Extensometers for Universal Testing Machine
Clip-on, long-travel, deflectometer and COD clip gauge: which to choose for each test and standard, integrated with Biopdi universal testing machines.
Biopdi extensometers measure strain directly on the test specimen, without the grip slack and frame deformation that are included in the crosshead displacement. They are supplied in four families: clip-on (L0 from 25 to 100 mm), long-travel (travel up to 250 mm), deflectometer (travel from 1 to 50 mm) and COD clip gauge, all integrated with Biopdi universal testing machines and the BioLab software, on up to 4 simultaneous channels.
This page helps you choose and order the right extensometer for your test. For the theory of strain measurement, strain gauges and accuracy classes, see the article extensometer and strain gauge.
Extensometer models
The four families of the M2 line, with code, L0 and travel.
Clip-on
Attached to the specimen, for modulus and Rp0.2. L0 from 25 to 100 mm, travel up to 50 mm.
Long-travel
For rubbers, films and ductile plastics. L0 from 10 to 100 mm, travel up to 250 mm.
Deflectometer
Measures specimen deflection in flexural testing. Travel from 1 to 50 mm.
COD clip gauge
Measures crack mouth opening displacement (CMOD) at the notch. L0 from 5 to 20 mm, travel up to 12 mm.
Available models: codes, L0 and travel
Four families for measuring strain directly on the test specimen, with up to 4 simultaneous channels in the BioLab software. L0 is the initial gauge length.
Clip-on
Attached directly to the specimen, for small strains such as modulus of elasticity and offset yield strength.
| Code | L0 | Travel |
|---|---|---|
| EXT-100-001 | 25 mm | −2.5 to +2.5 mm |
| EXT-100-002 | 25 mm | 0 to +12.5 mm |
| EXT-100-003 | 25 mm | 0 to +25 mm |
| EXT-100-003 | 50 mm | −5 to +5 mm |
| EXT-100-004 | 50 mm | 0 to +25 mm |
| EXT-100-005 | 50 mm | 0 to +50 mm |
| EXT-100-006 | 80 mm | 0 to +8 mm |
| EXT-100-007 | 80 mm | 0 to +40 mm |
| EXT-100-008 | 100 mm | 0 to +50 mm |
Long-travel
For materials that elongate a great deal before breaking, such as polymers, elastomers and films.
| Code | L0 | Travel |
|---|---|---|
| EXT-200-001 | 10 to 100 mm | 25 mm |
| EXT-200-002 | 10 to 100 mm | 50 mm |
| EXT-200-003 | 10 to 100 mm | 100 mm |
| EXT-200-004 | 10 to 100 mm | 150 mm |
| EXT-200-005 | 10 to 100 mm | 250 mm |
Deflectometer
Measures the deflection of the test specimen in flexural tests.
| Code | Travel |
|---|---|
| EXT-300-001 | 1 mm |
| EXT-300-002 | 4 mm |
| EXT-300-003 | 6 mm |
| EXT-300-004 | 12 mm |
| EXT-300-005 | 25 mm |
| EXT-300-006 | 50 mm |
COD clip gauge
Measures crack mouth opening displacement (CMOD) in tests on notched specimens, such as the notched 3-point flexural test.
| Code | L0 | Travel |
|---|---|---|
| EXT-400-001 | 5 mm | +2.5 mm |
| EXT-400-003 | 10 mm | +4 mm |
| EXT-400-004 | 12 mm | +7 mm |
| EXT-400-005 | 20 mm | +12 mm |
The four extensometers in four views
Renders of the M2 line extensometers in front, isometric, rear and trimetric views, showing the transducer housing, the arms and the contact point with the specimen. The images are not to scale relative to each other: the size of each family follows from the L0 and travel in the table above.
Clip-on
Long-travel
Deflectometer
COD clip gauge
The Biopdi universal testing machine
The extensometer is a machine channel: it adds the actual strain to the force and position the machine already measures.
