complete book on digital image

8/10/2019 complete book on Digital Image

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DIGIT L IM GE CORREL TION

ADEEL ZAFAR

1

CEE 498KUC Experimental Methods in Structures and Materials

March 14th, 2008


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DIGITAL IMAGE CORRELATION

ABSTRACT

Non-contact, optical technique for obtaining full-

field deformation

It uses image processing to go from severalimages of material, and to then calculate the

deformation at any point in the field , and to then

find deformation and strain values


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OUTLINE Introduction to DIC

History and Developments

Concept and Approach

Methodology

Theoretical background Calibration

DIC-3D

Various Tests and Results

Advantages and Disadvantages

Applications

Conclusion

3


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INTRODUCTION

What is DIC?

It is computer based process to obtain a 2-D full field

information by recording deformation and motion of

speckle patterns on a specimen surface before and

after deformation of the body

It takes advantage of the fact that applied stresseschange both the thickness and optical properties of

materials, to determine displacement


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INTRODUCTION

Why need DIC?

Although the bulk macroscale testing provides useful

information about materials but many process occur at

microscale and nanoscale. As a result testing methods

are required to have that resolution and capture results

at that level


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HISTORY AND DEVELOPMENTS


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Developed in the early 80s at the University of South

Carolina to measure full-field in-plane displacements

and displacement gradientsof a strained body at the

macroscale 1987Methodology to measure crack initiation mode

& stress intensity factor in brittle materials

1988Mixed-mode loading

19923D effects near a crack tip

1997Deformations measured in micron and

nanometer scales


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1997Plastic deformation patterns in ductile

materials

1998Method to evaluate large deformations

2002Professor Lambros constructed crack growthresistance curves for functionally graded materials

Contd.


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CONCEPT AND APPROACH


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EQUIPMENT

It is less demanding

optically-ordinary incoherent

light is sufficient and no

need for optical components

Requires one Hi resolutioncamera for 2D displacement

and for 3D displacement, 2

cameras

A computer and frame

grabbing circuit card to

digitize the output


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CONCEPT

It is very simple in concept

Digital camera takes picture

of surface of the specimen

Image is downloaded from

camera to a frame grabbingcircuit card

Analog signal from CCD

array are then digitized The data is stored for

subsequent processing


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APPROACH

Surface of specimen is sprayed

with target pattern This is photographed before and

after specimen is deformed

Digital image of specimen

contains intensitymeasurements at each pixel

location on CCD (charge couple

device-sensor) array before and

after Using target features and their

location, displacement field is

generated

Accuracy upto 0.02 pixels havebeen achieved


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METHODOLOGY


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LOCATING TARGET FEATURE

Camera with CCD array(Has small

photosensitive cells and high pixel count)

records intensity of light falling on a pixel.

Array in high resolution camera is

rectangular with thousand or more pixels

per line and a thousand or more lines perimage. (3x3 pixel)

Signal from CCD array is digitized and

gives a reading of the light intensity for

each pixel. Intensity readings are shownas 0 from dark pixels and 100 for light

pixel(Grey Scale)

Storage of image into pixel and

combination of pixel is calledCONVOLUTION

100 0 100 100 100

0 0 0 0 0

100 0 100 100 100

0

50

0


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LOCATING TARGET FEATURE

To locate the target from

intensity data on pixel array,

DIC uses intensity pattern

matching method called

CORRELATION This is done by defining the

subset of pixels that

surround the key feature on

target

Matching two patterns

establishes target

displacement

100 100 100 100 100 100 100 100 100

100 100 0 100 100 100 100 100 100

100 0 0 0 100 0 0 100 100

100 100 0 100 100 100 100 100 100

100 100 100 100 100 100 100 100 100

50 50

0

5050

50

50 50

0

50

0

50

100

100

100

100

50

100 100

100 100

100

100 100

100

100

50

100

50

100

100

100

0

100

100

0

50

100 100

100

100

100

50

100

100

Initial After


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TARGET FEATURE REQUIREMENT

If you have same type oftarget, then it would be

impossible to distinguish

between them

Need to have unique shape

for each point

This is achieved with

speckles Speckle should cover at

least 3x3 pixel area


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SPECKLE PATTERNS

Techniques in creating speckles are :-

For small speckles-Spraying white paint and sprinkling of carbon

particles

For moderate speckles-Spraying white paint and the black paint is

sprayed with light pass

For large speckles-Spraying white paint and brushing black paint For very high resolution- Fluorescent paint is used


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SPECKLE MATCHING

It is done based on

algorithm for matching

intensity field of many

speckles because ofrichness of intensity data

as compared to

individual speckle

matching


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SPECKLES WITH RESPECT TO SCALE

Macroscale Nanoscle

Speckles must be custom made to fit the scale of

observation. Thus, for the nanoscale, a smaller

speckle pattern must be used than for the

macroscale.


