Electrochemical

Strain Grid Marking

Moving Marking Technology Forward

Strain Grid Marking Equipment

Strain grid marking has become, over the years, the standard method for analysing stress in sheet metal forming.

We manufacture two power units capable of producing the large, geometric patterns required for analysis. These

patterns are laid down directly on to the surface of the sheet, prior to forming, by oxidising the material with

electro-chemistry. This process is favoured, due to the ease of marking and speed of operation. More importantly

however, the process is stress free, and ` forms a permanent mark without compromising the structure of the

material. Most metals form a dark oxide mark that can be measured with ease once the sheet metal is formed.

The ME96 and ME3000TSG Strain grid kits are designed specifically for strain grid marking applications and are

supplied as complete marking kits ready for use. The choice of machine will depend on the area that is to be

marked.

ME96 This is our heavy-duty strain grid marking kit. The machine can deliver 100amps output and can be used with our

largest strain grid stencils. Supplied with pistol roller with trigger operated output control.

Contents:-

Part Number Description Quantity

ME96 100 amp marking unit 1

UC10 & UC11 100 amp output cables (head cable UC10 is mounted into pistol

grip handle. EC11 Earth cable with clip. Cables 2m long

1 pr

UC8 output cables for hand marking. Cables 1m long. 1pr

R0ll280 280mm long single roller 50mm in diameter with pistol grip

handle and trigger

1

FP5D21p 280mm x 500mm pack of felt pads 1pk of 5

SG Strain grid stencil - customers choice 1

HM5 13mm x 31 mm hand marker 1

HM5/1 Spare felt / grid pads for HM5 1pk of 10

Mains cable and plug appropriate for country of use 1

ME8 1L of ME8 non-neutralising electrolyte for ferrous metal marking 1

ME5 lL of ME5 electrolyte for non-ferrous metal marking 1

MN2 lL neutraliser for ME5 electrolyte 1

ME3000TSG.

This machine gives a 20amp output and is

suited for relatively small strain grid

applications, typically 100mm wide marks. The

marking output can be set for continuous

output, or timed. The machine is supplied in a

high quality wooden instrument case with all

accessories required for marking including four

strain grid stencils. The kit also contains stencil

paper and an HM5 hand marker which are used

for part marking the test pieces.

Contents:-

Part Number Description Quantity

ME3000T20SG 20 amp marking unit 1

ME3000 kitbox Wooden kit box 1

UC8 1m long output cables 1pr

R0ll4 or

RM3

100mm wide roller or

100m wide rocker marker

1

FP2D21p 150mm x 200mm pack of felt pads 1pk of 5

SG77A Strain grid stencil 1

SG77B Strain grid stencil 1

SG77C Strain grid stencil 1

SG77D Strain grid stencil 1

HM5 13mm x 31 mm hand marker 1

HM5/1 Spare felt / grid pads for HM5 1pk of 10

FS8 Footswitch for timer control operation 1

Mains cable and plug appropriate for country of use 1

ME8 125 125ml of ME8 non-neutralising electrolyte for ferrous metal

marking

2 x 125ml

ME5 125ml of ME5 electrolyte for non-ferrous metal marking 1 x 125ml

MN2 125ml neutraliser for ME5 electrolyte 1 x 125ml

Note: Photo shows BRSK stencil tape as option for marking text on parts. Used with P-Touch printers.

Universal Marking Systems Application Information

This technique is used in many press-shops to reduce the time and costs of die try-out and to

transform press trouble-shooting from a series or random guesses into logical process. The

rudiments of this approach are described here.

Why circle grid analysis? In sheet metal forming we are looking at the permanent plastic

deformation of sheets by stretching, usually, in a complex way. If the sheet is marked in a non-

directional pattern (circles) then any deformation can be measured. If the pattern were, for

example, squares, then fruitful measurements would be impossible (fig.1).

It is necessary to cover a reasonable area - 250mm x 250mm, for example - when studying

sheet metal deformation, for the critical region of the stamping cannot be precisely pinpointed:

besides it can shift due to

variations in press setting and

lubrication, for instance.

How do we analyse what we

see?

A simple example is shown in

figure 2, considering only major

strain (or elongation).

It is now clear, even at this early

stage, why this approach is

termed circle grid analysis. But

one important point has not been settled. The circle size and pattern. The importance of circle

size is also demonstrated in figure 2.

If the original circle diameter had

been 50 units (assuming no space

between circles) then our analysis

would have shown 26%

elongation - and missed the

important peak of strain of 80%.

Widely spaced circles could also

miss peak strains, which could

occur between them.

An acceptable, well-tried design is

I 0

I 1

I 2

I 0

?

I 1 = M a j o r s t r e t c h

I 2 = M i n o r s t r e t c h

F i g u r e 1 .

U s e f u l i n f o r m a t i o n c a n b e d e r i v e d f r o m c i r c l e s , b u t n o t

s q u a r e s .

