ADVANCED MATERIALS & PROCESSES • APRIL 2011 17

E xtensometry involves the measurementand analysis of changes in linear dimen-sions during tensile testing. An exten-

someter measures test specimen elongation,characterizing strain. These instruments are di-vided into two main categories: contact andnoncontact extensometry. The range of appli-cations where extensometers are used is diverseand the technical requirements for these de-vices are multifaceted. As a result, there is nosingle device which satisfies all needs.

In recent years, significant development oc-curred in the area of noncontact extensome-ters, including those based on laser and videotechnology. Choosing the right extensometerfor a specific application requires an under-standing of the material being tested as well asthe strengths and weaknesses of both contactand noncontact instruments.

This article can help users determine themost suitable device for their testing require-ments; whether their need can be met using a

traditional clip-on extensometer, or whether itrequires the very latest in laser interferometry.

Role of test specimen properties in extensometer selection

The requirements for an extensometer aredetermined primarily by the characteristics ofthe material to be tested. This includes the ma-terial’s shape and dimensions, test require-ments, and the test standards that must befollowed. These conditions define the gagelength, accuracy, test sequence, and environ-mental conditions (Fig. 1).

The proper choice of extensometer cannotbe based on material characteristics such asspecimen dimensions, stiffness, strength, andplasticity alone. It is also necessary to decidewhether an extensometer can be connected di-rectly to the specimen without influencing theload measurement or mechanically damagingthe specimen itself. Very thin specimens suchas foils can be sensitive to clamping forces,while very small wire specimens, for example,do not provide enough visible area for reliablenoncontact measurements.

A high stiffness in the initial extensionrange, followed by high plasticity traditionallyrequires more than one extensometer. The firstmeasures small strains (typically up to 5 mm)very accurately in the elastic range, and the sec-ond measures very high extensions (typically≥500 mm). Materials with very smooth sur-faces, or those made of transparent materialsare not suitable for noncontact measurementswithout first fixing measuring marks onto thesurface of the specimen.

One very important consideration is the be-havior exhibited when the specimen fails. Met-als and hard plastics will slip through the knifeedges of a contact extensometer without dam-aging them, and rotatable knife edges should beused to further reduce the risk of damage if thesurface of the specimen is particularly rough.High extension or flexible specimens can dam-age or destroy the knife edges and even the ex-tensometer itself due to whiplash, splintering,or delamination of specimens. For these appli-cations, noncontact measurement is a must.Criteria for accurate measurements are dis-cussed below.

Choosing the Right Extensometerfor Every Materials Testing Application

Bill BeckerZwick USAKennesaw, Ga.

Manfred DripkeZwick/Roell GroupUlm, Germany

Recent advances inextensometertechnologyopened up opportunitiesin materialstesting previously notachievable.

Fig. 1 — Selection process for extensometers.

Specific test application

Extensometer requirements

Comparison and selection process criteria (must have/should have)

Extensometer properties

Extensometer portfolio

Material (component)

Test method,standard

Environmentalinfluences

Budget

Material properties

Specimen, test sequence,

test results

Temperature,light, noise

Purchasing,running costs

Measuring principle, properties

Functionality(auto/manual)

Environmentalconditions

Purchase, running costs,

ease of use

ADVANCED MATERIALS & PROCESSES • APRIL 201118

Measurement travel range and gage lengthWith contact extensometers, the measurement travel

is normally an engineered, fixed value, which is depend-ent on the range of the measurement transducer and, withfulcrum hinged-sensor arms, the leverage ratio. The ini-tial gage length is set manually with fixed steps or auto-matically over a defined range.

Noncontact extensometers that use a video camera orlaser interferometry system must have the field of viewlarger than the required range plus the initial gage length.Because the specimen portions that are outside the gagelength (and the machine components themselves) deformin the direction of loading, the position of the measuringmarks on the specimen changes during the test.

For extension and/or gage lengths that are expected tobe outside of the field of view, the objective lens must bechanged, or the distance between the specimen and thevideo camera must be increased. All these actions decreasethe accuracy of the measurement, and in addition, everychanged measurement configuration must be adjusted andcalibrated.

