Tensile test

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Logo of the German Institute for Standardization DIN 50125
Area Material testing
title Testing of metallic materials - tensile specimens
Brief description: Specimen geometry for tensile specimens
Latest edition 12.2016
ISO -

A tensile specimen is the test piece / test piece of the material test for a tensile test . This determines the tensile strength in particular , but also the first damage ( yield point or yield point ) and the elongation behavior of materials .

The tensile test specimens are defined for various materials in DIN  50125 and in DIN EN ISO 6892-1 (metallic materials - tensile test).

According to the standard, proportional rods (fixed multiple of the specimen measuring length  L 0 for the diameter  d ) are used for elongation at break . The proportionality factor of the rod shape is given in the formula symbol for the elongation at break as an index ( A 5 or A 10 ); if the samples deviate from the standardized shapes, the measuring length is indicated instead.

DIN 50125 defines samples with the following cross-sectional shapes:

  • circular (shape A, B, C, D)
  • rectangular (shape E)
  • Samples of flat products (form H, elongation at break A 50 or A 80 ).

The tensile specimen is produced as a shoulder, round or rod specimen for a tensile test, depending on the standard. In order to avoid notch effects and thus a falsification of the result, the surface of the test piece is finely turned or even polished . The measuring length is in a defined relationship to the diameter. The stress can be calculated on the basis of the cross-sectional area of the sample ; this is shown and evaluated in a stress-strain diagram (symbol for the area here S to avoid confusion with the elongation at break).

Sample shape

As a rule, the probe heads are wider at the ends of the sample than in the area of ​​the parallel length. The transition radius between the sample parts of different widths should not be less than 20 mm.

According to DIN EN ISO 6892-1, it can generally be said that the head width should be at least one to two times the initial width. You can find detailed information on this in the following under sample dimensions.

Sample dimensions

According to DIN EN ISO 6892-1:

  1. Parallel length:> L 0 + b 0/2 (L 0 = initial measuring length ; b 0 = initial width)
  2. In arbitration cases: = L 0 + 2 b 0 (if sufficient material is available)
  3. For strip samples with a width <20 mm, L 0 = 50 mm
    • In this case, the free length between the clamping points must be equal to L 0 + b 3 0 be

Sample production

The material properties must not be influenced during the production of the samples. This means that all areas that have been work hardened during manufacture through processes such as cutting or stamping must be machined. It should be noted that the rolling surfaces must not be damaged.

Manufacturing processes such as punching can bring about significant changes in the material properties. Specifically, the stretching / elongation limits may be affected.

According to DIN EN ISO 6892-1, it is extremely important that strongly hardening materials are always finished by milling or ideally by grinding .

In the case of particularly thin materials, it is advisable to process strips of the same width as a bundle. In addition, thicker cover sheets can provide stability.

The tensile specimen is produced as a shoulder, round or rod specimen for a tensile test, depending on the standard.

In order to avoid notch effects and thus a falsification of the result, the surface of the test piece is finely turned and even polished .

(DIN50125: surface roughness Rz6.3).

The surface roughness of the machined surfaces is given according to DIN 50125 with Rz 6.3:

Rz 6.3 surface roughness

Method for specimen preparation of flat tensile specimens

According to DIN EN ISO 6892-1, production by punching the tensile specimens and processing them using a specimen grinding machine has proven itself.

Process :

  1. First of all, the tensile specimen is punched by a machine optimized for this purpose. This process takes about 10 seconds from the sheet metal plate to the blank including inserting and removing.
  2. In the following, a stack of the samples is placed in the holder of a sample grinding machine. This type of machine grinds the samples within 30–60 seconds so that they can then be used directly for the tensile test .

Limits of the process:

  1. Thanks to highly developed machines, sheet metal thicknesses of up to 10 mm can now be punched and ground.
  2. The limits are at a ratio of 1.5: 1 (width to thickness) of the sheets.
  3. Damage to the sheet metal when heated to over 120 ° C can be excluded by using special grinding belts.

Tensile specimens from hardened sheet metal from the form hardening / press hardening / hot forming in the automotive industry:

The production of tensile specimens from hardened sheet metal poses a particular challenge for automotive manufacturers and suppliers. The manganese steels used are extremely difficult to machine in the hardened state (strength approx. 1200 - 1800 MPa) due to their extreme toughness using conventional milling technology. After just a few tensile specimens have been processed, the sharpness of the milling cutter cutting edges decreases rapidly. Carbide cutters also fail prematurely. As a result, the milling cutter creates a pinch at the processing point - a counterproductive strain hardening occurs and microcracks form. The results of the tensile specimens produced in this way are falsified and the required elongation at break of> 8% is not achieved.

Cut out the tensile specimen

Overall, there are only two relevant production processes available for sample production in this problem area.

