| 1. |
- Asnafi, Nader, 1960-
(author)
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On strength, stiffness and dent resistance of car body panels
- 1995
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In: Journal of Materials Processing Technology. - : Elsevier. - 0924-0136 .- 1873-4774. ; 49:1-2, s. 13-31
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Journal article (peer-reviewed)abstract
- There are cases in which practitioners wish to be able to predict the properties of a panel, should they replace one material by another. In this study, the yield strength, stiffness and dent resistance of double-curvature car body panels are treated both theoretically and experimentally. The results of the investigation show that the above-mentioned properties of a pressed panel can be predicted provided that the magnitude of the principal surface strains and the magnitude of the panel radii at the panel centre are known.
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| 2. |
- Asnafi, Nader, 1960-
(author)
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On stretch and shrink flanging of sheet aluminium by fluid forming
- 1999
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In: Journal of Materials Processing Technology. - : Elsevier. - 0924-0136 .- 1873-4774. ; 96:1-3, s. 198-214
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Journal article (peer-reviewed)abstract
- In this investigation, vertical stretch and shrink flanging of sheet aluminium by fluid forming are studied experimentally and theoretically. The theoretical part comprises both analytical modelling and finite-element simulations.The fracture limit in stretch flanging is determined by the plastic strain ratio, the strain hardening exponent, and the uniform strain. The greater the magnitude of these parameters, the greater will be the fracture limit.The maximum applied pressure determines the ’wrinkling’ limit in shrink flanging by fluid forming. The greater is this pressure, the greater ’wrinkling’ limit. This limit is certainly several times greater in magnitude than that in shrink flanging by conventional tools (a rigid punch and die).
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| 3. |
- Asnafi, Nader, 1960-, et al.
(author)
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Tubular hydroforming of automotive side members with extruded aluminium profiles
- 2003
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In: Journal of Materials Processing Technology. - : Elsevier. - 0924-0136 .- 1873-4774. ; 142:1, s. 93-101
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Journal article (peer-reviewed)abstract
- Side member left and side member right, which go from bumper to bumper in a car body, were at the focus in the present study. These side members were produced using straight round (hollow with a circular cross-section) extruded aluminium profiles as tube material. The tubes were bent and hydroformed. Rotary-draw bending yielded the best result. A spread within 8mm after bending was found to be acceptable provided that the bent tube was hydroformed with a high maximum internal pressure (1300bar in this study). Pressure-assisted tool closure (hydroforming tool) should be preferred. Such a tool closure prevents formation of buckles, which may be difficult to straighten out completely during hydroforming. Planeness and parallelity of the press tables and adapters play a significant role, as far as the spread and inplaneness of hydroformed components are concerned. The hydroforming tool must be matched in the press that actually will be used. Proper evacuation (of particularly air) is essential, especially in long hydroforming tools. All cross-sections must be deformed at least 2% (average perimeter enlargement) if the hydroformed components are to exhibit a reasonable spread. The critical (fracture) cross-sections predicted by finite-element simulation corresponded to those found in practice. However, the finite-element simulation was not able to predict formation of wrinkles at the tube ends caused by excessively large strokes. Such wrinkles were obtained in practice.
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| 4. |
- Bernspång, Lars, et al.
(author)
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Verification of an explicit finite-element code for the simulation of the press forming of rectangular boxes of coated sheet steels
- 1993
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In: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136 .- 1873-4774. ; 39:3-4, s. 431-453
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Journal article (peer-reviewed)abstract
- The industrial aim for shorter lead times in the development process for new products has encouraged a fast development of finite-element procedures for the simulation of sheet-metal forming. These procedures can shorten the design stage for new sheet-forming tools and the try-out period. In order to reach these goals robust calculation procedures and extensive verification of results are required. This paper presents a detailed validation of one promising type of finite-element code, namely the explicit code DYNA3D. Experiments for verification were performed on a zinc-coated sheet steel with a hot-dip galvanized coating. Stretch forming and deep drawing of cylindrical cups were performed. Rectangular boxes were formed from rectangular blanks and blanks with cut corners. Dies both with and without draw beads were used. Punch forces, flange draw-in and strain distributions were measured. The pressings with cylindrical shape were used to determine coefficients of friction with a fitting procedure based on comparison of data from pressings and from the DYNA3D calculations. These tribological data and constitutive data of the steels were used in the simulation of pressings of rectangular boxes. The calculations with DYNA3D gave a good description of flange draw-in and the strain distributions in the pressings. In most of the cases studied the punch-force curves were well reproduced. It is concluded that the present code is well suited for the simulation of sheet-forming operations.
