Preparation of a tensile specimen Tensile test ISO 6892 / DIN 50125
Purpose of different forms of tensile test
For the standard-compliant implementation of a tensile test The accuracy of the tensile test is just as important as the quality of the Tensile testing machine. If the tensile test specimen with incorrect dimensions oder durch incorrect processing This will likely lead to incorrect results. Even the best Tensile testing machine can then only provide inaccurate results. It is therefore important that exact dimensional accuracy is observed when producing the sample. In addition, the manufacturing method must be suitable. In general, the surfaces of the machined areas must have a surface roughness of less than 6,3R. The machined area is usually ground or even polished again after machining. This prevents premature cracking and thus provides the - real - elongation of a material. If the sample is produced with modern, fast-rotating processing machines with CNC control, the machined area meets the aforementioned requirements without subsequent grinding or polishing work. In particular, a flat tensile specimenthat with a stamping tool punched out of a sheet metal requires post-processing. If the punched edge is not reworked, this will lead to incorrect results in
- the yield strengths Rp0.01 / Rp0.2 / Rp1.0 / Rt0.5 etc.
- the uniform strain Ag
- the elongation at break A
- the determination of R+N values
The sample shapes are precisely defined in the current standards (ISO6892 in conjunction with DIN 50125). These standards can only be obtained from BEUTH Verlag - Berlin. The dimensions of the tensile samples are in an exact ratio to the cross-section. If the tensile sample is too short for your tensile testing machine (type of clamping tools), the measuring length (Lc or Lo) must not be changed during production under any circumstances. In this case, please be sure to extend the heads of the tensile sample. The dimensions of the tensile sample depend on the sample material that is available. In almost all cases, wire is tested in the delivery state. Only when screws or similar are made from this wire is it usual to process the wire and produce a tensile sample. In the case of sheet metal, the tensile sample is usually punched out and given a dumbbell shape (bone shape). However, if only a narrow strip (sheet metal strip) is available, which does not allow the production of a dumbbell-shaped tensile sample, the parallel strip is tested.
Note: In the case of samples with a homogeneous cross-section (wire, pipes, ribbed reinforcing steel, sheet metal strip samples), sample breaks often occur outside the measuring length L0 or at the clamping point (clamping break). An old trick used by experienced material testers can help here: slightly warming the sample with the warmth of your hand (with a large sample volume) or just two fingers (small sample volume) can influence the flow behavior of the material at this point so favorably that the flow, the constriction and the sample breakage almost certainly occur at the heating point. The very slight increase in temperature (e.g. +2° C due to the warmth of your hand) is irrelevant for the material properties.
Note: With parallel stripes a very favourable effect is achieved by placing the parallel stripe on the known sample grinding machine By grinding the sample (even just a little), the shear edge of the product (from the longitudinal division) is removed and a light dumbbell shape is created - a standard-compliant tensile specimen (since the head width is not exactly specified).
In order to achieve comparable results, the dimensions of a tensile specimen were precisely standardized. Different cross-sections were permitted. However, the measuring length (Lc or L0) of the specimen also increases with this cross-section. The reference length for the elongation values is calculated from a proportionality factor (similar to π to a circle). This proportionality factor is a ratio of volume to a measuring length L0. The extension (elongation) of a material takes place proportionally to its volume. This volume / elongation factor (ratio) was set at 5,65 (alternatively 11,3 and others). However, this flow behavior fails in flat specimens in particular with sheets with a thickness of less than 3,0 mm. Here, there is not enough material in the thickness of the sheet for flow to occur proportionally. In these cases, a "non-proportional" flat tensile specimen A80 / A50 or the new A30 (hot pressing / hot deforming) is used. For these tensile specimens, the measuring lengths are not calculated but are rigidly specified. The flow of a material does not occur proportionally, regardless of the cross-sectional area of the specimen.
