WHAT YOU SHOULD KNOW ABOUT THE BASIC RADIOGRAPHIC TESTING PRINCIPLES IN NDT
Radiographic testing is a method among the various types of NDT inspection. In its short format, it can be called Radiography. Radiography basically, is a non-destructive examination method that uses a beam of penetrating radiation such as X-rays and gamma rays.
When a beam passes through a component, some of the radiation energy is absorbed and the intensity of the beam is reduced. Variations in intensity are recorded in film and are seen as differences in shading that is typical of the types and sizes of any flaws present.
It may be seen from the above that three basic elements, a radiation source or probing medium, the test piece or object being evaluated, and a recording medium like film combined to make a radiograph.
There are sources of radiation during radiography. The two types of radiation used are;
Electromagnetic radiation and Particulate radiation.
For industrial radiography, X-rays and Gamma rays with relatively short wavelengths, and with the capability of penetrating opaque materials are mainly used. The major difference between the X-rays and the gamma rays is the manner in which they are produced.
Particulate radiation refers to a stream of sub-atomic particles that is capable of interacting with matter and thereby releasing energy. The most commonly known subatomic particles are alpha particles, beta particles, and neutrons, all of which are emitted from the nuclei of various atoms during radioactive decay.
x-ray equipment used for industrial x-ray generally ranges from 100KV to 450kV. While the modern types are available up to 450KV and 15mA.
During the radiographic testing, the examination of thicker sections is carried out using high-energy X-rays, whose energy value is 1 MeVor more. A number of machines such as synchrotron betatron, van-de-Graff type electrostatic generators, etc. are available of which electron linear accelerator (LINAC) is the most popular.
Gamma rays are electromagnetic radiation emitted from an unstable source, i.e. an isotope. Each isotope will have characteristic nuclear energy levels, which remain constant, and intensities that decay with time, as indicated by ‘half-life’.
In terms of recording, variations in the intensity of X-rays or gamma rays that pass through a material can be presented as; a visible permanent image, a visible real-time image, and a meter reading. The permanent image can be recorded in an X-ray film or radiographic paper. X-ray film is used more extensively than all other recording mediums.
The principles allow its application in the following areas to be possible;
Radiography can be used to inspect most types of solid material, both ferrous and non-ferrous alloys as also non-metallic materials and composites.
It is used extensively on castings, weldments, forgings, and parts when there is a critical need to ensure freedom from internal flaws. Though radiography still remained a vital method of the NDT inspection, it still has the following limitations:
Certain types of flaws are difficult to detect. Laminations are nearly impossible to detect with radiography; because of their unfavorable orientations.
The defect or discontinuity must be parallel to the radiation beam, or sufficiently large to register on the radiograph. A defect usually must be at least 2% of the thickness of the material before it can register on a radiograph with sufficient contrast to be detected.
The next important limitation is that the radiography is expensive when compared with other methods of NDT inspection.
