ASNT RT LEVEL II

ASNT : American Society for Nondestructive TestingRT : Radiographic Testing

Examination

Min Questions Min Passing Marks Average Passing MarksGeneral 40 70%

Specific 20 70% 80%

Practical 10 70%

Basic Principle of Radiographic Testing [ Figure 1,2 & 3]

Nondestructive testing

Nondestructive testing is a method of testing materials without destroying them. The material after inspection does not change in its size, shape, physical and chemical properties.

Basic principle of radiographic testing

Basic principle of radiographic testing is shown in figure. The transmitted radiation after passing through the object, is recorded on an x ray film. The image of an object produced on an x ray film shows optical density variation depending on the internal structure of the object.

Basic Radiation Physics

All materials in the universe are made of different elements. Ex. Hydrogen, oxygen, iron…

Atom

Atom is the smallest part of an element. An atom has a positively charged central portion called nucleus. It also has negatively charge electrons going around the nucleus in different orbits. The nucleus has two types of particles protons which are positively charged and neutrons which do not carry any charge. An atom is electrically neutral as the number of protons in its nucleus is equal to the number of electrons in the orbit.

Structures of some atoms are shown in figure 4. The number of protons in an atom is known as its atomic number Z. Every atom of particular element has definite number of protons eg. Aluminum has 13, cobalt has 27, iridium has 77. the mass of an atom is almost completely concentrated in the nucleus, as electrons have negligible mass. A proton or a neutron is about 1800 times heavier than an electron. Mass number A of an atom is the number of protons Z plus the number of neutrons N in its nucleus. A = N + Z

Isotope

Any atom having same number of protons Z, but having different number of neutrons N is called an isotope of the element. Isotopes of hydrogen are shown in figure 5

Radioisotopes

An unstable nucleus becomes stable by emitting radiations such as alpha particle, beta particle. This process is known as radioactivity. Therefore unstable isotopes are known as radioactive isotopes [ radioisotopes ]

Alpha particle

Alpha particle consists of 2 protons and 2 neutrons. It is positively charged. Alpha particles do not penetrate much in any material and they can be stopped by a sheet of paper or a thin foil of aluminum 0.05 mm. the nucleus which gives out alpha particle

loses two neutrons and two protons, the mass number decreases by four and the atomic number decreases by two.

Example

88Ra 226 ----------- 86 Rn 222

Beta particle

Beta particle is an electron. It has negative charge. After the emission of beta particle, the atomic number increases by one. But the mass number does not change. Beta radiations from a radioisotope have different energies from zero upto a maximum value. The beta radiation are more penetrating than alpha radiations ut beta radiations could be stopped by a few millimeters of aluminum. 5mm.

Example

15 P 32 ------- 16 S 32

Gamma radiation

If after the emission of a alpha or beta radiation, the nucleus has some extra energy , gamma radiation is emitted. Gamma radiation is highly penetrating and is similar to x ray in its properties. Hence gamma radiations are generally used for industrial radiography.

Property of gamma and x ray.

They are electromagnetic radiations like visible light.They travel at the speed of 300000 km per second.They affect photographic films.X rays have continuous energies, whereas gamma rays have discrete energies.They can be scattered and reflected.They can penetrate material, penetration is less if a. ] the absorber thickness is more, b] absorber atomic number is higher and c] absorber density is higher.They can damage living tissues.They can also cause ionization and excitation in the medium through which they travel

Naturally occurring radioisotopes &Artificially made radioisotopes

Potassium 40 , carbon 14, uranium 238 are some of the naturally occurring radioisotopes. Many other elements can be made radioactive by bombarding them with charged

particles and neutrons. These are called artificially made radioisotopes. Cobalt 60, iridium 192, cesium 137, are some of the artificially made radioisotopes.

Production of radioisotopes. : there are three methods of producing artificial radioisotopes.

1. activating elements with neutrons in a nuclear reactor2. processing the fission products from spent Uranium fuel rods from a nuclear

reactor3. bombarding elements with charged particles from a cyclotron or similar

accelerators.

Radioactivity :

Becquerel [Bq.] : One Becquerel corresponds to one disintegration per second. Formerly, the unit of activity was Curies [ Ci ] corresponding to 3.7 x 1010

Disintegrations per second.Specific Activity : Ci / gm

Energy of radiations :Energies of various radiations are usually specified in electron volts. One electron volt is the energy given to a single electron as it moves across a potential difference of one volt.

