Knowledge on the production of sheet metal tensile specimens

ISO6892 ASTM JIS GOST etc.

Why does special attention need to be paid to the production of tensile specimens?
Tensile test results are only as reliable as the quality of the tensile specimen allows!

It is not about punching a hole in a sheet of metal! Improper sample production leads to incorrect results, which usually go unnoticed until the customer complains. If the true material properties only become apparent during further processing or at the customer's site, the damage can be significant and lead to a lasting loss of reputation. The reliability of the test results depends primarily on the quality of the tensile test specimen. Even the best "gold-plated" Tensile testing machine cannot compensate for sample-related result errors.

Most of the methods are not suitable for standard-compliant production. The various methods and their advantages and disadvantages are presented below. It should be noted in advance that only 2 or 3 methods are actually suitable for producing flat tensile specimens from sheet metal in a high-quality, standard-compliant and efficient manner:

Punching + grinding – our know-how since 1970 Patent 2023: Fully automatic PSM2000-A
Shaping – in the roughing + finishing package (however, form-hardened sheets cannot be milled! – milling cutters fail during the first milling process)
laser cutting – (typical for sampling from hardened (3D) sheets, however, the melting edges / the entire heat affected zone must be removed by grinding / milling

Work hardening by punching coarse grain + stainless steel

Know-how for tensile specimen punching + grinding

... What use is a supplier who knows nothing about material testing? Many companies can build punches and punching tools. But who guarantees that the punching tools have a cutting gap optimized for material testing so that the material properties are not distorted by sample production? The manufacturers of punching tools optimize them for low punching burrs (fine punching). However, this is NOT the essential quality feature: The damage/deformation of the punching edge leads to material compaction/hardening through cold forming, the so-called "work hardening". This inflates the result for Rp0,2 and reduces the elongation extremely!

Fair advice since 1970: Rational sample preparation

Interesting facts about:

Standard sample dimensions with optimized punching allowance for final processing.
(Negative example: A tool manufacturer supplied the punching tools for a nominal sample width of b20 mm with an allowance of only 0,15 mm. As a result, the user does not test in accordance with the standard, since the samples would be b<19 mm after the work hardening was removed)
the minimum length for hydraulic clamping devices
Cutting gap optimization for steel, non-ferrous metals, stainless steels + deep-drawn sheets
Sampling longitudinally, transversely, diagonally to the rolling direction
Removal >100 mm to the rolling edge
Finishing: World's only sample grinding machine for sheet metal tensile specimens
a.) PSM2000-15, optional sample cooling from 2023
b.) PSM2000-A fully automatic machine patent application 2023

Extract ISO 6892-1 B.4 Sample preparation

"The samples must be manufactured in such a way that the material properties are not influenced. All areas that have to be work-hardened by cutting or punching during sample production - if they influence the properties - must be machined. These samples are mainly made from sheet and strip. If possible, the rolled surfaces should not be machined.

The production of these samples by punching can lead to significant Changes in the material properties, especially the yield/strain limits (due to the hardening). Materials that harden strongly should always be Milling, grinding etc.

Note: For materials that are strongly compacted at the edges by shearing (soft steels) and for tough materials (stainless steels) that have high toughness/elongation/deep-drawability, the work hardening/edge deformation is even more pronounced. In addition, microcracks are formed during compaction that initiate premature fracture, resulting in significantly lower elongation.

Do punched tensile specimens need to be ground?

We are asked this or similar questions by interested customers when it comes to explaining the purpose of the PSM2000 tensile specimen grinding machine. There is no doubt that damage to the punched edges (see photos of the structural changes at the edges of the image) must be eliminated. This requires reliable results and is ultimately also specified in the relevant standards (ISO 6892-1) is mandatory. If you compare the two images, it becomes clear that only punched samples produce an incorrect damage limit (yield limit / proof strength) and an "incorrect" (significantly lower) elongation at break. Unpunched samples show an elongation that is up to 1/2 lower.