reference length for the elongation L0 (initial measuring length)
The reference length or initial measuring length is calculated (with a proportionality factor of 5,65) according to the formula shown above. This will be illustrated using an example calculation of a tensile specimen with a circular cross-section and a diameter of 10 mm:
d = 10,0 mm (measured diameter of the round tensile specimen)
k = 5,65 proportionality factor
S0 = cross-section of the tensile specimen
L0 = initial measuring length of the tensile specimen (reference length for the elongation)
S0 – cross-section calculation
S0 = d² x π / 4
S0 = (10 x 10) x 3,14 / 4
S0 = 78,5
L0 – Calculation
L0 = 5,65 x √S0
L0 = 5,65 x √78,5
L0 = 5,65 x ≈ 8,86
L0 = 5,65 x ≈ 50,059
L0 = 5,65 x ≈ 50
L0 – simplified calculation (only for samples with circular cross-section)
L0 = d x 5
L0 = 10 x 5
L0 = 50
Whereby mean
So = calculated cross-section of a sample
5,65 = proportionality factor
measured sample diameter 10,0 mm
Otherwise, the proportionality factor must be specified: A11.3 or A4 (ASTM)
or the reference length of the non-proportional flat sample A80 / A50
or the reference length of the non-proportional wires A100 / A200
Note:
The elongation of different sample shapes cannot be directly compared: An elongation A5.65 of a proportional flat sample with a thickness of 3.01 mm, for example, is very different to the elongation A80 of a non-proportional flat sample with a thickness of 2.99 mm, for example. 25% elongation A5.65 is different to 25% elongation A80. The elongations can, however, be converted. Tensile specimens in different sample shapes are used for different products and materials. Their use and the advantages and disadvantages are explained below.
cylinder head test
Advantages:
- The samples are quite easy to produce on a lathe. However, this advantage is diminishing more and more, as a CNC lathe is usually used to produce the sample. A CNC lathe processes a sample automatically. In this case, other sample shapes (threaded / shoulder head) can easily be produced without the need for personnel.
Disadvantages:
- Compared to shoulder head / threaded head samples, expensive clamping device required (wedge clamping device / hydraulic clamping device)
- Despite carefully adapted clamping equipment, the sample may be subject to bending (different grip situation of the jaw teeth). Therefore, this form is usually not chosen for samples with low elongation (less than 5% elongation) because bending reduces the elongation and fracture occurs even earlier because these materials behave accordingly brittle (aluminium die-cast, gray cast iron without tempering).
- unfavourable for hard / unsuitable for very hard tensile specimens – see explanation below
- Samples must be relatively long because the grips have relatively long gripping surfaces
Note: The gripping surface of the jaw must be covered by at least 50% (better 2/3) otherwise the jaw can tip over. This can lead to premature damage to the clamping device
The samples require a clamping device that clamps the test piece with prismatic clamping jaws. A so-called wedge clamping device is commonly used here. With this operating principle, wedges with gripping surfaces / serrations are pulled deeper and deeper into the wedge clamping head by the sample. As the distance increases, an ever higher clamping force (wedge effect) is applied to the sample and it is held. Due to the wedge effect of the clamping jaws, the clamping force in relation to the tensile force increases by a factor of 1,5 - 2,0 (depending on the wedge angle of the clamping head). A wedge clamping device is less suitable for hard materials (e.g. for strengths above 1.200 MPa). This is particularly the case because the surface hardness of the test piece is so high that the (hard) teeth of the clamping jaw do not achieve a good initial bite. As a result, the sample slides along the teeth of the clamping jaws and these sometimes wear out extremely quickly. Clamping jaws for higher strengths are also available as an option.
This problem can be reduced by using a clamping device that uses an external force to close the wedges with a pre-force. These include, for example, wedge screw clamps (closing the wedge jaws using threaded adjustment) or pneumatic wedge clamps (a pneumatic cylinder generates a limited closing force).
Alternatively, hydraulic wedge clamps can be used. These allow a higher closing force. However, the closing force of a hydraulic, parallel closing clamp is not achieved.
