Extensometers: accurate and verifiable measurement from the start

Modulus values and slopes are determined at the initial stage of the test. An extensometer’s low measurement uncertainties are fundamental for reliable test results. How are these ensured?

Calibration confirms measurement accuracy—starting at the first measured value

Modulus values and slopes are determined at an early stage of the test. It is therefore important to verify that the extensometer provides the required level of accuracy for these early measuring points. If required by the test, ZwickRoell verifies reliable test results by setting the first measuring point at 10 µm for a calibration.

An example is slope mE in a tensile test on metals; it usually occurs in the range of 20 µm to 80 µm. Or the tensile modulus on plastics to ISO 527-1, where for the initial gauge length of 50 mm, the first measuring point is 25 µm.

The first calibration point is set where the first test value is measured, which if required, can be as early as 10 µm. This ensures reliable measured values in the initial measuring range of the test, for example when determining slope Me on metals or the tensile modulus on plastics.

The calibration report clearly states: in compliance with ISO 527-1 Annex C

Another important point is presented with the tensile modulus example: additional Calibration steps at the beginning and end of the tensile modulus verify reproducible tensile modulus values. Example: With an initial gauge length of L0 50 mm, the tensile modulus on plastics to ISO 527-1/-2 is determined at 25 µm and 125 µm measurement travel.

This is exactly where the additional calibration steps are set. This clearly demonstrates, according to Annex C, that the difference in the deviations measured there is <= 1 µm, which is also accounted for in the calibration report. Therefore, it is always possible to verify that the increased accuracy requirements according to ISO 527-1 Annex C are met and that reliable test results are determined.

videoXtens 2-120 HP: additional calibration steps confirm that the increased accuracy requirements in accordance with ISO 527-1 Annex C are met

High extensometer accuracy is a requirement

The measured values should be well within the required standard criteria—this is something on which ZwickRoell places great importance. Consequently, our extensometers are designed with the corresponding level of accuracy. Especially when it comes to extensometers with accuracy class 0.5 (ISO 9513), which are used for strain rate control on metals. Here, accuracy is defined by standards ISO 6892-1 Method A1 (closed loop) and ASTM E8 Method B. The result speaks for itself: instead of the permitted tolerance limit of ±20 % for the set strain rate, our extensometers are usually at ±5 %.

Within these limits, you are on the safe side, even in the case of unforeseen events. And the test results are not only reliable, but scatter is also significantly lower.

The multiXtens extensometer, for example, lies clearly within the required tolerance limits—throughout the entire, extraordinary measuring range of 700 mm.