Laser Testing

7/31/2019 Laser Testing

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1

A SEMINAR

REPORT

ON

LASER TESTING



2

INDEX

Page

Abstract 3

Introduction 4

Commonly used optical techniques 5

Electronic speckle pattern interferometry 6

Holographic interferometry 8 Optical interfeometry 10

Profilometry 11

Shearography 13

Advantages 15

Disadvantages 15

Application 16

Conclusion 17

Bibliography 18



3

ABSTRACT

The term ‘laser testing’ may cause confusion because several other NDT

methods make use of lasers in their inspection process. Lasers consist of

coherent electromagnetic radiation at various wavelengths. When the

wavelength is in the infrared part, it‘s considered to have thermal properties.

Lasers are used to produce and discover acoustic waves in several complex

method ultrasonic testing. Ultrasonic laser techniques have been topic of

discussion for many years but are not widely accepted.

Lasers generally funtion in the visible part of the spectrum, and the finest group

of laser techniques(shearography,profilometry,and holography)are used for

surface testing. The term laser NDT is usually means shearographic and

profilometric methods. Shearography is used commonly for the detection of

dents in airplanes and profilometry is used for detection of deposits in heat

exchanger piping.



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Introduction

In the field of Production and servicing, quality control is of primary importance,and

the application of non – destructive methods is of great significance .Though there are

many sophisticated methods of non destructive testing like eddy currents,X-ray and

ultrasound testing ,these are time consuming and can only be useful to a small area.so

the necessity for more improved method came to being and laser was the feasible

solution.

Optical methods do not come in contact with the subject.Due to the low penetration

depth of the radiation,these are typically indirect methods,that means that unlike other

techniques ,light based won’t detect defects directly instead they detect the defect

induced temperature or the deformation during loading.

In light based methods,there is no physical contact between the optical source and the

test subject.This point helps using this method while the test object is in production

line itself.,when the material is in intermediate or molten state.

This method is a point technique,ie. where ever the device is pointed ,the area is

scanned.if the whole area is to be scanned ,multiple probes in parallel should be used.

One of the most important area ofapplication of laser testing is aerospace industry

where very light and complex part are present.



5

Commonly used optical techniques

Electronic speckle pattern interferometry

Holographic interferometry

Optical interferometer

Profilometer

Shearography



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Electronic speckle pattern interferometry

Electronic speckle pattern interferometry is sometimes called as TV holography. It is a

method which uses laser along with video detection, recording and processing for

imaging the stationary and active displacements of components relative to an optically

rough surface. The imaging produced is in the form of fringes in which individual

fringe shows the displacement of half the wavelength of light used.

ESPI can be effectively used for the measurement of stress ,strain and NDT testing.

ESPI looks comparable to holographic interferometry in many aspects but it’s entirely

different.



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ESPI WORKING

In order to conduct ESPI the test object should have an optically rough surface; the

intension of using rough surface is that when the rough surface is illuminated by a

laser beam, the image formed will be of subjective speckle pattern type. The laser light

reaching a point in the speckled image is scattered from a limited area of the test

object. The phase, amplitude and intensity of the light depend on the microstructure of

material at that area.

Another light called the second light field or the reference beam is produced from the

same laser beam and is superimposed on the image produced by the video camera. The

two light fields produced interfere with each other and produce a light field which has

got a random phase, amplitude and intensity, and there is a speckle pattern. When

some type of deformation occurs to the body, there will be change in distance between

the object and the image, therefore the speckle pattern also alters. The relative phase

between the object beam and reference changes, hence the intensity of combined field

also gets changed. In cases where the phase of change is 2*3.14 or its multiples, the

relative change between the two fields remains unchanged. The overall intensity of the

image also remains unchanged.

To visualise this change the reference and image beam is combined by a video camera

and the images are recorded. Then the object is deformed from its initial position .the

new image produced is subtracted point by point from the initial image. The resultant

image produced is speckle pattern along with fringes indicating contours of 2*n*3.14.

http://en.wikipedia.org/wiki/File:ESPI.jpg



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Holographic Interferometry

Holographic interferometry is a method in which the dynamic and the static

displacements of the objects with an optically rough surface is measured to optical

interferometric precision. These measurements can be used for the measurement of

stress, strain and vibration and can be well applied in non-destructive testing. It can be

used for detecting optical path variations in the case of transparent materials, this

property helps in visualizing and analyzing the fluid flow. This can also be used for

producing the form of the surface.

