Main Seminar Report

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SEMINAR REPORT 2012 TV HOLOGRAPHY FOR THE NDE

Dept. of Production Engg, VAST 1

CHAPTER 1

INTRODUCTION

The industrial application of holographic nondestructive testing (HNDT) is well

established in the fields like tyre testing, bond testing of composite materials, early

detection of fatigue and vibration analysis of structure. But the technique has certain

limitations such as, stringent vibration isolation requirement, formation of extraneous

fringes due to rigid body motion, difficulty in obtaining in plane and out of plane

components, dark room facility and wet processing requirements, incapability of on-

line quality inspection under industrial condition, etc.

The newly emerged TV Holography technique, based on electronic speckle pattern

interferometry, is a very effective alternate NDE tool. Initially the speckle was

observed as a nuisance in laser holography but later it was observed that valuable

information on the properties of object surface are hidden in it. This aspect was later

used in the development of TV holography technique and its application for NDE.

In the present work, the TV holography technique is applied for the NDE of a low

modulus material used as insulator in solid rocket motor. This technique is most

suited since; it offers real time inspection for quality assurance and is less affected by

rigid body motion. TV holography technique using both mechanical and thermal

loading has been studied and a practical methodology has been developed for defect

sizing and characterization.


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CHAPTER 2

HOLOGRAPHY

Holography is a technique that allows the light scattered from an object to be

recorded and later reconstructed so that when an imaging system (a camera or an eye)

is placed in the reconstructed beam, an image of the object will be seen even when the

object is no longer present. The image changes as the position and orientation of the

viewing system changes in exactly the same way as if the object were still present,

thus making the image appear three-dimensional. The holographic recording itself is

not an image it consists of an apparently random structure of varying intensity,

density or profile.

When a coherent beam strikes an object it is reflected in many directions, and the

reflected wave fronts have a complicated shape. What we actually see is determined

by the particular part of the wave front that enters the pupil of our eye. As long as our

eye remains stationary the shape of the intercepted wavefront is unchanged, and the

effect looks exactly the same. But if we change our viewpoint our eye intercepts a

different portion of the wavefront, and the object looks different. Stereoscopic

photography (stereophotography) exploits this phenomenon. Two photographs taken

from the position of our two eyes about 7cm apart each is presented to the appropriate

eye, forming a stereoscopic image. Although each photograph records only part of the

information contained in the wavefronts this is still sufficient to provide; illusion of

depth. Nevertheless, it is only part of the information; the viewpoint is fixed. The 1

way to provide all of the information is to record the shape of entire wavefront - and

then to reconstruct the wavefront from the record.

A photograph cannot accomplish this; nor for that matter, can a video record, nor any

conventional device using a lens and light sensitive material. This is because the
http://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Three-dimensional_spacehttp://en.wikipedia.org/wiki/Three-dimensional_spacehttp://en.wikipedia.org/wiki/Light


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frequency of the light wave is so high that there is no way of recording their

fluctuations; only the time-averaged intensity is recorded. The phase information is

lost. But it is the phase relationships of the various parts of the wavefront reflected by

the object that carry the information about the distance of each point on the objectfrom the film - the third dimension and the wavefronts are traveling at 3 x 10

8m/s.

This is where holography comes into play. It mixes a second light beam with the

beam reflected from the object, or object beam. This second beam, known as the

reference beam, is derived from the same laser usually by a partly reflecting mirror or

beam splitter. It has the same wavelength, and it travels at the same speed, because the

two beams interact with one another to form an interference pattern, and although the

wavefronts continue to travel at the speed of light,- the pattern itself is stationary. A

hologram is a record of this pattern contains all phase information that is missing from

a photographic record.

2.1. INTERFERENCE

A coherent source produces a beam that appears to have originated from a single point

and is monochromatic. The lasers used in holography possess both aspects of

coherence to a high degree. The distance over which the wavefronts remain in phase

is called the coherence length; helium-neon (He-Ne) lasers used by semi professional

holographers have a coherence length of some 250 mm (10in) or more, and the argon-

ion lasers used by professional holographers have a coherence length of 3m (10ft) or

more. The latest generation of semiconductor-driven lasers (see p. 156) has a

coherence length of more than 3m. By comparison, the coherence length of sodium

light is only about 0.3 mm. Light sources with poor coherence qualities are notsuitable for holography.

