BMFB 4283 NDT & FAILURE ANALYSIS

BMFB 4283NDT & FAILURE ANALYSIS Lectures for Week 5 Prof. Qumrul Ahsan, PhD Department of Engineering Materials Faculty of Manufacturing Engineering

RADIOGRAPHIC TESTING

Issues to address 5.0 Radiology/Radiography 5.1 Introduction to X-rays and Gamma Rays 5.2 Radiation Fundamentals 5.3 Equipment and Testing 5.4 Techniques and Application 5.5 Radiation Safetyb

Introduction • This module presents information on the NDT method of radiographic inspection or radiography. • Radiography uses penetrating radiation that is directed towards a component. • The component stops some of the radiation. The amount that is stopped or absorbed is affected by material density and thickness differences. • These differences in “absorption” can be recorded on film, or electronically.

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

Electromagnetic Radiation • X-rays and gamma rays differ only in their source of origin. • X-rays are produced by an x-ray generator and gamma radiation is the product of radioactive atoms. • They are both part of the electromagnetic spectrum. Theyare waveforms, as are light rays, microwaves, and radio waves. • They can be diffracted (bent) in a manner similar to light. The radiation used in Radiography testing is a higher energy (shorter wavelength) version of the electromagnetic waves that we see every day. Visible light is in the same family as x-rays and gamma rays. Properties of X-Rays and Gamma Rays They 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 traveling through. They have enough energy to ionize matter and can damage or destroy living cells.

The part is placed between the radiation source and a piece of film. The part will stop some of the radiation. Thicker and more dense area will stop more of the radiation. = less exposure = more exposure General Principles of Radiography The film darkness (density) will vary with the amount of radiation reaching the film through the test object. X-ray film Top view of developed film

General Principles of Radiography • The energy of the radiation affects its penetrating power. Higher energy radiation can penetrate thicker and more dense 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

IDL 2001 Flaw Orientation Optimum Angle = easy to detect Radiography has sensitivity limitations when detecting cracks. = not easy to detect X-rays “see” a crack as a thickness variation and the larger the variation, the easier the crack is to detect. When the path of the x-rays is not parallel to a crack, the thickness variation is less and the crack may not be visible.

IDL 2001 0o 10o 20o Flaw Orientation (cont.) Since the angle between the radiation beam and a crack or other linear defect is so critical, the orientation of defect must be well known if radiography is going to be used to perform the inspection.

Radiation Sources Two of the most commonly used sources of radiation in industrial radiography are x-ray generators and gamma ray sources. Industrial radiography is often subdivided into “X-ray Radiography” or “Gamma Radiography”, depending on the source of radiation used.

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 transformation from which they originate Beta ParticlesA nucleus with an unstable ratio of neutrons to protons may decay through the emission of a high speed electron called a beta particle. This results in a net change of one unit of atomic number (Z). Beta particles have a negative charge Gamma-raysA nucleus which is in an excited state may emit one or more photons (packets of electromagnetic radiation) of discrete energies. The emission of gamma rays does not alter the number of protons or neutrons in the nucleus but instead has the effect of moving the nucleus from a higher to a lower energy state (unstable to stable).

Activity (of Radionuclides) • The quantity which expresses the degree of radioactivity or the radiation producing potential of a given amount of radioactive material 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 of radioactive 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 in which one atom is transformed per second. • Radioactivity is expressed as the number of curies or becquerels per unit mass or volume.

Isotope Decay Rate (Half-Life) • Each radionuclide decays at its own unique rate which cannot be altered by any 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, and 5.3 years for cobalt-60.

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 occurs depends 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 of returning 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.

Gamma Radiography • Gamma rays are produced by a radioisotope. • A radioisotope has an unstable nuclei that does not have enough binding energy to hold the nucleus together. • The spontaneous breakdown of an atomic nucleus resulting in the release of energy and matter is known as radioactive decay.

Gamma Radiography (cont.) • Most of the radioactive material used in industrial radiography is artificially produced. • This is done by subjecting stable material to a source of neutrons in a special nuclear reactor. • This process is called activation.

Gamma Radiography (cont.) Unlike X-rays, which are produced by a machine, gamma rays cannot be turned off. Radioisotopes used for gamma radiography are encapsulated to prevent leakage of the material. The radioactive “capsule” is attached to a cable to form what is often called a “pigtail.” The pigtail has a special connector at the other end that attaches to a drive cable.

