A Users Guide to Video Extensometers

Learn about the different types of extensometers, principle of operation, application range, features, capabilities and real world performance in this article about video extensometers.
Materials testing laboratories have used extensometers for years. In fact a device to measure changes in length was first described in the Journal of the Franklin Institute more than 130 years ago. And, while there are many different kinds of extensometers, they mainly fall into two categories: contact and non-contact.

Contact extensometers are further divided into two types: the clip-on and the automatic contacting extensometer.  The clip-on type comes in various sizes and is used to measure different levels of deflection from less than 1mm to more than 100mm.  Typically, clip-on extensometers are used for elongations ranging from 1mm to 10mm. 

Clip-on extensometers have numerous advantages, including low cost, ease of use and high accuracy. However, they also can involve a significant number of time-consuming steps as well as be subject to operator error.  Also, because of their contacting nature, they also may influence the results when testing small and fragile specimens.

Clip-on devices, however, have largely been supplanted by more advanced automatic contacting extensometers, which were developed to provide faster, more accurate and reliable materials testing results. By automating the process, these instruments greatly increase output, while reducing common operator errors.  They yield better, more accurate and reliable results. Many have resolutions of 0.3µm or better, and some devices are able to read values as low as 0.02µm. They also can remain on the specimen until failure and measure elongations up to 1000mm without any loss of accuracy.

Non-Contact Extensometers
Over the years, manufacturers of materials testing systems have worked extensively on developing innovative solutions for non-contact material testing strain measurement.  Among the solutions considered are those that incorporate laser scanners, which proved especially suitable for a range of materials that include plastics, film, rubber and textiles

Users of extensometers also have required more information, greater flexibility and increased versatility from their instruments.  Among the first non-contact devices to appear were laser-based extensometers that were used with traction machines.  While these instruments offered good accuracy measuring large strains, they lacked flexibility and were less accurate in measuring very low-level strains.

Laser extensometers typically are used for testing materials that might be damaged or affected by a traditional clip-on or contact extensometer.  They operate by illuminating the surface of a specimen with a laser and capturing the reflections made by the laser as force is applied. These reflections are then measured then using advanced imaging software incorporating complex algorithms.

Laser-based systems can be used with a wide range of materials and tests can be conducted on specimens at elevated as well as ambient temperatures, making them suitable for samples held in a thermal cabinet or environmental test chamber.  Laser extensometers also offer excellent accuracy and resolution and provide a certain level of safety, especially when testing specimens that may release large amounts of energy upon failure.

Video Extensometry

As video technology and computer software improved, a new generation of non-contact extensometers evolved: the video extensometer. Video extensometers have many advantages. They afford users greater flexibility as well as the best available accuracy across a broad range of applications. 

This non-contact solution is particularly well suited for brittle or thin materials as well as samples that release energy at failure or where there is a boost at break (such as cables, ropes, belts, etc.) as well as measurements inside a thermal cabinet or environmental test chamber. 

The technology also works well for tests where elongation and variation of section measurement are required and tests where transverse elongation and r & n values are required for ISO 10113 and ISO 10275. In addition, the instrument  can be used to test metals with the measurement of the Rp 0.2 limits according to EN 100002-1 and ASTM.

With recent improvements in video technology and image analysis software, materials testing system manufacturers have focused on video extensometers (VE) to provide accurate measurement for a number of specific applications, especially those where contact extensometers might have an adverse impact on test results or accuracy. 

The latest VE technology instruments are an attractive replacement for mechanical extensometers, laser-scanning systems and strain gauges.

They offer many benefits and provide a number of advantages compared with traditional contacting devices including:
  • No influence on the test specimen 
  • No problems with knife-edge slip 
  • No errors due to inertia of moving parts 
  • No errors due to worn or damaged parts 
  • No damage as a result of energy release at failure
 
However, VE technology’s greatest advantage is its versatility. The technology is simple and easy to use. One camera can be used for both longitudinal and transverse strain measurement. Only a simple adjustment and alignment with respect to the test axis is required. The technology permits multiple fields of view as well as multiple strain and material tests and real-time viewing and analysis.