I 0

I 1 I 1 I 1 I 1 I 1

1 0 1 3 1 8 1 2 1 0

0 0 . 3 0 . 8 0 . 2 0

0 % 3 0 % 8 0 % 2 0 % 0 %

I 0 = 1 0 u n i t s

M a j o r s t r e t c h

S t r a i n e = I 1 - I 0

E l o n g a t i o n I 0

[ s t r a i n x 1 0 0 ]

F i g u r e 2

that of 0.1” or 2.0mm diameter circles, close but not touching, in arrays of four, delimited in a

square matrix for easy identification.

Other patterns approximating this concept are also acceptable.

Summarising at this stage: to be able to identify regions of high strain is useful in itself, but

stain measurements from deformed small-diameter circles can provide far more useful

information. Firstly, we must consider both major and minor strains. For a given major strain,

e1, the minor strain e2, can be positive, zero or negative.

This is illustrated in figure 3.

The point of supreme importance here is that if, on a split

pressing, a deformed circle, close fracture, but not distorted

by it, is measured and the major and minor strains

(elongation’s) calculated and this is done for a wide range of

strain combinations. The resultant plot of e1 vs. e2 is termed

a Forming Limit Diagram (Figure 4) and it separates

successful strain combinations.

The FLD’s can be constructed in the

laboratory using a stretch-forming press

and sheet metal blanks of varying widths

to give different strain combinations at a

fracture.

Different metals naturally give different

FLD’s. FLD’s for low alloy steels,

aluminium alloys and titanium sheet, for

example, gives FLD’s of widely differing

shapes and levels.

Conveniently, the shape of the FLD’s for the type of sheet steel used in the motorcar, domestic

appliance, container, etc., industries are very similar and fairly represented by figure 4. The

level of FLD however changes with steel grades - the best grade giving the highest, or putting it

more correctly, the major strain (e1) at a minor strain of zero (e2=0) is greater for a better steel

grade (this strain state is known as plane

strain).

Studying figure 5 it is clear that major strain

alone is an inadequate parameter, for e1 = 40%

would represent success at e2 = 40%, failure at

e2 = 0 and not a whisper of a problem at e2 = -

20%.

+ e 1 + e 1 + e 1

- e 2 e 2 = 0 + e 2

M i n o r s t r a i n s c a n b e n e g a t i v e , z e r o , p o s i t i v e .

F i g u r e 4 .

A f o r m i n g l i m i t d i a g r a m ( F L D )

0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7

- 0 . 5 0 0 . 5

M a j o r s t r a i n

+ e 1

Typical of sheet

steel

Minor strain -e2 Major strain +e2

Succeed

Fail

Fail

Succeed

0

2 0

4 0

6 0

8 0

1 0 0

- 5 0 0 5 0

X Y

( S c a t t e r ) 1

% Elongation

Figure 5.

The importance of the minor strain

It is often instructive to represent the

strains in a panel by a plot of major

values taken in the critical area shown

in figure 6, with the values approaching

a ‘ ceiling’ - this will be the minor strain

limit from the FLD. Assume for a

moment that this is a die- try out

situation. It is now possible to see how

close a successful pressing is to failure.

If too close then action should be taken

before putting tools into production.

Take now a production splitting

problem represented in figure 8. The problem is now to move a ‘below the line’ and this can be

done in various ways by press adjustment, by lubrication, by changing the sheet metal. Until

the advent of FLD, it had now been

realised that failure could be

changed to success by increasing

the total strain, i.e. increasing e2

by the route A C.

The FLD is the most important

contribution to sheet metal forming

technology of the last 30 years and

now accepted in the press-shops of

the world as a method for firstly,

assigning a degree of severity to a

pressing and, secondly, as a logical

approach to remedying production stamping problems in the press shop.

LimitMajor

Strain

Only critical circles shown

Figure 6.

Severity point - how near is the part to failure?

Figure 7

0

10

20

30

40

50

60

70

80

-40 -20 0 20 40

B

A

C

Marking Information

The Universal Marking Systems strain grid marking equipment can be used on ferrous and non-

ferrous metals, producing either an oxide or etched mark on the test piece. The marking

process is quite simple and fast and is ideal for on-site testing. It is important to follow the

instructions of the equipment and solutions being used. Please contact us or your local Agent

or distributor for copies of our product sheets.

Ferrous metals.

There are a number of electrolytes that can be used, but ME8 is good choice due to its non-

neutralising properties. It will produce a good dark oxide mark on most grades steels. For a

very black mark on mild steels, then ME3 can be used but care must be taken to neutralise the

mark afterwards. For stainless steels, then ME6 is a good electrolyte to use, again neutralising

is important.

Non-ferrous metals.

ME5 is a good general-purpose electrolyte that will produce a white or grey oxide mark on

aluminium or can be used to etched mark. Neutralising must be done after marking.