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 radiographer’s exposure to radiation during use.

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 this opening when performing an exposure!

Gamma Radiography (cont.) A “drive cable” is connected to the other end of the camera. This cable, controlled by the radiographer, is used to force the radioactive material out into the guide tube where the gamma rays will pass through the specimen and expose the recording device.

X-ray Radiation • X-ray tubes produce x-ray photons by accelerating a stream of electrons to energies of several hundred kilovolts with velocities of several hundred kilometers per hour and colliding them into a heavy target material. • The abrupt acceleration of the charged 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.

X-ray Radiography Unlike gamma rays, x-rays are produced by an X-ray generator system. These systems typically include an X-ray tube head, a high voltage generator, and a control console.

X-ray Radiography (cont.) • X-rays are produced by establishing a very high voltage between two electrodes, called the anode and cathode. • To prevent arcing, the anode and cathode are located inside a vacuum tube, which is protected by a metal housing.

High Electrical Potential Electrons + - X-ray Generator or Radioactive Source Creates Radiation Radiation Penetrate the Sample Exposure Recording Device X-ray Radiography (cont.) • The cathode contains a small filament much the same as in a light bulb. • Current is passed through the filament which heats it. The heat causes electrons to be stripped off. • The high voltage causes these “free” electrons to be pulled toward a target material (usually made of tungsten) located in the anode. • The electrons impact against the target. This impact causes an energy exchange which causes x-rays to be created.

Inverse Square Law • Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. • The intensity of the influence at any given radius (r) is the source strength divided by the area of the sphere. • a point radiation source can be characterized by the diagram above whether you are talking about Roentgens, rads, or rems. All measures of exposure will drop off by the inverse square law. For example, if the radiation exposure is 100 mR/hr at 1 inch from a source, the exposure will be 0.01 mR/hr at 100 inches.

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 at some distance in a material can be expressed in the form of an equation as:

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 attenuation coefficient. If an incident energy of 1 and a transmitted energy is 0.5 is plugged into the equation it can be expressed as Approximate HVL for Various Materials when Radiation is from a Gamma Source

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 microfocus systems have spot sizes in the 30 micron range.

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 specimen thickness: Ug = f * b/a Where f = source focal-spot sizea = distance from the source to front surface of the objectb = the thickness of the object

Geometric Unsharpness • 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/objectb = distance from the front surface of the object to the detector

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, or brass. • 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 and is seldom used.

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 behind the film.

Radiation Undercut • Parts with holes, hollow areas, or abrupt thickness changes are likely to suffer from undercut if controls are 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 the transition area. • Undercut occurs due to scattering within the film. • The faster the film speed, the more undercut that is likely to occur • Masks are used to control undercut. • Sheets of lead cut to fill holes or surround the part • Metallic shot and liquid absorbers are often used as masks.

Imaging Modalities Several different imaging methods are available to display the final image in industrial radiography: • Film Radiography • Real Time Radiography • Computed Tomography (CT) • Digital Radiography (DR) • Computed Radiography (CR)

Film Radiography • One of the most widely used and oldest imaging mediums in industrial radiography is radiographic film. • Film contains microscopic material called silver bromide. • Once exposed to radiation and developed in a darkroom, silver bromide turns to black metallic silver which forms the image. • Putting emulsion on both sides of the base doubles the amount of radiation-sensitive silver halide, and thus increases the film speed. • The emulsion layers are thin enough so developing, fixing, and drying can be accomplished in a reasonable time. • A few of the films used for radiography only have emulsion on one side which produces the greatest detail in the image.

Film selection • Factors to be considered: • Composition, shape, and size of the part being examined and, in some cases, its weight and location. • Type of radiation used, whether x-rays from an x-ray generator or gamma rays from a radioactive source. • Kilovoltages available with the x-ray equipment or the intensity of the gamma radiation. • Relative importance of high radiographic detail or quick and economical results. if high resolution and contrast sensitivity is of overall importance, a slower and finer grained film should be used in place of a faster film.

Film Radiography (cont.) • Film must be protected from visible light. Light, just like x-rays and gamma rays, can expose film. Film is loaded in a “light proof” cassette in a darkroom. • This cassette is then placed on the specimen opposite the source of radiation. Film is often placed between lead screens to intensify the effects of the radiation.

