Extensometer verification
Checks that the extensometer reads displacement and gauge length correctly by comparing it with a precision extensometer calibrator at several displacements.
Why this is checked
Proof stress and modulus depend on accurate strain measurement. An extensometer that reads high or low shifts the 0.2% offset line and gives wrong proof stress values. Verification assigns a class (ISO 9513) or class letter (ASTM E83).
Typically every 12 months, after any knock or repair, and when changing gauge length settings, as defined in your SOP.
Learn how the UTM worksStandards & tools
- Extensometer calibrator (micrometer or laser-interferometer type) with a traceable calibration certificate
- Gauge-length setting fixture or calibrated gauge blocks
- Calibrated thermometer (calibrator and extensometer at stable room temperature)
Prerequisites
- Extensometer clean, knife edges undamaged
- Calibrator and extensometer acclimatised to room temperature
- Software set to the gauge length to be verified (e.g. 50 mm)
Step by step
- 1Check the gauge length using the setting fixture and record any deviation.
- 2Mount the extensometer on the calibrator spindles as it would sit on a specimen; zero both.
- 3Apply displacements in steps covering the range used (e.g. 0.020, 0.050, 0.100, 0.250, 0.500 and 1.000 mm for proof stress).
- 4At each step record calibrator displacement and extensometer reading.
- 5Repeat in a second series, remounting the extensometer between series.
- 6Calculate absolute and relative bias error at each step and assign the class.
Example observation table
| Calibrator displacement (mm) | Extensometer reading (mm) | Absolute error (µm) | Relative error (%) |
|---|---|---|---|
| 0.020 | 0.0203 | +0.3 | +1.50 |
| 0.050 | 0.0502 | +0.2 | +0.40 |
| 0.100 | 0.1004 | +0.4 | +0.40 |
| 0.250 | 0.2508 | +0.8 | +0.32 |
| 0.500 | 0.5015 | +1.5 | +0.30 |
| 1.000 | 1.0030 | +3.0 | +0.30 |
How to calculate the result
- 1At 0.020 mm: absolute error = 0.0203 − 0.020 = 0.0003 mm = 0.3 µm; relative error = 0.0003 ÷ 0.020 × 100 = 1.50%
- 2
Class 0.5 (ISO 9513) typically allows ±0.5% relative or ±1.5 µm absolute, whichever is greater. At 0.020 mm the relative error is 1.50% but the absolute error (0.3 µm) is within 1.5 µm, so the point passes.
- 3
At 1.000 mm: absolute error = 3.0 µm (above 1.5 µm) but relative error = 0.30% (within 0.5%), so it passes.
- 4
Every point passes at least one criterion, so the extensometer meets Class 0.5.
Pass if
- ISO 9513 Class 1 (typical minimum for proof stress under ISO 6892-1): bias within about ±1% relative or ±3 µm absolute, whichever is greater; Class 0.5: about ±0.5% or ±1.5 µm.
- Gauge length error and resolution within the limits for the class.
- ASTM E83: class B-2 or better is commonly required for yield strength, B-1 for modulus.
What to do
- Withdraw the extensometer and label it out of calibration.
- Check knife edges, arms and cable; have it repaired or adjusted and re-verified.
- Review proof stress results obtained with it since the last good verification.
Other UTM calibrations
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.