Introduction to Digital Image Correlation (DIC)

Ian McEnteggart,

Instron® Composites Marketing Manager

An Introduction to

Digital Image Correlation (DIC)

2

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’s surface

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• Non-contact strain measurement

• Full-field data (like thousands of tiny extensometers)

• Extract “conventional” 1D strain plots

• Identify strain hot spots over a large area

• Don’t need to know where to place the strain gauge/extensometer

before the test starts

• Validate correct specimen alignment

What are the Benefits of DIC?

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Sample Preparation

• Apply speckle pattern using:

• Airbrush

• Standard spray aerosol

• Brush—flicking

• Transferable stickers

• Rubber stamp

• Some materials can be left without speckling:

• Certain composites

• Concrete

• Textiles/fabric

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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

recognized

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

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 to points.

• Use virtual strain gauge for

calculating average strain over a

defined area.

Advanced Video Extensometer (AVE) 2

& DIC Replay

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AVE 2: Versatile and Capable • Doesn’t require operator to attach extensometer, reducing

operator influence and increasing consistency

• 1 micron accuracy—measure modulus to ISO 527

• Patented LED lighting and fan system eliminates

environmental influences

• Measures both tensile and compressive strain

• Can be used on chambers for cold and hot tensile tests

• Can be used for full-field strain measurement using

Digital Image Correlation Software

12

DIC Replay

• Streamlined full-field strain package tailored for the materials testing market

• Allows users to analyze advanced strain characteristics after the test

• No PhD degree required!

Simple integration & convenient mounting

•Consumes images saved by Instron® AVE camera

•No spaghetti cabling

•Synchronizes with data collected from the testing system

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Why Instron DIC?

Integrated and

synchronous

collection of all

data from

testing system,

e.g. force and

camera

Only 1 PC Integrated camera and

lighting unit sits on the

frame out of the way of

testing area. Polarized

light is used so ambient

light doesn’t matter.

Easy to use,

where users

can focus on

analyzing and

understanding

their results

rather than

assembling test

rigs.

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DIC Application Examples

• Multiple extensometers for the same sample

• Can decide location post test

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DIC Application Examples

• Component—Identify strain “hot spots”

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DIC Application Examples

• Foam specimen—Traditional strain measurement was not possible

• Speckled with felt pen • Split seen

in DIC

• Virtual extensometers at

25mm and 50mm GLs

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DIC Application Examples

• 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

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What Can We Do Next?

• If you’re interested in further discussions:

• Arrange a demonstration on site by our sales engineer

• Visit our applications laboratories and bring samples

• Send samples for us to test

Thanks for your time!