Extensometers are used to measure the engineering strain ,  (L-Lo)/Lo ,during tensile experiments, Lo being the gauge length of the extensometer.

The true strain at a given stretching can be obtained by  ln(L/Lo)

The raw data you get from an extensometer is the extension over the gauge length.   

The best bit is what you do with this extension during and after the test.  Of course you need to record the data.  At least load versus extension. Although logging time, load and extension frequently during the whole test (100s of data triplets per test)  is better. For most people they will then use these raw data to calculate engineering stress and engineering strain and then determine the proof stresses (0.1, 0.2, 1% etc...) strain at the tensile strength etc.  Indeed, to determine an accurate proof stress to the ISO standard it is absolutely necessary to use an extensometer.   The results will be more accurate, reliable and reproducible than simply using the displacement of the cross head.  

If you have a closed loop controlled test machine you should conduct the tensile test in strain control at a fixed strain rate.  This is better than just running a tensile test at a fixed cross head speed.  

Next with all the time, extension and load data triplets you can now calculate true stress and true strain and determine work hardening rates, provide data for FEA modelling and lots of interesting research activities.  Perhaps to research dynamic strain ageing or to identify the onset of cracking in cast iron or the onset of necking.

I would always recommend using an extensometer.    Indeed, I would say it was an absolute must.    

Extensometer is device that is used for measuring a small deformations of specimens and based on that deformations (strain) and the effective stress you can calculate the modulus of elasticity . 

With extenso meter we can plot stress strain curve,which gives proof stress and modulus of elasticity.

Dear Arvind, The extensometer is a device to measure the extension in the tensile test mainly in the linear part of the load -extension (stress-stain) curve. Usually the extensometer is removed after getting sufficient readings to calculate the youngs modulus. However, there are old types of contact type and advanced with laser beams.

e test mainly in the linear part of the load -extension (stress" -- the linear part or nonlinear doesn't matter. The extensometer simply fixes deformations

Examples of use of the extensometer

The main advantage of using the extensometer is that you gather the data from the sample itself. This will exclude the extension of the machine and/or the grips. In other words the extensometer gives you the "real" data.   

IN addition, each extensometer has a length or gauge length should be known to calculate the strain based on that length.

from extensometer we can get the strain value.....that is tensile strain, unifrom elongation, strain at break etc.

Extensometer is a device that is used for precise measure of  the specimen deformation - strain, basing on it and the effective stress you can calculate the modulus of elasticity. Extensometer is very usefull tool, especially non-contact.

What if extensometer is not attached directly to gauge length of a tensile specimen instead it is applied at cross head position? What about the accuracy and reliability of yield strength & ultimate tensile strength. Does the position of extensometer matter with the accuracy of % elongation as well?

@Javed - Tensile specimen is specifically designed to the dogbone shape that  you get almost constant state of stress (uniaxial) and maximum elongation at the gauge length. Hence it is important to put the extensometer at the gauge length to get correct readings.  Placing the extensometer somewhere else will give you some reading but that will not give to actual strain specimen is going under. Hope this helps you.

@ Anshul Faye- Thanks. Could you please provide some references which show the difference in % elongation depending upon the position of extensometer i.e at the guage length or somewhere else?

@Anshul... It is not always necssary to have dogbone specimen. For example, in composites, the specimen has constant width except some tests (short beam notch test). But, I am agreed that it is better to have extensomer installed on the gauge length. 

Since extensometer is removed before failure, its not possible to obtain strain at failure then?

so if we use extensometer, then we should find stain from raw data as (extension / gauge length) or from (displacement/ gauge length) ?

thanks

change in length, ductility, reduction in area

FORCE VERSUS ELONGATION

One interesting piece of information is to compare the quadrature encoder based displacement data with the data from an extensiometer. The disparity reflects how poorly most instruments perform because they do not correctly measure displacement in the sample. The problems are resolution limits of the encoders and mechanical problems with the drive in the instrument (lead screws, bearings, etc.). Quadrature encoders only work well for higher speeds and larger displacements with mechanically 'tight' instruments. We use cable potentiometric distance encoders, and that removes the problem.

displscement

what is the difference between the UTM (MPa) values of the aluminium alloy specimens while tensile testing are performed with and without extensometer..??

Dear researchers,

When we use an extensometer, it means that we will measure the real strain of the material because when we use the traverse for measuring the strain the slip between the material and gauge length is not eliminated. However, with an extensometer, the slip is not included

To respond to San Ty: if we want to calculate the strain until failure, and high elongation is expected, we can use a video extensometer technique.

Best wishes

The reason for extensiometers is the fairly poor mechanicals of the analytical instrument itself. Quadrature encoders just measure the rotation of the lead screws-not the actual movement of extension of the sample between its gripped parts or ends. The uprights of typical mechanical testers deform and stretch under significant load, and this puts error into the quadrature information about displacement. A distinct and inexpensive improvement is the use of a high performance cable distance encoder connected to between the crosshead and the holder at the far end of the sample. This approach eliminates all the drive and frame stretch inaccuracies which arise from using the lead screw encoder approach.

Dear Arvind,

Sensor arm extensometers is easy device to set up and offer the advantages of automatic operation and a large measurement range with high measurement accuracy.

Testing

(1) Change in the length of an object. It can be useful for testing advanced composites, rigid plastics, soil, metals to measure in tension and compression, cyclic testing, creep compliance and dynamic resilient modulus testing. Extensometers may be used to measure specimen elongations up to 30 in. There are various types of extensometer available in scientific shop such as; magnetic, electrical, mechanical and soil extensometer.

Informations

(1) It is useful for stress-strain measurements and tensile tests

(2) By measuring elongation it helps in finding out the proof stress at the required elongation percentage.

(3) Can measure strain in highly extensible materials.

(4) In civil engineering, It can useful for finding linear displacement/relative displacement(using mechanical extensometer), calculating the displacement of retaining walls, used for accurate measurements of relative deformations of multiple segments and measure and monitor the movement in a slope or foundation.

Ashish

Dear all, Thank you for your answers.

Just a question, should the size of the extensometer be exactly equal to the gauge length? I mean if I have a gauge length of 25 mm, should I use an extensometer exactly with the size of 25 mm or I can use 10 mm also?

Thank you

The extensometer during the tensile test records the deformations ɛ of the tested specimen.