension tests were carried out as per ASTM standard E 8M-04 (2004) on INSTRON 4482, 100KN machine, in strength of materials laboratory at DTU Delhi. The HS sheets of 0.85, 1.15 and 1.55mm thickness were tested for the mechanical properties. The tension test specimens were cut from the sheet metal at 0° i.e. parallel to, inclined at 45° and perpendicular at 90° with respect to the rolling direction of the sheet metal. The tensile testing of material was carried out with standard size specimen as shown in Fig.4. Figure 4: tensile specimens cut as per ASTM-E8M in direction parallel to, perpendicular to and inclined at 45° to the rolling direction The typical stress strain curve obtained from the tests is shown in Fig.5 to Fig. 7. Since the departure from the linear elastic region cannot be easily identified, the yield stress was obtained using the 0.2 % offset method. UTS was determined for the maximum load and original cross section area of specimen. The tensile properties of the specimens show that the sheet metal is slightly anisotropic. Hence it can be regarded as isotropic material. 521
  • 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Table 2: Mechanical properties of High Strength steel sheet Young’s Direction Strength Strain Sheet thickness σy yield Modulus wrt. Coefficient hardening metal (mm) (MPa) (MPa) rolling (k) coefficient(n) 0˚ 355 791.39 0.188 High 0.85 210000 45˚ 367 762.0 0.179 strength 90˚ 376 785.4 0.179 0˚ 312 744.7 0.199 High 1.15 210000 45˚ 320 730.18 0.190 strength 90˚ 314 741.7 0.188 0˚ 315 735.9 0.196 High 1.55 210000 45˚ 319 732.0 0.192 strength 90˚ 324 716.51 0.184 Stress- Strain curve for 0.85mm sheet 600 engg stress stress in MPa 400 strain curve X-0-1 200 0 stress strain 0 0.2 0.4 curve:x-90- -200 strain in mm/mm 1 Figure 5: engineering stress strain curve of 0.85mm thick sheet as obtained from tensile test on UTM, depicting that the metal is almost isotropic. 522
  • 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME stress -strain curve for 1.15mm sheet 500 400 stress stress in MPa strain 300 curve Y-0-1 200 100 stress 0 strain -100 0 0.2 0.4 curve: y- 45-1 strain in mm/mm Figure 6: engineering stress strain curve of 1.15mm thick sheet as obtained from tensile test on UTM, depicting that the metal is almost isotropic. stress strain curve for 1.55mm sheet 600 engg stress strain Stress in MPa 400 curve z-0-1 200 engg stress 0 strain 0 0.2 0.4 curve z-45- -200 strain in mm/mm 1 Figure 7: engineering stress strain curve of 1.55mm thick sheet as obtained from tensile test on UTM, depicting that the metal is almost isotropic. The strain hardening exponent (n) and the strength coefficient (K) values are calculated from the stress strain data in uniform elongation region of the stress strain curve. The plot of log (True stress) versus log (True strain) which is a straight line is plotted. The power law of strain hardening is given as: σ = K. εn (2) Where, σ and ε are the true stress and true strain. Taking log on both sides: log (σ) = log (K) + n. log (ε) (3) This is an equation of straight line the slope of which gives the value of ‘n’ and ‘K’ can be calculated taking inverse natural log of the y-intercept of the line (i.e. ln (K)) as shown in Fig.8. 523
  • 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME log stress strain curve n=0.188, K=791.39MPa 6.8 Ln(true stress) y = 0.188x + 6.673 6.6 R² = 0.999 6.4 log stress strain curve 6.2 6 5.8 Linear (log stress strain -4 -2 0 2 curve) Ln (true strain) Figure 8: calculation of n and k value of high strength steel 3.2. Fabrication of Bending Tools As discussed earlier two sets of dies and punches with the punch corner radius 7.5 mm and 10 mm were required for the experimental setup of V-bending in addition to other accessories. The included angle for dies and punches were kept as 90°. The D-2 tool steel for cold working was selected for the bending dies. The drawings of tooling were made in CATIA-V5 as shown in the Fig.9 and Fig.10. The DXF file was used in EDM-wire cut to fabricate the dies and punches. A total of six dies and two punches were designed with two different punch corner radii. The clearance between dies and punch was made equal to sheet thickness to avoid the localized compressive stresses during bending operation. The dies and punches were designed with a holding shank of 25mm length and 12.5mm thickness for easy holding and proper alignment in UTM. The setup was designed for UTM for capturing the data for load and deflection. After fabrication the dies and punches were hardened and tempered in Metallurgy laboratory. D-2 steel dies and punches were heated to 910°C in a muffle furnace for 4hrs. And hardened in air and then tempered at 250°C. After air hardening the hardness was 65Rc and after tempering the hardness was 62HRc. Figure 9: a CAD drawing of the v-die showing various dimensions 524
