Different strain gauges / extensometers
Purpose / Field of application: What is each strain gauge suitable for?
The standardized performance of tensile tests according to ISO6892 / ISO527 requires (in addition to a Tensile testing machine the use of an extensometer / strain gauge. In addition to the strain results, this additional equipment is used for the safe and standard-compliant determination of the E-modulus and the parameters derived from it (metal: yield strength Rp0,2 ... / yield point | Plastic: yield stress + yield strain) are absolutely necessary. These differ fundamentally:
1. Tactile (contacting) extensometers Clamping on the sample by cutting
- Clamp-on extensometer “clothespin” short measuring path: E-modulus, yield strength Rp / Rt
- Long-distance extensometer high resolution < 0,1 µm large measuring range: E-modulus, yield strength, uniform strain, fracture strain
- Long-distance extensometer low resolution < 10µm: Elongation elastomer | rubber, not usable for E-modulus
2. Contactless, optical extensometers Contactless tracking of target marks / speckle patterns
- Video extensometer high resolution <0,1 µm large measuring range: E-modulus, yield strength, uniform strain, fracture strain
- Video extensometer low resolution >10 µm large measuring path: strain on elastomers | rubber, not for E-modulus
- Laser Speckle Extensometer high resolution <0,1µm large measuring range: E-modulus yield strength uniform strain, fracture strain
clamp-on strain gauge
special extensometer
Templates
video strain gauge
high-temperature extensometer
transverse strain gauge
MFA2
- extremely precise (DMS technology)
"always" more accurate than long-distance extensometers - very robust against premature sample breakage (aluminum die-cast / GFK / CFK)
- Le 30 + 50 (Option: Special Le 20 to 100 mm)
- inexpensive, easy to use
- no values Ag, Agt, A: manual measurement
MFA2 double-sided
- as before
but double-sided clamp-on extensometer:
Electro-mechanical compensation of bending influence (NATCAP etc. required)
MFA2mini
- extremely precise (DMS technology)
"always" more accurate than long-distance extensometers - Le 10 – 80 mm
- slightly cheaper than MFA2
- Easy to use
- no values Ag, Agt, A: manual measurement
- not as robust as MFA2
MFA2mini double-sided
- as before
but double-sided clamp-on extensometer:
Electro-mechanical compensation of bending influence (NATCAP etc. required)
clamp-on strain gauge
They are attached to the tensile specimen before the start of the test and have a limited measuring path (typically < 2 mm). This type of extensometer is removed after the determination of the Young's modulus and the damage limits ReH / Rp0,01 / Rp0,2 / Rp1,0 / Rt0,5 during the test. In principle, these devices are not extensometers for determining the plastic strain until fracture, but rather Young's modulus and Rp0,2 evaluation devices. Since these extensometers only have a limited measuring path, strain in the true sense of the word is not recorded.
In order to determine the elongation at break in accordance with the standard, a metallic tensile specimen must be strained before the test using a partial installation marked with a grid or the L0. After the tensile test, the broken sample is put together and the elongation Lu is measured with a caliper and entered into the test software. The standard ISO 6892 Tensile testing on metals allows the calculated L0 to be increased by the next higher step size of 5 or 10 (e.g. L0 52,1 mm –> L0 55 mm). With this procedure, our dividing device with fixed dividing increments can be used for simplified manual elongation measurement for efficient marking of the tensile test can be used to determine the elongation at break in a matter of seconds.
Exception: Elongation at break measurement with MFA2: This clamp strain gauge is very robust. In our experience, this clamp strain gauge can remain on the sample until it breaks in special cases (no guarantee - we only recommend this procedure "unofficially" - but we are not aware of any damage caused by sample breakage).
Requirements for the job
- Elongation <2% (at L0 50 mm) e.g. aluminum die-casting | glass fiber plastic GFK | carbon fiber plastic CFK
- Use of exact Le (e.g. Le55 for L055 mm)
- Protection against falling out by thin chain
This type of strain gauge cannot usually be used for tensile tests on plastics, as plastic tensile specimens exhibit a much higher plastic strain. According to current standards, the strain until fracture is measured with a (long-distance strain gauge) taking into account the elastic component.
