From fragile wires to structural steel components, metals producers are faced with a myriad of testing challenges. Metals manufacturers producing products such as plate, rebar, sheet metal and wire/cable, must not only meet advertised strength and performance ratings, but also comply with a wide range of ASTM, EN, JIS, SAE and other international standards.
Typical Properties Determined from a Tensile Test
Tensile testing of metals is typically performed to determine such characteristics as yield strength, yield point elongation (YPE), ultimate tensile strength, plastic strain ratio (‘r”) value and the strain-hardening exponent (‘n’) value. One common requirement needed for all these calculations is the ability to accurately measure the strain of the material in question.
What is Strain?
In practical terms, strain is a measurement of the deformation in a defined, uniform, section of a test specimen (known as the gauge section) as it is subjected to a load. Engineering strain is the extension divided by the original gauge length; it is usually expressed as a %.
Measuring of Strain
Strain measurement in materials testing is traditionally carried out using some form of contacting extensometer. A typical clip-on extensometer, for example, attaches to the specimen with clips or elastic bands and uses knife-edges to accurately track deformation in a specimen during testing. The use of an extensometer eliminates the measurement of extraneous compliance in the loading system, which would otherwise result in a reported deformation from the crosshead extension that is greater than the actual deformation of the specimen. Moreover, when testing typical “dog bone” shaped specimens, crosshead extension is often an inadequate way to measure strain, as it will record the deformation of the entire specimen between the grips along with the deflection of the test machine, load cell and grips instead of just the narrow, parallel section which is the area of interest (the gauge length).
Measurements, such as yield strength; YPE; r & n; and ultimate tensile strength, place unique demands on extensometers. Depending on the required results, axial strain, transverse strain, or both must be measured. High resolution axial strain measurement is required to determine the initial slope for an accurate yield strength determination. However, a long travel axial strain measurement is necessary to measure the total elongation when sheet metal is being tested. The “r” value calculations require both axial and transverse strain measurement.
Most industries that perform metals testing have used traditional contacting extensometers and trust the reliability of the results. Developments in the technology of extensometery now offer metals testing customers other options, such as non-contacting video extensometers based on high resolution digital cameras and real-time image processing.
Disadvantages of Contact Extensometers
While providing accurate strain measurement in numerous applications, contact extensometers have a number of disadvantages. Knife-edge contact points can create stress concentrations on the specimen, which may cause premature specimen failure. Results may also show evidence of extensometer contact point slippage as knife-edges become dull over time. For thin foils and wires, the contact force of the extensometer can increase the apparent stiffness of the test specimen and the weight of the contact extensometer itself can distort delicate specimens thereby causing erroneous results. The process of attaching a clip-on type of contacting extensometer, in a repeatable fashion, requires a level of operator skill.
Certain delicate specimens cannot be tested using a contact type extensometer because they would be ruined by the attachment of the extensometer before the test even began. Examples of these include fine wire and thin film. The high energy released during specimen failure can damage contact extensometers. In order to prevent this, the test must be stopped at some point before specimen failure to remove the extensometer. This procedure can potentially introduce even more variability in the test results.
These drawbacks are inherent to contact extensometers and while most are designed to minimize these effects as much as possible, they cannot be completely eliminated. In order to truly eliminate these sources of error, an extensometer is needed that does not contact the specimen.
In the past, the clip-on extensometer has been thought of as the more accurate and reliable strain measurement device. However, as imaging technology has advanced over recent years, a new generation of video extensometers has emerged that takes advantage of high-resolution digital camera technology, offering all the advantages of non-contact extensometry while meeting accuracy requirements of mainstream standards.
How Video Extensometers Work
This new advanced video extensometer measures strain by tracking contrasting gauge marks placed on the specimen. The gauge marks can be in the form of dots or lines and are applied to the specimen by a wide variety of methods. Prior to running a test, the software uses a neural network to search the image for marks that are compatible with a library of acceptable marks. Advanced image processing algorithms then track the center of the marks, which ensures accurate strain measurement even as the gauge marks distort during a test involving high elongation.
Recent Advances with Video Extensometers
Most non-contact extensometers can only perform relative measurement, i.e. the measurement of change in displacement relative to an initial distance. In contrast, the newer advanced video extensometers are capable of absolute measurement, allowing extension to be measured in absolute displacement units in addition to percentage strain. Before the start of each test, the advanced video extensometer automatically measures the gauge length (in absolute displacement units) and uses this for calculating strain. This eliminates errors introduced by inaccurate specimen marking.
The next generation video extensometers are also capable of transverse strain measurement, making it useful for applications such as plastic strain ratio (r value) in sheet metal testing. This makes the next generation video extensometer a Type 1 extensometer system as defined by the ASTM E 83 standard.
Overcoming Limitations of Video Extensometers
One of the main barriers of using video extensometry has been the need to make the measurements insensitive to the wide variation in the surface finish of metal specimens. The surface finish of metal products varies from the matt black of a hot rolled steel to the bright finish of stainless steel or aluminum. The variability of ambient lighting conditions further complicates this problem. These barriers can be overcome by the use of dedicated illumination systems. Another potential problem, which can be overcome by careful design, is noise caused by refraction of light from unwanted air currents between the camera and the specimen.
The next generation non-contacting video extensometer is now able to replace traditional contacting extensometers in a wide range of material tests. Furthermore, non-contact extensometry has many advantages over contacting extensometry in terms of productivity, ease of use and performance.