The requirements for an extensometer are determined primarily by the characteristics of the material to be tested. This includes its shape and dimensions, test requirements, and the formal Standards that must be met. These define the gauge length, accuracy, test sequence and environmental conditions (see Figure 1).
Having said this, the right choice of extensometer cannot be limited to the basic material characteristics such as specimen dimensions, stiffness, strength and plasticity alone. It is also necessary to decide whether an extensometer can be connected directly to the specimen without influencing the load measurement or mechanically damaging the specimen itself.
Thin specimens, such as foils, can be sensitive to clamping forces, while small wire specimens do not provide enough visible area for reliable non-contact measurements.
A high stiffness in the initial extension range followed by high plasticity traditionally requires more than one extensometer. The first measures small strains (typically up to 5 mm) accurately in the elastic range, and the second measures high extensions (typically > or = to 500 mm).
High extension or flexible specimens can damage or destroy the knife edges and even the extensometer itself because of whiplash, splintering or de-lamination of specimens (for example steel rope). For these applications, non-contact measurement is a must.
Criteria for accurate measurements
Measurement travel (range) and gauge length
With contact type measuring extensometers, the measurement travel is normally an engineered and fixed value that is dependent on the range of the measurement transducer and, with fulcrum hinged sensor arms, the leverage ratio. The initial gauge length is set manually with fixed steps or automatically over a defined range.
Non-contact extensometers that use a video camera must have the field of view larger than the required range plus the initial gauge length. Because the specimen portions are outside the gauge length and the machine components deform in the direction of loading, the position of the measuring marks on the specimen changes during the test. For extension and gauge lengths that are expected to be outside the field of view, the objective lens must be changed or the distance between the specimen and the video camera must be increased. These actions decrease the measuring accuracy, and every changed measurement configuration must be adjusted and calibrated.
Measurement accuracy
Figure 3. Combination of fully automatic feeler arm extensometers with dual averaging axial and transverse strain extensometer.
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“Accuracy” is a commonly used, qualitative term. To qualify the integrity of a measured signal, Standards use quantitative terms such as “resolution,” “deviation,” or “uncertainty,” and definitive values are given for these. Requirements for the accuracy of extension measurements are normally given in application-specific test requirements and International Standards. Many test Standards, ie. tensile test on metals and plastics, refer to Standards for calibrating extension measurement systems and the required accuracy classes contained therein (see Figure 2). Ergonomics and economy
Easy-to-setup devices and automated sequences reduce personnel time and effort while improving test result quality because subjective influences are minimized. Higher initial acquisition costs can be quickly amortized, especially when the extensometer can be used for a range of applications.
When using a video extensometer, the time and costs for marking the specimen must also be considered, as well as any potential human error introduced in attaching or aligning the marks.
Contact-type measurement extensometers
Figure 4. Schematic diagram showing the operating principle of various extensometers.
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Clip-on extensometers are, as the name implies, mounted directly onto the specimen. The mechanical parts that transfer extension, via knife edges, from the specimen to the internal transducer are short and stiff. There is practically no relative movement between the specimen and the extensometer, and for this reason the measurement accuracy is high. The range of a clip-on extensometer is limited to a few millimeters, and it applies a load directly to the specimen. Some extensometers are available with counter-balance weight, and double-sided measuring systems are used to compensate for superimposed bending stresses. The application and removal is normally manual. To minimize setting errors, however, certain products are equipped with motorized application and removal systems (see Figures 3 and 4).
Feeler arm extensometers offer the advantage of automatic operation and a large measurement range with high measurement accuracy. They are also suitable for many different applications. Precision designs with a smooth and balanced mechanical operation apply minimum loading to the specimen (as little as the measurement marks used for non-contact extensometers). Because the feeler arms are in contact with both sides of the specimen, superimposed bending strains are compensated.
Clip-on and feeler arm extensometers are in direct mechanical contact with the specimen via knife edges, which are perpendicular to the gauge length. The small contact force from the knife edges can cause a microscopic indentation in the specimen surface, which gives a light form-fit and thereby a precisely positioned contact point. This is an important factor for the large measurement accuracy and small scatter band width of the measured values. Because of direct contact with the specimen, feeler arm extensometers can be damaged or even destroyed by whiplash at the failure point of high elasticity / high extension specimens.
Non-contact extensometers (video and laser-scanning)
The main advantage of non-contact video and laser-scanning extensometers is that they can be used up to break without damage even when testing specimens that exhibit whiplash. They require measurement marks to be attached to the specimen that are optically distinct from the surrounding area of the specimen.
The measurement marks are clipped, tacked, or glued onto the specimen, or the specimen is marked with a colored pen. In every case, this introduces additional sources of error as the marks can become indistinct, move or fall off the specimen surface as it deforms during loading. The application of the measurement marks is also an additional process by the operator and can introduce higher costs as well as inaccuracies to the test results (Figure 4).
The position of the measurement marks on the specimen is evaluated by software algorithms, which determine a certain area around an Optical Center Point. This becomes the gauge length and, as the specimen is loaded, the movement of the marks is converted to extension values. Special lighting for surface or background illumination of the specimen optimizes the contrast to the measurement mark. During the deformation of the specimen, the lighting changes on the measurement marks as well as specimen, and surrounding influences (reflections etc.) can influence the optical center point. This is often the cause of scatter in the test results.
Non-contact extensometers (laser interferometry)
Figure 5. Zwick’s laserXtens non-contact extensometer operating without
specimen marks.
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The Zwick laserXtens is a non-contacting device that does not require measurement marks. It uses the unique structure of a specimen’s surface as a fingerprint to generate a virtual measurement mark. Laser light directed on these measurement positions is reflected in various directions corresponding to the surface structure and creates a specific pattern of speckles. Selected measurement points are constantly followed and converted to direct extension values. The change in the surface structure, which is the basis of the speckle pattern, is continuously evaluated during specimen deformation (see Figure 5). Summary
Contact-type extensometers measure extension accurately and are cost effective. However, clip-on extensometers require more manual intervention and without care can introduce scatter in the test results. Feeler arms extensometers offer high accuracy and repeatability and ease of use because of fully automatic operation, which includes setting variable gauge lengths.
Non-contact extensometers are required when the specimen is sensitive to notching knife edges or when the extensometer might be damaged at specimen break. They are also still relatively expensive and time consuming to set up and calibrate especially when testing different specimen types.
In short, there is no such device as a universal extensometer. The range of applications demands various devices with different functions and characteristics, and the extensometer must be selected for each application.
Zwick offers a range of extensometers, as well as expert advice and consultation. The company can also carry out pre-testing in an applications laboratory before purchase to make sure the optimum device has been chosen for each testing application.
For more information, contact Manfred Dripke, Zwick GmbH & Co KG, via info@zwickroell.eu or 07305-10-0, or visit the company’s Web site at www.zwick.com.
