X-rays and gamma radiation have wavelengths shorter than 100 nanometres (nm). Energy, at these wavelengths, will penetrate solid material. The shorter the wavelength, the greater the penetration. Like visible light, X-rays and Gamma radiation also have a photochemical effect on silver halide and can therefore produce an image on film. Thus, by passing penetrating radiation through an object, and recording the emerging radiation on a film, a two dimensional picture of the differences in thickness or density of the object can be obtained. Hence, flaws in the object can be detected.
This process was discovered by William Roentgen in 1895 and was soon applied to medicine and then to industrial components. It is based on the principle that radiation is absorbed and scattered as it passes through an object. If there are variations in thickness or density (e.g. due to defects) in an object, more or less radiation passes through and affects the film exposure. Flaws show up on the film, usually as dark areas.
With training, an inspector can tell, from the shape of the dark areas on the film, what and where the flaws are. If the flaw in the object makes little difference to the through thickness of the object, it is unlikely to show on the radiograph. A lamination can, therefore, be difficult to detect by radiography. Cracks parallel to the beam, porosity, slag inclusions and root defects show very well.
The biggest disadvantage is that short wavelength radiations are ionising. This means they can cause chemical changes in the human body. No ionising radiation is safe, as small amounts can cause genetic damage and increase the likelihood of cancers. Stringent safety precautions are needed when using radiography and this makes it rather time consuming, disruptive and expensive.
Industrial radiography uses two sources of penetrating radiation:
X-ray sets, of varying power, that run from an electric mains or on-site generators
Radioactive isotopes, carried in shielding containers, which do not need a supply of electricity
Books on Radiography:
R Hamshaw, Introduction to the Non-Destructive Testing of Welded Joints, 2nd edition, Abington Publishing, Cambridge, UK, 1996 (ISBN 1 85573 314 5)
R Halmshaw, Industrial Radiography, published by Agfa Gevaert, Mortsel, Belgium, 1986
Radiography in Modern Industry, 4th Edition, published by Kodak Ltd, Rochester, New York, USA, 1980
Alan Martin and Samuel A Harbison, An Introduction to Radiation Protection, published by Chapman and Hall, 1986, 3rd edition, 1986 (ISBN 0-412-27810-3)
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