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| 5. |
- Edberg, Jonas, et al.
(author)
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The wedge rolling test
- 1994
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In: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136 .- 1873-4774. ; 42:2, s. 227-238
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Journal article (peer-reviewed)abstract
- A method for the evaluation of friction models is described. A wedge is rolled to uniform thickness, a range of reductions being investigated thereby in one experiment. Finite-element simulations are performed in order to estimate the friction parameters that can be used in the simulation of hot rolling. The influence of the material parameters and the friction parameters on the calculated results are investigated and the latter are compared with experimental results. It is shown that it is possible to separate the influence of the material parameters and the friction parameters, thus enabling the friction parameters to be evaluated from a minimum number of experiments.
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| 6. |
- Ervasti, Esa, et al.
(author)
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Behaviour of longitudinal surface cracks in the hot rolling of steel slabs
- 1999
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In: Journal of Materials Processing Technology. - 0924-0136 .- 1873-4774. ; 94:2-3, s. 141-150
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Journal article (peer-reviewed)abstract
- The behaviour of a longitudinal V-shaped crack, on the surface of a continuously cast steel slab, is studied during hot rolling. The analysis is carried out by means of the commercial FE-code LS-DYNA3D. Process parameters obtained from industry are used as a reference. The slab of initial width 1000 mm and thickness 220 mm is rolled down to 30 mm. It is assumed that the material can be treated as rigid-perfectly plastic and that the cracks do not propagate. The latter assumption is in agreement with industrial observations for a steel grade similar to that analysed here. The aim of the study is to investigate the possibility of controlling the plastic deformation so that the cracks disappear or so that their deteriorating effects are minimised. The analysis is focused upon the influence of friction, roll radius and rolling schedule on the change in the shape of a crack of initial depth 20 mm and a crack angle of 6 degrees. The reliability of the simulations is checked by pilot-plant experiments using aluminium as the model material for steel. The results indicate that it is not possible to prevent the bottom side surfaces of the crack from coming into contact, especially not for small reductions/pass and small roll radii. The influence of friction was found to be marginal. Contact between the crack surfaces is found already at the beginning of the rolling, as the V-shaped crack is being changed to Y-shape. Considering the upper part of the crack, this remained open during the majority of the schedules studied. However for heavy reduction/pass and a large roll radius, this part of the crack is closed also, but not before the final passes. If the bottom side surfaces of the crack are in complete contact, they are prevented from further oxidisation. It is assumed that for such conditions the deteriorating influence of the bottom part of the defect decreases during subsequent rolling. During the elongation of the workpiece, the oxide flake of the Y-crack bottom is broken into splinters with oxide free material in between, making the creation of a high performance weld possible. Provided that this supposition is correct, the best results should be obtained for Light reductions/pass at the beginning of the rolling, resulting in an early closure of the crack bottom, followed by heavy reductions/pass enabling also the closure and oxide splintering of the upper part of the folded crack.
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| 7. |
- Ervasti, Esa, et al.