The two models in the image are examples. On the single-column MBIOI-10-M2, up to 10 kN, the typical use is the clip-on and the long-travel extensometer on plastics, rubbers and films; on the MBIOII-50-M2 dual-column machine, up to 50 kN; the clip-on extensometer for metals and composites; and the deflectometer and COD clip gauge for flexural tests. The entire M2 line, from 200 N to 1,000 kN, accepts the extensometers on this page. In a test with an extensometer, each part of the machine has its own role:
- 1
Moving crosshead applies the force at the rate set by the standard; the optical sensor measures its position with a resolution of 0.0001 mm
- 2
Load cell measures the force; load cells are interchangeable to match the test range
- 3
Grips or fixture hold or support the test specimen aligned with the load axis
- 4
Extensometer measures strain over the gauge length L0, on up to 4 channels with automatic recognition
- 5
BioLab Software combines force, position and strain, plots the curve and calculates the results
- 6
Control panel and emergency stop positions the crosshead so the extensometer can be mounted, and stops the test
How the extensometer is mounted on the test specimen
Each family contacts the test specimen in its own way, and that is what determines which test it is suited for.
Clip-on
Two knife edges rest on the test specimen, held by springs or elastic bands, separated by the gauge length L0. The setting pin fixes L0 during mounting and is removed before the test. Models without fracture protection are removed before failure.
Long-travel
Two arms clamp the specimen at the L0 marks and slide along a rod attached to the machine frame, following elongations of several hundred percent up to the fracture of rubbers, films and ductile plastics.
Deflectometer
Mounted on the flexural fixture, with the probe touching the specimen face at midspan. It measures the deflection of the bar itself, without the deformation of the fixture and frame that is included in the crosshead displacement.
COD clip gauge
Fitted between two knife edges attached at the notch mouth, it measures the crack mouth opening displacement (CMOD) in notched beams under bending, as in fiber-reinforced concrete and fracture toughness tests.
From test specimen to test report
The extensometer signal enters the machine controller together with force and position, and BioLab calculates the properties that depend on the actual strain.
Force and position
The load cell measures force and the crosshead sensor measures displacement. That is enough for maximum force, strength and force × displacement curves.
Actual strain
The extensometer measures only the gauge length L0 of the test specimen. It is what makes modulus of elasticity, Rp0.2 and elongation under load possible with the accuracy the standard requires.
Results in BioLab
The software combines the channels, plots the stress × strain curve, calculates the results required by the standard and accepts custom calculations using the extensometer channels.
Which extensometer for each test
The family follows from the type of test; the gauge length (L0), from the standard; the travel, from the expected elongation.
| Test | Common standards | Family | Starting point |
|---|---|---|---|
| Tensile testing of metals: modulus, Rp0.2 and Agt | ISO 6892-1 (ABNT NBR ISO 6892-1 in Brazil); ASTM E8/E8M | Clip-on | L0 of 50 mm (standard flat and round specimens); ±5 mm travel for modulus and Rp, longer travel to follow up to Agt |
| Tensile testing of rigid and semi-rigid plastics: modulus | ASTM D638 (types I to III); ISO 527 | Clip-on | L0 of 50 mm for D638; for ISO 527, check the specimen L0 in the current edition |
| Tensile testing of rubber, elastomers and highly ductile plastics | ASTM D412; ISO 37 | Long-travel | L0 set to the specimen gauge marks (25 mm on the die C dumbbell) and 250 mm travel |
| Flexural testing of plastics and composites: deflection | ASTM D790; ISO 178 | Deflectometer | Travel greater than the deflection at the test limit (about 7 mm for the 3.2 mm bar in D790) |
| Crack mouth opening (CMOD) on notched specimens | Fiber-reinforced concrete (EN 14651); fracture toughness (ASTM E399) | COD clip gauge | Travel above the maximum opening expected in the test (EN 14651 goes up to 4 mm CMOD) |
Starting points for the technical discussion; the final choice takes into account the actual test specimen and the edition of the standard used by the laboratory.
How to size the travel and the gauge length
Three calculations cover most requests.