MFA25
- Determination of E-modulus, Rp0,2, yield stress, yield strain
- Large measuring range < 25 mm: Suitable for hard / medium-hard plastics for determining
Elongation at break (< 50% at Le 50) - Option MFA12,5 = linear
- easy handling (less convenient than MFA2)
- less linear than MFA2 due to scissor movement
- slightly more expensive than MFA2
MFA20
- circularly arranged cutting edges for wire / reinforcing steel
- large, linear measuring range < 20 mm: determination of Ag, Agt, A100, A200
- cheapest solution for reinforcing steel (!): measurement of E-modulus, ReH, Rp0,2
- variable Le adjustable through plug-in system
- Option: Le extension for Le 100 or Le 200
- comfortable handling
- more expensive than MFA2 / MFA25
- only suitable for large Le from Le 50 (option Le 40)
MFS
- Strain gauge specially designed for SCHÜTZ+LICHT, used to measure the extension of screws
- Specifically, the extension of the entire screw (mm) is measured
(to measure % elongation on turned screws we recommend using MFA2 / MicronXT - the screws must have a countersink in the head / foot (like countersink for lathe)
- special clamping device with "window" above / below the screw required
MFI for ropes
- designed for strain measurement on ropes
- secure clamping on ropes due to circular arrangement of the cutting edges
- Ropes tear with (extreme) energy – device must be removed after the transition to plasticity
PDF MFA25
PDF MFA20
PDF MFS
PDF MFI
Micron 2
- world's cheapest extensometer A%
- high-resolution measuring path 0,05µm
- automatic closing / opening of measuring arms
- Manual Le setting 3 seconds
- Determination of E-modulus Rp0,2 AL
- Determination of Ag, Agt, A5,65, A80…
- Mandatory @ Transverse strain gauge R+N
MFX200
- reference class of HQ long-travel extensometers
- high-resolution measuring path 0,1µm
- automatic closing / opening of the measuring arms
- automatic Le setting
- automatic centering of the clamping heads
- Determination of E-modulus, Rp0,2 …, Reh, AL
- Determination of Ag, Agt, A5,65, A80…
- measuring range 200 mm (minus Le)
- Required if R+N value is required and a manual transverse strain gauge is used
probe arm extensometer
- reference class of HQ long-travel extensometers
- high-resolution measuring path 0,1µm
- automatic closing / opening of the measuring arms
- automatic Le setting
- automatic centering of the clamping heads
- Determination of E-modulus, Rp0,2 …, Reh, AL
- Determination of Ag, Agt, A5,65, A80…
- measuring range 500 mm (minus Le)
- Required if R+N value is required and a manual transverse strain gauge is used
MFL
- “Top of the Top” HQ long-path extensometer
- high-resolution measuring path 0,1µm
- automatic closing / opening of the measuring arms
- automatic Le setting
- automatic centering of the clamping heads
- Determination of E-modulus, Rp0,2 …, Reh, AL
- Determination of Ag, Agt, A5,65, A80…
- measuring range 200 mm (minus Le)
- Required if R+N value is required and an automatic transverse strain gauge MFQ-A is used
long-distance strain gauge
Measurement of E-modulus, Rp0,2 and elongation at break
PDF MicronXT
PDF – MFX200 Extensometer
PDF MFX500
Top product of the reference class long-travel extensometer
These devices have a similarly high measurement resolution as the clamp-on strain gauges (1 µm or even 0,05 µm) and remain on the sample for much longer.
Depending on the clamping device, these measuring devices remain on the sample until Ag is exceeded or even "until fracture" and can therefore be used to accurately determine the elongation at break. When testing tensile specimens, it is often required or believed that an extensometer should remain on the sample until fracture in order to be able to accurately measure the elongation at break. However, this is neither necessary nor sensible. Some manufacturers (GALDABINI and some competitors) offer strain gauges for which the statement is made (in essence): "Suitable for measuring strain at break - remains on the sample until breakage". However, the strain gauges must be opened shortly before breakage so that the elastic strain (springback of the elastic part) or even parts of the broken sample do not damage the cutting edges of the strain gauge or even the arms.
Why can the arms of the extensometer be opened and the results are still correct? The cutting edges/measuring arms of the extensometer must remain on the sample until Ag (uniform elongation) or, better, well after the uniform elongation. From the maximum force, the elongation behavior changes: The sample is no longer stretched equally at all points (uniform elongation), but begins to form a waist at one point. If the sample is stretched further, a constriction is usually visible to the naked eye. As soon as this clear constriction elongation occurs, it no longer has to be measured with the extensometer, but the crosshead travel measuring system can be used for this elongation, since it can be assumed that this elongation only occurs in the L0 range.
Video-XT camera
Video-XT video strain gauge
Video-XT on machine
Video-XT video strain gauge
- indestructible because it does not contact
- complete strain measurement until break: E-modulus, Rp0,2, Ag, A (<50%)
- Le / L0 selectable: Delivery includes various Le markings + markers for easy marking + automatic measuring mark recognition
- optional R+N value software module for EN 10113, EN 10275 or Poisson module (**)
- No force influence (weight / actuation force) on sample as it is not tactile: Even for the thinnest samples / foils / wires ...