Film Radiography (cont.) • In order for the image to be viewed, the film must be “developed” in a darkroom. The process is very similar to photographic film development. • Film processing can either be performed manually in open tanks or in an automatic processor.

Development of Radiography film • Development - The developing agent gives up electrons to convert the silver halide grains to metallic silver. Grains that have been exposed to the radiation develop more rapidly, but given enough time the developer will convert all the silver ions into silver metal. Proper temperature control is needed to convert exposed grains to pure silver while keeping unexposed grains as silver halide crystals. • Stopping the development - The stop bath simply stops the development process by diluting and washing the developer away with water. • Fixing - Unexposed silver halide crystals are removed by the fixing bath. The fixer dissolves only silver halide crystals, leaving the silver metal behind. • Washing - The film is washed with water to remove all the processing chemicals. • Drying - The film is dried for viewing.

Film Radiography (cont.) Once developed, the film is typically referred to as a “radiograph.”

Radiographic Image Quality

Subject contrast • Subject contrast is the ratio of radiation intensities transmitted through different areas of the component being evaluated • dependent on: • the absorption differences in the component • the wavelength of the primary radiation • intensity and distribution of secondary radiation due to scattering • The larger the difference in thickness or density between two areas of the subject, the larger the difference in radiographic density or contrast. • low kilovoltage will generally result in a radiograph with high contrast • low energy radiation is more easily attenuated • Hence, the ratio of photons that are transmitted through a thick and thin area will be greater with low energy radiation • In turn will result in the film being exposed to a greater and lesser degree in the two areas

Subject contrast • As contrast sensitivity increases, the latitude of the radiograph decreases. • Radiographic latitude refers to the range of material thickness that can be imaged. • more areas of different thicknesses will be visible in the image. • The goal is to balance radiographic contrast and latitude so that there is enough contrast to identify the features of interest but also to make sure the latitude is great enough so that all areas of interest can be inspected with one radiograph. • In thick parts with a large range of thicknesses, multiple radiographs will likely be necessary to get the necessary density levels in all areas.

Film contrast • Film contrast: • density differences that result due to: • the type of film used • how it was exposed, and • how it was processed. • Since there are other detectors besides film, this could be called detector contrast, but the focus here will be on film. • Exposing a film to produce higher film densities will generally increase the contrast in the radiograph.

A typical film characteristic curve, which shows how a film responds to different amounts of radiation exposure, is shown to the right. From the shape of the curves, it can be seen that when the film has not seen many photon interactions (which will result in a low film density) the slope of the curve is low. In this region of the curve, it takes a large change in exposure to produce a small change in film density. Therefore, the sensitivity of the film is relatively low. It can be seen that changing the log of the relative exposure from 0.75 to 1.4 only changes the film density from 0.20 to about 0.30. However, at film densities above 2.0, the slope of the characteristic curve for most films is at its maximum. In this region of the curve, a relatively small change in exposure will result in a relatively large change in film density. In general, the highest overall film density that can be conveniently viewed or digitized will have the highest level of contrast and contain the most useful information. Film contrast

Radiographic Image Quality • Radiographic definition is the abruptness of change in going from one area of a given radiographic density to another. • Like contrast, definition also makes it easier to see features of interest, such as defects, Since radiographic contrast and definition are not dependent upon the same set of factors, it is possible to produce radiographs with the following qualities: Low contrast and poor definition High contrast and poor definition Low contrast and good definition High contrast and good definition

Radiographic Density • Radiographic density (or film density) is a measure of the degree of film darkening. • Technically it should be called "transmitted density" when associated with transparent-base film since it is a measure of the light transmitted through the film. • Radiographic density is the logarithm of two measurements: the intensity of light incident on the film (I0) and the intensity of light transmitted through the film (It). This ratio is the inverse of transmittance. Industrial codes and standards typically require a radiograph to have a density between 2.0 and 4.0 for acceptable viewing with common film viewers. Film density is measured with a densitometer.

Image Quality • Image quality is critical for accurate assessment of a test specimen’s integrity. • Various tools called Image Quality Indicators (IQIs) are used for this purpose. • There are many different designs of IQIs. Some contain artificial holes of varying size drilled in metal plaques while others are manufactured from wires of differing diameters mounted next to one another.

Image Quality (cont.) • IQIs are typically placed on or next to a test specimen. • Quality typically being determined based on the smallest hole or wire diameter that is reproduced on the image.

BMFB 4283 NDT & FAILURE ANALYSIS