1. Gauge length (L0)
It comes from the standard and the test specimen. In ISO 6892-1, the proportional test specimen has L0 = 5.65 × √S0: for a 10 mm round bar, S0 = 78.5 mm² and L0 = 50 mm. In ASTM D638, types I to III, the gauge length is 50 mm.
2. Travel
Required travel = expected elongation × L0. A rubber with 600% elongation at break, measured between 25 mm gauge marks, opens 25 × 6 = 150 mm: the 250 mm travel (EXT-200-005) covers it with margin, while the 150 mm one is at the limit.
3. Deflection in flexure
In ASTM D790, the deflection for 5% strain in the outer fiber is D = 0.05 × L² / (6 d). For a 3.2 mm bar with a 51.2 mm span: D = 0.05 × 51.2² / 19.2 ≈ 6.8 mm. The 12 mm deflectometer (EXT-300-004) covers the test.
The accuracy class also comes from the standard. ABNT NBR ISO 6892-1:2024 (the Brazilian adoption of ISO 6892-1) requires a class 1 extensometer (ISO 9513) for the proof strength (offset yield strength) and class 0.5 for the modulus of elasticity; above 5% extension it accepts class 2. State the standard in your request so the proposal includes a compatible extensometer and class.
Frequently asked questions
What people ask most before ordering an extensometer.
What types of extensometers are there for universal testing machines?
The main ones are the clip-on, attached to the specimen for small strains; the long-travel extensometer, for rubbers, films and ductile plastics; the deflectometer, which measures deflection in flexural tests; the COD clip gauge, which measures crack opening; and the optical (video) extensometer, with no contact. Biopdi supplies the first four families in the M2 line and the optical one on request.
Does the Biopdi extensometer work on a universal testing machine from another brand?
No. Biopdi extensometers are accessories for Biopdi universal testing machines: the signal goes into the machine controller and is processed by the BioLab software, so they are not sold for machines from other brands.
Can I buy just the extensometer for the Biopdi machine I already have?
Yes. The extensometer can be ordered together with a new machine or added to a Biopdi universal testing machine already in your lab. State the machine model in your quote request.
How many extensometers can the machine read at the same time?
Up to 4 strain channels in addition to force and position, with automatic recognition of the connected transducer, for extensometers and LVDTs.
Which extensometer should I use for the modulus of elasticity of steel?
A clip-on attached to the specimen, with a gauge length that matches the standard (50 mm on the ASTM E8 standard specimen and on the 10 mm round specimen of ISO 6892-1) and a short travel, such as ±5 mm. ABNT NBR ISO 6892-1:2024 calls for an ISO 9513 class 0.5 extensometer for the modulus; confirm the class in the proposal.
Which extensometer should I use for rubber and elastomers?
The long-travel extensometer, which follows elongations of several hundred percent up to fracture. The gauge length is set to the specimen marks, from 10 to 100 mm, and the travel reaches 250 mm.
Do I need an extensometer for every tensile test?
No. It is required when the standard calls for modulus of elasticity, offset yield strength (Rp0.2) or elongation under force. Elongation after fracture is usually measured on the two fitted halves of the specimen, and maximum force does not depend on an extensometer.
What is a COD clip gauge?
It is an extensometer mounted at the notch mouth that measures the crack mouth opening displacement (CMOD) during flexural testing of notched specimens, used for fiber-reinforced concrete and fracture toughness tests.
Does Biopdi supply a video extensometer?
The optical extensometer (video extensometer) is developed on request. For large elongations with contact, the line already includes the long-travel extensometer, with up to 250 mm of travel. Describe your test in the quote request.
What information do I need to provide to receive a proposal?
The Biopdi machine model (or whether it is a new machine), the material, the test standard, the specimen type and dimensions, the properties to be measured and the expected elongation at fracture.
Request your quote
Price and lead time for an extensometer for your Biopdi universal testing machine, or with a new machine. Tell us the material, standard and test type.
Reply within 24 business hours
A proposal covering capacity, fixtures and the standards met, not just the price.
Configured for your test
Family, gauge length (L0) and travel selected based on your test specimen and the standard.
In-house manufacturing, ISO 9001
Installation, training and technical support provided by the people who designed the machine.