This presentation looks at traditional methods of strain measurement and the latest developments in automatic contacting and non-contacting extensometers.

1. Ian McEnteggart
Composites Market Manager
Strain Measurement Techniques for
Composites Testing

2. 2
Topics
• Introduction
• Composite Materials
• Testing of Composite Materials
• Strain Measurement Techniques for
Composite Coupon Testing
• Strain Gauges
• Contacting Extensometers
• Non-contacting Extensometers
• Full-field Strain Measurement
• Examples of Tests
• Tensile
• Compression
• In-Plane Shear (IPS)

3. Introduction
Composite Materials and Properties

4. 4
Composite Materials
• Two or more distinct phases
• Matrix and Reinforcement
• Common Matrix materials
• Polymer Matrix Composites (PMC)
• Ceramic Matrix Composites (CMC)
• Metal Matrix Composites (MMC)
• Many others e.g. wood
• Common Reinforcements
• Continuous fibers (carbon, glass, basalt)
• Discontinuous fibers / Particles
Today’s focus is on testing of Continuous Fiber
Polymer Matrix Composites (CFRP, GFRP or simply
Carbon/Glass Fiber) Coupons

5. 5
• 3 Poisson’s ratios υ12, υ23, υ13
3
1
2
• 3 Tensile moduli/strengths E1t , E2t , E3t , S11t , S22t , S33t
• 3 Shear moduli/strengths G12, G23, G13 ,,S12, S23, S13
• 3 Compressive moduli/strengths E1c, E2c , E3c , S11t , S22t , S33t
Composite Material Properties
• Most metals and plastics are isotropic - properties
independent of direction
• Composites are anisotropic - properties depend on direction
• Composite Intralaminar properties
• Composite Interlaminar fracture properties are important, too!

6. 6
Tension: Fiber dominant property. Dependant
on the tensile stiffness and strength of the fiber
Compression: Matrix dominant property.
Dependant on the stiffness and adhesion
qualities of the resin being able to maintain the
fibres as straight columns and not buckle.
Shear: Matrix dominant property, transferring
stresses across the composite.
Flexure: Combination of above three:
upper=compression; lower=tension; middle=shear
Composites Require Many Different Tests
to Characterise
Also a range of “structural tests” on Coupons e.g. open hole tension
& compression, bearing load, Compression after Impact (CAI)

7. Strain Measurement Techniques for
Coupon Testing

8. 8
Strain Measurement for Coupon Testing
• Require high accuracy/resolution
• Extensometers: ASTM E-83 class B-1/2
• Strain Gauges: Resolution < 50µε (my estimate)
• Non-contacting Video / DIC great for research but not generally used for standard tests
• Most strain measurements on coupons made using either strain gauges or clip-on
extensometers
• Use of averaging Axial strain measurement in order to correct for and/or monitor
specimen bending is common
• Use of Biaxial (Axial + Transverse) strain measurement to determine Poisson’s ratio
and Shear strain
• Majority of composites testing is done at temperature, mostly between -80 to 200 °C
(-112 to +392 °F) but the upper temperature limit is increasing as resins improve
• Composite coupons are often “conditioned” in hot/wet environments or liquids e.g. water,
hydraulic fluids, fuel prior to testing - this can make bonding of gauges difficult
• Explosive failures are common, extensometers often removed prior to failure

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Strain Measurements for Composites Testing
Non-contacting
DIC
Video
Strain gauges
Contacting Extensometers
Bi-axial clip-on
Axial clip-on
Automatic

10. 10
Strain Gauges
• Use of strain gauges is very common in composites testing.
• In some cases e.g. measurement of local shear strain,
there is no extensometer solution available
• In other cases the test specifications mandate the use of
strain gauges
• Use of multiple strain gauges is usual, strain readings are
typically combined to generate
• Average axial / width strain
• Difference in axial strain (bending)
• Shear strain (axial - transverse strain)

11. 11
Electrical Interface to Single Strain Gauge
• Strain gauges used for strain measurement are usually used
singly and have an absolute calibration (i.e. a Gauge Factor
relating change in resistance to change in strain).
• Using a single strain gauge with an strain channel requires:
• Bridge completion (Quarter Bridge – see above)
• A method of providing an absolute calibration.
• NOTES:
• A single active gauge in a voltage driven bridge has an inherent non-
linearity which is acceptable (<1%) at low strain levels (<1% Strain).
• A 3-wire connection to the strain gauge (see above) provides
compensation for changes in wire resistance due to temperature
changes

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Verification of Strain Gauge Measurements
• No accepted primary standard for
strain (primary standard for
extensometers is displacement)
• Gauge manufacturer checks
gauges on a sample basis using
bending beam rig or similar and
supplies a certificate stating the
Gauge Factor
• Strain gauge Gauge Factor is
influenced by the Poisson's ratio
of the material—this can be
important when making
measurements on composites
• Verification of strain gauge data
acquisition channels is performed
using a traceable strain gauge
simulator