Holography helps to record and replay the light field scattered from the objects .when

the recorded field is superimposed on the live field scattered from the test objects the

two fields will be identical but if some deformation is given to the object the fields

won’t be identical and this relative phase change can be used to measure the

interference .This method of measuring the interference is called live interferometry.

Another method of holography is the fringe holography in this fringe is obtained by

building two recordings of the scattered light by using a single recording medium. The

produced light fields can then produce fringes which can be used to understand the

displacements of the surface.

The fringe model depends on the changes in the surface position and air compaction.

Many methods of analysing such patterns automatically have been developed in the

recent years.



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11

Profilometer

Profilometer is device that is used for calculating a surface’s profile,i.e surface

roughness. Profilometer is used to measure the vertical depth of the material along a

specified horizontal length. The use of this method includes measuring depth, surface

roughness and deposited film thickness. The obtained results can be easily displayed

on a printable graphical interface.

Unlike surface profilometers or stylus profilometers optical profilometers are non

contact instruments and provides sufficient information about the material surface

texture. An optical profilometer can be effectively used to measure wear analysis,

fingerprint, material analysis and surface finish. Optical profiliometer can be

effectively be applied on paper, plastics, paint, metals etc. Another important merit of

using optical profilometer is the scan speed. Scan speed are not restricted in the case of

optical profiliometer, other merits include less surface wear and less maintenance.



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Common classification of profilometer is

Contact profiliometer

Non contact profiliometer



13

Shearography

Shearography is an optical measuring technique which uses coherent light for the

interferometric inspection of optical and technical surfaces. The test object to be tested

is illuminated by the use of laser and the object is imaged on a CCD camera (TV

shearography) by the use of an optical shearing element. The shearing elements

provide coherent superposition of 2 different laterally displace images of the test

object on the image sample. The lateral displacement is produced on the plane is called

the shear of the images, the superposition of the two images is termed as shearogram

and it’s the interferogram of the of the object wave and sheared object wave which is

the reference wave.

Two images are taken for the test object under different loading conditions the purpose

of loading is that the loading should induce some deformation into the test object or

should alter the deformation state on the surface of the material. The common loadings

provided are mechanical, thermal or acoustical. The load can be applied to the objectin the dynamic or static way.

The absolute difference between the two shearograms produced during different

laoding conditions on the test object produce an interference fringe pattern which is

related to the change in the deformation pattern. The image thus produced is then

termed as D-image. Unlike holographic interferometry the fringes produced in the

shearography shows the slope of the deformation. The defects inside the body may

interfere the deformation produced on the surface of the material and thus cause

disturbances in the loading fringes this permits the detection and classification of

defects based on the shearographic images produced.

Shearography is a non contact, non destructive testing method and have wide

application in the manufacturing industry especially in the case of composites and



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bonded structures. Shearography is an optical strain method because the strain

produced during the loading process is used to know the defects in the materials.

Another merit of laser shearography is that this method does not require any surface

preparation.

Thermography can be applied only to a small area , holography is highly sensitive to

surrounding disturbances. These demerits could be overcome by the usage of

shearography .some of the other main advantages

Non contact method

Non-destructive method

Less sensitive to external noise.



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Advantages

Effective

Less time consuming

Can be used along with production process

High speed scan

High resolution image

Less Cost

Disadvantages

Skilled labour

Complex parts

Sensitive to external noise



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Application

Space application

Automobile fields

Used in wind power

Used in aerospace industry



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Conclusion

Laser testing in NDT is gaining acceptance worldwide. Though it will take some time

for laser testing to get accepted as conventional methods such as Ultrasonic, X – ray

and eddy current inspections. It is much faster process and can scan a large area in a

small time period. The method is serving its primary purpose of NDT, detecting

structural properties, defect detection. They use more tactile techniques. Shearography

has advantages such as excellent lateral resoultion and good penetration depth.

Shearography testing is gaining huge importance in the field of aviation, with

components of lightweight or component structures.A better defect visibility can be achieved by using better image processing and

examination methods.



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Bibliography

Wikipedia-NDT methods

NDT-lasers.com Industrial-lasers.com

Intopsys.com

Laser Testing

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