National Physical Laboratory

Measurement of strain - extensometers, strain gauges, Fibre Bragg Gratings (FBGs)

Measurement of strain 1 Measurement of strain 2 

Strain measurement is an integral part of materials characterisation and a wide variety of techniques are available ranging from conventional strain gauges and extensometry to non-contact full field 3D techniques. NPL has expertise and capability in a number of areas, detailed below.

Strain Gauges

General strain gauge measurements on a variety of components according to customer requirements. Portable multichannel data logging facilities are available, together with a variety of mechanical testing machines to carry out in-situ loading measurements.

  • Facilities for strain gauge installation and multichannel logging (up to 32 channels at frequencies of 5 Hz)


A range of conventional single-side and averaging extensometry is available to complement the mechanical test machines available.

  • Room temperature designs with a range of gauge lengths
  • High temp extensometry to 1000 ºC

Digital Image Correlation (DIC)

Full field non-contact technique based on the changes in optical images of a testpiece or component. Uses advanced correlation algorithms to track the movement of blocks of pixels, from which vector displacement and strain components can be calculated.

  • Laboratory based 3D system using 2 or 5 Megapixel cameras
  • Portable 3D system for use off site
  • Real time point tracking using DIC 'videogauge' technology
  • High resolution digital SLR (14 Megapixel) and medium format (65 Megapixel) cameras and lenses available
  • In house DIC software
  • Facility for analysing customer supplied images

To learn more about DIC click here Adobe pdf file (671 KB)

Electronic Speckle Pattern Interferometry (ESPI)

The technique is particularly suitable for monitoring and measuring non-uniform strain fields at high resolution. Full field imaging of deformation with a resolution of 0.1 microns.

Measurements can be done on a variety of materials and the deformations under mechanical and thermal loads can be measured along three perpendicular axis without contact with the sample. Test samples require little or no preparation. Validations of FEA and other analysis predictions are also possible.

Fibre Bragg Gratings (FBGs)

FBGs are fibre optic based strain measurement sensors that require the measurement of wavelength to measure strain, typically with a resolution of 10 microstrain. They are intrinsically safe and can have 50-100 sensors on one length of fibre optic. NPL has evaluated temperature effects on strain transfer, calibration techniques and mounting techniques for both embedded sensors in composites and for use in civil engineering structures.

  • FBG interrogators
  • FBG model bridge and full size civil engineering demonstrator
  • Guidelines and advice on FBG installation and applications


Customer Service tel: +44 20 8943 8681

Last Updated: 30 Mar 2012
Created: 26 Jul 2007


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