During compression, tensile, and also flexure tests, varying product properties need the measurement of force as well as deformation (compressive deformation, extension, deflection, strain) for a specimen under load.

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A distinction is made between direct and indirect extension measurement.

Indirect extension Measurement

Indirect extension measurement involves determining specimen expansion by measure up the sports in the distance between the testing device crossheads. Thus, it consists of the deformations that all systems within the testing an equipment load frame. The is crucial that the amount of these individual deformations is negligible contrasted to the extension to it is in measured, arguing that either it should be listed below the permit measurement error, or it deserve to be partially eliminated through a computed correction curve.

It is possible to determine this mediate curve because that a particular machine configuration and use the to exactly the measured extension values, provided that the deformation the the test arrangement is adequately reproducible.

Zwick materials trial and error machines record the change in crosshead take trip (and consequently in crosshead speed) making use of digital crosshead encoders with really high resolution (better than 0.2 μm because that all types of machines).

Extension measurement through crosshead travel may be suitable for the complying with cases:

Compression test on specimens with high deformation (for example, indigenous 30 come 50 mm)Compression tests whereby it is feasible to compensate because that the results of the test plan through a mediate curveNon-flowing, stable materialsStrip specimens and also parallel clamped specimen grips which ensure defined grip-to-grip separations and the specimen has actually non-flowing, secure characteristicsCharacteristic values with high strains (strain in ~ break)

Direct expansion Measurement

Direct expansion measurement on the specimen removes all undesirable next effects, such as load-cell deformation, load-frame deformation (columns, lead-screws, crossheads), deformation of the specimen grips, and also any specimen slippage.

Zwick provides measurement equipment for the following:

Deformation measurement for flexure and also compression testsTransverse strain measurement (measurement perpendicular come the tensile direction)Axial stress, overload measurement (measurement in tensile direction)

Direct extension measurement is always essential if:

Elastic deformation of device components should not distort the resultThe an outcome could be distorted by the specimen behavior in the clamping area, particularly with soft materialsRequired through the appropriate test standardSpecimen deformation exterior come the gage size is not to be includedSelf-clamping specimen grips space employed and movement that the jaw inserts is no to it is in included

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Direct expansion measurement

Relevant material Properties

Extension Measurement

Based top top the product properties to it is in determined, expansion measurement during loading up to specimen break deserve to be classified into various ranges:

Determination of counter yield native the start of irreversible deformationFine strain dimensions in the elastic variety and in ~ the start of the permanent deformation rangeDetermination of uniform strain and also strain in ~ break

Fine stress, overload Measurement

This is greatly used to identify the technological elastic border (0.01 % proof strength) and Young’s modulus because that metals and also Young’s modulus for plastics. The usual strain range to it is in measured below is in the range of 0.05% come 0.25% (also as much as 1% because that films/foils).

These material properties necessitate the measure of very small extensions in ~ proportionately high resolution and extremely small errors. In addition to clip-on and sensor-arm extensometers (multiXtens, makroXtens), laserXtens and also videoXtens are right according ISO 9513 (see the picture below).

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Range of application of Zwick extensometers based on resolution

Determination of offset Yield (Proof Strength)

When trial and error plastics or metals, if the change from the elastic come the plastic variety is consistent in the stress-strain diagram, counter yields are figured out for characterizing the materials.

Offset yield (see the image above) can be established by making use of all sensor-arm extensometers, analog and digital clip-on extensometers, and also non-contact measurement systems, because that example, videoXtens, laserXtens, and lightXtens.

Uniform Strain and also Strain in ~ Break

Uniform strain, which is the non-proportional stress, overload under loading in ~ maximum force, is identified for metals. Straight extension measure up allows consistent testing indigenous the elastic selection up to specimen break.

Strain at break is the permanent extension, relating to the early stage gage size on the specimen complying with break. In bespeak to recognize strain at break, the is necessary to style an extensometer such that it has a long measurement take trip for testing up to specimen break. The many suitable types for this are non-contact measure up systems and also sensor-arm extensometers through swiveling knife edges. Clip-on extensometers are suitable only come a minimal extent because that the decision of strain at break.

