RADIOGRAPHIC TESTING

Radiography is a non-destructive testing method and the purpose of radiography is to show the presence and nature of defects or other structural discontinuities in the interior of the materials under examination.

BASIC PRINICIPLE -

This technique makes use of the ability of short wavelength electromagnetic radiations, such as X-rays or gamma rays, to penetrate objects. In general, the shorter the wavelength, the greater is the penetrating power. The radiation that enters through the material, some being absorbed in the material itself and amount of absorption is a function of the density and thickness of the material. Should there be a cavity or discontinuity in the interior of the material, the beam of radiation will have less material to pass through than in solid material. Consequently, there will be a variation in the absorption of the rays by the material in the defective area. The variation, if measured or recorded on a film sensitive to X- or gamma radiation, produces an image that will indicate the presence of the defect. The image is an X-ray shadow of the interior of the material. Thus, radiography is essentially based on the principle of shadow projection and such a shadow picture is called a radiograph. Variations in the darkness may be interpreted to provide information concerning the internal structure of the material.

The basic setup essentially consists of a source of radiation, the object to be radiographed and a detector, which is normally a sheet of photographic film.

High energy X-ray source –

Examination of thicker sections is carried out using high energy X-rays whose energy value is 1 MeV or more. Using high energy X-rays, possibilities of large distance to thickness ratios with correspondingly low geometrical distortion, short exposure times and high production rate can be achieved. Also, small focal spot size and reduced amount of high angle scattered X-rays reaching the film result in radiographs with good contrast, excellent penetrameter sensitivity and good resolution. A number of machines such as Betatron, Linear Accelerators (Linac) and Van De Graff type electrostatic generators are available.

Gamma ray sources –

Radiography with gamma rays has the advantages of simplicity of the apparatus used, compactness of radiation source, and independence from outside power. This facilitates the examination of pipe, pressure vessels and other assemblies in which the access to interior is difficult.

Gamma rays are electromagnetic radiation emitted from an unstable nucleus. The gamma ray energy levels remain constant for a particular isotope but the intensity decays with time as indicated by the half-life.

The four most popular radiographic sources are – Cobalt 60 (Co-60), Iridium 192 (Ir-192, Cesium 137 (Cs-137) and Thulium 170 (Th-170).

Cobalt-60 and Iridium-192 are available in high specific activities and thus tiny sources of these radioisotopes giving intense radiation, have found popular use. The radioactive isotopes most often used in gamma radiography are Ir-192 and Co-60.

APPLICATIONS OF RADIOGRAPHIC INSPECTION

a) Can be used with most materials

b) Provides a permanent visual image record of the test specimen on film.

c) Reveals the internal nature of material

d) Discloses fabrication errors

e) Reveals structural discontinuities

LIMITATIONS

a) The defect or discontinuity must be parallel to the radiation beam, or sufficiently large, to register on the radiograph.

b) Impracticable to use on specimens of complex geometry.

c) Compared to other NDT methods of inspection, radiography is expensive.

d) Two-side accessibility of the specimen is required.

e) Inspection of thick sections is a time consuming process.

f) Safety requirements impose both economic and operational constrains on the use of radiography for inspection.