Principle of Operation
Video extensometry incorporates a high-resolution digital camera with advanced real-time image processing to make highly precise strain measurements of a variety of specimen types.  The video camera captures the image, which is transferred via an IEEE 1394 (Firewire) interface to a PC or laptop computer.

Longitudinal strain is determined by measuring the change in distance between two line markers that are applied to the test specimen with a color market, sticker or clamp. A variety of optional marking pens and target applicators are available.

The video extensometer determines the position of the markers through changes in the brightness of the light/dark edges of the markers.  The camera digitizes the image, and image analysis algorithms measure the change in grayscale along one image line on the specimen surface.  These algorithms allow the instrument to measure edge positions with sub-pixel accuracy. 

The transverse deformation is calculated from the measured change in the width of the specimen. Gauge length is automatically measured at the beginning of each test and used for strain calculation, eliminating errors due to inaccurate specimen marking.

The measured values are transferred from the video extensometer to a tensile test machine through a digital or analog interface.  The tensile testing machine can then record and post process every measurement in the same way as other strain sensors or gauges. Typically the E-modulus and Poisson’s ration are calculated.

Application Range 
VE lends itself to a wide range of applications. The technology is especially well suited for testing rigid materials such as metals and composites. However, a wide range of materials lend themselves to non-contact tensile strain measurement, including plastics, textiles, paper, thin sheets and foils, and wires.

The technology is also appropriate for conducting a variety of test procedures such as the measurement of material properties, true strain controlled tensile tests, exploration of cracks, investigation of strain behavior on dynamic tensile tests, dynamic and high-speed tests, vibration analysis and more.

Features and Capabilities
The current generation of VE equipment feature modern, configurable and intuitive user interfaces as well as a wide range of options for data communications, management and export.  Its multithread-analysis-kernel supports multi-core processors to achieve low processor load.

A variety of templates are available for different measurement tasks. Connecting a video extensometer to a tensile testing machine also allows for fully automatic operation.

Multi-camera systems permit simultaneous measurement of different specimen sides up to 360o along with small and large field of views for accurate E-modulus and full stress-strain curves.

Depending on type of camera (high-accuracy versus high-speed), VE is capable of significantly different accuracy and sampling rates.  With a high-accuracy, 2-megapixel camera, accuracy is 0.002% strain (1µm @ 100mmFow) and the  sampling rate is up to 100Hz. 

Real World Performance
In one real-world example, an R&D lab performs tension and compression tests on a variety of materials ranging from metal and wood to elastomers and plastics. In addition, it has several different traction machines with different interfaces and multiple gauges that utilize different ranges and technologies.

Of all the available extensometer solutions, only VE offers the versatility and adaptability to meet all of the laboratory’s needs.  With it, the lab can conduct tests that measure only a few millimeters with an accuracy of 0.1µm up to 100 millimeters with only two focal lengths.  With its digital and analog interfaces, VE is compatible with all of the lab’s tensile test machines.

VE also solves many of the problems than can occur when a specimen has relatively soft edges like a thermoplastic or has features that can rupture or break and cause damage to a clip on extensometer. Overall, VE provides users with a high-performance, high-accuracy and reliable solution for a host of test and measurement applications.

Conclusion
After more than 130 years, extensometers continue to play an important role in materials testing. The capabilities of these devices have improved significantly so there is now a greater range of technologies available.  Selecting the right technology depends on a number of factors, including cost, accuracy and ease of use as well as the specific nature of the material and test being performed.

The best choice for a particular material or application might not be the right choice for another.  Keeping that mind, given recent improvements in both hardware and software, extensometer users need to seriously consider video extensometers based on their flexibility, versatility, reliability and cost-effectiveness over a wide range of materials and applications.