(author)
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Transversal cracks and their behaviour in the hot rolling of steel slabs
- 2000
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In: Journal of Materials Processing Technology. - 0924-0136 .- 1873-4774. ; 101:1-3, s. 312-321
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Journal article (peer-reviewed)abstract
- The behaviour of transversal cracks in reverse rolling of steel slabs is analysed. For this purpose a commercial FE-program, LS-Dyna3D has been utilised. Process parameters from an industrial rolling mill have been used as a reference. One basic assumption is that the cracks do nor propagate for the steel considered. This is in agreement with observations in industry. Furthermore the material is treated as elastic-plastic. The aim of the study is to investigate the possibility of controlling the plastic deformation so that the cracks disappear at the same time as their boundary surfaces are prevented from getting into contact with each other. If they should get into contract, a defect in the form of an oxide flake should be created. The optimum situations are assumed to be found when the depth of the crack decreases more rapidly than the height of the slab at the same time as the crack width increases. For such rolling conditions it should be possible to find the crack bottom on the workpiece surface without any trace let by the original crack. During rolling two phenomena are present, counteracting each other. At the entrance to the roll gap, the crack widens, which is regarded as beneficial. The explanation to the phenomenon is that when the front edge of the crack gets into contact with the roil it is dragged towards the gap at the same time as its velocity is rapidly changed from horizontal to become parallel to the tangential velocity of the roll. If this phenomenon did not occur, the risk for the creation of a permanent oxide Rake should be big. During the passage of the gap the widened crack angle is decreased. The decrease of the angle is explained by the high hydrostatic pressure that is associated with the roll/workpiece contact surface. At the exit the angle is slightly increased once again which might be explained by the fact that the back edge of the crack is still in contact with the roll and its movement is retarded because of friction. Thus the final result depends on which of the two phenomena, widening or contraction that has been dominating. The analysis shows that light reductions/pass, small roll radius and high friction are preferred. A deep crack of V-shape is easily transformed to Y-shape. This means that a remaining defect should be formed at the bottom of the crack. Reverse rolling is found to be beneficial because this way of rolling implies that the crack does nor tilt so easily which should result in the formation of an oxide flake.
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| 8. |
- Gunnarsson, Lars, et al.
(author)
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In-process control of blank holder force in axi-symmetric deep drawing with degressive gas springs
- 1998
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In: Journal of Materials Processing Technology. - : Elsevier. - 0924-0136 .- 1873-4774. ; 73:1-3, s. 89-96
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Journal article (peer-reviewed)abstract
- A new blank-holder system with degressive gas springs has been developed and evaluated for axi-symmetric deep drawing. In contrast to most published works in this field, in which hydraulic systems are utilized, the new system is integrated with the tool body instead of with the press, the advantage with this approach being that the system is independent of press facilities. The new system consists of inter-connected active and passive gas springs, moving parallel during forming. With the new system, degressive, constant and progressive blank-holder force (BHF) trajectories can be obtained. The BHF-trajectories were pre-determined with a specially developed software. Two materials, an ordinary deep drawing steel and a high-strength rephosphorized steel, have been analysed. The thickness was 0.7 mm and a water emulsion was used as lubricant. The blank holder force, punch force, total force and the separation of the blank holder (wrinkle detection) were recorded continuously during the pressing operation. With a degressive BHF-trajectory the process window between fracture and wrinkling in axi-symmetric deep drawing is larger compared to those obtained with constant and progressive trajectories, and also, the LDR is higher.
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| 9. |
- Hansson, Lars, et al.
(author)
-
The effect of microwave drying on Norway spruce woods strength: a comparison with conventional drying
- 2003
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In: Journal of Materials Processing Technology. - 0924-0136 .- 1873-4774. ; 141:1, s. 41-50
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Journal article (peer-reviewed)abstract
- The purpose of the present work was to investigate whether the drying method itself affects strength of wood apart from fibre direction, density, temperature in the wood, moisture content and with which angle the microfibril is placed in the middle layer at the secondary cell wall S2. The drying methods compared were microwave drying and conventional air-circulation drying, and the species tested was Norway spruce. The result shows that it is not possible to demonstrate any difference between the two drying methods with respect to the strength of the wood. What affects wood strength are such variables as moisture content, number of annual rings and the density properties weight, width and thickness
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| 10. |
- Högberg, Dan, 1965-
(author)
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Use of finite element method in trailer deck design
- 2001
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In: Journal of Materials Processing Technology. - : Elsevier. - 0924-0136 .- 1873-4774. ; 117:1-2, s. 238-243
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Journal article (peer-reviewed)abstract
- The employment of the finite element (FE) method in conceptual and detail design of a secondary deck platform for trailer bodies is presented. The main objective of the project is to design a versatile, light and cheap secondary deck. To assist the designer with sufficient means for the concept generation and evaluation process, the FE method is used in order to gain fair structural analysis data. Assistance from the computer software stimulates creativity, reduces the time required in the design process, and eventually increases probability to gain customer satisfaction. This approach is precise enough for comparison purposes at this stage of the design process. The structural data gained is compared mainly with weight and access criteria.
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