Ian McEnteggart

Composites Market Manager

Strain Measurement Techniques for

Composites Testing

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)

Introduction

Composite Materials and Properties

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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

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• 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!

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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)

Strain Measurement Techniques for

Coupon Testing

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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 TestingNon-contacting

DIC

Video

Strain gauges

Contacting Extensometers

Bi-axial clip-on

Axial clip-on

Automatic

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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)

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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

Non-contacting Video Extensometers

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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 timeUse 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?

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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

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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 = 𝐿𝑡 − 𝐿0

𝜀 =∆𝐿

𝐿0

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Analysis of Various Strain and Displacement Data

Axial StrainTransverse 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

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

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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.