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Contacting Extensometers – Clip-on
• General
• Manually attached
• Strain gauged types - temperature range -70 to
200 ºC
• Capacitive types – temperatures up to 600 ºC
• Averaging Axial
• Corrects for specimen bending
• Versions with independent axial outputs allow for
measurement of average and PBS (Percentage
Bending Strain)
𝑃𝐵𝑆 =
∈ 𝑓−∈ 𝑏
∈ 𝑓+∈ 𝑏
× 100
• Biaxial
• Versions with Transverse Strain measurement
allow for determination of Poisson’s ratio (∈ 𝑇
∈ 𝐴
)

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Automatic Extensometer
• Automatic contacting
extensometer
• 1 micron accuracy
• Capable of testing
multiple gauge lengths
• Suitable for tension
and compression
• Measures strain
through failure
• Automatically closes
on specimen to test

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AutoX750 for Composites Testing
• 1µm accuracy
• Robust - arms
can be left on
until failure
• Repeatable
positioning and
attachment
ensures
consistent
results.
• 1µm accuracy
• Low clamping
force does not
damage
specimen.
• Low drag force
minimizes
specimen
bending.
• 1 um accuracy
• Less expensive
than strain
gauges
• Robust - arms
can be left on
until failure
• Easy to use
Tow – Tensile
ASTM D4018
Laminate – Tensile
ASTM D3039 &
ISO527-4/5
Laminate
Compression
ASTM D695
Laminate
Flexure
ASTM D790/7264
EN2562/2746
ISO178/14125
• 1 um accuracy
• Robust - arms
can be left on
until failure
• Easy to use

16. Non-contacting Video Extensometers

17. 17
Instron AVE 2 Non-contacting Video
Extensometers
• High accuracy strain measurement meets
most composites standards
• The 1 micron accuracy allows measuring
modulus to ISO 527-4/5
• 490 Hz frame rate prevents missing fast
events such as break
• Patented LED lighting and fan system
prevents environmental influences
• Doesn’t require operator to attach
extensometer, reducing operator influence
and increasing consistency

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Applications of Non-Contacting Video
Extensometers
• Can be used to measure tensile and compressive strain
• Can be used on chambers for cold and hot tensile tests
• Can be used with any test machine with +/- 10V input
• Can be used for Full-field strain measurement using
Digital Image Correlation software

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What is Digital Image Correlation?
Images Displacement Strain
Analysis of image
surface over time
Use of cross correlation to
determine displacement
Strain calculated
from displacement
An optical method to measure deformation on an object surface.

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How Does it Work?

21. 21
Calculating Full-field Displacement
• Repeated for each subset over the entire surface
• The result is a regular map of displacements over the
entire specimen surface
Specimen
Surface
image
Split into
small
subsets
Pattern
recognised for
each subset
As the specimen deforms, axial (x)
and transverse (y) displacements
for each subset are calculated

22. 22
Calculating Strain
• Strain at each location is
calculated using central
differencing
• Strain calculated in the x and y
directions separately
• For the x direction:
∆𝐿 = 𝐿 𝑡 − 𝐿0
𝜀 =
∆𝐿
𝐿0

23. 23
Analysis of Various Strain and Displacement Data
Axial Strain
Transverse Strain Shear
Strain
Poisson’s
Ratio
Minimum
Normal Strain
Maximum
Normal Strain
Axial
Displacement Transverse
Displacement

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Extracting 1D Plots
• Use virtual extensometer for
calculating strain/displacement
between two points
• Use virtual strain gauge for
calculating average strain over a
defined area

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DIC Example 1 - Vee-notch Shear
• Test to determine shear properties
• V – notched specimen
• Approximately uniform shear stress distribution in notch
• Traditional approach is to use strain gauges mounted at
+/- 45º required to measure shear strain (see below left)
• DIC allows determination of strain distribution (see below
right)
ASTM D 5379
ASTM D 7078

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DIC Example 2 – Open Hole Tension Test
• Composite Laminate – Open Hole Tension
• Complex 2D strain distribution
• Measure all components of 2D Strain
Tensor (Axial, Transverse, Shear) along with
Maximum and Minimum Principle Strains
Shear Strain
Axial Strain

27. Examples of Strain Measurement in
Composite Tests

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Strain Measurement for In-Plane Tensile
• In-plane (laminate) tensile
• Specimens may have different
orientations (e.g. 0/90º)
• Biaxial extensometer or axial + transverse
strain gauges required for determination
of Poisson's ratio
ASTM D 3039
ISO 527-4/5
EN 2597
n = - etrans / elongitudinal

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Strain Measurement for Compression Testing
• Use of strain gauges is common as short
unsupported gauge sections and support
fixtures provides little room for
extensometers
• Some specialized extensometer solutions
are available
• Independent measurement of strain on
both sides of the specimen is required
(to allow measurement of bending)
Unsupported Gauge Section
Supported Gauge Section

30. 30
In-Plane Shear Properties by +/- 45 Degree
Tension Test
• Test specimen has fiber directions of +/- 45 degrees
• Test set up similar to tensile test
• Axial and Transverse Strain measured using biaxial
extensometer or axial + transverse strain gauges
• Simple test but not a pure shear stress state
(shear + axial tension)
• Shear Strain = Axial Strain – Transverse Strain
ASTM D 3518
ISO 14129
AITM 1-1002
prEN 6031

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Thank you for your time!
Please contact Instron® with any questions.
Visit www.instron.com
for more information.