Wherever two coherent wavefronts are superposed, an interference pattern is created.

At each point the resultant intensity represents the sum of the two waves at that point.

Thus where the waves are in phase (peaks coincide with peaks and troughs with


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troughs) there will be a maximum of intensity. This is known as constructive

interference. Where the wavefronts are in antiphase (peaks from one source coincide

with troughs from the other and vice versa) there will be a minimum of intensity.

This is known as destructive interference. The light and dark regions of the patternthus correspond to the in phase and anti phase states of the two wave-fronts,

respectively. In between these maxima and minima there will be a gradual change in

intensity as the relative phases of the two wavefronts change.

If the amplitude and phase of the two wavefronts are known the amplitude and phase

of the resultant can be deduced. Furthermore, if the amplitude and phase of just one of

the component wavefronts is known and a record of the amplitude and phase of the

resultant available, the amplitude and phase of the other component wavefront can be

calculated.

2.1.1. ENCODING PHASE AND AMPLITUDE

Holography does provide such a record. We can demonstrate this using a laser beam

and a piece of optical equipment called a Fresnel biprism. This is a pair of very thin

prisms made to back from a single piece of glass, and it is often used to demonstrate

interference patterns large enough to be visible to unaided eye. If we expand a laser

beam by means of a concave lens and place the Fresnel biprism symmetrically across

the disk of light we will be able to see the parallel bands of the interference pattern.

These are known as interference fringes. If we place a piece of white card in the beam

and then angle the card, we can spread out fringes so that we can see them quite

easily. The dark fringes correspond to regions where the wavefronts interfere

destructively, the light fringes correspond to regions where they interfereconstructively. The fringes may have some messy marks on them, caused by dirt in

the optical system, but they will be straight, because both halves of the beam consist

of plane waves


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Now take a fine sewing needle or a straight piece of fuse wire and set it in a piece of

modeling clay so that it sticks up obliquely in the path of one of the beams, between

the biprism and the screen. We will see that a small distortion has appeared in the

fringe pattern. This distortion represents an alteration in phase in part of the objectwavefront due to the presence of the object. If we now replace needle by a glass

object such as a glass animal, the pattern become much more fragmented;

nevertheless, it has still recorded the precise disturbance in the object

wavefront. To the permanent record of this, all we need to do is to substitute lens

with photographic film for the screen and, when it has received sufficient exposure,

remove and process it. We now have a record which contains all the information

about the object wavefront-a hologram. If we do not have access to a Fresnel biprism,

we can produce the same effect with an optical arrangement known as Lloyd's mirror.

This uses a large sheet of glass placed at a very shallow grazing angle in one half of

the beam, directing it into the other half on the screen. We won't be able to get the

angle between the two parts of the beam as small as we can with the Fresnel biprism,

and we may have to turn the card to a very steep angle to the beam in order to spread

the interference fringes out sufficiently to be viewed properly.

2.2. RETRIEVING THE IMAGE

The process of capturing the information carried in the shape of the object-beam

wavefront is a fairly simple. All we need is a reference beam to mix with the object

beam and a photographic film to record the resulting interference pattern. Having

developed the film, all we need to do to recover the visual information is to put the

film back in the reference beam in its original position, and it will re-create the beam

as it came from the object.


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2.2.1. DIFFRACTION

Diffraction occurs whenever a wavefront encounters an obstacle. It shows itself as a

change in direction, a change in the shape of the wavefront.The iridescent coloring of

dragonflies, beetles and tropical butterflies, the 'eyes' in a peacock's tail feathers, the

flashing hues of a CD record, the lustre of a pearl, all are produced by diffraction. All

these objects contain orderly rows of microscopic scales, grooves or layers and they

cause light transmitted or reflected by them to be spread out into a spectrum; this is

how the colors are produced.

The simplest possible grating is called a cosine grating and if we pass a laser beam

through it three beams emerge One of these beams is just the undiffracted beam. The

other two, one each side, emerge at an angle which depends on the wavelength of the

light and on the spatial frequency of the grating. The angle is greater for longer

wavelengths and higher spatial frequencies.