- Also ideal for experiments in thermal chambers (viewing window)
In which Video-XT is a contactless video extensometer developed by GALDABINI for the high-precision, bi-directional recording of longitudinal strain (option: transverse contraction). This meets the requirements of ISO EN 9513 (grade 0,5) and ASTM E83 B1 (depending on the lens). Since this extensometer does not contact the sample, it is a truly universally applicable device. It is suitable for all types of materials or components. The bi-directional strain gauge can be used for all types of tests (tensile tests, compression tests, bending tests as well as cycle and hysteresis tests). In addition to the routine for evaluating longitudinal strain, a software module for transverse contraction measurement is optionally available. The system is suitable for all materials such as steel, plastic, rubber, resins, wires, ropes, wire ropes, polyurethane foam, semi-finished products and finished products (ductile and high-strength materials, low and high strains (*).
MFHT5 high-temperature strain gauge for tensile tests at temperatures up to 1200 degrees (ISO6892-2)
- HQ strain gauge for high-temperature strain measurement
- first-class measuring system with DMS strain gauges (significantly better linearity compared to inductive HT strain gauges)
- Attachment directly to the HT oven (no separate holding arm – better handling)
- for temperatures up to 1200 °C
- for Le 25 and Le 50 mm
- Pressure system with integrated "rocker" to compensate for curved samples
- only applicable for E-modulus, Rp0,2 (Rp1,0) no measurement of Ag / Agt / A% (manual measurement)
- Ceramic arms – sensitive handling required
thinning during deep drawing
For forming processes - especially deep drawing processes - the determination of the transverse strain is very important. These transverse contraction gauges are used to measure the waist formation / necking or R+N value. For example, a sheet must not thin out too much during the deep drawing process in order to avoid a crack.
MFQ H2
- inexpensive clamp-on device for standard-compliant measurement of R+N values
- 2 measuring systems – averaging via Y-cable
- highly accurate – superior to other systems
MFQ H1
- like MFQ H2 – but only 1 measuring system
MFQ A
- comfortable automatic clamping
- standard-compliant measurement R+N values
- 2 measuring systems: averaging @ Y-cable
- highly accurate – superior to other systems
PDF MFQ
- Measurement Width Reduction Tensile Test
PDF MFQ-HR
- Measurement Width Reduction Tensile Test
The position of the probe arms on the sample depends on the type of tensile testing machine and the type of clamping device
In a hydraulic tensile testing machine with a plunger piston, it is basically never possible for the probe arms to remain in place until they break: As soon as a tensile specimen breaks (particularly when only slight elongations have been achieved and the specimen hardly/not at all constricts), the energy stored in the specimen is suddenly released. This causes the plunger piston to move at high speed and the probe arms to be dragged along with it. It is only a matter of time before the probe arm extensometer is destroyed. In addition, an older tensile testing machine usually only has a wedge clamp that no longer fixes the tensile specimen after the tensile test - the tensile specimen falls out or is even thrown away. This also accelerates the extensometer, which is still in contact, to such an extent that at least the cutting edges become blunt prematurely. Damage cannot be ruled out either.
Furthermore, the tensile specimen must be held securely in position by a suitable gripping device during/after fracture.
This is generally only achieved with:
- (cost-intensive) hydraulic clamping device
- other rigid clampings
These suitable, rigid clamping devices can be, for example, clamping devices for
- samples with shoulder head
- samples with threaded head
These clamping devices must use a lock nut / a pneumatically operated ram to fix the sample in position in the event of sample breakage. In addition, the crosshead of the machine must not accelerate too much (this excludes hydraulic testing machines with plunger pistons / modern hydraulic universal testing machines with double piston – piston side A + B – are suitable).
Increased wear of the cutting edges (probe arms clamped until sample breaks)
If the measurement is to be carried out using an extensometer until it breaks, a higher level of wear on the blades of the extensometer is to be expected. Wear can begin after just a few tests. This wear is unavoidable because the sample is deformed both plastically and elastically during the test...
If the sample breaks at the end of the test, the elastic part relaxes within a fraction of a second. The sample is accelerated strongly in both directions under the blades of the strain gauge. This retraction/relaxation (for steel usually around 0,3% of the total strain) occurs so quickly that it is completely impossible to open the arms of the strain gauge beforehand (in relation to the fracture event). The sample therefore "shoots" past the clamped blades and blunts them more and more.
Why is it useful and harmless for the measurement results to open the strain gauge according to Ag?
The elongation (in steel) consists of 3 parts:
- the elastic part (contracts again after breaking)
- the stretching to the maximum force (all parallel areas Lc stretch evenly = uniform stretching Ag + Agt)
- Elongation in the necking area Z (from Rm onwards, elongation only occurs in the flow area “Z”)
The elastic component (strain) is ultimately stored energy (similar to a tensioned spring) that is discharged when the sample breaks. If a sample breaks without any constriction, it does so explosively. This is typical, for example, with cast materials (grey cast iron, cast aluminum) or hardened materials and glass fiber plastics (GFK). With these materials, the elongation at break is often only approx. <2%. These materials do not constrict (Z). Therefore, the force does not drop. With these samples, the shock to the strain gauge is usually extreme.