Cut out the tensile specimen by means of a laser cut and then remove the melt edge by grinding

Since the edges of the 3D molded parts have to be reworked in almost all cases after the hot forming (trimming of excess edge areas), the laser cutter used for this work is also used to cut out a tensile specimen. This is also possible in small areas that would otherwise not allow a strip to be removed. Since mold hardening is only common for sheet metal up to 3.0 mm, the heat influence of the melting edge of the laser cut is limited to only approx. 0.3 - 0.4 mm. The resulting melt edge including the heat affected zone must be removed. Since milling fails, the only adequate means is to use a sample grinding machine.

Cutting out the tensile specimen using punching technology (hard cutting) and then removing the punched edge by grinding

A segment is first removed from the hardened 3D part. A hand cut-off machine (angle grinder / "Flex") with a very thin cutting disc is used for this. The very thin separating sheet saves separating volume / the narrow separating cut enlarges the sample piece. Then the bone-shaped / dumbbell-shaped sample is punched out on a punching tool. The punching creates an edge compression / deformation / break edge. The resulting work hardening must be removed. The most effective method of finishing is to use a sample grinder.

Sample forms

Specimen forms for tensile specimens from hardened sheet metal

Hardened sheet metal is usually only used at certain points on the vehicle body (A-pillar, B-pillar, rocker panel, etc.) Due to the complex geometry of these 3D parts, the typical tensile specimens (A80) are too large as there is usually no sufficiently large sampling point available . Therefore, a tensile specimen A50 (total length 165 mm) or a specimen in the special shape A30 (total length 120 - 136 mm) is used. Length 136/165/250 mm

Problems with tensile testing

Tensile test problems:

Conventional wedge grips cannot be used for this type of testing of hardened, extremely short specimens, as the hardness of the sheets prevents the teeth of the jaws from penetrating. Even special clamping jaws with file teeth fail prematurely due to the lack of initial clamping force. Wedge screw clamping devices are somewhat cheaper to handle and wear - but are not a permanent solution.

In the case of hardened specimens, the specimens should be clamped with a hydraulic clamping device (the teeth do not penetrate / only slightly) - clamping is achieved by clamping. Conventional, hydraulic grips with horizontal pistons are unsuitable for extremely short specimens because the wall thickness of the clamping cylinder requires a large minimum length of the specimen. On the right is a photo of a clamping device specially developed for short (and hard) tensile specimens: the wedges are closed with high hydraulic force / the construction allows the tensile specimen to be clamped without the probes exceeding the center line of the clamping jaw (constructive avoidance of the risk of tipping) .

Problems with determining the R + N value in tensile tests on sheet metal (up to 3.0 mm thick)

Micrograph of tensile specimen, sample grinding machine PSM2000

Depending on the system, the transverse strain gauge (transverse contraction gauge) glides with the contact points of the measuring line over the side edge of the sample to be processed (depending on the manufacturer and type). Even good CNC milling machines often leave chatter marks (even visible to the naked eye) in these areas. But all other processing methods (punching, water jet cutting, laser cutting, etc.) alone do not condition the flank quality in such a way that the true parameters can be determined. For this type of test, the samples should be ground on the edges.

With the sample grinding machine, the laser fused edge / punched edge damage is removed on both sides by means of contour grinding

Principle of grinding technology
Grinding head with tensile specimen and specimen holder free

Alternative methods of separating metal work differently. The advantages and disadvantages of these methods are shown below.

High pressure water jet cutting (water jet cutting)

Advantages of water jet cutting

  • cuts all materials
  • Processing of sheet metal over 10 mm thick

Disadvantages of water jet cutting

  • sometimes extremely long processing times
  • Steel samples can corrode (if no anti-rust emulsion is used)
  • Cutting particles are added to the water jet which, together with the removed material, form a metal sludge that has to be disposed of in a complex manner
  • Only high-quality waterjet cutters can be used with which the cutting head can be tilted in order to avoid a taper of the flanks
  • the processed edge hardly reaches the quality of 6.3Rz required by the standard (DIN 50125 - drawings of the samples)

Laser cutting of tensile specimens

Advantages of laser cutting

  • universal, cuts all metals
  • Processing of hard sheets with high strength> 1,700 MPa possible

Disadvantages of laser cutting

  • high investment costs
  • the sample has to be reworked after production, as the thermal processing with the laser creates a heat-damaged zone that falsifies the results. The heat input depends on the thickness and can therefore only be used for sheets up to approx. 3.0 mm - heat influence / melting edge up to approx. 0.4 mm per side

Conventional milling with manual control

Advantage of conventional milling

  • low investment costs as used equipment can be purchased

Disadvantage of conventional milling

  • the advantages of the cheap purchase are quickly used up by the personnel costs
  • precise samples can only be produced with increased effort