1 eV = 1 electon volt1 KeV = 1 kilo electron volt1 MeV = 1 million electron volt

Radioisotope Half life Main Gamma Energy [MeV]

Specific activity

Cobalt 60 5.27 years 1.17 & 1.33 MeV[Average 1.3 MeV]

Up to 150

Iridium 192 74.5 days 0.24 - 0.67 MeV[Average 0.5 Mev]

Up to 500

Thulium 170 [Tm 170] 129 days 0.08 and 0.05 Mev Up to 22Cesium 137 [ Cs 137] 30.1 years 0.66 Mev

Half life

As the time passes, a given radioisotope continues to disintegrate and hence, its activity reduces.The half life of a radioisotope is the duration in which its activity reduces to one half of its initial value. [ see the figure ]

A x 1/2n

I = Io e-µt

I = activityIo = Initial activityµ = linear disintegration coefficient [µ = 0.693/half life]t=time

Gamma Radiography Equipment

X ray technology

X rays

They are electromagnetic radiations like visible light.They travel at the speed of 300000 km per second.They affect photographic films.X rays have continuous energies, whereas gamma rays have discrete energies.They can be scattered and reflected.They can penetrate matter, penetration is less if a. ] the absorber thickness is more, b] absorber atomic number is higher and c] absorber density is higher.They can damage living tissues.They can also cause ionization and excitation in the medium through which they travel

X and Gamma rays have similar properties. Gamma rays are emitted by the nucleus, whereas x rays are generated outside the nucleus when high speed electrons interact with atoms. Gamma rays have definite, discrete energies, whereas x rays have continuous energy . the maximum energy of x rays depends on the incident electron energy

Advantages : x ray machines have higher radiation output they have small focal spot size, which gives sharper images. The use of x rays results in better image contrast due to its continuous spectrum X ray units ensure complete radiation safety, when the unit is switched off.

Disadvantage : They are bulky, for use at complicated and inaccessible locations. They require electric power for operation. They require high capital investment.

Production of x rays

X rays are produced when a beam of high energy electrons collides with any material.The essential requirements for the production of x rays are

A source of electrons [ heated tungsten filament]o Current is applied to filament and free electrons clouds are generated due

to heating of filament and this works for a source of electrons. High voltage supply to accelerate the electrons A target, usually tungsten, to stop the electrons and to convert their energy to x

rays.o Selection of the target material is based on the following properties.

It should have a high melting point. It should possess a high atomic number. It should possess high thermal conductivity [ to dissipate the heat

quickly]

X ray production increases with increase in atomic number of the target atom and with increase in the incident electron energy. In a x ray machine, less than one per cent of the electrical power supplied is converted to x rays and the remaining power appears as heat if this heat is not removed efficiently the target material may melt. The target is usually of small dimension, say 2-3mm. when the electrons hit the target, much of the energy

appears in the form of heat and it has to be rapidly removed. Copper is used for the purpose of heat removal. certain anodes are hollow in construction, so that primary coolants can be circulated through the same to remove the generated heat. Mineral oil is also sometimes used around the x ray unit, to remove heat and serve as electrical insulator.

The penetration of x ray beam depends on the applied kilovoltage, whereas, the intensity is decided by the current flowing through the filament [ miliampearage]

In an x ray machine, if the potential difference between the filament and the target or the applied kilovoltage is 200 kv, then the energy of the electrons hitting the target is 200 kev and the maximum energy of the x rays would be 200 kev.

Interaction of radiation with matter

Photoelectric effect

Photoelectric effect : is an interaction between a gamma ray [ photon] and an orbital electron. The gamma radiation looses complete energy to the electron and the electron is removed from its orbit. This electron is called photoelectron. This electron being a charged particle gets absorbed in the medium. it increases with increase in atomic number of the object.. hence, higher atomic number materials, like lead, uranium, tungsten, etc.

absorb gamma radiations better than lower atomic number materials, like plastic , aluminum, iron etc.

Compton effect :

Compton effect : gamma radiation interacts with a free electron [ electron of external orbit] . The gamma radiation gives a part of its energy to the electron and travels in a different direction [gets scattered] with decreased energy.

Half value thickness [ HVT] : the thickness of the material, needed to reduce the intensity of radiation to half its initial value is known as the half value thickness.

• Density, Atomic number and thickness increases absorption increases and penetration decreases.

