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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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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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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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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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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.


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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


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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


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.

BMFB 4283 LFW - 5a

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