2.2.2 RECONSTRUCTING THE OBJECT BEAM

The interference pattern produced by the reference beam and the object beam, as

recorded by a film, has also the transmitter profile of a cosine grating, though an

irregular one because of irregularities of the object-beam wavefront. So when we

direct the original reference beam onto the hologram it is also split in to the beams.

One is the undiffracted beam, which passes straight the hologram. The second is the

more diffracted beam. The third is the diffracted beam on the other side this is usually

feeble. Because there are irregularities in the grating, the main diffracted beam is

irregular too, but it is spread out purposefully, not randomly: it is a replica of the

object beam. If we put our eye into this beam and look through the-grating we will see

a view identical with that of the original subject, as if through a window.

The grating produced is much too fine a pattern to be recorded in ordinary films and

so there is a special ultra-high resolution film made for holography


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CHAPTER 3

PRINCIPLE

In TV holography, the object and reference beams of a speckle interferometry set up

is imagined by a CCD camera and the interference pattern is grabbed and stored as a

digital image in a computer. Two speckle interferometry patterns of the test specimen

are recorded in the initial state and after loading the object (IA,IB). The two images are

electronically processed to produce correlation fringes, which are contours of constant

phase changes corresponding to object movement. The deformation caused by defects

in the material produce fringe anomaly, since there will be stress/strain concentration

close to the defect. Even the subsurface and inner defects can generate a fringe

anomaly if the loading is sufficient.

The light intensity recorded in the image plane is a function of the reference and

object wave fronts and can be expressed as

IA = I1 + I2+ 2 I1 I2Cos (3.1)

where IA is the intensity distribution of the object in its initial state and is therandom phase difference between the waves.

If the object receives a static displacement, a phase change due to object

deformation is introduced into the wave front and the intensity distribution can be

expressed as

IB = I1 + I2+ 2 I1 I2Cos(+ ) (3.2)

The change in intensity due to the deformation can be obtained as

I = IA- IB =4 I1 I2Sin(+ /2)Sin /2 (3.3)

The image corresponding to I can be obtained by digital subtraction. The intensity of

images displayed in the monitor is proportional to I and is maximum when =


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(2n+l) and is minimum when =2n where n is an integer. This intensity change

will appear as dark and bright fringes on the monitor.

The fringes are formed due to the change in optical path length when the specimen is

deformed. The phase change due to deformation of specimen is given as

= 2 / X [w(1+Cos) + uSin] (3.4)

If the angle of illumination is small, equation is simplified to = 4w/, where w

is the out of plane displacement component.

When is equal and opposite, the in-plane displacement component is producing the

phase change and the corresponding fringe pattern.

= (4 uSin)/, where u is horizontal in plane displacement. (3.5)

= (4vSin)/, where v is vertical in plane displacement. (3.6)


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CHAPTER 4

EXPERIMENTAL SETUP

The experimental set up used for this study are shown in Fig. 4.1 and Fig.4.2 for the

out of plane and in-plane displacement configurations. These methods are selected,

since the setup is very compact and can be easily arranged for industrial application.

The light from a continuous wave He-Ne laser of 25 mW is split into two using a

plate beam splitter. One of the beams is used to illuminate the specimen and the other

is made to fall on the reference surface. The object is imagined onto a CCD camera,

which is connected to a host computer for processing the grabbed images. Fig. 4.1

shows the optical configuration sensitive to out of plane displacement.

Fig. 4.1 Optical Configuration for ESPI for out of plane displacement


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The optical configuration of TV Holography for in plane displacement is shown in

Fig.4.2. Here the two beams from the beam splitter is made to illuminate the specimen

at an equal and opposite angle to the optic axis.

Fig. 4.2 Optical Configuration for ESPI for in plane displacement

4.1. CONTINUOUS ACQUISITION OF SPECKLEGRAMS

The data flow diagram developed for continuous acquisition of images is shown in fig

4.3. The frame grabber card properly installed in the system is operated for enabling

the frame grabber item on the menu. This item on the frame grabber window has been

selected for displaying live video images. After controlling frame grabber parameters

such as grab size and input channel selection, the individual frames of the images are

grabbed.


