Radiography Techniques

By-Vertika Nigam

Introduction

The discovery of X-rays and the phenomenon of radioactivity and their application to the examination of objects provided the starting point for the advancement of industrial radiography. This technique is one of the most widely used for the detection of internal defects such as voids and porosity. Planar defects can also be detected by radiography with some proper orientation. Radiography is also suitable for detecting changes in material composition, thickness measurements and locating unwanted or defective components hidden from view in an assembled part.

PrincipleRadiography makes use of the ability of short wavelength electromagnetic radiations such as x-rays or gamma rays to penetrate objects. The shorter the wavelength, the greater is the penetrating power. The amount of radiation that is absorbed by the material is a function of density and thickness of the material. In the event of the presence of a cavity or discontinuity the beam of radiation will have less material to pass through than in solid material. As a consequence there will be variation in the absorption of rays by the material in the defective area. This variation is measured and recorded on a film sensitive to x-rays or gamma rays. An image is produced on the film that indicates the presence of the defect. The image obtained is an X-ray shadow of the interior of the material under examination. In summary, radiography is based on the principle of shadow projection and such a shadow picture is known as a radiograph. The basic setup consists of a source of radiation, the object to be radiographed and a detector which is normally a sheet of photographic film.

Inspection Techniques

The choice of the appropriate technique is made on the basis of geometry, size, sensitivity requirements and available and appropriate testing space. The techniques for various engineering components for radiographic inspection are given below:• Single Wall Single Image Technique: This technique

is used when both sides of the specimen are accessible. This is used for plates, cylinders, shells and large diameter pipes. The source is kept outside and the film inside or vice versa and the weld is exposed part by part.

• Double Wall Penetration Technique: This technique is used when the inside surface of the pipe is not accessible. The source of radiation and the film are kept outside. The radiation penetrates both the walls of the pipe.

• Fluoroscopic Techniques: Fluoroscopy involves the real time viewing of X-rayed parts. The quality of the inspection technique depends on the resolution of the system and skill of the operator/interpreter. The X-ray fluoroscope is a fixed machine operating under typical dark room lighting conditions. To obtain satisfactory results geometry, scatter radiation, and source kilo-voltage must be carefully controlled. Advantages of fluoroscopic inspection include the immediate viewing of the subject from a multitude of angles, the illusion of a three dimensional presentation and the ability to study moving parts in action.

Applications

• Radiography can be used to inspect most types of solid materials both ferrous and non-ferrous alloys as well as nonmetallic materials and composites.

• Radiography is well suited for the inspection of semiconductor devices for detection of cracks, broken wires, unsoldered connections, foreign material and misplaced components. Other NDT methods are limited in their abilities to inspect semiconductor devices.

• Radiography is extensively used for checking castings, weldments and forgings which need to be free from internal flaws.

• Radiography is used for liquid level measurement in sealed components.

Merits and demerits of radiography• The basic advantage of radiography is that

objects that range in size from a miniscule electronic part to a mammoth missile or a power plant structures can be examined.

• Further the method can be used on a variety of materials.

• No prior preparation of the specimen surface is necessary unlike other NDT methods.

Radiographic methods however have certain limitations:

• Certain types of flaws cannot be detected. For example cracks cannot be detected unless they are parallel to the radiation beam.

• Tight cracks in thick sections usually cannot be detected at all even when the radiation beam is properly oriented.

• Minute discontinuities such as flakes, micro porosity and micro fissures cannot be detected unless they are sufficiently large in size.

• The defect or discontinuity must be parallel to the radiation beam or sufficiently large to register on the radiograph. A defect must be at least 2% of the thickness of the material before it can register on a radiograph with sufficient contrast, thus to be detected.

• Certain areas in many items cannot be radiographed because of the geometric considerations involved. Often it is difficult if not impossible to position the film and source of radiation so as to obtain a radiograph of the area desired.

• Protection of personnel for not only those engaged in radiographic work but also those in the vicinity of the radiographic inspection site are of major importance. Safety requirements impose both economic and operational constraints on the use of radiography for inspection.

• Compared to other methods of NDT, radiography is expensive. Capital costs can be low only when portable X-ray or gamma ray source is used.

• An inspection of thick sections is a time consuming process.

• Radioactive sources also limit the thickness that can be inspected primarily because high activity sources require heavy shielding for protection of personnel.