Measurement accuracy“Accuracy” is a qualitative term. To qualify the integrity

of a measured signal, standards use quantitative termssuch as resolution, deviation, and uncertainty, and theseterms are given definitive values. Requirements for the ac-curacy of extension measurements are normally given inapplication-specific test requirements and internationalstandards.

Many test standards, for example, tensile tests on met-als [1] and plastics [2] refer to standards for calibration of ex-tension measurement systems and the required accuracyclasses [3,4] contained therein (Fig. 2).

Ergonomics and economyDevices that are easy to set up and sequences that can

be automated reduce personnel time and effort, and, at thesame time, improve the quality of the test results because

subjective influences are minimized. Higher initial acqui-sition costs can be quickly amortized, especially when theextensometer can be used for a wide range of applications.Many applications can be handled using either a video orlaser extensometer. The higher initial cost of the laser ex-tensometer, which requires no specimen marking or at-tachments, can often be justified by the savings inpersonnel time and cost and the reduction in human error.

Contact-type measurement extensometersClip-on extensometers are, as the name implies,

mounted directly onto the specimen. The mechanicalparts that transfer extension, via knife edges, from thespecimen to the internal transducer are short and stiff.There is practically no relative movement between thespecimen and the extensometer, resulting in a high level ofmeasurement accuracy.

The range of a clip-on extensometer is limited to a fewmillimeters, and it applies a load directly to the specimen.Extensometers with counter-balance weight and double-sided measuring systems are used to compensate for su-perimposed bending stresses. The application and removalis normally manual. However, to minimize setting errors,some clip-on extensometers are equipped with motorizedapplication and removal systems (Figs. 3 and 4).

Sensor arm extensometers offer the advantages of au-tomatic operation and a large measurement range withhigh measurement accuracy. Precision designs with a verysmooth and balanced mechanical operation apply mini-mum loading to the specimen (as little as the measurementmarks used for noncontact extensometers). Because thesensor arms are in contact with both sides of the specimen,superimposed bending strains are largely compensated.Clip-on and sensor arm extensometers are in direct me-chanical contact with the specimen via knife edges that areperpendicular to the gage length. The extremely small con-tact force from the knife edges can cause a microscopic in-dentation into the specimen surface, which enablesprecision in positioning at the contact point.

Because of the direct contact with the specimen, sen-sor arm extensometers can be damaged or even destroyed

Fig. 2 — The required measuring points for the calculation of elastic modulus for metal and plastic shown in comparison to a human hair highlights the accuracy requirements for extensometers.

Fig. 3 — Zwick’s multiXtens extensometer combining fully automatic sensor arm extensometers with a dual averaging axialand transverse strain extensometer.

Str

ess

s, G

Pa Metal (A)

Aluminum (B)

Plastics (C)

20 40 60 80 100 120 100 μm

Strain ΔL, μm

400

300

200

100

ADVANCED MATERIALS & PROCESSES • APRIL 2011 19

at the failure point of high elasticity/high extension speci-mens. An example from the automotive industry is thetesting of safety belts. At the point of failure, the materialwill exhibit backlash or whiplash characteristics that coulddamage the testing equipment.

Noncontact video extensometersA primary advantage of noncontact video extensome-

ters is that they may be used up to the material breakingpoint without damage, even when testing specimens thatexhibit whiplash. Another advantage is the capability tomore accurately determine strain and use strain as a con-trol loop mechanism for test samples undergoing large de-formations. An example of this may be found incharacterization of biomaterials and medical-grade poly-mers, where video extensometry supports measurementof large strains. Additional applications include testing ofmedical components in solution, where attachment of atraditional extensometer would not be practical (Fig. 5).

Noncontact video extensometers require measurementmarks to be attached to the specimen,which are optically distinct from the sur-rounding area of the specimen. Themeasurement marks are clipped, tacked,or glued onto the specimen, or the speci-men is marked with a colored pen. Theapplication of the measurement marksadds a step to the test cycle, potentiallyreducing throughput, increasing the costof testing, and introducing inaccuraciesthrough human error (Fig. 4).