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Fig. 4.3 Data flow diagram for continuous acquisition of images

The data flow diagram developed using different Imaging Graph operators is executed

for continuous acquisition of live images under different loading conditions. This

method of continuous acquisition is very effective for fixing the optimum load for

getting visible fringe pattern after subtraction. The start operator is used as the starting

point of an Imaging Graph, because most operators require at least one input. The

acquire operator grabs a currently stored image from the frame grabber. The number

of images for grabbing is decided by fixing the frames parameters. It is seen that if

frames are set to one, then a single image is the output. If the number of frames

selected is greater than one the sequence of images are grabbed and the output is

obtained as a vector sum of the images. In this case, the delay parameter sets the time

interval between images grabbed in sequence. The display operator creates a pop-up

frame containing the data of the object received at its input port. If the object is an

image, then the pop-up is presenting a rendered image. If the object is not an image,

then display will pop-up a frame with text showing all the pertinent data contained

within the object. The Charged Couple Device used is an interline-transfer colour

video camera module designed for RGB image processing. It has high performance

horizontal resolution of 570 TV lines with a signal to noise ratio of 58dB and

sensitivity of 2000 Ix. The video capture card is capable of digitizing live images at


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CHAPTER 5

RESULTS AND ANALYSIS

5.1. MECHANICAL LOADING

A circular disc of 40mm diameter has been used as the specimen fixed at its periphery

and subjected to a central mechanical load of 40mm. This loading condition has been

arrived by trial and error, in order to get sufficient fringe density to view the

interferogram. The speckle interferometry fringes are obtained by digital subtraction

of speckle grams before and after loading. The electronic subtraction is done using the

Image Compact software. Fig.5.1 shows the out of plane displacement fringes

corresponding to a defect free specimen after image processing.

Fig. 5.1 Speckle Interference Pattern for Defect free specimen - Mechanical Load

A tight crack of 5mm length is introduced in the specimen and repeated the

experiment. In this case the central loading required was only 10 mm to get the same

order of fringes as in the defect free case. Fig 5.3. Shows the results of defective

specimen after image processing. The location of crack is clearly visible as an


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anomaly in the fringe pattern. It was noticed that number of fringes increases from the

crack site due to larger displacement.

Fig. 5.2 Speckle Interference Pattern for Defective specimen - Mechanical Load

5.2. THERMAL LOADING

The experiment has also been carried out using thermal loading. The specimen is

heated using an IR lamp to produce a change in temperature about 10C above room

temperature. The fringes corresponding to the specimen in the initial condition and

after introducing a tight crack of 5mm length are given in Fig.4.3 and Fig.4.4. The

fringes seen outside the specimen dimension are spurious fringes.


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Fig. 5.3.Speckle Interference Pattern for Defect free specimen with Thermal Load

Fig. 5.4. Speckle Interference Pattern for Defective specimen with Thermal Load


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CHAPTER 6

CONCLUSION

The results of ESPI for NDE of low modulus materials used for rocket systems have

been presented. Experimental results show that out of plane ESPI configuration can

give an indication of even inner defects if they can influence the surface on loading.

This method is highly sensitive, whole field and real time. The result shows that the

technique as such can be applied to aerospace components such as solid rocket

propellant, composite material, graphite etc.


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REFERENCES

1. A. Mujeeb, V. R. Ravindran and V. U. Nayar, "Studies on TV holography for

the Non-destructive Evaluation (NDE) of space vehicle components", Proc.Fourteenth Kerala Science Congress, 2002. Pp 498-501.

2. Graham Saxby (1991), Manual of Practical Holography, Butterworth-Heinemann Ltd. Pp 4-17.

3. P.Hariharan (1996), Optical Holography Principles, techniques andapplications. 2

ndEdition, Cambridge University Press. Pp 163-177.

4. Applications of Holography, Plenum press, NY-London,1971. Pp 105-1155. A. Mujeeb, V.U. Nayar and V.R. Ravindran, A Method for Real Time and

Continuous Acquisition of Interferogram of ESPI for Non-Destructive

Evaluation , National Seminar on NDE , Hyderabad, 2006.

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