Technique.

1. Use a large flat workspace for the marking process

allowing room for the test piece and the equipment

to be set-up and used safely.

2. Lay the test piece on the flat surface and degrease

and wipe dry.

3. Position the equipment close enough to the test

piece so the output cables reach easily.

4. Place the stencil onto the test piece in the correct area where the analysis is to be taken

after pressing or forming.

5. Lay the felt pad on top of the stencil

6. Soak the felt pad in fresh electrolyte. Use the roller marker to distribute the electrolyte over

the complete area of the felt pad.

7. Using a tissue, soak up any excess electrolyte that may over spill onto the test piece.

8. Set the marking unit for marking. According to oxide or etch marking and for the electrolyte

that is being used.

9. Place the roller marker on the felt pad close to the edge.

10. Depress the trigger on the pistol grip (ME96 roller marker) and gently using the weight of

the roller. Roll over the length of the stencil pattern several times.

11. Gently, lift the corner of the stencil to see if the

mark is clear. If not roll over the stencil again

until the desired mark is obtained.

12. Remover the felt and stencil

13. Place them in a tray if they are to be used again

or wash them out in cold running water and

store carefully.

14. Dry the test piece with clean tissue.

15. If required, neutralise with the appropriate

neutralising solution (MN1 for ferrous metals or MN4 as an immersion neutraliser or MN2

for non-ferrous metals).

Press or form the test piece

and measure the results.

Measuring the results.

The simplest way is to use our Universal Marking Systems Mylar rules. These are graduated in

the percentage of stretch of the circle. Optical measuring systems are also available.

Universal Marking Systems Strain Grid stencil sizes

Universal Marking Systems Strain grid stencils are produced in three standard sizes.

Size Stencil size Pattern size

A 150mm x 300mm 100mm x 280mm

B 300mm x 300mm 250mm x 280mm

C 500mm x 300mm 400mm x 280mm

Strain grid ref. No. Pattern style. Example of Pattern

SG01 6.1mm circles

SG01 5mm circles

SG02

SG03 20mm circles

SG04 0.25 inch circles

SG05* 2.5mm circles

SG5a 1mm circles (I.D)

SG06 2mm increments

SG07

SG08 5mm circles in 12.5 mm squares

SG09 10mm circles

SG10 2mm circles

SG11 2.5mm circles

SG12 0.5inch circles

SG13* 6.4mm circles in

15mm squares

SG14 1mm squares

SG15 20mm circles

SG16 5mm circles

SG17 15mm circles overlapping

SG18* 3/4 inch squares with 1 inch circles

SG19 10mm circles

SG20* 2mm circles in 6mm squares

SG21 2mm circles in 6mm squares

SG22 0.1 inch circles in 0.25 inch squares

SG23 10mm circles

SG24* 10mm circles on 8mm squares

SG25 20mm squares

SG26* 0.1inch circles

SG27* 8mm circles

SG28 5.5mm increments

SG29* 5 mm circles

SG30* 5mm circles

SG31 5mm circles

SG32 5mm circles with 2.5mm squares

SG33-500 5mm squares

SG34* 0.1 inch circles

SG35* 0.1 inch circles As above

SG36* 0.1 inch circles As above

SG37 0.1 inch circles As above

SG38* 0.1 inch circles As above

SG39 10mm circles in 5mm squares

SG40 13/64 inch circles in 1/2 inch squares

SG41* 5mm circles with 2.5mm squares

SG42* 10mm circles with 5mm squares

SG43 2mm squares

SG44 5mm circles in 12.5mm square

SG45 1mm circles

SG46 0.25mm dot

SG47 5mm grid – special 50mm x 50mm pattern size

SG48 5mm grid – special 50mm x 35mm pattern size

SG49 2.5mm grid – special 50mm x 50mm pattern

size

SG50 2.5mm grid – special 50mm x 35mm pattern

size

SG51 5mm circles

SG52 4mm grid

Please note:

Maximum width of strain grid stencil now available is 300mm

It is possible to manufacture additional grid designs. Please submit specification

for confirmation

Strain grid stencils for ME3000TSG strain grid marking kits

Strain grid

ref. No.

Marking size Stencil size Pattern style

SG77A 159mm x 97mm 198mm 137mm 6mm circles

SG77B 125mm x 90mm 165mm x 130mm 2.5mm circles

SG77C 131mm x 90mm 171mm x 103mm 5mm circles

SG77D 135mm x 92mm 175mm x 132mm 2.5mm circles in 6mm

squares

Marking size

Stencil size

Universal Marking Systems Ltd. Head Office: Mount Road, Feltham, Middx. TW13 6AR. ENGLAND

Sales - Tel. 020 8898-4884/5 Fax. 020 8898-9891. e-mail: sales@ums.co.uk www.ums.co.uk