  • 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Figure 10: a CAD drawing of the v-punch showing various dimensions 4. FEA SIMULATIONS: RESULTS AND DISCUSSIONS The FEA simulations for the above experimental procedure were carried out using ABAQUS 6.10 in CAD lab. The material model for 2D deformable blank was prepared as isotropic hardening following power law of hardening, with young’s Modulus of Elasticity as 210000MPa and Poisson ratio of 0.3 with structure insensitive elastic constants for steel. The plastic data of the steel was directly taken from data acquisition of UTM were true stress and strain. The plastic strain at stress level 350MPa was 0.0, and at 445MPa strain was 0.058mm/mm. The dies and punches were modelled as 2D analytical rigid requiring no meshing properties. The friction condition between the punch and blank was frictionless and a friction value of 0.1 was used between blank and the die. Table 3: Different part attributes used in the FEA Model PARTS 2D DIE ANALYTICAL RIGID PUNCH ANALYTICAL RIGID BLANK DEFORMABLE The simulation results are listed as below: Case-1.Bending of HS steel 0.85mm thick with punch corner radii 7.5mm. Case-2.Bending of HS steel 1.15mm thick with punch corner radii 7.5mm. Case-3.Bending of HS steel 1.55mm thick with punch corner radii 7.5mm. Case-4.Bending of HS steel 0.85mm thick with punch corner radii 10mm. Case-5.Bending of HS steel 1.15mm thick with punch corner radii 10mm. Case-6.Bending of HS steel 1.55mm thick with punch corner radii 10mm. 525
  • 10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME CASE 1: High Strength steel 0.85mm thick with punch corner radii 7.5mm. Figure 11: overlay plot showing springback of HS 0.85mm thick sheet with punch corner radius of 7.5mm. CASE 2: High Strength steel 1.15mm thick with punch corner radii 7.5mm. Figure 12: overlay plot showing springback of HS 1.15mm thick sheet with punch corner radius of 7.5mm CASE 3: High Strength steel 1.55mm thick with punch corner radii 7.5mm. Figure 13: overlay plot showing springback of HS 1.55mm thick sheet with punch corner radius of 7.5mm 526
  • 11. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME CASE 4: High Strength steel 0.85mm thick with punch corner radii 10mm. Figure 14: overlay plot showing springback of HS 0.85mm thick sheet with punch corner radius of 10mm CASE 5: High Strength steel 0.9mm thick with punch corner radii 10mm. Figure 15: overlay plot showing springback of HS 1.15mm thick sheet with punch corner radius of 10mm CASE 6 High Strength steel 1.55mm thick with punch corner radii 10mm. Figure 16: overlay plot showing springback of HS 1.55mm thick sheet with punch corner radius of 10mm 527
  • 12. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Table 4: Comparison of springback values in experimental and simulation results by bending with punch corner radius of 7.5mm when no compensation was done. SHEET CONFORMING ROLLING EXPERIMENTAL BY SIMULATION THICKNESS BENDING DIRECTION (IN DEGREE) (IN DEGREE) (mm) LOAD (N) 0.85 0˚ 200 4.61˚ 4.12˚ 0.85 45˚ 200 5.66˚ 4.46˚ 0.85 90˚ 200 4.81˚ 4.33˚ 1.15 0˚ 400 3.21˚ 2.41˚ 1.15 45˚ 400 3.90˚ 2.21˚ 1.15 90˚ 400 3.64˚ 2.10˚ 1.55 0˚ 800 1.92˚ 1.62˚ 1.55 45˚ 800 2.78˚ 2.13˚ 1.55 90˚ 800 2.12˚ 1.92˚ Table 5: Comparison of springback values in experimental and simulation results by bending with punch corner radius of 10mm when no compensation was done. SHEET CONFORMING BY ROLLING EXPERIMENTAL THICKNESS BENDIG SIMULATION DIRECTION (IN DEGREE) (mm) LOAD(N) (IN DEGREE) 0.85 0˚ 200 5.76˚ 5.45˚ 0.85 45˚ 200 5.99˚ 5.15° 0.85 90˚ 200 5.76˚ 5.54˚ 1.15 0˚ 400 4.14˚ 3.48˚ 1.15 45˚ 400 4.49˚ 3.18˚ 1.15 90˚ 400 4.91˚ 3.79˚ 1.55 0˚ 800 2.18˚ 2.56˚ 1.55 45˚ 800 2.94˚ 2.44˚ 1.55 90˚ 800 2.98˚ 2.17˚ 528