Transverse stress, overload Measurement

Poisson’s ratio (μ)

A measure of the deformation ratio between axial and also transverse strain in a tensile check is described as Poisson’s proportion μ. Trial and error long-fiber reinforced plastic is the desired use that Poisson’s ratio. 2 strain measurement solution with the capacity to operate in both axes in ~ the very same time are essential to measure up Poisson’s ratio.

The solutions offered by Zwick encompass video-based non-contact transverse stress, overload measurement mechanism (videoXtens transverse strain extensometer) or digital and analog transverse stress, overload extensometers. The transverse stress, overload extensometers are used in combination with a sensor eight extensometer (multiXtens, makroXtens).

Vertical Anisotropy (r-Value)

Vertical anisotropy entails characterization of the cold workability of slim sheet metal in relationship to the habits of the material at the time of deep drawing. The r-value to represent the resistance of sheet metal to a diminish in thickness throughout single-axis tensile loading. Transverse strain should be measure on a dumbbell specimen in stimulate to identify these values.

A mix of a digital or analog transverse stress, overload extensometer with the makroXtens is ideal for this.

The videoXtens transverse stress, overload extensometer, Zwick’s video-based non-contact transverse strain measurement system deserve to be provided as one alternative.

The biaxial digital clip-on extensometer is an especially proposed for vertical anisotropy test and also is designed because that transverse and axial stress, overload measurement.

Selection Criteria

A an easy decision to be taken is whether direct extension measure is needed, or whether it is enough to measure up indirect extension via crosshead travel.

It is essential to pick an extensometer with the suitable characteristics in order to use direct measurement. Few of the criteria that recognize this an option are offered below, based on the material to it is in tested and also the results to be determined.

Initial Gage size (Le)

Depending on the shape and also dimensions that the to-be-tested specimen, a selection of early gage lengths room recommended by test standards. In a majority of the cases, the gage length is comparatively tiny for higher strain levels and also quite large for reduced strain levels. In the situation of metals testing, the early stage gage size is in straight association with the cross-section the the specimen (for proportional specimens).

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Range of application of Zwick extensometers based on initial gage length

Measurement Travel

It is crucial that the extensometer measurement travel is enough for the to-be-tested specimen. For an unknown specimen strain, it can be approximated for different materials and characteristic values (see the photo below).

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Range of application of Zwick extensometers based upon measurement travel

Type of Loading

The type of loading also has an impact on the measure up range. The needs for cyclic or compression tests and for tensile trial and error are different, because that instance, it is necessary for extensometers employed for cyclic tests to have actually zero mechanical hysteresis.

Resolution and also Accuracy

It is vital to watch the resolutions and also accuracy level stipulated in the separation, personal, instance test.

Specimen Break

The extensometer have to be defended from high specimen resilience in ~ break and the occurring high acceleration forces.

Specimens susceptible to a whiplash result at break, consisting of straps, ropes, and elastomers, face this risk. Non-contact measurement equipment are proper for together tests.

Sensor arm extensometers native Zwick come through swiveling knife edges and sensor arms. Throughout the specimen break, they relocate aside and also out of the means to prevent damage to the extensometer.

Notch and Flexure Sensitivity

Furthermore, the notch and flexural sensitivity that the specimen material decides the choice of extensometer. The load or possible weight compensation has an affect on the style of a clip-on extensometer (specimen loading via torque), i beg your pardon is attached directly to the specimen, conversely, the way in which the is attached impacts the test. Because that instance, the application of optional counter-rollers come specimens through thin cross-sections could result in distortion that the test results.

Drag Forces

It is vital that the drag pressures of clip-on and also sensor-arm extensometers space as short as feasible to avoid any type of effect ~ above the specimen — guaranteed with Zwick extensometers.

Edge-Fiber Strain

The material and the specimen shape offer indicators of the possible existence of different edge fiber strain, which should be allowed during stress, overload measurement by averaging. For instance, differing edge fiber strains take ar as a result of imprecise axial clamping the the specimen or loading specimens through flexural emphasize (with single-sided clip-on extensometers with long levers or with just one counter-roller).

Testing in Temperature Chambers

Non-contact extensometers or extensometers with extended sensor arms should be used for deformation measurement in temperature chambers.

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This information has been sourced, the review and adjusted from materials provided by ZwickRoell Ltd (UK & Ireland).

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