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DIGITAL IMAGE

Corresponding

intensity profile

as a surface plot

Typical digital

image captured

by CCD camera


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DIGITAL IMAGE PROCESSINGTHEORETICAL BACKGROUND


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THEORETICAL BACKGROUND

2-D deformation In-plane displacement

Strain fields

Known discretefunctions

f(x,y)Undeformed

g(x,y)Deformed


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THEORETICAL BACKGROUND

up

vp

Components of displacement vector of point p in

x and y direction:


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USING G(X,Y)

Use above equations to solve for displacement

vector :

Position of any point can be defined by

correlation functions:


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MAPPING DISPLACEMENT(MATCHING)

Correlation involves comparison of intensity data of

two pixel fields. Mapping the coordinates between the

two fields is accomplished by using:-

X = x+u(x,y)

Y= y+v(x.y) Matching process involves minimizing coefficient that

measure how well both configuration compare


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MATCHING PROCESS

Two different coefficient have been used to compareaccuracy Least-square correlation coefficient

Cross-correlation coefficient

is a light intensity value (grayscale level)

is corresponding intensity

value of the same point q after deformation


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LEAST-SQUARES


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CROSS-CORRELATION


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SMOOTHING THE INTENSITY DATA

Digital Image contains thousands of pixels with

different gray scale values. Usually they changeabruptly

This produces mathematical difficulties when

determining mapping parameter Therefore data is smoothed over the entire field in

both images. Process is called INTERPOLATION


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SMOOTHING THE INTENSITY DATA

Interpolation over the intensityvalues of deformed configuration

is needed to determine these

sub-pixel location intensity

values

Following interpolation schemes

are generally used;-

Bilinear

Cubic

Both interpolations approximates

intensity value of any point that

lies between pixels


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BICUBIC INTERPOLATION

BILINEAR INTERPOLATION

INTERPOLATIONS


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DETERMINATION OF DISPLACEMENT VECTOR

Two methods exist:- Coarse-Fine method. Brute force approach in which

minimization coefficients are computed for a wide range of

values and are minimized independently. Time consuming

Newton-Raphson method . All coefficients are minimized

simultaneously. Faster

The process involves in obtaining optimum solution by

searching over given range for the deformationparameters


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CALIBRATION


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CALIBRATION

Determination of Magnification factorRelates to dimension on specimen to a

corresponding dimension on the image plane of

camera

Location of lens center

Center of camera lens should coincide with center

of area of interest

Lens Distortion

Lens with long focal lengths are employed to

minimize lens distortion


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DIC - 3DOUT OF PLANE DISPLACEMENT


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STEREO IMAGING TECHNIQUE

Measurement of 3D

component on aspecimen is much more

difficult

2 CCD cameras areemployed

Procedure is to get

image from both cameras

and then develop stereoimaging equations before

correlation process


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TESTS AND RESULTS


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PROCESS

Deformation vector field Strain fieldLive field


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TRANSLATION TEST


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ROTATION TESTS


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TENSION TESTS

Contour plot of displacement


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FRACTURE TEST


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FRACTURE TEST


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FRACTURE TEST

Deformation vector field Strain fieldLive field


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FRACTURE TEST


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ADVANTAGES AND DISADVANTAGES

ADVANTAGES


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ADVANTAGES

Preparation of the specimen is relatively simple as the speckle

patterns used as unique targets are made by spray painting its

surface. Ordinary white light is used to illuminate the surface of the

specimen.

Specimen size is not an issue as the method can be applied to

both large and small specimens. Specimen size isaccommodated by changing the magnification used in

recording the image.

The method is not overly sensitive to vibration and a vibration

isolation table is not necessary. However, it is essential thatboth the specimen and the camera be stable during the time

required to record the image.

Large strains or significant rigid body movement does not

cause difficulties if the specimen does not move out of the fieldof view of the camera.


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DISADVANTAGES

Sensitive to fluctuation and nonuniformity of light

intensity

Measuring out of plain displacement requires more

equipment and expertise

Mathematics involved is challenging

The calibration process is tedious


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APPLICATIONS

Professional development

LaVisions StrainMaster

Correlation Solution in Biomechanics

Research development

Velocity vectors in displacement zone

of steel during cold-rolling process


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SUMMARY

DIC is a non-contact method for measuring whole field

displacement Method involves interpolation to smooth gray scale levels and

then applies coefficient to map the parameters to find strains

Mathematics involved is challenging, but if computer code is

available, mapping process becomes automatic Accuracy is often quoted as 0.02 pixels for each displacement

component. Accuracy depends on:-

Interpolation schemes

Lens distortion Uniformity of light distribution

Quality of speckle pattern


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REFERENCES

http://www.airframer.com/news_story.html?release=108

http://en.wikipedia.org/wiki/Digital_image_correlation

Thesis by JAY K. Patel on DIC for microscale and nanoscale

deformation measurements

Thesis by Jorge Luis Abanto-bueno on Fracture Modeling

Structural Engineering Seminar by Prof Lambros
http://www.airframer.com/news_story.html?release=108http://en.wikipedia.org/wiki/Digital_image_correlationhttp://en.wikipedia.org/wiki/Digital_image_correlationhttp://www.airframer.com/news_story.html?release=108


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QUESTIONS

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