Punched edge tensile specimen a 7,1mm: The micrograph of the sample grinding machine PSM2000 proves: ReH + Rp0,2 will be correct

Important: Read why most manufacturing processes are unsuitable

Some customers did not follow our recommendation. They cut tensile samples with water jet, plasma torch, wire EDM etc. – which in retrospect turned out to be turns out to be a big mistake.

The production of tensile specimens by punching and grinding has proven itself millions of times and is the only adequate method of producing a high-quality tensile specimen in less than 3 minutes. Our systems are used in sheet metal processing (slitting/cross-cutting lines), in rolling mills, strip galvanizing plants and steel service companies for the efficient production of tensile specimens of unrivalled quality.

For sample preparation, we recommend that you first test the tensile specimen on a specimen specimen that is optimized for this task. stamping tool punched out (tensile specimen production). From the (non-precut) sheet metal to the punched tensile specimen, this only takes about 15 seconds, including the insertion and removal of the tensile specimen and the specimen sheet. This or a stack of specimens is then placed in the specimen holder of the specimen grinding machine and ground within about 120 seconds so that it can be used for testing immediately afterwards - without further processing. In a quality that is unsurpassed and "indispensable" when it comes to determining R+N values.
The undisputed advantage of the punching technique: the sheets do NOT have to be pre-cut, you can take a sample from a huge sheet.

But there are also limits to the system – sheets >12 mm cannot be punched:

Today, tensile specimens are typically produced from sheet metal <10 mm by punching. Thanks to the further development of our punches and the PSM2000 tensile specimen grinding machine, sheets up to 12 mm can now also be punched and ground. The limits are a width-thickness ratio of approx. 1,5:1.
By using special grinding belts, damage to the sample caused by heating above 120°C is impossible - numerous tests prove this. Measurements have been carried out several times and the temperature never exceeded 50 - 60 °C. Active cooling of the grinding process is not necessary for tensile specimens made of steel and is also not advisable.

New in 2023: Patented tensile specimen grinding machine PSM2000-A or Hybrid PSM2000 with active cooling.
The situation is different for tensile specimens made of aluminum and other non-ferrous metals as well as special steels such as bake hardening steels. For these, we developed the patented version of the sample grinding machine in 2022/2023. This cools the samples to below 45 degrees and works fully automatically. Especially against the background that it is becoming increasingly difficult to find qualified personnel, automation has become even more important in order to reduce the use of personnel to a minimum.

Even if the samples are not to be ground but milled, punching is useful:

For milling, a rectangle must first be produced using a shearing machine. It is better to punch out a tensile specimen with the appropriate allowance "near the final shape" and then mill it in a stack. This significantly reduces the handling effort (shearing machines are no longer required) and the milling time.

If you need to produce several tensile specimen shapes (ISO / ASTM / JIS), it makes sense to punch a blank in "bone shape" from which all the different tensile specimens can be milled - we will be happy to recommend the rough shape that best suits your requirements.

The following device combinations occur:

Punches, punching tools + sample grinding machine < 20 – 500 samples / day)
punching + CNC milling >400 samples/day or various sample types

eccentric presses / hydraulic punches

Punching is now almost standard when producing flat samples from sheet metal up to a thickness of approx. 12 mm. And this is where the first uncertainty arises: all national and international standards (including ISO 6892-1) prescribe the post-processing of the punched edge.
However, the standard does not say how far the work hardening extends laterally into the material. In practice, eccentric presses (which, by their nature, have a high, sudden punching speed) are expected to have a work hardened edge zone of up to 35% per side (35% of the sheet thickness). Eccentric presses would therefore have to work with a punching allowance of 4 mm per side in order to remove the work hardening. This can only be achieved economically by milling.

