laser testing
TRANSCRIPT
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A SEMINAR
REPORT
ON
LASER TESTING
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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
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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.
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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.
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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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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
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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