Strain and Extensometers

When Do You Need to Verify Your Extensometry?

When you use extensometry to determine strain measurements for your test results you need to have confidence that the data is sound. An Instron calibration of your strain measuring system gives you a statement of how well your system is performing.

Many testing methods that are based on ASTM, EN or ISO standards stipulate that the materials testing machine meets particular performance requirements and has current calibration certification to show proof of compliance.

Instron strain calibrations are carried out using ISO or ASTM methods and calibration equipment traceable to national measurement institutes (NMIs) by a fully trained and accredited calibration staff. Such calibrations provide a high integrity independent calibration report that fully meets ISO 9000 and ISO 17025 needs.

Strain calibrations provide a check on the soundness of the gauge length measurement as well as the strain displacements. The displacement verifications can be tailored to suit the working range of the extensometer.

Undertaking a Realistic Verification

When choosing a calibration provider, it is essential to determine whether the provider has the equipment necessary to fully assess the capability of your extensometer. Some extensometers are fixed directly on the specimen and are fully supported by it. Others are mounted on the testing machine and measure specimen displacement by contact with arms or by optical or video techniques. Some calibration providers' equipment will move only one arm of your extensometer, keeping the other stationary. This approach is suitable for specimen mounted extensometers, but may not be ideal for machine mounted equipment. Moving both arms more closely replicates how the extensometer is used in real life and may uncover weaknesses hidden by the single arm displacement method.

Instron® designs their calibration equipment to meet the requirements of a variety of specialized extensometers. Depending on your application and the type of your extensometer, Instron has the expertise and the necessary equipment to undertake the most suitable calibration to properly evaluate the capability of your extensometry.

Strain Verification Standards -Which to Choose

There are two internationally recognized standard methods for verifying extensometry systems in materials testing. They are:
  • ISO 9513: Metallic materials - Calibration of extensometers used in uniaxial testing
  • ASTM E 83: Standard Practice for Verification and Classification of Extensometer System

While both standards are similar there are small differences in their verification and classification methods. The verification method should be chosen to best suit your needs and testing requirements. Sometimes this choice is defined by the testing program where the test method will dictate the calibration class or grade of extensometer to be used. As a general guideline, in North America most users prefer strain verifications to the ASTM E 83 method, while Europe generally require ISO 9513 calibrations.

Instron can provide calibrations to either method to suit your needs.

For non-metallic materials such as rubbers and plastics, the ISO 9513 standard is not the most suitable verification method to use as it concentrates on low strain measurement.  A more appropriate standard to use for high strain measurements is ISO 5893. While this document is not specifically a verification standard, it does detail strain requirements for rubbers and plastics extensometry. ASTM E 83 can be used for both low and high strain applications.

IPS Strain Gauge Channel Calibration