If samples with strengths above 1200/1400 MPa are frequently clamped, a hydraulic clamping device with parallel closing jaws should be used. The enormous closing force of the hydraulics also clamps high-strength samples securely (more by clamping than by biting). However, such a clamping device is very cost-intensive.
thread head sample
Advantages:
- the clamping device is relatively inexpensive (threaded nuts and mounting bells)
- The gimbal function integrated into the clamping device means that any bending influence on the sample is almost completely excluded
- the tensile test specimen can also be made from a small amount of (short) raw material
Disadvantages:
- Compared to a cylinder head sample, this sample shape is more complex to produce. If the machining is carried out with a CNC lathe, this disadvantage is irrelevant because the processing is unmanned. A CNC lathe processes a sample automatically (after clamping). In this case, tensile samples with a threaded head / shoulder head can also be easily produced without the need for personnel.
shoulder-head test
Advantages:
- The clamping device is relatively inexpensive, mostly split shoulder head nuts and receiving bells (easy handling)
- laterally open heads for direct insertion of the tensile specimen (horizontal insertion)
- a bending influence on the sample is almost completely excluded because the clamping device has a cardanic function
- the tensile test specimen can also be made from a small amount of (short) raw material
- Due to the simple sample geometry (no thread), even high-strength and extremely hard materials can be processed into a tensile sample (e.g. with CBN turning tools)
Disadvantages:
- The production of samples is relatively complex (compared to a cylinder head sample). However, this disadvantage is disappearing more and more, as a CNC lathe is usually used to produce the sample. A CNC lathe processes a sample automatically (after clamping). In this case, tensile samples with a threaded head / shoulder head can also be easily produced without any personnel expenditure.
Non-proportional flat sample (sheet) thickness <3.0mm
Please note:
- Samples with a thickness of less than 3,0 mm can no longer flow proportionally to the volume (because the thickness is so small). The small thickness prevents further flow because the thickness does not result in a flow volume. Therefore, these samples are classified as non-proportional samples. The L0 of a non-proportional tensile sample is not calculated but set to fixed dimensions.
– Sample form 1 is usually used for a sample made of non-ferrous metals
– usually sample form 2 is used for a sample made of steel / iron
– For comparability with Japanese samples JIS standard, sample form 3 has been added - The samples are most often produced by punching. However, the damaged punched edge must be reworked to remove the resulting punched edge damage caused by compaction. The PSM 2000 tensile specimen grinding machine safely removes this edge damage (work hardening, microcracks, cold forming, burrs).
- Alternatively, the samples can be produced by milling. A CNC milling machine can mill a package and is therefore as efficient as the aforementioned punching technique
Due to the development of steels (particularly their micro-alloying or, for example, through press hardening), high-strength sheets from the automotive sector have had to be tested in recent years. Due to the high strength (hardness) in combination with high toughness, the production of tensile specimens from such sheets is very difficult. Even high-quality milling cutters fail after a surprisingly short service life. The production of these samples from sheets in the area of hot deforming, hot forming / warm forming / press hardening can be carried out by:
- Punching: We have developed a special concept for punching (hard cutting) high-strength sheets for strengths up to 1.600 MPa
- Laser beam cutting: As an alternative, these high-strength sheets are produced on a laser cutting system. However, as the extremely high temperatures cause a melting edge on the punched edges, this punched edge damage must be removed. This has been done for years at VOLKSWAGEN in Wolfsburg (in the World Central Research and in the bodywork plant) with our unique sample grinding machine PSM2000.
- Waterjet cutting system: The conicity of the waterjet creates a slanted edge, which must be ground off using the PSM2000, for example.
Proportional flat sample (sheet metal) thickness >3.0mm
Please note:
- Samples with a thickness greater than 3,0 mm flow proportionally. Therefore, the reference length is calculated here with a factor of 5,65.
- The samples are often produced by punching (up to approx. 8 or even 12 mm).
- However, the damaged punching edge must be reworked in order to remove the resulting punching edge damage caused by compaction.
- The PSM2000 tensile specimen grinding machine safely removes these edge damages (work hardening, microcracks, cold forming, burrs).