The position of the measurementmarks on the specimen is evaluated bysoftware algorithms, which determine acertain area around an optical centerpoint. This becomes the gage length, andas a load is applied to the specimen, the

Fig. 4 — Schematic diagrams showing the operating principle of various extensometers.

Fig. 5 — Zwick’s videoXtens noncontact extensometer supporting testing ofa medical component in solution.

Clip-on extensometer

Force

ΔL Hinged knife edge

Measuring transducerL0

Fixed knife edgeTest specimen(= reference)

Testing machine

Sensor arm extensometerForce

Feeler arm Measuring

L0 + transducerΔL

L0

Test specimenMachine(= reference)

Video extensometerForce

Field of view

Image sensorL0 + ΔL

L0Video cameraMeasuring markTest specimenMachine(= reference)

Fig. 6 — Zwick’s latest laserXtens noncontact extensometer operating without

specimen marks.

ADVANCED MATERIALS & PROCESSES • APRIL 201120

movement of the marks is converted toextension values. Special lighting for sur-face or background illumination of thespecimen optimizes the contrast to themeasurement mark. During deformation,the ambient lighting changes on themeasurement marks as well as on thespecimen, and surrounding influences(such as reflections) can influence the op-tical center point. This is often the causeof scatter in the test results.

Noncontact laser extensometers (laser interferometry)

The latest in extensometer technol-ogy uses a noncontacting device thatdoes not require measurement marks.The laser extensometer uses the uniquestructure of a specimen’s surface as a“fingerprint” to generate a virtual meas-urement mark. Laser light directed onthese measurement positions is re-flected in various directions correspon-ding to the surface structure and createsa specific pattern of speckles. Selectedmeasurement points are constantly fol-

Fig. 7 — Measuring strain on a micro-wire using the laserXtens noncontactextensometer.

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ADVANCED MATERIALS & PROCESSES • APRIL 2011 21

lowed and converted to direct extension values. Thechange in the surface structure, which is the basis of thespeckle pattern, is continuously evaluated during speci-men deformation (Fig. 6).

This laser interferometer-based method of noncon-tact extensometry allows test labs to characterize materi-als, components, and even subassemblies making itwell-suited for quality control and R&D applications. Ad-ditionally, this novel approach to extensometry supportstests on microspecimens with small gage lengths that re-quire exceptional accuracy in strainmeasurement (Fig. 7). Such tests wouldnot be possible using traditional exten-sometry. Speeding up throughput anddelivering the utmost accuracy in strainmeasurement, laser interferometer non-contact extensometers offer significantvalue for high volume test labs.

SummaryContact-type extensometers meas-

ure extension accurately and are cost ef-fective. However, clip-on extensometersrequire more manual intervention and,without care, can introduce scatter inthe test results. Sensor arm extensome-ters offer extremely high accuracy, excel-lent repeatability, and ease of use due tofully automatic operation, which in-cludes the setting of variable gagelengths.

Noncontact extensometers are re-quired when the specimen is sensitive tonotching from knife edges, or when theextensometer may be damaged when thetest specimen fails. In addition, noncon-tact extensometers can be relatively ex-pensive to purchase and time consumingto set up and calibrate compared to con-tact type extensometers.

There is no such device as a universalextensometer. The large range of applica-tions demands various devices having dif-ferent functions and characteristics, andthe extensometer must be selected for eachspecific application.

References1. DIN EN ISO 6892: Metallic Materials –Tensile Tests.2. DIN EN ISO 527: Thermo-plastics andduro-plastics – Determination of TensileCharacteristics.3. DIN EN ISO 9513: Calibration of Exten-sion Measurement Systems for Testing WithSingle Axis Loading.4. ASTM E 83-02: Standard Practice for Ver-ification and Classification of ExtensometerSystem.

For more information:Bill Becker is managing director of Zwick USA, 1620 Cobb In-ternational Blvd., Kennesaw, GA 30152; tel: 800/546-9757; fax:770/420-6333; email: info@zwickusa.com; www.zwick.com.Manfred Dripke is the former Head of Product Developmentfor Zwick/Roell AG.

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