Ways to Measure Strain

Automatic Contacting

The need for increased specimen throughput has created a demand for faster, more reliable, and more accurate materials testing routines. Automatic Contacting Extensometers provide this capability by enhancing the productivity of testing laboratories that can take advantage of the automatic gauge length positioning feature, as well as the automatic attachment to the test specimen.

Suitable For

• Almost all materials and a variety of geometries, such as, strip, round, hexagonal and flat shape.

Overview

Common Standards

ASTM D638, ISO 527-2 ASTM D412, ISO 37ASTM D790, ISO 178 ASTM D882, ISO 527-3 ASTM D3039, ISO 527-4 ASTM F2516

Averaging Axial and Biaxial

Averaging Axial and Biaxial Extensometers are used for testing advanced composites, rigid plastics and metals. Average axial strain is determined from independent strain measurements on opposite sides of the specimen. The use of average strain eliminates errors due to specimen bending and allows the determination of precise modulus values. By integrating a transverse strain transducer the extensometer can simultaneously measure average axial and transverse strain allowing the determination of Poisson’s ratio. Generally, this type of extensometer is intended for measuring small strains such as those used for the determination of elastic properties.

Suitable For

• Static tests to determine elastic properties (modulus and Poisson’s ratio) of Composites, rigid plastics, and metals

Overview

Dynamic Clip-On

Clip-on extensometers for cyclic fatigue testing offer a versatile contacting solution for precise strain measurement. Their lightweight rugged construction and high natural frequency makes them ideal for high frequency and fatigue testing applications. The low operating force arms reduce the possibility of knife-edge slippage when testing hard or smooth surfaced materials.

Suitable For

Dynamic clip-on extensometers are used for cyclic strain measurement in tension, compression or reverse-stress through zero measurements on a wide range of materials and components including metals, plastics, and composites.

Overview

Long Travel

The long-travel (XL) extensometer is a precise device for measuring strain in highly extensible materials . Designed for use with electromechanical testing instuments, long-travel extensometers can be used to measure specimens elongation's up to 10 in or 250 mm. The XL is designed to clamp directly onto to a specimen quickly and easily. It will adjust to incremental gauge lengths ranging from 0.5 in to 5 in or 10 mm to 200 mm.

Suitable For

• Elastomers, semi-rigid plastics, and films

Overview

Common Standards

ASTM D412, ISO 37 ASTM D882, ISO 527-3

Static Clip-On

Clip-on extensometers for static testing offer speed of attachment and ease of use. Their light-weight, rugged cross brace design eliminates errors caused by physical distortion, while built-in protection ensures that damage is not caused by over-extension. The low operating-force arms of the extensometer reduce the possibility of knife-edge slippage when testing hard or smooth surfaced materials.

Suitable For

• Static clip-on extensometers are used for static strain measurement on a wide range of materials and specimens including metals, plastics, and composites.

Overview

Common Standards

ASTM D638, ISO 527-2 ASTM E8, ISO 6892, and ASTM D3039

Transverse

Transverse clip-on extensometers are used to measure transverse (or width) strain. Versions with small travels (< 0.5 mm) are used for the determination of elastic properties versions with larger travels (> 5 mm) are suitable for the determination of plastic properties.

Suitable For

• Static tests to determine r-value (plastic strain ratio) on sheet metal, Poisson’s ratio on composites, plastics and metals.
  NOTE: An axial extensometer is also required for these tests

Overview

Video

A video extensometer is a non-contacting extensometer that can measure deformation by tracking the movement of two attached markers on the specimen, using high-resolution digital camera technology. Key advantages of video extensometers are that they do not exhibit any influence on the test specimen, problems with knife-edge slippage are eliminated, and errors due to inertia of moving parts is also eliminated.

Suitable For

• Almost every material

Overview

Common Standards

Conforms to the most rigorous testing standards, including ISO 527 and ASTM D638

DIC

Digital Image Correlation (DIC) is a strain measurement technique that works by capturing a series of images throughout a test and analyzing them afterwards. A system that includes a camera system, lighting, and a software package to control image capture and to conduct post-test analysis. Recently integrated DIC systems, which use existing video extensometers, have been introduced to provide a streamlined package tailored for the materials testing market. 

Suitable For

• Almost all materials (flat, planar is best)

Overview

DIC can provide:

• Full field strain and displacement maps with video export capability
• Virtual extensometers and strain gauges for plot generation
• Automatic synchronization with material testing data

Crosshead Displacement

Although crosshead extension can be an acceptable and common method of measuring strain, it is often an inadequate way as it will record the deformation of the entire specimen between the grips instead of just the narrow, parallel section which is the area of interest (the gauge length).

Suitable For

• Limited to typical dumbell or "dogbone"-shaped specimens

Overview

Bonded Strain Gauges

Bonded strain gauges are small metallic circuits which are glued to the surface of a specimen. As the circuit deforms the resistance changes, and this change in resistance is then turned into a strain recording. At the end of the test the bonded strain gauge is thrown out at a cost of around $5 per test. Bonded strain gauges are very accurate but are limited to strains under 10%. They take a highly skilled operator several minutes to apply, and require using curing ovens for hours at a time to set the adhesive.

Suitable For

• Composites, component, and metals testing

Overview

Common Standards

ASTM D3039, ISO 527-4

Deflectometer

The Deflectometer on its own is not a strain measuring device – it is simply a spring loaded plunger that is used in conjunction with an extensometer, whether it be a clip-on, an automatic contacting extensometer (AutoX750), or a video extensometer (AVE2). It is primarily used for flexural tests.

LVDT

Linear Variable Differential Transformer (LVDT) displacement transducers are measuring devices that measure the displacement of the load frames of materials testing systems. They feature an LVDT as the active element, and are designed for use on both electromechanical and hydraulic Instron^® testing systems.

Suitable For

• Almost all materials – dependent on travel

Overview

Common Standards

ISO 178, ASTM D1621, ASTM D575-91, ASTM A370,

High-Temperature Uncooled Clip Ons

Most clip-on contacting extensometers use strain gauges to measure strain on the specimen. These devices use a high-temperature capacitive sensor allowing them to be used up to maximum temperature of most environmental chambers used in materials testing.

Suitable For

• Static and dynamic tests on composites, rigid plastics, and metals

Overview

High-Temperature Uncooled Clip Ons

Most clip-on contacting extensometers use strain gauges to measure strain on the specimen. These devices use a high-temperature capacitive sensor allowing them to be used up to maximum temperature of most environmental chambers used in materials testing.

Suitable For

• Static and dynamic tests on composites, rigid plastics, and metals

Overview