For this reason, a probe arm strain gauge is generally not recommended for these samples. A clamp-on strain gauge is usually more suitable here. The strain gauge type STRAIN 25 - 50 is the preferred device here. This can measure a breaking strain of up to approx. 2 mm measuring travel (3 mm on request). It is not officially approved by the manufacturer for this purpose - but we know from many customers (and from this manufacturer) that this special strain gauge can be left on the sample until it breaks (2 or 3% strain) without being damaged. However, the prerequisite here is that the sample is not accelerated too much / the sample is not released from the clamping device (fixed sample) and that the only clamped strain gauge does not fall down due to the impact and onto the machine bed. Falling down can be prevented by a safety device (small chain).
Touch arm devices: Due to the sensitivity, the general recommendation is:
The probe arms should be opened before the fracture
This is possible without distorting the strain values. However, it should be noted that the measuring arms should only be opened (significantly) after Ag.
Why are the strain results still accurate after opening the strain gauge?
This is explained by the possibility of different recording and combination of measured values from different sensors (strain gauge / crosshead travel) and their different evaluation: Up to the maximum force, a sample stretches evenly across the entire parallel area. However, since the strain does not have to be related to the entire parallel area (Lc) but to the initial length (Lo of the sample), it is essential that the strain gauge remains until Ag (uniform strain across the entire parallel area).
After the maximum force (Fm or measured value Rm) is exceeded, the sample begins to slowly and steadily contract (Z). Any extension of the sample now occurs exclusively in the yield area (which is then the weakest point and the later breaking point).
Numerous tests at the manufacturer GALDABINI have confirmed this procedure: It is irrelevant (if it is certain that Ag has been exceeded) which measuring system records this extension. This can be done using the strain gauge or alternatively using the crosshead travel measuring system of the tensile testing machine. Below is an illustrative sketch: The yield range begins at Rm. The strain gauge should not be opened too early but only when the stress gradient becomes clearly negative.
When can remaining on the sample cause erroneous results?
In some cases, the necking and fracture of the sample do not occur exactly in the middle but near the sample heads. In extreme cases, this even occurs outside the cutting edges of the extensometer... (outside Lo or the cutting edge distance Le). If the fracture occurs near the cutting edges, part of the strain "flows under" the extensometer cutting edges. If the fracture occurs outside the cutting edges, the entire strain is lost to Ag.
– In the graphic shown, the strain measurement is carried out uniformly up to Fm (Rm) (uniform strain Ag or Agt within Lc or Lv).
– From the maximum force onwards, further stretching occurred outside the strain gauge: The graph does not undergo any further extension and falls vertically immediately after Ag / Rm. In the graph it is even clear that this runs backwards (the tap inside the cutting edges contracts elastically).
This test must be rejected (Reh/Rp0,2 and Rm can be used / the breaking strain A could only be measured manually. If in this case the arms of the strain gauge had been opened after Ag had been exceeded (channel switching to the traverse path), the strain from Rm in the necking area would have been recorded. Where this necking area is located is then irrelevant. The test could probably have been assessed as valid ...
Is the result of the elongation at break an essential criterion of a tensile test?
It is true that elongation is mentioned as a test result in ISO6892-1 and its calculation is described precisely. If a design or the statics of a component were based on elongation at break results, the designer would have to rely on values that are determined long after the first damage limit of the material. In practice, therefore, elongation is "only" an indicator of the general ductility of materials. The most important element of any metal tensile test is therefore the damage limit (Reh or yield strength Rp0.2). On this basis, the designer can calculate the load up to which the material can be loaded without damage occurring.
Elongation is only an important criterion in a forming process. But here too, the elongation at break A is not required as a good/bad criterion, but rather the value Ag: up to what elongation can a material (e.g. deep-drawn sheet metal) be stretched without partial, unwanted elongation occurring. Elongation in a partial area would weaken the workpiece at that point and, for example, cause the wall thickness of a car sheet to fall below tolerance. The results of the elongation at break are therefore often overestimated (if one only follows the numerical values). The only goal is then to meet the minimum requirements of a steel grade in terms of elongation at break. Please remember that ISO6892-1 provides for the elongation values to be rounded to 0,5%...
Procurement of extensometers / strain gauges
for GALABINI universal testing machine / modernization of universal testing machines
Do you need an extensometer / strain gauge?
We are happy to offer you this equipment.
However, the connection of this equipment to third-party machines is not possible: Competitors do not support this with electronic plans or the necessary passwords for the software integration. Therefore, the integration is only possible for
- the new machines GALDABINI offered by us
- the modernization we offer for hydraulic / mechanical universal testing machines
- DOLI electronics with DOLI software
