bmfb 4283 lfw - 5a
TRANSCRIPT
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BMFB 4283
NDT & FAILURE ANALYSIS
Lectures for Week 5
Prof. Qumrul Ahsan, PhDDepartment of Engineering Materials
Faculty of Manufacturing Engineering
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RADIOGRAPHIC TESTING
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5.0 Radiology/Radiography5.1 Introduction to X-rays and Gamma Rays5.2 Radiation Fundamentals5.3 Equipment and Testing5.4 Techniques and Application5.5 Radiation Safetyb
Issues to address
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Introduction
This module presents information on the NDTmethod of radiographic inspection or radiography.
Radiography uses penetrating radiation that isdirected towards a component.
The component stops some of the radiation. Theamount that is stopped or absorbed is affected bymaterial density and thickness differences.
These differences in absorption can be recorded
on film, or electronically.
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Outline
Electromagnetic Radiation
General Principles of
Radiography
Sources of Radiation Gamma Radiography
X-ray Radiography
Imaging Modalities Film Radiography Computed Radiography Real-Time Radiography
Direct Digital Radiography Computed Radiography Radiation Safety Advantages and Limitations Glossary of Terms
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Electromagnetic RadiationThe radiation used in Radiography testing is a higher energy (shorterwavelength) version of the electromagnetic waves that we see every day.Visible light is in the same family as x-rays and gamma rays.
X-rays and gamma rays differ only in theirsource of origin.X-rays are produced by an x-raygenerator and gamma radiation is theproduct of radioactive atoms.They are both part of the
electromagnetic spectrum. They arewaveforms, as are light rays, microwaves,and radio waves.They can be diffracted (bent) in a mannersimilar to light.
Properties of X-Rays and Gamma RaysThey are not detected by human senses (cannot be seen, heard, felt, etc.).They travel in straight lines at the speed of light.Their paths cannot be changed by electrical or magnetic fields.They can be diffracted to a small degree at interfaces between two different materials.They pass through matter until they have a chance encounter with an atomic particle.Their degree of penetration depends on their energy and the matter they are travelingthrough.
They have enough energy to ionize matter and can damage or destroy living cells.
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General Principles of Radiography
Top view of developed film
X-ray film
The part is placed between the radiation sourceand a piece of film. The part will stop some of theradiation. Thicker and more dense area will stopmore of the radiation.
= more exposure
= less exposure
The film darkness (density) willvary with the amount ofradiation reaching the filmthrough the test object.
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General Principlesof Radiography
The energy of the radiation affects its penetrating power.Higher energy radiation can penetrate thicker and moredense materials.
The radiation energy and/or exposure time must be
controlled to properly image the region of interest.
Thin Walled Area
Low Energy Radiation High Energy Radiation
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IDL 2001
Radiography hassensitivity limitationswhen detectingcracks.
X-rays see a crack as a thickness variation and the larger thevariation, the easier the crack is to detect.
OptimumAngle
Flaw Orientation
= easytodetect
= not easyto detect
When the path of the x-rays is not parallel to a crack, the thickness variation isless and the crack may not be visible.
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IDL 2001
0o 10o 20o
Since the angle between the radiation beam and a crack or other lineardefect is so critical, the orientation of defect must be well known ifradiography is going to be used to perform the inspection.
Flaw Orientation (cont.)
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Radiation Sources
Two of the most commonly used sources of radiation inindustrial radiography are x-ray generators and gamma raysources. Industrial radiography is often subdivided intoX-ray Radiography or Gamma Radiography, dependingon the source of radiation used.
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Gamma Radiation Gamma radiation is one of the three types of natural radioactivity.
Gamma rays are electromagnetic radiation, like X-rays. The other two
types of natural radioactivity are alpha and beta radiation, which are in
the form of particles. Gamma rays are the most energetic form of
electromagnetic radiation, with a very short wavelength of less than
one-tenth of a nanometer
Alpha ParticlesCertain radionuclides of high atomic mass (Ra226, U238, Pu239) decay by the emission of alpha
particles (two neutrons and two protons each).Alpha particles are emitted with discrete energies characteristic of the particular transformationfrom which they originateBeta ParticlesA nucleus with an unstable ratio of neutrons to protons may decay through the emission of a highspeed electron called a beta particle.This results in a net change of one unit of atomic number (Z).Beta particles have a negative chargeGamma-raysA nucleus which is in an excited state may emit one or more photons (packets of electromagneticradiation) of discrete energies.The emission of gamma rays does not alter the number of protons or neutrons in the nucleus butinstead has the effect of moving the nucleus from a higher to a lower energy state (unstable to
stable).
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Activity (of Radionuclides) The quantity which expresses the degree
of radioactivity or the radiation producing
potential of a given amount of radioactivematerial is activity.
The curie was originally defined as that
amount of any radioactive material that
disintegrates at the same rate as one
gram of pure radium. (as a quantity ofradioactive material in which 3.7 x 1010
atoms disintegrate per second.)