Methods for exposure controlThere are three basic methods to control exposure when working with radiography sources :• Time: Limit personnel time. Time of handling a radiation source can be

made low by proper training, adopting fast work technique, mock up practices and efficient administrative control.

• Distance: Maximize personnel distance. The radiation intensity decreases with distance on the basis of inverse square law. Therefore equipment design and operating procedure must take into account this factor.

• Shielding: A third important way to reduce exposure is to place a shield between the operator and the source of radiation. The denser the shielding materials the more effective it is in bringing down the levels of exposure to X-rays and gamma rays. The commonly used shielding materials in radiographic installations and equipment are steel, lead, concrete and depleted uranium.

Radiograph Interpretation - Welds Interpretation of radiographs takes place in three basic steps: (1) detection, (2) interpretation, and (3) evaluation.

All of these steps make use of the radiographer's visual acuity. Visual acuity is the ability to resolve a spatial pattern in an image.The ability of an individual to detect discontinuities in radiography is also affected by the lighting condition in the place of viewing, and the experience level for recognizing various features in the image. The following material was developed to help students develop an understanding of the types of defects found in weldments and how they appear in a radiograph.

Discontinuities

Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the base metal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of the codes or specifications used to invoke and control an inspection, are referred to as defects

General Welding Discontinuities

The following discontinuities are typical of all types of welding.• Cold lap is a condition where the weld filler metal does not properly

fuse with the base metal or the previous weld pass material (interpass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into the base material without bonding.

• Porosity is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters, or in rows. Sometimes, porosity is elongated and may appear to have a tail. This is the result of gas attempting to escape while the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material and it will have a higher radiographic density than the surrounding area.

• Cluster porosity is caused when flux coated electrodes are contaminated with moisture. The moisture turns into a gas when heated and becomes trapped in the weld during the welding process. Cluster porosity appear just like regular porosity in the radiograph but the indications will be grouped close together.

• Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld and base metal. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative of slag inclusions.

• Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change in thickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines. Cracks can sometimes appear as "tails" on inclusions or porosity.

• Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint. It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress riser from which a crack may propagate. The appearance on a radiograph is a dark area with well-defined, straight edges that follows the land or root face down the center of the weldment.

• Incomplete fusion is a condition where the weld filler metal does not properly fuse with the base metal. Appearance on radiograph: usually appears as a dark line or lines oriented in the direction of the weld seam along the weld preparation or joining area.

• Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited is less than the thickness of the base material. It is very easy to determine by radiograph if the weld has inadequate reinforcement, because the image density in the area of suspected inadequacy will be higher (darker) than the image density of the surrounding base material.

• Internal concavity or suck back is a condition where the weld metal has contracted as it cools and has been drawn up into the root of the weld. On a radiograph it looks similar to a lack of penetration but the line has irregular edges and it is often quite wide in the center of the weld image.

• Internal or root undercut is an erosion of the base metal next to the root of the weld. In the radiographic image it appears as a dark irregular line offset from the centerline of the weldment. Undercutting is not as straight edged as LOP because it does not follow a ground edge.

• External or crown undercut is an erosion of the base metal next to the crown of the weld. In the radiograph, it appears as a dark irregular line along the outside edge of the weld area.

• Offset or mismatch are terms associated with a condition where two pieces being welded together are not properly aligned. The radiographic image shows a noticeable difference in density between the two pieces. The difference in density is caused by the difference in material thickness. The dark, straight line is caused by the failure of the weld metal to fuse with the land area.

Sources of Attenuation• A single interaction event between a primary x-ray photon and a

particle of matter does not usually result in the photon changing to some other form of energy and effectively disappearing. Several interaction events are usually involved and the total attenuation is the sum of the attenuation due to different types of interactions. These interactions include the photoelectric effect, scattering, and pair production.

• The four types of interactions are: photoelectric (PE), Compton scattering (C), pair production (PP), and Thomson or Rayleigh scattering (R).

Since most industrial radiography is done in the 0.1 to 1.5 MeV range, it can be seen from the plot that photoelectric and Compton scattering account for the majority of attenuation encountered.

• Photoelectric (PE) absorption of x-rays occurs when the x-ray photon is absorbed, resulting in the ejection of electrons from the outer shell of the atom, and hence the ionization of the atom. Subsequently, the ionized atom returns to the neutral state with the emission of an x-ray characteristic of the atom. This subsequent emission of lower energy photons is generally absorbed and does not contribute to (or hinder) the image making process. Photoelectron absorption is the dominant process for x-ray absorption up to energies of about 500 KeV. Photoelectron absorption is also dominant for atoms of high atomic numbers.

