gh ratio of surface area to thickness. Sheet metal forming is
basically conversion of a flat sheet metal into a product of desired shape without defect like fracture or
excessive localised thinning.
In automobiles the sheet metal is deformed into the desired and brought into the required form to get
autobody pressings like bonnet, bumpers, doors, etc. In aircrafts sheet metal is used for making the entire
fuselage wings and (body). In domestic applications sheet metal is used for making many parts like washing
machine 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 from
simple bends to double curvatures with shallow or deep recesses. Typical examples are metal desks, appliance
bodies, aircraft panels, beverage cans, auto bodies, and kitchen utensils. In many cases while deforming the
sheet metal, the component fractures at certain point. The causes of failure are parameters related to forming
process.
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. These
tests 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 tests
subject 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. CONCEPT
Strain analysis by grid marking is a useful method, which has been used effectively to solve the problems in
metal forming. When sheet metal is formed, its surface is subjected to different stresses. This results into non
uniform strains to be developed in the formed part. Thus there will be regions of high strains as well as low
strains, which may lead to wrinkling or fracturing of the material. By the grid marking method the areas of
high 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 of
strain will be found by FLD (forming limit diagram) and control can be planned by varying the forming
parameters.
GRID
Many types of circle grid patterns have been used, such as square arrays of contacting or closely spaced non
contacting circles and arrays of overlapping circles. With small closely spaced circles, it is possible to
determine strain gradients accurately. After deformation the circle is transferred into ellipse. The direction of
the strains is indicated by the major and minor axis of the ellipse. Circles of 2.5mm diameters have been found
to be a good size.
2.5 mm

Patterns of Circle Grids GRID MARKING METHODS
There are various techniques available for applying the grids. Circular grids are normally made in two
different ways. They can be made electro-chemically or photo-chemically, both processes having particular
advantages and disadvantages.
1) Photochemical etching - This is an accurate method of grid marking. The following steps are involved in
marking the grid by this method.
a) cleaning of metal surfaces.
b) Covering with photo resist.
c) Illuminating with UV light.
d) Developing
e) Etching
f) Surface Cleaning.
Cleaning of metals is achieved with toluene (C
6
H
5
CH
3
) or trichlorethylene (CHCl : CCl
2
) but can also be done
in an acid bath for a shorter time. Photo resist emulsion is applied on the blank. Then the emulsion is covered
with a photographic negative and exposed to strong ultra-violet radiation. The image of the negative is
developed like a photographic print. Very fine, sharp lines can be printed on the blank in this way. The
photogrid should be placed in close contact with the metal surface. This can be achieved by creating vacuum
between the surface and the grid. A dark room is required for the development. The photogrid prepared in this
way is removed by chemicals and rubbing. This difficulty can be overcomed by further etching the metal
surface.
After developing the resist already illuminated, the etchant is applied to the metal surface for etching
uncovered metal. The different acid solutions used for etching are HCl, HNO
3
, HF, etc.). After etching the
metal surface is cleaned with toluene or trichlorethylene.

UV Light
Photochemical technique with vacuum between photogrid and the metal surface. 2) Electrochemical Marking - This method is the most preferred method for applying grids since it is easy
and quick. In this process an electric stencil is placed on the cleaned blank. A felt pad soaked with
electrolyte is placed on the top of the blank and an electrode (flat or roller type) is placed above the felt
pad. A wooden block is kept above as shown in the figure. Leads from a 14 V power source are attached to
the 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 through
stencil and comes in contact with blank etching the grid pattern electrochemically into the blank. After
etching the blank is washed with a neutralizing solution.
Power pack
200 A 10 V A.C
Wooden block
Metal electrode
Felt pad
Stencil
Steel
Floor
Set Up for Electrochemical Marking
Electrolyte Proper electrolyte should be used for marking a specific metal. A basic solution of electrolyte is
composed approximately as :-
potassium chloride
: 80 gms
Sodium chloride
: 90 gms
Nitric acid
: 100ml
Hydrochloric acid
: 100ml
Water
: 4.5litre
This solution has been found satisfactory on ferrous and non ferrous metals STRAIN MEASUREMENT
After sheet metal is formed the marked circles will deform into ellipses of different sizes. Strain is calculated
from the following formula.
Major strain = (major axis length original circle dia ) X 100
original circle dia.
Minor strain = (minor axis length original circle dia ) X 100
original circle dia.
1) Dividers and steel rule - This is the most simple and quick method. This method is suitable for
measurement on more or less flat surface. On curved surface the measured dimension will be less i.e. it
will 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 scaled
to read directly in percent strain. This scale is produced by photographic printing from a negative on to
film. The scale is placed over an ellipse over a sharp radius and then shifted until the diverging lines line
up with the major axis of the ellipse. The percent strain is measured directly from the scale. The scale is
next turned 90 degrees to read the minor strain.
Mylar Tape 3) Travelling microscope - This is the most widely used method for measuring the changes in the dimension
of grid circles. There are two right angle slides on which work is mounted. The work is positioned under
the microscope. Cross wire is aligned at one end and the measurement is taken. The cross wire is then
aligned on the other end by moving the work table and the measurement is taken. The difference between
the two readings gives the absolute measurement. This is an accurate method. Two persons can get
different readings because of error in aligning the two axes.
4) Grid Circle Analyser (GCA) - They use a solid state digital array camera with a built in light source, a
computer, keyboard, and CRT display. The image of given deformed circle is displayed on the CRT and a
least squares curve fitting program selects the most suitable ellipse, which is displayed simultaneously.
The major and minor strains computed from the equation for ellipse and the diameter of the original circle
are displayed on the screen.
GCA Lay Out
FORMING LIMIT DIAGRAM CONCEPT
To reduce experimentation through trial and error method which is both expensive and time consuming,
Keeler and Goodwin proposed grid strain analysis. This involves etching a pattern of fine circles on the sheet
metal before pressing. After pressing the circles will be deformed into ellipses which can be measured to
indicate major and minor strains produced in the component. An estimate of how close the metal is to failure
is obtained by reference to the FLD, which is a plot of the major and minor strains at fracture over a wide
range of conditions. Forming limit diagrams indicate the limiting strains that sheet metals can sustain over a
wide range of major to minor strain ratios.
camera
Light source
sample
Interface
Oscilloscpe display
Computer
Keyboard
Trigger unit
Curve fit Display Two types of tests are used to determine these limiting strains. The first category of test involves stretching
test specimens over a punch for example the hemispherical punch method producing some out of plane
deformation. The hemispherical punch method for determining FLD uses circle gridded strips of the test
material ranging in width from 25.4 to 203 mm that are clamped in a die ring and stretched to fracture by a
steel punch (102 mm dia). The strains are measured in and around regions of vis