- Alternatively, (thicker) samples can be produced by milling. A CNC milling machine can mill a package and is therefore just as efficient as the punching technique mentioned above.
In recent years, more and more high-strength sheets from the automotive sector have had to be tested. Due to the high strength (hardness), the production of tensile specimens from such sheets is very difficult. Even high-quality milling cutters fail after a surprisingly short service life. The production of these specimens from sheets in the area of hot deforming, hot forming / warm forming / press hardening can be carried out by:
- Punching technology: Our company has created a special concept for punching (hard cutting) of high-strength sheets
For strengths up to 1.600 MPa – get advice - Laser beam cutting: Alternatively, high-strength sheets are cut out on a laser cutting system. This is particularly useful if the production technology requires trimming after press hardening. Since the punching edge is worked at extremely high temperatures, this punching edge damage must be removed. VOLKSWAGEN in Wolfsburg has been using our unique PSM2000 sample grinding machines for this purpose for years (in the World Central Research and in the bodywork plant).
- Water jet cutting: The conicity of the water jet creates a slanted edge, which must be cut by, for example, PSM2000 be sanded down.
The most important terms + descriptions Metal tensile test ISO 6892-1
|
abbreviation |
unity |
designation / simplified declaration |
|
A0 |
mm |
Initial thickness of a flat sample or wall thickness of a pipe |
|
B0 |
mm |
Width of a flat sample in the test length / average width of a pipe strip sample / a profile wire |
|
D0 |
mm |
outer diameter of a pipe |
|
L0 |
mm |
Initial measuring length (reference length or starting length for the strain) |
|
Lc |
mm |
test length (parallel part of the measuring length between the radii) |
|
Lt |
mm |
total length of the sample |
|
Lu |
mm |
Measuring length after fracture (L0 stretched) |
|
S0 |
mm² |
initial cross-section of the tensile specimen |
|
Su |
mm |
smallest cross-section after fracture (for calculating the necking) |
|
K |
- |
Proportionality factor – ratio of the cross section to Lo |
|
Z |
% |
Fracture reduction – ratio between So and Su |
|
R (?) |
MPa |
Stress – force divided by the initial sample cross-section |
|
A (?) |
% |
Elongation at break – extension of the sample relative to the reference length Lo |
|
At |
% |
like A, but this value also includes the elastic strain |
|
Ag |
% |
Gleichmaßdehnung – die gesamte parallele Messlänge dehnt sich gleichmäßig (bis Fm bzw. Rm). Ab dort erfolgt die Dehnung nur noch im Bereich der Einschnürung. Die Gleichmaßdehnung ist für die Umformbarkeit eine sehr wichtige Aussage. Die Gleichmaßdehnung kann nur mit einem Extensometer gemessen bleiben der bis nach Ag (Rm an der Probe verbleibt. Der Dehnungswert Ag beinhaltet keine elastische Dehnung. Diese wird abgezogen. |
|
Agt |
% |
like Ag, but this value also includes the elastic strain |
|
A5,65 |
% |
Elongation at break with respect to a calculated Lo |
|
A11,3 |
% |
Elongation at break with respect to a calculated Lo |
|
A50 |
% |
Elongation at break of a flat sample with a thickness of 0.1 – 3.0mm (most common sample shape for non-ferrous materials such as aluminum, copper, etc. for measuring, among other things, the non-proportional elongation in samples under 3.0mm thick) |
|
A80 |
% |
Elongation at break of a flat sample with a thickness of 0.1 – 3.0mm (most common sample shape for steel flat tensile samples for the measurement of non-proportional elongation, among other things, only a small amount of material is available, but the sample shape A50 can also be selected |
|
A100 |
% |
Elongation of a wire at break: Wire often breaks at an undefined point. If you were to measure an A5,65 elongation, the elongation/breakage would usually occur outside the cutting edges of the strain gauge. You therefore set the L0 and the cutting edge distance from 100 or 200 mm and thus determine the elongation A100 / A200. |