The situation is different with slow-cutting hydraulic punches/presses. Due to the low cutting speed, the work hardening (with a cutting gap optimized for the sheet thickness) only penetrates to a maximum of 10% of the sheet thickness per side. Numerous structural investigations confirm this. For example, 12 mm thick sheets are punched with an allowance of just 1,2 mm per side in the measuring length. The sample, which is now 22,4 mm wide, can be easily finished with the PSM2000 sample grinding machine described here (recommendation: fully automatic PSM2000-A).

Sample punches, punching tools + sample grinding machine

Advantages of punching and grinding technology

Even large sheets, e.g. 1800 x 400mm, do NOT need to be pre-cut: saves the purchase of guillotine shears, avoids wasted working time.
Note: For milling, a sample must always be pre-cut, e.g. 300 x 30mm
fastest sample preparation (individual sample <3 minutes if the truck waits)
cost-effective production of a high number <800 per operator / shift (with fully automatic sample grinding machine PSM2000-A)
highest quality through longitudinal grinding (PSM2000)
always significantly higher elongation (compared to other methods)
Results of ReH / ReL and Rp0,2 are guaranteed correct
hardly any operating costs:
Maintenance-free machines: Oil change 5 years / Sharpening tools >20.000 – 100.000 samples
Overhaul (simple) of the grinding machine every 10 years
Can be automated (robot system: from the sheet metal to the punched, labeled tensile test specimen)
To operate the system, NO professional staff required, even semi-skilled workers can produce 1A tensile specimens

Disadvantage of punching + grinding

Uneconomical if there are only a few samples per day / week –> have samples milled
Automation is costly

CNC-controlled milling machine

For <20 samples/week, milling is the better choice (purchase of service)

Advantage CNC milling

Flexible for different sample shapes
Using the milling machine for other tasks
for high quantities, unmanned operation is “worth it”

Disadvantage of CNC milling

Disadvantages: Strips ~300 x 40mm must be cut (time, requires guillotine shears).
(Sheet metal panels 1.800 x 400mm cannot be milled (see advantages of punching)
High cost CNC + Guillotine shears
Production time (logistics / waiting times for urgent testing)
operating and maintenance costs
Wear of milling cutters…
Operating the system requires skilled personnel, which is difficult to find these days.

Wire EDM of tensile specimens

unsuitable if the machined edge is not reworked

Advantage of wire EDM

Even extremely hard sheets can be cut
Wire EDM machines work fully automatically after loading
Batches of samples can be processed

Disadvantage of wire EDM

Very high acquisition costs
Extremely long processing time (waiting time for urgent samples)
Edge roughness does not meet the standard
Extremely expensive consumables – cutting wire can easily cost >10.000 / 50.000 € and more per year
Operating the system requires skilled personnel, which is difficult to find nowadays

high-pressure water jet cutting

Water-jet cutting is unsuitable for producing tensile specimens: slow, expensive, rough flank, expensive service

Advantages of waterjet cutting

Very universal, cuts all materials
Processing of all sheets, including high-strength, difficult-to-punch sheets (strength > 1.600 MPa)
Processing of sheets over 12 mm thick

Disadvantages of waterjet cutting

Partially extremely long processing times (waiting times for urgent examinations)
Customer experience (purchase error): Only approx. 4 (four!) samples / day manufactured in order to achieve a flank quality that conforms to standards!
Qualified personnel are required to operate the system
(which in today's time hard to find is)
High operating and maintenance costs arise
In addition to water, abrasive granulate is required – complex, costly disposal of the sludge
The samples are rusted the next day (if no rust protection emulsion is used)
The separation takes place transversely to the pulling direction – the surface roughness of the cutting surface must be improved by post-processing (grinding) for roughness <6,3µ Rz
Expensive purchase because more than 3 axes of movement are required (inclined position of the cutting beam to avoid conicity of the sample
Different programs must be created / selected for different sheet thicknesses so that the conicity compensation does not lead to undefined sample widths (sample width e.g. b = 20mm)
To operate the system, skilled personnel is required, which in today's is difficult to find

without compensation the sample becomes conical

Laser beam cutting of tensile specimens

suitable if the edges are reworked by grinding / milling, very suitable for hard sheets, very flexible sample shapes possible

