Extensometers for material testing machines - Zwick - #9

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9 5. Relevant Material Properties 5.1 Extension measurement Extension measurement during loading up to specimen break can be divided into different ranges according to the material properties to be determined: • Fine strain measurements in the elastic range, and at the beginning of the permanent deformation range. • Determination of offset yield from the start of permanent deformation • Determination of uniform strain and strain at break. Fine strain measurement This is primarily used to determine Young’s modulus and the technical elastic limit (0.01% proof strength) for metals and Young’s modulus for plastics. The strain range to be recorded here is typically between 0.05 and 0.25 % (also up to 1 % for foils / films). These material properties require measurement of extremely small extensions at correspondingly high resolution and very small errors. Clip-on and sensor-arm extensometers (multiXtens, Macro extensometer) plus laserXtens, videoXtens and optiXtens are suitable, according to EN ISO 9513 (see diagram). Determination of offset yield (proof strength) Offset yields are determined for characterization of materials if the transition from the elastic to the plastic range is continuous in the stress - strain diagram when testing metals or plastics. All analog and digital clip-on extensometers, sensor-arm extensometers and non-contact measurement systems such as lightXtens, videoXtens and laserXtens can be used for determination of offset yield (see diagram). Uniform strain and strain at break Uniform strain is the non-proportional strain under loading at maximum force and is determined for metals. Direct extension measurement enables continuous testing from the elastic range through to specimen break. Strain at break is the permanent extension, with reference to the initial gage length, on the specimen after break. To determine strain at break, an extensometer must be designed to have a long measurement travel and for testing up to specimen break. Sensor-arm extensometers with swiveling knifeedges and non-contact measurement systems are the most suitable types for this. Clip-on extensometers have only limited suitability for determination of strain at break. Range of application of Zwick extensometers based on resolution