The Principals of
Digital Image
Correlation
Motivation
Problem: Static / dynamic measurement
Position
Dynamic displacement
Dynamic deformation
Speed / velocity
Acceleration
Strain
Previous Solution:
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Displacement Sensors
Laser trackers
LVDT
Draw Wire Sensors
Accelerometers
Strain Gages
Extensometers
Clip gages
Profilometer
LVDT
Motivation
Collecting data at a single point
Setup (Displacement Sensor):
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Reference Frame
Mounting the Sensor
Wire up Sensor
Data Acquisition
Data Analysis
Displacement Sensor
Reference
Data Acquisition
Motivation
Collecting data at several points
Setup (Displacement Sensor):
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Reference Frame
Mounting the Sensor
Wire up Sensor
Data Acquisition
Data Analysis
Reference
Reference
Reference
Reference
Data Acquisition
Reference
Reference
Still only 1D
Motivation
Industry requires comprehensive analysis tools
Imagine a 3D setup
Motivation
Design Criteria
Measure & Visualize Entire Structure’s Response including:
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3D displacements & strain gradients
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including complex material types & geometries
Motivation
Design Criteria
Verify Finite Element Models with 10,000+ measurement points
Reduce the Number of Required Prototypes ($$$)
DIC Overview
DIC is a powerful method for detecting deformation on the surface
of a material or component and is most commonly used in applications
that involve:
Materials testing and Characterization
 Failure and Fatigue Studies
 Long Term Health Monitoring
 Materials that have a complex composition or shape
 Static and Dynamic Measurements of Strain or Motion
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DIC Overview
Theory
- Digital Image Correlation (DIC) systems use the
principals of photogrammetry, digital image
processing, and in most cases stereo imaging to
track features in space and assign their position to a
predetermined coordinate system
- The measurement is made by the comparison of an
image series that is captured over timescales from
microseconds to years
- 2D measurements (in-plane) and 3D measurements
(in and out of plane) are possible
- Analysis is done in post-processing
DIC Overview
Theory
- Much like a strain gage is zeroed to a condition
that is said to be “undeformed” and then used as
reference for future deformations the first image taken
by DIC is used as the undeformed reference condition.
- DIC is a non-contact optical measurement system that
measures surface displacements of an object
subjected to a driving force
- Provides experimental data that is directly comparable
to Finite Element simulations
- Triangulation between the stereo camera pair is used
to determine location in z-direction (out of plane)
DIC Overview
Advantages:
 Non-Contact Measurement
 Rich data set compiled from over 10,000 pts on the surface
 Analysis is done in post-proccessing (place gages on after
test)
 Provides information for shape, position, displacement, and
strain
 Calibration Technique ensures high accuracy
 Not affected by rigid body motion
Disadvantages:
 Cannot Measure Existing Damage
 Must have clear line of sight to part by both cameras
DIC Hardware Today
DIC Hardware Today
Cameras
• High Resolution CCD Cameras (no
internal moving parts) 2MP – 12MP
• Larger Sensors than point and shoot
cameras provide better pixel quality
with less pixels
• High Light Sensitivity
• Typically Monochrome (Black and
White)
• Capable of Image Acquisition Rates
from 15Hz to 1M fps
DIC Hardware Today
Sensor Controller and Computer
• Data Acquisition Controller that triggers cameras to take
pictures
• Synchronizes with Test machines and records analog-todigital signals
•Load, and Displacement
•Temperature
• Uploads images and AD information to computer
DIC Hardware Today
Calibration Artifacts
• Provide Scale information for the
field of view and used to create a
calibrated volume within which the
3D coordinates are known
• Calibration Objects have a dense
grid of points on them which are at a
known location by sensor supplying
in-plane coordinate information
• By moving the calibration object
closer or further from the camera,
information can be provided to the
sensor relating depth of field and
out of plane coordinates
Using Photogrammetry
To track a single point
The center point of an
ellipse or a target can be
tracking in calibrated space
by interpolating to find its
location
The perimeter of the ellipse
where the color changes
from black to white is
traced by some n number
of pixels
The more pixels that trace
the ellipse the more
accurate its location can be
tracked
Sub Pixel Interpolation
DIC Procedure
Using Facets to create 3D coordinates from a 2D image
L
R
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L
R
Undeformed Specimen
L
Deformed Specimen
Image acquisition by stereo
camera pair
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Speckle Pattern on the specimen
tracked in both camera images by
regularly spaced facets as part
deforms
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DIC Procedure
3D Results
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Image processing
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3D coordinates
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3D displacements
and velocity
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Strain tensor
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Major and Minor strain
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Strain in X, Y, shear strain
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Thickness reduction
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Strain rates for all strain
values
DIC Procedure
Sensitivity
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Displacement Sensitivity out of plane is 1/30,000
the field of view regardless of camera resolution
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In Plane sensitivity is 10x higher and increases
with resolution
Field of View
Sensitivity
Noise Floor for 135mm FOV is 0.16 Microns.
Total Displacement of 0.6 Microns is Clearly
Visible
Displacement
10 x 8 mm
0.3
Microns
100 x 80 mm
3.0
Microns
30.0
Microns
1 Meter x 800 mm
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Strain Sensitivity is Constant; 50-100 Microstrain but
resolution for strains increase as field of view
decreases (pixel scaling factor)
Thank you for
your attention
Trilion Quality Systems
500 Davis Drive, Suite 200,
Plymouth Meeting, PA 19462
Office: (215) 710-3000
Fax: (215) 710-3001
Email: Info@Trilion.com
Web Site: www.Trilion.com
www.Trilion.com