In steel/sheet metal plants where other samples have to be produced in addition to tensile samples, a system for laser cutting sheet metal is probably the most sensible (multi-stage) production method.
Frequently required sample shapes

Tensile specimens according to ISO 6892 (20 x 80)
Tensile specimens other specimen shapes according to ISO 6892 (12,5 x 50), JIS, ASTM, GOST
discs for zinc coating thickness measurements
quadrilaterals for roughness measurements
Rectangular specimens for bending stiffness measurements
plates as reference samples
Note: Tensile specimens must be finished by milling or grinding!

Advantages of laser cutting

universal, cuts all metals
Processing of hard sheets with high strength > 1.600 MPa possible
Processing of sheets over 12 mm thick
In the automotive industry, a laser cutting system is often already available for trimming pressed parts.

Disadvantages of laser cutting

For the production of tensile specimens only suitable for sheets < 6,0 mm (the heat affected zone is limited to approx. 0,6 mm depth and can be sample grinding machine PSM 2000 can be easily removed – the combination of laser cutting and sample grinding machine is used very frequently.
Press hardening in particular: Anyone who has ever tried to mill manganese steel will despair - the toughness destroys any milling cutter. So the only options left are laser cutting and grinding.
Very high investment costs
The operation of the system requires qualified personnel
High operating and maintenance costs
The sample must always be reprocessed after production, since thermal processing with the laser creates a thermally damaged heat zone, which severely distorts the results (double processing).
To operate the system, skilled personnel is required who are familiar with theis difficult to find

When cutting with a laser, a heat-affected zone of approx. 10% of the sheet thickness is created if a fiber laser or C0² laser is used (others larger)

plasma cutting of tensile specimens

extremely unsuitable because the enormous heat input changes the structure, the edges have to be reworked at great expense

Plasma cutting of sheet metal generates extreme heat. The sample even glows at the edges, and the heat input reaches very deep into the material at the sides. The heat input changes the material properties extremely. All areas that have been heated above 120 degrees must be removed by milling or grinding. This type of sheet metal processing is extremely uneconomical, as it requires an extreme amount of post-processing.

Advantages of plasma cutting

universal, cuts all metals
Processing of hard sheets with high strength > 1.800 MPa possible
Processing of sheets over 12 mm thick

Disadvantages of plasma cutting

High investment costs (if only needed for sample preparation)
The edge roughness (unprocessed) is far from the required roughness of 6,3 Rz!
High operating and maintenance costs arise
The sample must always be reworked after production, as processing with the plasma torch creates a thermally damaged heat zone that extremely distorts the results (double processing).
Operating the system requires skilled personnel, which is difficult to find these days.

Plasma cutting creates an extreme heat-affected zone of approximately 30% of the sheet thickness

nibbling machine for the production of tensile specimens

"completely" unsuitable because the sheets flutter and the edges always have to be reworked

In our opinion, a nibbling machine is the worst and completely unsuitable method for producing tensile specimens. The nibbling/punching edge must be heavily reworked because (as with punching) cold forming (work hardening) occurs. Without rework, the ReH/Rp0,2 results are greatly distorted (exaggerated) and the elongation at break is greatly reduced. The edge cut by nibbling alone is in no way compliant with the standard. In addition, the cut edge is usually not linear and always requires rework (grinding or milling).

Advantage Nibbling Machine

High working speed
(which, however, creates a 3-fold larger work hardening zone due to the high punching speed – see eccentric punching)

Disadvantage of nibbling machine

High investment costs
The edges must be reworked as with punching
The sheet metal may flutter (depending on the constantly changing sheet thicknesses) (unsuitable for different sheet thicknesses)
Operating the system requires skilled personnel, which is difficult to find these days.