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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

    517

    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.

    INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING

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

    IJMET

    I A E M E

  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

    6340(Print), ISSN 0976 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) IAEME

    518

    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:

  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

    6340(Print), ISSN 0976 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) IAEME

    519

    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(

    )

    3 + 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

  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

    6340(Print), ISSN 0976 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) IAEME

    520

    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 enginee

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