The International System (SI) unit for
activity is the Becquerel (Bq), which is
that quantity of radioactive material inwhich one atom is transformed per
second.
Radioactivity is expressed as the number
of curies or becquerels per unit mass or
volume.
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Isotope Decay Rate (Half-Life)
Each radionuclide decays at its own
unique rate which cannot be altered byany chemical or physical process.
Half-life is defined as the time required for
the activity of any particular radionuclide
to decrease to one-half of its initial value. one-half of the atoms have reverted to a
more stable state material.
Half-life of two widely used industrial
isotopes are 74 days for iridium-192, and5.3 years for cobalt-60.
i i
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Ionization As penetrating radiation moves from point to
point in matter, it loses its energy through
various interactions with the atoms it
encounters.
The rate at which this energy loss occursdepends upon the type and energy of the
radiation and the density and atomic
composition of the matter through which it is
passing.
The term "excitation" is used to describe an interaction where
electrons acquire energy from a passing charged particle but are not
removed completely from their atom. Excited electrons may
subsequently emit energy in the form of x-rays during the process ofreturning to a lower energy state.
The term "ionization" refers to the complete removal of an electron
from an atom following the transfer of energy from a passing charged
particle.
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Gamma Radiography
Gamma rays are produced bya radioisotope.
A radioisotope has anunstable nuclei that does not
have enough binding energyto hold the nucleus together.
The spontaneous breakdownof an atomic nucleusresulting in the release of
energy and matter is knownas radioactive decay.
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Gamma Radiography (cont.)
Most of the radioactivematerial used in industrialradiography is artificiallyproduced.
This is done by subjectingstable material to a sourceof neutrons in a specialnuclear reactor.
This process is calledactivation.
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Gamma Radiography (cont.)
Unlike X-rays, which are producedby a machine, gamma rays cannotbe turned off. Radioisotopes usedfor gamma radiography are
encapsulated to prevent leakage ofthe material.
The radioactive capsule is attached toa cable to form what is often called a
pigtail.The pigtail has a special connector atthe other end that attaches to a drivecable.
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Gamma Radiography (cont.)
A device called a camera is used to store, transport and
expose the pigtail containing the radioactive material. The
camera contains shielding material which reduces the
radiographers exposure to radiation during use.
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Gamma Radiography (cont.)
A hose-like device called
a guide tube is connected
to a threaded hole called
an exit port in the
camera.
The radioactive material
will leave and return to
the camera through thisopening when
performing an exposure!
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Gamma Radiography (cont.)
A drive cable is connected to the
other end of the camera. Thiscable, controlled by theradiographer, is used to force theradioactive material out into theguide tube where the gamma rayswill pass through the specimen andexpose the recording device.
X R di ti
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X-ray Radiation
X-ray tubes produce x-ray photons by
accelerating a stream of electrons to
energies of several hundred kilovoltswith velocities of several hundred
kilometers per hour and colliding
them into a heavy target material.
The abrupt acceleration of thecharged particles (electrons)
produces Bremsstrahlung photons.
X-ray radiation with a continuous
spectrum of energies is produced
with a range from a few keV to a
maximum of the energy of the
electron beam.
Target materials for industrial tubes
are typically tungsten.
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X-ray Radiography
Unlike gamma rays, x-rays are produced by an X-raygenerator system. These systems typically include an X-raytube head, a high voltage generator, and a control console.
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X-ray Radiography (cont.)
X-rays are produced by establishing a very high voltagebetween two electrodes, called the anode and cathode.
To prevent arcing, the anode and cathode are locatedinside a vacuum tube, which is protected by a metalhousing.
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X-ray Radiography (cont.)
The cathode contains a smallfilament much the same as in a lightbulb.
Current is passed through thefilament which heats it. The heatcauses electrons to be stripped off.
The high voltage causes these freeelectrons to be pulled toward atarget material (usually made oftungsten) located in the anode.
The electrons impact against thetarget. This impact causes an energyexchange which causes x-rays to becreated.
High Electrical Potential
Electrons
-+
X-ray Generatoror RadioactiveSource CreatesRadiation
Exposure Recording Device
RadiationPenetratethe Sample
I S L
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Inverse Square Law Any point source which spreads its
influence equally in all directions
without a limit to its range will obeythe inverse square law.
The intensity of the influence at any
given radius (r) is the source
strength divided by the area of thesphere.
a point radiation source can be
characterized by the diagram above
whether you are talking aboutRoentgens, rads, or rems.
All measures of exposure will drop off by the inversesquare law. For example, if the radiation exposure is 100mR/hr at 1 inch from a source, the exposure will be 0.01mR/hr at 100 inches.
I i B P i R di i d M
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Interaction Between Penetrating Radiation and Matter
When x-rays or gamma rays are directed into
an object, some of the photons interact with
the particles of the matter and their energy
can be absorbed or scattered.
