Extensometers have become commonplace in material testing labs, measuring deformations or strains in test specimens as they are subjected to loads. They also let engineers accurately characterize mechanical properties, such as stiffness, yield strength, and ultimate elongation. Although traditional clip-on style and other contacting extensometers work well in most testing applications, new materials and delicate specimens are uncovering a few shortcomings.

A few problems

Crosshead extension, for example, is often inadequate for measuring strain. System compliance in the test frame, load cell, and grips may introduce error in measurements. Also, strain in dog-bone shaped specimen or those without a constant cross-sectional area between the grips is not constant. Therefore, crosshead motion as a strain measurement device is only an average of the total strain in the specimen and not an accurate measure of strain in the gage length.

Measuring material strain is usually done with contacting extensometers. This traditional equipment attaches to a specimen with clips or elastic bands and uses knife-edges to track specimen deformation. Although this yields results, contacting extensometers also have their disadvantages.

For example, knife-edge-contact points create stress concentrations in specimens, which can cause premature failure. The energy suddenly released when a specimen fails can damage contacting extensometers, as many nitinol suppliers will attest. To side step damaging breaks, technicians have the option of removing the extensometer before specimen failure. But this potentially introduces even more variability in results for ultimate elongation. When the extensometers are not removed, knife edges may dull over time and slip, requiring more frequent calibration and verification to ensure their compliance with international testing standards.

In addition, contact forces created by the extensometer can actually increase the apparent stiffness of test specimens. The weight of the contact extensometer can distort delicate specimens thereby generating erroneous results. Also,attaching contacting extensometers in a repeatable fashion requires a high level of operator skill.

Delicate specimens present other problems. They cannot be tested using contacting extensometers because the attachment would ruin them even before the test began. Delicate materials include soft biological tissues, fine wire, and thin films.

Although many techniques exist to minimizes these effects, they are not completely eliminated. Removing sources of error calls for extensometers that do not touch the specimen.

A solution

A video extensometer, a non-contacting device, measures by tracking movement between two markers attached to a specimen. Previously, clip-on extensometers were considered to produce more accurate and reliable strain-measurement devices than video style extensometers. However, as imaging technology advances, Instron's third generation video extensometers are those that take advantage of high-resolution, digital-camera technology, offering all the advantages of noncontact extensometers while meeting accuracy requirements of mainstream standards.

Standard Video Extensometers (SVE) and Advanced Video Extensometers (AVE) from Instron measure strain by tracking contrasting gauge marks on specimens. These marks can be dots or lines and can be applied to specimens many ways. Paint and ink are frequently used.

Prior to testing, the software searches for images of the marks matching those in a library of acceptable marks. Advanced image-processing algorithms then track each mark's center for accurate strain measurement even if the gauge marks are distorted by elongation.

The AVE and SVE also compensate for poor lighting. A previous challenge in video extensometry was providing enough light for the camera to recognize gauge marks on specimens. This gets worse with the variability of ambient lighting. The AVE solves the problem with an array of high-intensity pulsed red LEDs that ensures optimal lighting under all conditions. Filters eliminate bright reflections from certain materials for good mark contrast and accurate measurements.

Many noncontacting extensometers only perform relative measurements, that is, measuring changes in displacement relative to an initial distance. However, the AVE and SVE take absolute measurements, recording extensions in absolute displacement units in addition to percentage strain. (This makes the AVE a Type 1 extensometer system as defined by the ASTM E 83 standard.)

Before each test, the extensometer measures the gauge length (in absolute displacement units) and uses this to calculate strain. This measurement eliminates errors introduced by inaccurate specimen marking. The AVE can also handle transverse strain measurements, making it useful for applications such as calculating the plastic strain ratio (r value) from sheet-metal tests.

Testing in liquids

For many in the biomedical industry, it is necessary to test in a variety of environmental conditions, which presents its own set of challenges. For example, some medical devices, biological tissues, replacement tissues, and biomaterials should be tested under physiological conditions, otherwise their mechanical properties change. In these cases, the SVE works with Instron's BioPuls Bath, letting it accurately measure strain in a fluid bath set to 37°C. Although some submersible clip-on extensometers may provide acceptable solutions for testing rigid specimens, such as bone and cartilage, video extensometry is the only other solution for most other biomedical applications.

The SVE and the bath have been used to test hydrogels, soft tissues, sutures, and medical-grade plastics and tubing. System set-up and calibration is quick and easy, and it's also possible to verify and classify the extensometer to some industry standards. Although specimen marking in solutions is often application specific, several customer-proven techniques have been successful, including water-soluble and water-insoluble inks, superglue, adhesive putty and permanent marker.

Testing nitinol

The recently issued ASTM standard, F 2516-05: Standard Test Method for Tension Testing of Nickel-Titanium Superelastic Materials, suggests that strain in nitinol specimens can be determined either by machine crosshead motion, a clip-on extensometer, or a video extensometer. Comparing results between these three methods shows that the video and clip-on extensometers provide comparable and most accurate results. However, the video extensometer has several advantages.

For example, video extensometers eliminate slipping of clip-on extensometer's knife edges when they dull. Slipping significantly affects residual elongation and modulus calculations. Knife edges may also cause premature failure by generating stress concentrations at high loads. This lowers values of tensile strength and uniform elongation, especially when testing thin wires and tubing.

Another concern of nitinol suppliers and users: It is often important to conduct tests in a heated chamber to ensure the correct phase transformation states. The SVE can be mounted on a chamber that generates temperatures from about -70 ° to 600°C for a variety of requirements. For high-temperature measurements however, the marking technique must be rated for the given temperatures.

Testing medical-grade plastics

It's no surprise that plastics are the most widely used material in medical devices. That means a vast array of plastics and plastic products get tested to customer-specific R&D and QC standards, and those defined by industry, such as ASTM D 638, D 882, and ISO 527. This often requires an extensometer. Video work well with plastics because most molded specimens or thin sheets are easily marked with paint or ink. And unlike clip-on extensometers, gauge marks can be tracked through specimen failure. Further, the AVE lets technicians measure axial and transverse strains necessary for calculating Poisson's ratio.

Tubing applications, modulus, yield strength, and ultimate elongation all require accurate strain measurements. SVE and adhesive tape marks can measure strain through break in even when the tubing stretches a lot.

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What extensometers can measure

At first glance, extensometers seem to measure just strain. But when teamed with calculation software, they can produce enough data to sufficiently characterize new and advanced materials. For example:

• Residual strain, uniform elongation, and upper and lower-plateau strength at a variety of temperatures are needed by designers of nitinol parts.

• Medical-grade plastics and thin films require a modulus and Poisson's ratio.

• Designers working with medical tubing and wires need yield strength, modulus, and ultimate elongation.

• Some biological tissues call for modulus and ultimate elongation. Such tissues include skin, tendons, ligaments, bone, along with replacement tissues and scaffolds, such as hydrogels. These must be tested under ambient or physiological conditions duplicated by baths.