RADIOGRAPHY TESTING

Introduction

  • 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
  • Radiation Safety
  • Advantages and Limitations
  • Imaging Modalities
    • Film Radiography
    • Computed Radiography
    • Real-Time Radiography
    • Direct Digital Radiography
    • Computed Radiography

Electromagnetic Radiation

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.

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General Principles of Radiography

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.
The film darkness (density) will vary with the amount of radiation reaching the film through the test object.

  • 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.

radio6 = less exposure,    radio7 = more exposure

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Flaw Orientation

Radiography has sensitivity limitations when detecting cracks. 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.
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.
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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.
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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.
  • 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.

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.
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.
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.
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!

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X-ray Radiography

  • radio24Unlike 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-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.
  • 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.

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

  • 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.
  • 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.
  • 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.
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Simple Arrangement of RT

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