Contact-type Measurement Extensometers
Non-contact Video Extensometers
Noncontact Laser Extensometers (Laser Interferometry)
is the science of the measurement and analysis of changes in a
materials linear dimensions during tensile testing.
is an instrument that measures test specimen elongation to characterize
strain. The range of applications where extensometers are used is
diverse and the technical requirements for these devices are
multifaceted. There is no single device that meets all needs for
The requirements for an extensometer are determined by the
characteristics of the material to be tested including shape and
dimensions, test requirements, and the test standards that must be
It is also necessary to decide whether an extensometer can be
connected directly to the specimen. Very thin specimens such as foils
can be sensitive to clamping forces, while small diameter wire
specimens, for example, lack sufficient visible area for reliable
A high stiffness in the initial extension range, followed by high
plasticity requires more than one extensometer. The first measures
small strains (typically up to 5 mm) very accurately in the elastic
range, and the second measures very high extensions (typically 500 mm).
Materials with very smooth surfaces, or those made of transparent
materials are not suitable for noncontact measurements without first
fixing measuring marks onto the surface of the specimen.
To qualify the integrity of a measured signal, standards use
quantitative terms such as resolution, deviation, and
uncertainty. Requirements for the accuracy of extension
measurements are normally given in application-specific test
requirements and international standards.
Devices that are easy to set up and sequences that can be automated
reduce personnel time and effort. Simplified set up routines can
improve the quality of the test results by minimizing subjective
Contact-type Measurement Extensometers
extensometers are, as the name implies, mounted directly onto the
specimen. The mechanical parts that transfer extension, via knife
edges, from the specimen to the internal transducer are short and
stiff. There is practically no relative movement between the specimen
and the extensometer, resulting in a high level of
Image 1 : A Zwick Roell Clip-on
Extensometer - Measuring Strain on a Metal Specimen.
The range of a clip-on extensometer is limited to a few
millimeters, and it applies a load directly to the specimen.
Extensometers with counter-balance weight and double-sided measuring
systems are used to compensate for superimposed bending stresses.
To minimize setting errors, some clip-on
extensometers are equipped with motorized application and removal
arm extensometers offer the advantages of automatic operation and a
large measurement range with high measurement accuracy. Precision
designs with a very smooth and balanced mechanical operation apply
minimum loading to the specimen (as little as the measurement marks
used for noncontact extensometers). Because the sensor arms
are in contact with both sides of the specimen, superimposed bending
strains are largely compensated.
Because of the direct contact with the specimen, sensor
arm extensometers can be damaged or even destroyed at the failure
point of high elasticity/high extension specimens. An example from the
automotive industry is the testing of safety belts. At the point of
failure, the material will exhibit backlash or whiplash characteristics
that could damage the testing equipment.
Image 2: Zwick’s multiXtens extensometer
combining fully automatic sensor arm extensometers with a dual
averaging axial and transverse strain extensometer.
Non-contact Video Extensometers
A primary advantage of noncontact
video extensometers is that they may be used up to the material
breaking point without damage, even when testing specimens that exhibit
whiplash. Another advantage is the capability to more accurately
determine strain and use strain as a control loop mechanism for test
samples undergoing large deformations. An example of this may be found
in characterization of biomaterials and medical-grade polymers, where
video extensometry supports measurement of large strains. Additional
applications include testing of medical components in solution, where
attachment of a traditional extensometer would not be practical.
video extensometers require measurement marks to be attached to the
specimen, which are optically distinct from the surrounding area of the
specimen. The measurement marks are clipped, tacked, or glued onto the
specimen, or the specimen is marked with a colored pen. The application
of the measurement marks adds a step to the test cycle, potentially
reducing throughput, increasing the cost of testing, and introducing
inaccuracies through human error.
Image 3: Zwick’s videoXtens noncontact
extensometer with the unique flexible illumination system
The position of the measurement marks on the specimen is evaluated
by software algorithms which determine a certain area around an optical
center point. This becomes the gage length, and as a load is applied to
the specimen, the movement of the marks is converted to extension
values. Special lighting for surface or background illumination of the
specimen optimizes the contrast to the measurement mark. During
deformation, the ambient lighting changes on the measurement marks as
well as on the specimen, and surrounding influences (such as
reflections) can influence the optical center point. This is often the
cause of scatter in the test results.
Non-contact Laser Extensometers (Laser
The latest in extensometer technology uses a noncontacting device
that does not require measurement marks. The laser
extensometer uses the unique structure of a specimens surface as a
fingerprint to generate a virtual measurement mark. Laser light
directed on these measurement positions is reflected in various
directions corresponding to the surface structure and creates a
specific pattern of speckles. Selected measurement points are
constantly followed and converted to direct extension values. The
change in the surface structure, which is the basis of the speckle
pattern, is continuously evaluated during specimen deformation.
This laser interferometer-based method of noncontact
extensometry allows test labs to characterize materials,
components, and even subassemblies making it well-suited for quality
control and R&D applications. Additionally, this novel approach to
extensometry supports tests on microspecimens with small gage lengths
that require exceptional accuracy in strain measurement.
Such tests would not be possible using traditional extensometry.
Speeding up throughput and delivering the utmost accuracy in strain
measurement, laser interferometer noncontact extensometers
offer significant value for high volume test labs.
Image 4: Zwick’s laserXtens noncontact
extensometer operating without specimen marks.
This article was written using material supplied by Zwick Roell.
Zwick is one of the top-ranking manufacturers of static and dynamic
materials testing systems worldwide. With our extensive portfolio of
testing machines, systems, and software we supply customized solutions
for almost all test tasks required in the area of modern mechanical
For more information please contact Zwick Roell