RADIOGRAPHY TEST
Industrial radiography involves exposing a test object to
penetrating radiation so that the radiation passes through the object being
inspected and a recording medium placed against the opposite side of that
object. For thinner or less dense materials such as aluminum, electrically
generated x-radiation (X-rays) are commonly used, and for thicker or denser
materials, gamma radiation is generally used.
Gamma radiation is given off by decaying radioactive
materials, with the two most commonly used sources of gamma radiation being
Iridium-192 (Ir-192) and Cobalt-60 (Co-60). IR-192 is generally used for steel
up to 2-1/2 - 3 inches, depending on the Curie strength of the source, and
Co-60 is usually used for thicker materials due to its greater penetrating
ability.
The recording media can be industrial x-ray film or one of
several types of digital radiation detectors. With both, the radiation passing
through the test object exposes the media, causing an end effect of having
darker areas where more radiation has passed through the part and lighter areas
where less radiation has penetrated. If there is a void or defect in the part,
more radiation passes through, causing a darker image on the film or detector,
as shown in Figure 8.
RT Techniques
Film Radiography
Film radiography uses a film made up of a thin transparent
plastic coated with a fine layer of silver bromide on one or both sides of the
plastic. When exposed to radiation these crystals undergo a reaction that
allows them, when developed, to convert to black metallic silver. That silver
is then "fixed" to the plastic during the developing process, and
when dried, becomes a finished radiographic film.
To be a usable film, the area of interest (weld area, etc.)
on the film must be within a certain density (darkness) range and must show
enough contrast and sensitivity so that discontinuities of interest can be
seen. These items are a function of the strength of the radiation, the distance
of the source from the film and the thickness of the part being inspected. If
any of these parameters are not met, another exposure ("shot") must
be made for that area of the part.
Computed Radiography
Computed radiography (CR) is a transitional technology
between film and direct digital radiography. This technique uses a reusable,
flexible, photo-stimulated phosphor (PSP) plate which is loaded into a cassette
and is exposed in a manner similar to traditional film radiography. The
cassette is then placed in a laser reader where it is scanned and translated
into a digital image, which take from one to five minutes. The image can then
be uploaded to a computer or other electronic media for interpretation and
storage.
Computed Tomography
Computed tomography (CT) uses a computer to reconstruct an
image of a cross sectional plane of an object as opposed to a conventional
radiograph, as shown in Figure 9. The CT image is developed from multiple views
taken at different viewing angles that are reconstructed using a computer. With
traditional radiography, the position of internal discontinuities cannot be
accurately determined without making exposures from several angles to locate
the item by triangulation. With computed tomography, the computer triangulates
using every point in the plane as viewed from many different directions.
Digital Radiography
Digital radiography (DR) digitizes the radiation that passes
through an object directly into an image that can be displayed on a computer
monitor. The three principle technologies used in direct digital imaging are
amorphous silicon, charge coupled devices (CCDs), and complementary metal oxide
semiconductors (CMOSs). These images are available for viewing and analysis in
seconds compared to the time needed to scan in computed radiography images. The
increased processing speed is a result of the unique construction of the
pixels; an arrangement that also allows a superior resolution than is found in
computed radiography and most film applications.
