11 Types of Extensometers – How they Work and their Uses

Extensometers are devices used in the measurement of length of objects. It has a high precision and used in stress-strain measurements and also in tensile strength tests. There are basically two major types of extensometers based on their principle of operation: They are the contact extensometers and the Non-contact extensometers. The contact extensometer is of various types of which the clip-on extensometer is the most common. The non-contact extensometers are the laser extensometer and the video extensometer. The Extensometer was invented by Charles Huston in 1879.

Laser extensometer is used in elongation measurements or strain measurements on certain materials or specimen. The working principle behind the Laser extensometer is that a laser beam is directed at the specimen and the resultant reflections from the surface of the specimen are then received by a Charge-Coupled device(CCD) whose information are then processed by very complex software program or algorithm.

Video extensometer work by capturing continuous images of the specimen using a digital camera that is attached to a computer. The pixel distance can then be measured in real time and then mapped accurately against a calibration value to give a direct strain measurement of the specimen. Resolution of even less than one micrometer can be achieved with a good calibration value and nice image processing data.

11 Types of Extensometers

There are 11 well known types of extensometers used in measurements both in the laboratory and in the industry.

These extensometers include: 

1. Automatic contacting Extensometer

2. Average axial Clip-on Extensometer

3. Axial Clip-on Extensometer

4. Biaxial Clip-on Extensometer

5. Circumferential Extensometer

6. High Temperature Extensometer

7. Long Travel Extensometer

8. Non-contacting Video Extensometer

9. Transverse Clip-on Extensometer

10. Deflectometer

11. Crack Opening Displacement (COD) Gauges

In mining, the extensometer is used in the measurement of displacement on highwalls. Geologists can then plot a graph of displacement against time to ascertain if there is an imminent fracture or failure on the walls. Complicated wall failures can be determined by the use of laser beams or radar to enable a 3-dimensional or 4-dimensional analysis.

Choosing Extensometers for Materials Testing

Extensometers measure the extension, compression or shear deformation of a test specimen during a materials test. They can provide key properties such as offset yield, total elongation and complete stress-strain curves.

Choosing the right one depends on the material, test standard and anticipated deformation. Many test standards set a specific gauge length requirement. Avoiding damage to a fragile specimen is also important. Swivelling knife edges on contact extensometers reduce risk even for rough specimen surfaces.

The right choice of extensometer for a test is dependent on several factors. The type of material and any test standard requirements will help focus your search, especially when considering different stiffness classes (elastomers vs. metals), expected elongation, and whether you need to measure before or at yield. Other considerations include the clamping force required by a contact extensometer, specimen thickness, or the requirement for non-contact measurements. Very thin specimens such as foils may be sensitive to clamping forces, while very small wire specimens do not provide enough visible area for reliable non-contact measurements.

How Do Extensometers Work? 

Depending on the application, extensometers are available in both contact and non-contact models. In the case of contact, the two most common forms are probe (magnet) and rod extensometers. A probe extensometer consists of a hollow flush PVC tube with a series of equal-length, spaced-out magnets embedded inside a donut shape. A sprung leg is threaded over the PVC tube and pushed to its required depth by an insertion tool. When the sprung leg reaches its required length, the insertion tool is removed and the instrument signals an audible signal indicating a load has been applied.

Rod extensometers, on the other hand, consist of hydraulically operated anchors and connecting rods. These are connected to the measuring head by a flanged housing which is grouted concentric with the borehole. The measurement head itself consists of a hydraulically driven reference head and a connecting rod with a calibrated tip.

During a test, the extension values are transferred to a machine controlling computer in tab-delimited format, making them easy to import into post test analysis software such as Messphysik. In addition, all results are displayed in real time on the video monitor in the test machine.

Specimens that require the use of contact extensometers are first marked with measuring marks which are either clipped, tacked, or glued onto the specimen surface. These measurement marks can become indistinct, fall off, or deform under load and are therefore a significant source of error. The latest generation of non-contact extensometers, such as the laserXtens from Zwick, eliminate this source of error by using the unique structure of a specimen’s surface as a fingerprint to generate virtual measurement marks. These measurement positions are illuminated with laser light and detected by a camera that is integrated into the testing system. This allows the measured strain to be converted into direct extension value.

More On Types Of Extensometers

There are a number of different types of extensometer available as explained at the beginning of this post. The choice will be influenced by the type of material and the test standards that must be met. In general, a “class” or required measurement accuracy will be specified in the test standard and this should be taken into account when choosing an extensometer. Expected elongation (before yield or at fracture) is another important factor to consider when selecting an extensometer.

There is a wide range of both contact and non-contact extensometers. A contacting extensometer attaches physically to the specimen and can be of the ‘clip-on’ variety or larger devices that attach directly to the test frame. Contacting extensometers are generally suitable for most tests, however there are some exceptions that need to be taken into consideration.

