Background Knowledge sheet metal tensele specimen preparation

... it is not about to get a hole into a sheet metal plate. And also not to choose the cheapest available press...

What use is a supplier who offers a punch without the necessary knowledge of material testing? Manufacturer of blanking tools and punches - you are able to find several different solutions and different low and high quality suppliers. Our goal is to meet your requirements and the very special necessity of specimen preparation for tensile tests. We are able to state: Our technology meets the requirements of the standards - without influence of the properties of the material.

We have seen several systems on the market - with incredible mistakes:
- incredible thickness ranges (too less numbers of tools)
- big burr caused by wrong gap between punching die and punching plate
- big zone of damaging the edges in gage length
- cold hardening of the edges
- micro craks inside the damaged edges

This mistakes will cause wrong results of sheet metal tensile test
- wrong yield point (ReH / Rp0,2 / Rt0,5)
- wrong (too low) uniform elongation
- wrong (too low) elongation at rupture (up to 1/3)

At specimen preparation our solutions respects:

• your range of thickness (customising will reduce the number of tools)
• optimising the gap for very weak material or material with high toughness (high elongations)
• taking care about your specimen direction (0 / 45 / 90 ° to rolling direction)
• finishing blanked specimen by using our unique specimen grinding machine PSM2000
  without finishing your results of
  - ReH, Rp0.2 / Rt0.5 will be wrong
  - elongation at rupture will decrease up to 1/3
• meeting the standards of ISO6892 / ASTM A370, JIS, GOST and other means:
  You have to follow the requirements of the standards by removing all damaged edges

We know about. We supplied hundreds of presses, tools and grinding machines. This theme is our core competence since 1970 and no one around the world has deeper knowledge on specimen preparation for sheet metal tensile specimens by blanking / punching. We are able to support this knowledge based not only on our experience - we studied the metallographic properties and made several researches on the inner structure of the materials and deformed edges. These researches confirmed our experience we collected since more than 40 years: If you are using a slow acting hydraulic press (never an eccentric press) the damage of the edge has a ratio of 1/10 corresponding to the thickness (each side). For example: At sheet metal of 3.0 mm you have to remove 0.3 each side. Tensile specimen preparation form sheet metal plates specimens according ISO6892, DIN 50125, ASTM A370 / E8, GOST, JIS and other standards. Each result of a tensile test is as reliable as the quality of tensile specimen allows. If a specimen is prepared in a not suitable way also the highest quality tensile testing machine can’t compensate this mistake which occurs from specimen preparation. For tensile specimen preparation from sheet metal plates punching / blanking today is nearly standard. And here appears the first uncertainty. All national and international standard including ASTM, GHOST, ISO, EN, DIN, JIS - advises: The damage of the specimen at the edges (cold hardened effect by deforming at blanking / punching) has to be removed completely. The standards of course do not give a hint how deep this effect occurs. In the past, specimen blanking was done by high speed eccentric presses. This caused unacceptable damage of the edges of the specimens because the deforming and compression of the material is moving into the specimen very deep: The cold hardening effect of eccentric presses are up to approximately 35 % of the specimen thickness. To remove this very big area from each side of the specimen the specimen will be blanked with oversize of nearly 2 mm. After blanking both side of the specimen has to be milled in first and second milling process. A time-consuming method and often the specimen do not meet the advised roughness of 6.3 p R / Rz. This fine surface you only will reach by a high quality CNC milling machines.

Hydraulic blanking / punching presses of SCHÜTZ+LICHT
Depth of deep drawing effect is completely different if a slow moving hydraulic press will be used. The work hardening phenomenon was investigated by working together with experts in the sheet metal industry. This leads to the development of a low speed specimen blanking system which greatly reduced the amount of cold hardening effect. Even because of using a special shape of punching die the cold hardened zone is <10 % of the thickness of the sheet metal plate. This is approved and confirmed by numerous metallurgical researches.

The photos beside was taken through a microscope with 50 / 100 times magnification. In this photos you easily can recognize where the structure of the material is affected by deforming / deep drawing effect. It is a fact: Deforming of structure = cold hardened effect = wrong results. The manufacturers of tensile test machine do follow the increasing requirements of tensile tests performance and reliability of results or reduction of labour costs. State of the art is to have electronic controlled machine with HQ software for evaluation and HQ load cells and extensometers class 0.5. To perform a tensile test the extensometer will automatically adjust the distance of knives (L0) and will follow the elongation up to rupture (minimum up to Ag). Within the last years more and more a contactless measuring system occurs (video measuring system or Laser Speckle extensometer).

Sometimes the systems run as a full automatic robotised system (without operator) with specimen magazine, robot and at the end with data export to a host system. The testing technician will be released from standard procedures including the documentation of the results.But the fundamentle requirement to receive correct results is the correct perpared sheet metal tensile specimen itself.

This is how we are asked by interested customers if we are to show the purpose of the PSM2000 sheet metal tensile specimen grinding machine. Without a doubt, punch edge damage (see first photo) must be removed. This is required for reliable results and ultimately this is also binding in the relevant standards. Comparing the two photos it becomes clear: With only punched sample, only a "wrong" (too low) elongation can be determined. Un-sanded samples have less elongation up to 1/3.