Radiographic Testing

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RADIOLOGY is the general term given to material inspection methods that are based on the differential absorption of penetrating radiation either electromagnetic radiation of very short wavelength or particulate radiation by the part or testpiece (object) being inspected. Because of differences in density and variations in thickness of the part or differences in absorption characteristics caused by variations in composition, different portions of a testpiece absorb different amounts of penetrating radiation. These variations in the absorption of the penetrating radiation can be monitored by detecting the unabsorbed radiation that passes through the testpiece. The term radiography often refers to the specific radiological method that produces a permanent image on film (conventional radiography) or paper (paper radiography or xeroradiography). In a broad sense, however, radiography can also refer to other radiological techniques that can produce two-dimensional, plane-view images from the unabsorbed radiation. Recently, the American Society of Testing and Materials (ASTM) defined radioscopy as the term to describe the applications when film or paper is not used and defined radiology as the general term covering both techniques.

However, the term radioscopy has not received wide acceptance yet, and this article considers the following two techniques as radiographic inspection (with x-rays or -rays):

· Film or paper radiography: A two-dimensional latent image from the projected radiation is produced on a sheet of film or paper that has been exposed to the unabsorbed radiation passing through the testpiece. This technique requires subsequent development of the exposed film or paper so that the latent image becomes visible for viewing

· Real-time radiography (also known as radioscopy): A two-dimensional image can be immediately displayed on a viewing screen or television monitor. This technique does not involve the creation of a latent image; instead, the unabsorbed radiation is converted into an optical or electronic signal, which can be viewed immediately or can be processed in near real time with electronic and video equipment The principal advantage of real-time radiography over film radiography is the opportunity to manipulate the testpiece during radiographic inspection. This capability allows the inspection of internal mechanisms and enhances the detection of cracks and planar defects by manipulating the part to achieve the proper orientation for flaw detection. Moreover, part manipulation in real-time radiography simplifies three-dimensional (stereo) dynamic imaging and the determination of flaw location and size. In film radiography, however, the position of a flaw within the volume of a testpiece cannot be determined exactly with a single radiograph; depth parallel to the radiation beam is not recorded. Consequently, other film techniques, such as stereoradiography, triangulation, or simply making two or more film exposures (with the radiation beam being directed at the testpiece from a different angle for each exposure), must be used to locate flaws more exactly within the testpiece volume.

Although real-time radiography enhances the detection and location of flaws by allowing the manipulation of the testpiece during inspection, another important radiological technique with enhanced flaw detection and location capabilities is computed tomography. Unlike film and real-time radiography, computed tomography (CT) involves the generation of cross-sectional views instead of a planar projection. The CT image is comparable to that obtained by making a radiograph of a physically sectioned thin planar slab from an object. This cross-sectional image is not obscured by overlying and underlying structures and is highly sensitive to small differences in relative density. Moreover, CT images are easier to interpret than radiographs (see the article "Industrial Computed Tomography" in this Volume). All of the terms and techniques in the preceding discussion refer to radiological inspection with penetrating electromagnetic radiation in the form of x-rays or -rays. Other forms of radiation include subatomic particles that are generated during nuclear decay. The most commonly known subatomic particles are particles, particles, and neutrons, all of which are emitted from the nuclei of various atoms during radioactive decay. Beta particles and neutrons are sufficiently penetrating to be useful for radiography, but neutrons are more widely used. More information on neutron radiography is available in the article "Neutron Radiography" in this Volume.

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