For example, very thin specimens such as foils can be sensitive to clamping forces and small wire specimens may not provide enough visible area for reliable non-contact measurements. Similarly, the testing of very stiff or highly plastic materials often requires the use of a specially designed extensometer which has been developed to resist these forces.

Some of the main types of contact extensometers include sensor arm, incremental and analogue clip-ons and shear pin style devices.(see images above). These are all capable of being used for tensile, compression and shear testing as well as cyclic testing. They offer a range of gauge lengths to suit the various test requirements and most will also work in both axial and transverse direction.

As explained earlier, there are also several types of non-contact extensometers which use optical technology to measure the strain in a specimen. The main types of non-contact optical extensometers are based on a laser or an infrared beam and offer various options including a variety of measuring lengths, an integrated camera for localized deformation measurements, multiple measurement points along the specimen and even a dedicated flexure extensometer which can be used to test shear samples.

Generally, it is best to select the highest specification extensometer that is within your budget as it will give the best data quality and allow you to meet the most test standards without having to invest in more than one device. However, there are occasions when a more specialized or expensive extensometer will be vital for specific applications (torsion, biaxial, high temperature, cyclic fatigue etc).

Uses and Applications Of Extensometers

Many different types of extensometers exist to suit a wide variety of applications. When selecting an extensometer, it is important to consider the specific material and testing requirements of each application. For example, some materials such as thin films are best tested with non-contact instruments to avoid damage from contact. Additionally, some samples may need to be inspected at elevated temperatures and require non-contact extensometers to maintain measurement accuracy.

Watch the Video version of this post here👇

What To Look Out For When Selecting An Extensometer

When selecting an extensometer, it’s also crucial to consider the ease of operation and ergonomics of the device. This is especially true if a lot of series testing is conducted, as it’s important to minimize strain and fatigue on the operator’s hands and arms. Some advanced models of extensometers include features such as swiveling knife-edges that help to mitigate the risk of damage and improve user safety.

It’s also vital to determine how you will calibrate and verify the accuracy of your extensometer in advance. This can include electrical calibration (for strain-gaged extensometers) and visual inspection for displacement measurements. Additionally, it’s important to plan for preparing and storing your extensometer for use. Many models come with protective coverings and cases to ensure that they remain in good condition when not in use.

Lastly, it’s important to select an extensometer that will integrate with your testing machine. This will ensure that the data is transmitted accurately and the measurements can be displayed on-screen for easy interpretation. Some advanced models of extensometer can also be operated using a computerized interface to make the setup and test process much faster and more convenient.

Selecting an Ideal Extensometer

To select an ideal extensometer for your testing needs, start by determining the maximum extension or strain that you want to measure. This will allow you to select an extensometer that can handle the required loading conditions. Additionally, it’s important to consider any other unique requirements, such as the need for a particular measuring range or temperature conditions. If necessary, it’s also recommended to obtain a Tech Note that details the specifications of your testing standard. Finally, it’s vital to check whether your equipment technicians have the training needed to operate and maintain an extensometer.

Repair and Maintenance of Extensometers

The initial costs of purchasing a testing equipment like an extensometer may be substantial, but the real cost is the cost of maintaining it. The longer your test equipment sits idle, the more it is subjected to environmental conditions that can affect performance. These conditions include humidity, water exposure and temperature extremes. In addition, jarring vibrations can damage or disturb the extensometer and cause erroneous readings. This type of maintenance and protection is an ongoing effort that requires everyone involved with the testing process.

The first step in restoring an extensometer to calibration is to exercise it two to three times by running the rod from its initial position (zero) through its full range of travel and back to its initial position. This process resets the sensor to its correct zero position and also calibrates any electronics that might have been affected by hysteresis.

When an extensometer is not being used, it should be stored in a protective case or other device to prevent environmental effects from damaging the sensitive sensor. Moisture, especially, can damage the strain gages used in many models of an extensometer. Short term condensation normally does not cause permanent damage, but long periods of exposure can degrade the gages and wires. If an extensometer must be exposed to moisture, it is recommended that the user dry it as soon as possible and store it in a container with desiccant.

Watch the Video on Extensometers  Here

Optical extensometers can also suffer from out-of-plane motion, which causes an error in the measurement signal. To overcome this, scientists have proposed effective methods to compensate this error, but they have the drawback of requiring that the reference specimen is changed periodically during the testing procedure.

For geotechnical applications, a fixed extensometer is often used to monitor settlement or heave between points without the use of a removable probe. This is done using a riser settlement rod that is either buried at the embankment fill level or grouted inside borehole as a deep benchmark in a firm soil. Alternatively, an optical levelling extensometer can be used to monitor changes in surface elevations.