the principals of digital image correlation
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The Principals of Digital Image Correlation. Motivation. Problem: Static / dynamic measurement Position Dynamic displacement Dynamic deformation Speed / velocity Acceleration Strain. Previous Solution :. Displacement Sensors Laser trackers LVDT Draw Wire Sensors Accelerometers - PowerPoint PPT PresentationTRANSCRIPT
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The Principals of Digital Image Correlation
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MotivationPrevious Solution:Problem: Static / dynamic measurement
PositionDynamic displacementDynamic deformationSpeed / velocityAccelerationStrain
• Displacement Sensors• Laser trackers • LVDT• Draw Wire Sensors• Accelerometers• Strain Gages• Extensometers• Clip gages• Profilometer• LVDT
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MotivationCollecting data at a single point
Setup (Displacement Sensor):
– Reference Frame– Mounting the Sensor– Wire up Sensor– Data Acquisition– Data Analysis
Data Acquisition
Displacement Sensor
Reference
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MotivationCollecting data at several points
Reference
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Setup (Displacement Sensor):
– Reference Frame– Mounting the Sensor– Wire up Sensor– Data Acquisition– Data Analysis
Data Acquisition
Still only 1D
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Imagine a 3D setup
MotivationIndustry requires comprehensive analysis tools
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Measure & Visualize Entire Structure’s Response including:
• 3D displacements & strain gradients
• including complex material types & geometries
MotivationDesign Criteria
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Verify Finite Element Models with 10,000+ measurement points
MotivationDesign Criteria
Reduce the Number of Required Prototypes ($$$)
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DIC is a powerful method for detecting deformation on the surfaceof 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
DIC Overview
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DIC OverviewTheory
- 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
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- 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 OverviewTheory
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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 Overview
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DIC Hardware Today
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• 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 TodayCameras
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• Data Acquisition Controller that triggers cameras to take pictures
• Synchronizes with Test machines and records analog-to-digital signals
• Load, and Displacement• Temperature
• Uploads images and AD information to computer
DIC Hardware TodaySensor Controller and Computer
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• 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
DIC Hardware TodayCalibration Artifacts
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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
Using PhotogrammetryTo track a single point
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Undeformed Specimen
Deformed Specimen
• Speckle Pattern on the specimen tracked in both camera images by regularly spaced facets as part deforms
DIC ProcedureUsing Facets to create 3D coordinates from a 2D image
• Image acquisition by stereo camera pair
L
RL
R
R
R
L
L
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DIC Procedure3D Results
• Image processing• 3D coordinates
• 3D displacements and velocity
•Strain tensor– Major and Minor strain– Strain in X, Y, shear strain– Thickness reduction– Strain rates for all strain
values
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DIC ProcedureSensitivity
• Displacement Sensitivity out of plane is 1/30,000 the field of view regardless of camera resolution
• In Plane sensitivity is 10x higher and increases with resolution
Field of View Displacement Sensitivity10 x 8 mm 0.3 Microns
100 x 80 mm 3.0 Microns
1 Meter x 800 mm 30.0 Microns
• Strain Sensitivity is Constant; 50-100 Microstrain but resolution for strains increase as field of view decreases (pixel scaling factor)
Noise Floor for 135mm FOV is 0.16 Microns.Total Displacement of 0.6 Microns is Clearly Visible
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