  • 13. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Table 6: Table depicts final spring-back values after spring-back compensation due to localisation of compressive stresses. INITIAL FINAL SHEET ANGLE ANGLE SPRING-BACK BY ROLLING Load THICKNESS EXPERIMENTAL SIMULATION DIRECTION (N) ( ɵi ) (in (ɵf) (in (mm) (IN DEGREE) (IN DEGREE) degree) degree) 0.85 0˚ 1200 44.95˚ 44.57˚ 0.3805˚ Nil 0.85 45˚ 1200 44.95˚ 44.25˚ 0.7009˚ Nil 0.85 90˚ 1200 44.95˚ 43.43˚ 0.5212˚ Nil 1.15 0˚ 3000 44.92 44.71 0.2101˚ Nil 1.15 45˚ 3000 44.92˚ 44.48˚ 0.4416˚ Nil 1.15 90˚ 3000 44.92˚ 44.33˚ 0.5868˚ Nil 1.55 0˚ 4000 44.96˚ 45.6˚ -0.6351˚ Nil 1.55 45˚ 4000 44.96˚ 45.46˚ -0.4966˚ Nil 1.55 90˚ 4000 44.96˚ 44.72˚ -0.2416˚ Nil Discussions: when a sheet metal is bent as discussed above, it will be in compression on punch side and tension on die side, and at the mid surface it has elastic stress component which primarily depends on bend angle. When localised compression is applied then the neutral surface vanishes forcing the sheet section in compression only and springback is compensated by localised compression. 5. CONCLUSIONS With reference to the above studies and results following conclusions are drawn: 1. When the bending load was low and enough to conform to the shape of the die then considerable value of springback was seen experimentally and by numerical simulations. 2. It was confirmed by numerical simulation that as the sheet thickness increases the spring- back decreases. 3. It was determined that as the punch corner radii increases the spring-back effect increases significantly keeping other factors same. The result is in agreement with some of the literatures [14]. 529
  • 14. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME 4. Experiments show that the spring-back can be effectively compensated by localization of compressive stresses by the increase in the load in the final phase of bending when the sheet is conforming to the shape of the die fully. Experiments show negligible springback value but these values should be carefully used while designing the toolings for v-bending operation. Thicker sheets showed negative springback but the same was not reflected in the numerical simulation results. 6. ACKNOWLEDGEMENTS The Authors thank the Management of Delhi Technological University for their continuous support and constant encouragement to carry out this research work. REFERENCES [1]. R Hill, the mathematical theory of plasticity Oxford, London, 1950. [2]. J. Lubahn et al, Bending of an ideal plastic metal, Transactions of the ASME, Volume 72, 1950, 201-208. [3]. R. Crafoord, Steel sheet surface topography and its influence on friction in a bending under tension friction test, International Journal of Machine Tools and Manufacture, Vol. 41, issues 13-14, 2001, 1953-1959. [4]. Z. Tan et al, An empiric model for controlling springback in V-die bending of sheet metals, Journal of Materials Processing Technology, 34, 1992, 449-455. [5]. S. Kalpakjian and R. Schmid, manufacturing processes for engineering materials 4th edition( Pearson Education Inc., 2003). [6]. K. A. Stelson and D.C. Gossard, An Adaptive Pressbrake control using an Elastic-Plastic Material Model, ASME, Journal of Engineering for Industry, 104, 1982, 389-393. [7]. S. Kim and K. A. Stelson, Real time identification of workpiece material characteristics from measurements during brakeforming, ASME Journal of Engineering for Industry, 105, 1983, 45-53. [8]. A. K. Stelson, An Adaptive Pressbrake Control for Strain Hardening Materials, ASME Journal of Engineering for industry, 108, 1986, 127-132. [9]. K. J. Weinmann and R.J. Shippell, Bending of HSLA steel Plate, 6th North America Metal Working Research, Conf. Proc., April-1978, Florida, USA. [10]. ZaferTekiner, An experimental study on the examination of springback of sheet metals with several thicknesses and properties in bending dies, Journal of Material Processing Technology, 145, 2004, 109-117. [11]. J. Yanagimoto and K. Oyamada, Springback of High-Strength Steel after Hot and Warm Sheet Formings, Annals of CIRP, Vol. 54/1, 2005, 213-216. [12]. De Cicco J.M., Projected fuel savings and emmissons reductions from light vehicle fuel economy standards, Transportation Research, Part-A: Policy and Practice, Vol. 29, no.3, 1995, 205-228. [13]. Auto Steel Partnership, Tailor welded blank design and manufacturing manual, Southfield, MI, Report, 2001. [14]. W.D. Carden, R. H. Wagoner et al, Measurement of springback, International journal of Mechanical sciences, 44, 2002, 79-101. 530