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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
1(Department of Mechanical Engineering, Delhi Technological University, Road, Delhi-110042, India, [email protected])
2(Department of Mechanical Engineering, Delhi Technological University, Road, Delhi-110042, India, [email protected])
3(Department of Mechanical Engineering, Delhi Technological University, Road, Delhi-110042, India, [email protected])
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.
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 –
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 –
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
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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.
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
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 4
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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.