This absorption and scattering is called
attenuation.
Other photons travel completely through the
object without interacting with any of the
material's particles. The number of photons transmitted through
a material depends on the thickness, density
and atomic number of the material, and the
energy of the individual photons.
For a narrow beam of mono-energetic photons, the change in x-ray beam intensity atsome distance in a material can be expressed in the form of an equation as:Where
:I = the intensity of photons transmitted across some distance x
I0 = the initial intensity of photons
s = a proportionality constant that reflects the total probability of a photon being scattered or
absorbed
= the linear attenuation coefficientx = distance traveled
Half Value Layer
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Half-Value Layer The thickness of any given
material where 50% of the
incident energy has been
attenuated is know as the half-value layer (HVL).
The HVL is inversely proportional to the attenuationcoefficient. If an incident energy of 1 and a transmittedenergy is 0.5 is plugged into the equation it can beexpressed as
Approximate HVL for Various Materials when Radiation isfrom a Gamma Source
Half-Value Layer, mm (inch)
Source Concrete Steel Lead Tungsten Uranium
Iridium-192 44.5 (1.75) 12.7 (0.5) 4.8 (0.19) 3.3 (0.13) 2.8 (0.11)
Cobalt-60 60.5 (2.38) 21.6 (0.85) 12.5 (0.49) 7.9 (0.31) 6.9 (0.27)
Geometric Unsharpness
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Geometric Unsharpness Geometric unsharpness refers to the loss of
definition that is the result of geometric factors
of the radiographic equipment and setup.
It occurs because the radiation does not
originate from a single point but rather over an
area.
The three factors controlling unsharpness are source size, source to object distance,
and object to detector distance.The source size is obtained by referencing manufacturers specifications for a given X-ray or gamma ray source.Industrial x-ray tubes often have focal spot sizes of 1.5 mm squared but microfocussystems have spot sizes in the 30 micron range.
Geometric Unsharpness
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Geometric Unsharpness
For the case, such as that shown
to the right, where a sample of
significant thickness is placed
adjacent to the detector, the
following formula is used to
calculate the maximum amount
of unsharpness due to specimenthickness:
Ug = f * b/a
Where
f = source focal-spot sizea = distance from the source to
front surface of the object
b = the thickness of the object
Geometric Unsharpness
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For the case when the detector is
not placed next to the sample,such as when geometric
magnification is being used, the
calculation becomes:
Ug = f* b/a
Where,
f = source focal-spot size.
a = distance from x-ray source to
front surface of material/object
b = distance from the front
surface of the object to the
detector
Geometric Unsharpness
Filters in Radiography
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Filters in Radiography At x-ray energies, filters to absorb the lower-energy x-ray photons emitted
by the tube before they reach the target.
The use of filters produce a cleaner image by absorbing the lower energy x-
ray photons that tend to scatter more.
The total filtration of the beam includes the inherent filtration (composed of
part of the x-ray tube and tube housing) and the added filtration (thin sheets
of a metal inserted in the x-ray beam).
Filters are typically placed at or near the x-ray port in the direct path of the
x-ray beam.
Placing a thin sheet of copper between the part and the film cassette has
also proven an effective method of filtration.
For industrial radiography, the filters added to the x-ray beam are most
often constructed of high atomic number materials such as lead, copper, orbrass.
The thickness of filter materials is dependent on atomic numbers,
kilovoltage settings, and the desired filtration factor.
Gamma radiography produces relatively high energy levels at essentially
monochromatic radiation, therefore filtration is not a useful technique andis seldom used.
Secondary (Scatter) Radiation
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Secondary (Scatter) Radiation Secondary or scattered photons create a loss of
contrast and definition.
Secondary radiation striking the film reflected from
an object in the immediate area
Control of side scatter can be achieved by moving
objects in the room away from the film, moving the
x-ray tube to the center of the vault, or placing a
collimator at the exit port.
Backscatter when it comes from objects behind the
film. Industry codes and standards require a lead letter
"B" be placed on the back of the cassette to verify
the control of backscatter.
If the letter "B" shows as a "ghost" image on
the film, a significant amount of backscatter
radiation is reaching the film.
The control of backscatter radiation is achieved
by backing the film in the cassette with a sheet
of lead that is at least 0.010 inch thick.
It is a common practice in industry to place a
0.005" lead screen in front and a 0.010" screen
Radiation Undercut
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Radiation Undercut Parts with holes, hollow areas, or
abrupt thickness changes are likely
to suffer from undercut if controlsare not put in place.
Undercut appears as a darkening
of the radiograph in the area of
the thickness transition.
This results in a loss of resolution or blurring at thetransition area. Undercut occurs due to scattering within the film.
The faster the film speed, the more undercut that is likely tooccurMasks are used to control undercut.
Sheets of lead cut to fill holes or surround the partMetallic shot and liquid absorbers are often used as masks.