TVT = Tenth Value Thickness

1 TVT = 3.3 HVT

Radioisotopes Material (mm)Concrete Steel Lead

HVT TVT HVT TVT HVT TVTIridium 192 46 140 12.5 40 5 16Cobalt 60 66 218 20 66 12 40X Rays100 kV 16 55 1.2 3.8 0.25 0.84150 kV 22 70 2.3 7.6 0.29 0.96200 kV 26 86 3.8 12.7 0.42 1.4250 kV 28 90 5.4 17.7 0.86 2.9300 kV 30 100 6.5 21.5 1.7 5.7

Io x 1/2n

I = Io e -µt

I = IntensityIo = Initial Intensityµ = linear absorption coefficient [µ = 0.693/HVT ]t= thickness

Safety Calculations

• Radiation effect– Certain effect

– Uncertain effect

Time

Distance [ I ∞ 1/d2 ] I1d12 = I2d2

2

Shielding [ HVT, TVT ]

Construction of film

Latent image formation

Screens

When an object is radiographed without using screens, there are two disturbing factors, There is lesser absorption of transmitted radiation intensity in film, which results

in longer exposure time. Low energy scattered radiation is absorbed more, which spoils the image quality.

Hence, to reduce exposure time and improve image quality of radiographs, radiography screens are used.

Types of radiography screens : there are two types of radiography screens, commonly used in industrial radiography practice, metallic foil screens and fluorescent screens,

Metallic foil screens

Metallic screens help in reduction of scattered radiation from the object. The x ray film is sandwiched between a pair of lead screens.

Reduction of scattered radiation : absorption of incident radiation in the object results in production of low energy scattered radiation and this falls on the film, along with the transmitted primary radiation. As high atomic number elements have more absorption capacity for low energy radiations, if a metal foil of high atomic number is placed between object and film, it will absorb most of the low energy radiation, before it falls on the film. Hence, effect of the low energy, scattered radiation will be reduced. This metallic foil will serve additional purpose of intensification action also. Similarly, to reduce low energy back scattered radiation a high atomic number sheet is placed behind the film. Hence, a double coated h ray film is sandwiched between a pair of metallic foil

screens to reduce the effect of scattered radiation on the film, thus resulting in an improved definition of radiograph.

Intensifying action : intensifying action of a radiography screen is expressed in terms of intensification factor which is defined as

IF = exposure time required to produce a certain film density without screen Exposure time required to produce same density with screen

Intensification action occurs due to the photoelectrons, emitted as a result of interaction of x or gamma rays with the metallic foil.

Salt or fluorescent screens : intensifying action of a fluorescent screen is due to additional glow appearing as a result of interaction of x or gamma rays with the salt screen material. A material which has its light emission in blue or ultraviolet region, is used for this purpose, since the film is highly sensitive to blue light. The most commonly used material for this purpose is calcium tungstate.

During x or gamma ray exposure, each crystal in salt screen emits light and this light diverges in all directions and gives image unsharpness. So these screens give poor quality image. Hence these screens are not much used in industrial radiography.

Handling of films : X ray films should be handled carefully to avoid physical strains such as pressure

and friction on it. Films should be held by edges in order to prevent finger or pressure marks in

loading and unloading process. The expiry date on a film pack means that the film should be used before the

given date. X ray film should be protected from x rays and gamma rays to avoid fogging. Film should not be kept in a chemical storage room or in any location where there

is leakage of chemical vapours, high humidity and in high temperature.

Films may be stored for long periods in a cool place like refrigerator.

Film processing :

After exposure to the radiation, a latent image of the object is formed in the x ray film. The purpose of the film processing is to convert this invisible image to visible and permanent image. There are five main stages involved in film processing.

1. development, creation of the visible image.2. stop bath, removal of unwanted developer products.3. fixation, removal of unwanted, unexposed silver bromide.4. washing, removal of unwanted fixation products.5. drying, removal of unwanted water.

Latent image formation : on exposure to x or gamma radiation, the silver bromide in the films is converted into metallic silver which is black in colour.

Development : the function of developer solution is to reduce only exposed grains of silver bromide into metallic silver and leave unexposed, unaffected grains.

The electrons for the reaction is donated by developer solution.

Stop bath : after developing, if no action is taken, the developer will continue to work producing high density, fog or irregular developer stains. The unwanted developer is removed with stop bath.there are two types of stop baths generally employed.

1. water rinse : the excess unwanted developer is removed from the film simply by immersing it in a running water bath.

2. acetic acid rinse : this bath is static and 2 to 3 per cent solution of acetic acid is employed. This is more efficient in action and required films to be immersed in it for about 5 seconds with agitation.

Fixing bath : the fixer reacts with undeveloped silver bromide in the film and transforms it into water soluble compound which gets dissolved in washing operation. In addition, it enables the image to become permanent. since the silver bromide is still sensitive to light to a small extent, and so will slowly darken if it is allowed to remain on the film.

Washing and drying :washing and drying is carried out to eliminate unwanted products from the emulsion after the fixing operation. The excess water is then removed by drying, since a wet radiograph is not convenient to handle and easily got damage.

