Limits, advantages/disadvantages of different types of production of tensile specimens

The production of tensile specimens by means of tensile specimen punches and specimen grinding machines has been proven a hundred times over and is now generally accepted. Especially for the processing of sheet metal (slitting lines, cut-to-length lines) in steel service companies and in small to medium-sized enterprises, this type of efficient sample production cannot be replaced by anything else: If a high-quality tensile sample is to be ready for testing within 3 minutes, there is no system that is faster than the sample preparation systems described on our pages.

For sample preparation, we recommend first punching the tensile specimen on a tool optimised for this task (tensile specimen production). From the sheet metal plate to the blank, this takes only about 10 seconds including the insertion and removal of the sample plate (without tool change - about 30 seconds). Afterwards the blank or a stack of samples is inserted into the sample holder of the sample grinding machine and within approx. 30 - 60 seconds it is ground so that it can be used for testing directly afterwards - without further processing. And this with a quality that is not reached or exceeded by any other system.

However, there are some limits to this system:

Typical today is the punching of sheets up to approx. 8 or even 10 mm. Due to the further development of our punching machines and the PSM2000 tensile specimen grinding machine, sheets up to 10 mm can now also be punched and ground. The limits are at a ratio of 1.5 : 1 (width to thickness) of the sheets. By using special grinding belts, damage to the sample by heating above 120° C is excluded - numerous tests have proven this. Measurements have been carried out several times and temperatures of 50 - 60°C have never been exceeded. Active cooling of the grinding process is not necessary or useful.

But also if you do not want to grind the specimens but rather mill them: Punching a tensile specimen also makes sense

• no need to pre-cut a strip on guillotine shears or similar, the blank can be inserted into the milling machine in a compact stack one above the other
• by punching "close to the final shape" the machining time with a milling machine is minimised as only the measuring length has to be machined

So it comes to the device combinations:

• tensile test punch, tensile test punching tools + sample grinding machine (sample volume < 200 - 400 / day)
• Tensile sample punch and CNC milling machine (sample throughput > 200 / day)

Again and again we are also asked what alternatives are conceivable for sample production. In the following we have listed these methods and collected arguments for and against them.

Advantages of water jet cutting

• very universal, cuts all materials
• Processing of all sheet metals, including high strengths that cannot be punched (strength > 1,800 MPa)
• Processing of sheet metal >10 mm thick

Disadvantages of water jet cutting

• sometimes extremely long processing times (waiting time for urgent testing)
• qualified (expensive) specialist personnel is required to operate the system
• high operating and maintenance costs are incurred
• abrasive granulate is required in addition to water - complex, cost-intensive disposal
• the samples are already rusted the following day (if no rust protection emulsion will be used)
• the cutting is carried out crosswise to the direction of tensile test - the surface roughness of the cut surface must be improved by finishing (grinding) for roughness of <6.3µ Rz
• expensive acquisition as only more than 3 movement axes are required (inclination of the cutting beam to avoid conicity of the sample)
• for different sheet thicknesses, different programs must be created / selected so that the taper compensation does not lead to undefined sample widths (sample width b = 12.5 / 20 / 25 mm)

Advantages laser cutting

• universal, cuts all metals
• Processing of hard sheets with high strength > 1,800 MPa possible
• Processing of sheet metal over 12 mm thick
• iin the automotive sector, a laser cutting system for trimming pressed parts is often already available (also used for tensile tests specimen)

Disadvantages of laser cutting

• For the production of tensile specimens only suitable for sheets < 3.0 mm (the heat-affected zone is limited to a depth of approx. 0.4 mm and can be easily removed with the PSM 2000 sample grinding machine - the combination of laser cutting and sample grinding machine is used very often. For larger material thicknesses >3.0 mm, the heat introduced is so great that the melting edge extends too far into the material. The destroyed, burnt microstructure must be removed at great expense. Anyone who has ever tried to mill magan steel (press hardening) will "despair" of it - the toughness destroys every milling cutter.
• high investment costs (if only the sample preparation should be purchased)
• long processing times (waiting time for urgent tesnile test)
• the operation of the system requires qualified technical personnel
• High operating and maintenance costs are incurred
• the sample must be reworked after production in any case because the thermal processing with the laser creates a heat-damaged heat zone which extremely falsifies the results (double processing)