CNC-controlled milling machine

Advantage of CNC milling

  • flexible use also for different sample shapes

Disadvantage of CNC milling

  • high investment costs
  • long manufacturing time
  • Qualified personnel are required to operate the system
  • some materials cannot be milled, as even the slightest edge compression (work hardening ) cannot be ruled out with new milling cutters
  • Manganese steels (used for form hardening / press hardening ) can not be milled due to their extreme toughness - the sharpness of the milling cutter decreases rapidly after just a few millings

Nibbling machine

Advantage nibbling machine

  • fast sample preparation

Disadvantage of the nibbling machine

  • high investment costs
  • the edges must be reworked , as with punching
  • the sheet metal flutters (depending on the sheet thickness) (unsuitable for different sheet thicknesses)

Sample punch + sample grinding machine

Advantage of punching + grinding technology

  • Fast sample production: punching 10 seconds / grinding (multiple samples) 30 - 60 seconds
  • low operating costs
  • automatable
  • often high elongations can be achieved
  • relatively cheap in large numbers
  • high quality through longitudinal grinding
  • Maintenance typically less frequent than alternative methods
  • possible use of robotics
  • can be easily integrated into most processes
  • Flexibility in the choice of material

Disadvantage of punching + grinding technology

  • uneconomical for a few samples (investment costs for punch, punching tools, sample grinding machine)

Calculation of the cross-sectional area

  • Cuboid (shape E) or flat specimen (shape H):
with thickness  a , width  b
  • Round specimen (form A, B, C, D):
with diameter  d
with mass  m , density  ρ , length  l
with outside diameter  d a , inside diameter  d i , wall thickness  d w
with the titer (length-related mass).

literature

  • M. Hörbinger. (2015). Water jet cutting: process options and comparison with alternative industrial cutting processes. Hamburg. Bachelor + Master in Publishing.
  • A. Fritz, G. Schulze. (2010). Manufacturing engineering. 9th edition. Berlin. Springer publishing house. Pp. 386-393
  • F. Klocke, W. König. (2007). Manufacturing process 3. Berlin. Springer publishing house.

Web links

Individual evidence

  1. a b DIN EN ISO 6892-1 - 2017-02 - Beuth.de. Retrieved May 4, 2018 .
  2. a b Material processing with high pressure water jet . In: Manufacturing process 3 (=  VDI book ). Springer, Berlin, Heidelberg, 2007, ISBN 978-3-540-23492-0 , pp. 321–332 , doi : 10.1007 / 978-3-540-48954-2_8.pdf ( springer.com [accessed May 4, 2018]).
  3. a b c Manufacturing technology | SpringerLink . doi : 10.1007 / 978-3-642-12879-0.pdf ( springer.com [PDF; accessed on May 4, 2018]).
  4. a b Michaela Hörbinger: Water jet cutting: process options and comparison with alternative industrial cutting processes . Bachelor + Master Publication, 2015, ISBN 978-3-95820-401-0 ( google.de [accessed on May 4, 2018]).
  5. a b DIN 50125 - 2016-12 - Beuth.de. Retrieved May 4, 2018 .
  6. a b c Laser technology for manufacturing | SpringerLink . doi : 10.1007 / b137581.pdf ( springer.com [PDF; accessed May 4, 2018]).
  7. a b J. Franke, W. Schulz, D. Petring, E. Beyer: The role of the exothermic reaction in laser cutting . In: Lasers in Technology / Lasers in Engineering . Springer, Berlin, Heidelberg, 1994, ISBN 978-3-540-57444-6 , pp. 562-567 , doi : 10.1007 / 978-3-662-08251-5_123.pdf ( springer.com [accessed May 4, 2018]).
  8. a b Reinhart Poprawe: Tailored Light 2: Laser Application Technology . Springer Science & Business Media, 2011, ISBN 978-3-642-01237-2 ( google.de [accessed on May 4, 2018]).
  9. a b Björn Olaf Assmann: Manufacture of high-precision prototypes using milling as a quasi-generative rapid prototyping process . Haltern June 2013, p. 28-29 .
  10. a b Eva Ponick, Alexander Stuckenholz: LoraWan - Wireless communication for industry 4.0 . Ed .: Hamm University of Applied Sciences - Lippstadt. Hamm.
  11. a b c d e f g h i D. Veeramani, S. Kumar: Optimization of the nibbling operation on an NC turret punch press . In: International Journal of Production Research . tape 36 , no. 7 , 1998.
  12. a b c d e Weiss, Michael: Seamless joining of FV structures . 2006, doi : 10.3929 / ethz-a-005198693 ( ethz.ch [accessed on May 15, 2018]).