STRAIN STUDIES IN SHEET METAL STAMPINGS
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presented by:
Amit Mukund Joshi

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STRAIN STUDIES IN SHEET METAL STAMPINGS
INTRODUCTION
Sheet metal is one of the most important semi finished products used in the steel industry, and sheet metalforming technology is therefore an important engineering discipline within the area of mechanicalengineering. Sheet metals are characterized by a high ratio of surface area to thickness. Sheet metal forming isbasically conversion of a flat sheet metal into a product of desired shape without defect like fracture orexcessive localised thinning.In automobiles the sheet metal is deformed into the desired and brought into the required form to getautobody pressings like bonnet, bumpers, doors, etc. In aircraft’s sheet metal is used for making the entirefuselage wings and (body). In domestic applications sheet metal is used for making many parts like washingmachine body and covers, iron tops, timepiece cases, fan blades and casing, cooking utensils etc.The products made by sheet-forming processes include a large variety of shapes and sizes, ranging fromsimple bends to double curvatures with shallow or deep recesses. Typical examples are metal desks, appliancebodies, aircraft panels, beverage cans, auto bodies, and kitchen utensils. In many cases while deforming thesheet metal, the component fractures at certain point. The causes of failure are parameters related to formingprocess.Traditional evaluation of formability is based on both intrinsic tests and simulative tests.The intrinsic tests measure the basic characteristic properties of materials that can be related to their formability. Thesetests provide comprehensive information that is insensitive to the thickness and surface condition of the material.Examples of intrinsic tests are Uniaxial tensile test, Plane strain tensile test, Marciniak Biaxial Stretching test,Hydraulic Bulge test, Marciniak In-Plane Sheet torsion test, Miyauchi shear test, Hardness test. The simulative testssubject the material to deformation that closely resembles the deformation that occurs in a particular forming operation.Examples of these tests include Ericksen , Olsen, Fukui, Swift tests.
CONCEPTStrain analysis by grid marking is a useful method, which has been used effectively to solve the problems inmetal forming. When sheet metal is formed, its surface is subjected to different stresses. This results into nonuniform strains to be developed in the formed part. Thus there will be regions of high strains as well as lowstrains, which may lead to wrinkling or fracturing of the material. By the grid marking method the areas ofhigh strain can be easily identified. The sheet is marked with the grid before forming process is carried out.After the sheet metal is deformed into desired shape, strain distribution can be visualized and critical areas ofstrain will be found by FLD (forming limit diagram) and control can be planned by varying the formingparameters.
GRIDMany types of circle grid patterns have been used, such as square arrays of contacting or closely spaced noncontacting circles and arrays of overlapping circles. With small closely spaced circles, it is possible todetermine strain gradients accurately. After deformation the circle is transferred into ellipse. The direction ofthe strains is indicated by the major and minor axis of the ellipse. Circles of 2.5mm diameters have been foundto be a good size.
GRID MARKING METHODS
There are various techniques available for applying the grids. Circular grids are normally made in twodifferent ways. They can be made electro-chemically or photo-chemically, both processes having particularadvantages and disadvantages.
1) Photochemical etching - This is an accurate method of grid marking. The following steps are involved inmarking the grid by this method.a) cleaning of metal surfaces.b) Covering with photo resist.c) Illuminating with UV light.d) Developinge) Etchingf) Surface Cleaning.Cleaning of metals is achieved with toluene (C6H5CH3) or trichlorethylene (CHCl : CCl2) but can also be donein an acid bath for a shorter time. Photo resist emulsion is applied on the blank. Then the emulsion is coveredwith a photographic negative and exposed to strong ultra-violet radiation. The image of the negative isdeveloped like a photographic print. Very fine, sharp lines can be printed on the blank in this way. Thephotogrid should be placed in close contact with the metal surface. This can be achieved by creating vacuumbetween the surface and the grid. A dark room is required for the development. The photogrid prepared in thisway is removed by chemicals and rubbing. This difficulty can be overcomed by further etching the metalsurface.After developing the resist already illuminated, the etchant is applied to the metal surface for etchinguncovered metal. The different acid solutions used for etching are HCl, HNO3, HF, etc.). After etching themetal surface is cleaned with toluene or trichlorethylene.
Electrochemical Marking - This method is the most preferred method for applying grids since it is easyand quick. In this process an electric stencil is placed on the cleaned blank. A felt pad soaked withelectrolyte is placed on the top of the blank and an electrode (flat or roller type) is placed above the feltpad. A wooden block is kept above as shown in the figure. Leads from a 14 V power source are attached tothe electrode and the blank. Current varies from 15 – 200 A depending on stencil size and line density.After applying pressure over the electrode the felt pad will squeeze, the electrolyte will pass throughstencil and comes in contact with blank etching the grid pattern electrochemically into the blank. Afteretching the blank is washed with a neutralizing solution.
Dividers and steel rule - This is the most simple and quick method. This method is suitable formeasurement on more or less flat surface. On curved surface the measured dimension will be less i.e. itwill measure the chord length rather than arc length. The accuracy is also limited.
2) Mylar Tape – this is a transparent scale to measure the strain directly. This tape has diverging lines scaledto read directly in percent strain. This scale is produced by photographic printing from a negative on tofilm. The scale is placed over an ellipse over a sharp radius and then shifted until the diverging lines lineup with the major axis of the ellipse. The percent strain is measured directly from the scale. The scale isnext turned 90 degrees to read the minor strain.
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