Tensile specimen preparation
sheet metal plates
0.05 - 8.0 mm (0.002 - 0.3 inch)
Specimens according standard ISO6892, DIN
50125, ASTM, GOST, JIS and others
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 a high 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 a first and second milling process. A
time-consuming method and often the specimen does not meet the advised
roughness of 6.3 p R / Rz. This fine surface you only can reach by a
high quality CNC milling machine.
Hydraulic blanking / punching presses of SCHÜTZ + LICHT
The 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 only 10 % of the thickness of the sheet metal plate thickness. This
is approved and confirmed by numerous metallurgical researches.
The photos below were taken through a microscope with 50 / 100 times
magnification. In this photo’s you easily can recognize where the
structure of the material is effected by deforming / deep drawing
effect.
It is a fact:
Deforming of structure = cold hardened
effect
The manufacturer 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 knifes (Lo) 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 runs as a full automatic robotised system (without operator)
with specimen magazine, roboter 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 basic requirements to receive correct results is the specimen itself
If you are think that specimen preparation by hydraulic presses (and
additional
specimen grinding machine PSM2000) is not the best
solution you may look into our page to show you
advantages
/ disadvantages of different specimen preparation systems | 