Characteristics of films :

Film Density : Blackening of the film is called optical density [D] and is given by the expression,

D = Log [ Io/It]

Here Io = Intensity of incident light It = Intensity of transmitted light

Film density is measured by an instrument called Densitometer.

The factors which control the optical density are : type of film energy of radiation amount of exposure and processing conditions

Geometry of image formation : the radiographic image on the film is governed mainly by basic principles of the shadow formation a sharp and undistorted image is obtained by controlling various geometrical factors such as source size, source to film distance, film to object distance alignment of radiation beam with respect to the plane of the object and film.

Both x and gamma rays produce a certain unsharpness on the radiograph due to the finite size of the focal spot or the source. This unsharpness is called geometrical unsharpness [Ug] which is calculated as

Ug = f x t / d

Where, f = effective diameter of the source / focal spot t= object to film distance d= source to object distance

SFD = Source to film distance SFD = d + t

It is clear from this expression , that Ug can never be zero. The magnitude of Ug can only be minimized by proper selection of exposure variables. Considering this fact, various codes permit a certain Ug value related to object thicknesses.

Recommendations for optimum results are

1. source or focal spot size should be as small as practicable.2. source to object distance should be as large as possible3. the film should be in close contact with object and4. source location should be such, that radiation pass normal through object

thickness.

Radiographic contrast : is the density difference in two areas of a radiograph. It depends upon the subject contrast and film contrast.

Subject contrast : is defined as the ratio of x or gamma ray intensities transmitted through two different areas of a specimen. It depends upon the radiation quality, intensity distribution and scattered radiation. Higher the radiation energy lower would be the subject contrast.

Film contrast : refers to the slope of characteristic curve of the film at a given density. It depends upon the type of film. Processing conditions, and optical; density. It is independent of radiation quality.

Film contrast or gradient is defined as the change in density recorded on a film for a given change in radiation intensity.

The slope of the characteristic curve at a given density is the measure of the film gradient, Gd at that density.

Slope of straight line joining two points on the curve is called average gradient of a film. It can be expressed as

Gd = ∆D [Density difference] / ∆ V [ log relative exposure]

Film Speed : a film which requires less exposure to produce required density is faster.

Film factor : the amount of exposure of a given radiation energy required on a film to produce a given film density [ usually 2.0] under specified conditions of processing is called film factor.

Film Graininess : fine grain films have lower graininess and speed but higher film contrast at a given density, and coarse grain film have higher graininess and speed but lower film contrast at a given density.

Exposure time calculations :

Radiographic Sensitivity

Radiographic Sensitivity is a general qualitative term referring to the size of the smallest detail which can be seen on a radiograph. It is governed by the contrast and definition of the image appearing on the radiograph.

Image quality indicators [ IQI ]

The quality of radiograph is expressed in terms of IQI or penetrameter sensitivity. These are small devices placed on the surface of the specimen during exposure and visibility of their image on the radiograph, forms the criteria of sensitivity.

Quality levels of IQI

1-1T 2-1T 4-1T

1-2T 2-2T 4-2T

1-4T 2-4T 4-4T

n = IQI thickness shown in % of job thickness.

Wire Type IQI [ ASTM / DIN ]

Placement of IQI

Radiographic Techniques.

Double wall single image, pipe dia. More than 3.5 in.

Double wall double image, pipe dia. less than 3.5 in.

Defect Indication

Weld defects :

Arc strike [ due to localized heat ]

Burn through [ coalescence of weld metal beyond root.]

Cold cracks [ rupture of metal under stress below 400ºF]

Hot cracks [ rupture of metal under stress above 400 ºF, usually associated with grain boundary]

Incomplete fusion [ improper fusion between weld and parent metal usually with edge or between two passes ]

Incomplete joint penetration [ failure of penetration due to poor gap. Low current.]

Gas inclusions [ entrapment of gas due to wet electrode, inadequate cleaning of weld or inadequate protection from atmospheric gases ]

Concavity [ normally in the root due to improper movement of electrode or too much heat in II pass over I pass ]

Slag inclusion [ entrapment of nonmetallic solid materials]

Tungsten inclusion [ entrapment of tungsten from tip of electrode in TIG process.]

Undercut [ Use of high current which burns edges on parent metal ]

Casting defect

Cold shuts [ failure of molten metal to fuse with solid metals.]

Hot tears [ due to stresses near solidification temperatures when metal is weak. ]

Shrinkage [ lack of enough molten metal to fill up the shrinkages.]

Gas and blow hole [ discrete cavities of 1.6 mm dia. or more due to entrapment of gas ]

Inclusions [ entrapment of materials such as dross and sand. ]