Digital Image Correlation Presentation

The Principals of

Digital Image

Correlation

Motivation

Previous Solution:Problem: Static / dynamic measurement

Position

Dynamic displacement

Dynamic deformation

Speed / velocity

Acceleration

Strain

• 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 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

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 Photogrammetry

To 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

Sensitivity

10 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: [email protected]

Web Site: www.Trilion.com

www.Trilion.com