• Compton scattering (C) occurs when the incident x-ray photon is deflected from its original path by an interaction with an electron. The electron gains energy and is ejected from its orbital position. The x-ray photon loses energy due to the interaction but continues to travel through the material along an altered path. Since the scattered x-ray photon has less energy, it, therefore, has a longer wavelength than the incident photon. The event is also known as incoherent scattering because the photon energy change resulting from an interaction is not always orderly and consistent. The energy shift depends on the angle of scattering and not on the nature of the scattering medium.

• Pair production (PP) can occur when the x-ray photon energy is greater than 1.02 MeV, but really only becomes significant at energies around 10 MeV. Pair production occurs when an electron and positron are created with the annihilation of the x-ray photon. Positrons are very short lived and disappear (positron annihilation) with the formation of two photons of 0.51 MeV energy. Pair production is of particular importance when high-energy photons pass through materials of a high atomic number.

• Thomson scattering (R), also known as Rayleigh, coherent, or classical scattering, occurs when the x-ray photon interacts with the whole atom so that the photon is scattered with no change in internal energy to the scattering atom, nor to the x-ray photon. Thomson scattering is never more than a minor contributor to the absorption coefficient. The scattering occurs without the loss of energy. Scattering is mainly in the forward direction.

• Photodisintegration (PD) is the process by which the x-ray photon is captured by the nucleus of the atom with the ejection of a particle from the nucleus when all the energy of the x-ray is given to the nucleus. Because of the enormously high energies involved, this process may be neglected for the energies of x-rays used in radiography.

Effect of Photon Energy on Attenuation

Absorption characteristics will increase or decrease as the energy of the x-ray is increased or decreased. Since attenuation characteristics of materials are important in the development of contrast in a radiograph, an understanding of the relationship between material thickness, absorption properties, and photon energy is fundamental to producing a quality radiograph. A radiograph with higher contrast will provide greater probability of detection of a given discontinuity. An understanding of absorption is also necessary when designing x-ray and gamma ray shielding, cabinets, or exposure vaults.

Gamma radiationAdvantages 1. A permanent record of defects, in a welded object is obtained. 2. Reference standards for defects are available. 3. Low initial cost. 4. This is a very good method for testing at the site.

Disadvantages 1. Trained operator is required. 2. The method involves radiation hazards. 3. γ-ray source loses strength continuously. 4. γ-ray radiography possesses lower sensitivity and definition than X-ray radiography.

Applications 1. Pressure vessels and boilers. 2. Penstocks and pipe-work. 3. Ship building. 4. Structural steel work.

Precautions for Radiation Safety• (a) shielded enclosures are constructed and exposure devices are manufactured in• compliance with authorizations;• (b) all safety systems and components of shielded enclosures and exposure devices• are of the required quality;• (c) personnel are trained and competent to operate the exposure devices safely;• (d) approved operational procedures are being followed;• (e) exposure devices and shielded enclosures are appropriately surveyed and• maintained;• (f) sources are sealed and leak free;• (g) dosimetric and medical surveillance of the workers are carried out correctly;• (h) the response to incidents follows the agreed emergency plan or regulatory• requirements;• (i) exposure devices and shielded enclosures are maintained in a safe and secure• condition at the end of use or are decommissioned safely;• (j) the source inventory is properly maintained.

Radiographic Film• X-ray films for general radiography consist of an emulsion-gelatin containing radiation

sensitive silver halide crystals, such as silver bromide or silver chloride, and a flexible, transparent, blue-tinted base. The emulsion is different from those used in other types of photography films to account for the distinct characteristics of gamma rays and x-rays, but X-ray films are sensitive to light. Usually, the emulsion is coated on both sides of the base in layers about 0.0005 inch thick. 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.

• When x-rays, gamma rays, or light strike the grains of the sensitive silver halide in the emulsion, some of the Br- ions are liberated and captured by the Ag+ ions. This change is of such a small nature that it cannot be detected by ordinary physical methods and is called a "latent (hidden) image." However, the exposed grains are now more sensitive to the reduction process when exposed to a chemical solution (developer), and the reaction results in the formation of black, metallic silver. It is this silver, suspended in the gelatin on both sides of the base, that creates an image.

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