The Principals of Digital Image Correlation

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

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

MotivationCollecting data at several points

Reference

Reference

Reference

Reference

Reference

Reference

Setup (Displacement Sensor):

– Reference Frame– Mounting the Sensor– Wire up Sensor– Data Acquisition– Data Analysis

Data Acquisition

Still only 1D

Imagine a 3D setup

MotivationIndustry requires comprehensive analysis tools

Measure & Visualize Entire Structure’s Response including:

• 3D displacements & strain gradients

• including complex material types & geometries

MotivationDesign Criteria

Verify Finite Element Models with 10,000+ measurement points

MotivationDesign Criteria

Reduce the Number of Required Prototypes ($$$)

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

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

- 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

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

DIC Hardware Today

• 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

• 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

• 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

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

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

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

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

Thank you for your attention

Trilion Quality Systems500 Davis Drive, Suite 200, Plymouth Meeting, PA 19462

Office: (215) 710-3000 Fax: (215) 710-3001

Email: Info@Trilion.comWeb Site: www.Trilion.com

www.Trilion.com