What is NDT? 

Radiography uses X-rays or gamma-rays to produce an image of an object on film. The image is usually natural-size. X-rays and gamma-rays are very short wavelength electromagnetic radiation which can pass through solid material, being partly absorbed during transmission. Thus, if an X-ray source is placed on one side of a specimen and a photographic film on the other side, an image is obtained on the film of the thickness variations in the specimen, whether these are surface or internal.
This is a well-established technique which gives a permanent record and is widely used to detect internal flaws in weldments and castings and to check for mis-constructions in assemblies.

The source of radiation is either an X-ray tube or a pellet of radioactive material emitting gamma-radiation. X-ray equipment is usually described by the electrical voltage across the X-ray tube: thus, 300 kV X-rays. The higher the voltage, the greater the penetrating power of the radiation; industrial X-ray equipment ranges from about 20 kV to 20 MV and the most powerful equipments can be used to radiograph up to 500 mm (20") steel.

Nearly all gamma-radiography is done with either cobalt-60 or iridium-192 sources; there are a few other radioactive isotopes suitable for gamma-radiography, for special applications.

To obtain images with good definition it is desirable to have small-diameter radiation sources and the effective source size of typical X-ray and gamma-ray sources is in the range 1 to 4 mm diameter. After the radiographic film has been exposed, it has to be photographically processed (develop, wash, fix, dry) and is then placed on an illuminated screen for visual interpretation of the image. X-rays and gamma-rays are dangerous and radiographic equipment must be used either inside a protective enclosure, or with appropriate barriers and warning signals, to ensure that there is no radiation hazard to personnel. Qualified staff must be employed.

Radiography on film is a relatively expensive NDT method, due to the cost of the film. The larger X-ray equipments (for thick specimens) are also costly and require costly protective enclosures. Gamma-ray sources are much less expensive and are widely used on site work because of their greater portability. Generally, exposure times in gamma radiography are much longer than with X-rays.

High-definition radiography

Special X-ray tubes have been developed in which the source of X-rays is much smaller than in a conventional tube. In microfocus X-ray tubes the source size may be as small as 10 micrometres and this permits projected, enlarged images to be obtained which are adequately sharp. Such X-ray tubes have a very low output of X-rays and this is a technique which has not yet been fully developed.

By using a collimated radiation detector to scan the specimen, in register with a suitable radiation source, a measure of the transmitted radiation can be obtained. Whilst the advantage of having a radiographic image is lost, there are nevertheless several advantages to be gained by using radiometric techniques:
a) The radiation source may be of much lower activity than a radiographic source for the same item, typically up to 6 orders of magnitude lower. Thus, it is potentially much less hazardous.
b) Gamma-ray sources are most frequently used being highly portable and of very stable radiation output (long-term decay factor apart).
c) In the testing of shielding for buildings and containers holding radioactive materials, radiometry yields directly quantitative data on shielding defects, which radiography does not do.

X-ray tomography
By measuring the X-ray absorption through a specimen in a large number of different directions, digitising and collecting the data, a computer programme can be used to calculate and display a new form of X-ray image. The image produced is as if a thin slice has been cut through the specimen along the plane of the X-ray beam and this slice then radiographed.

The usual method used in industrial X-ray tomography is to move the specimen transversely and rotationally in a series of steps across a fixed X-ray beam measuring the absorption at each position. The computer requirements are quite large, and for industrial applications this is still a new technique.

Dynamic radiography/radioscopy

Radiography and radioscopy can be applied to the study of objects in motion inside opaque casings, or other situations where the object is obscured from direct visual observation. These techniques can give valuable information to Design and Development Engineers on the performance of gas turbines, reciprocating engines, 2-phase flow and a wide range of transient phenomena. The radiation sources used may be constant potential or pulsed X-ray generators, gamma-ray and neutron sources. The recording medium may be X-ray film, cine film or video systems, the latter two in conjunction with X-ray image intensifiers.

The type of source used, and the image recording system, radiographic film, image intensifiers, cine film, video recording, will depend on the type of event that is required to be studied, and the thickness and density of the subject.

X-ray sources may be constant potential, (half-wave generators are of limited use); repetitively pulsed eg linear accelerators; or single pulse eg flash pulsers. Gamma-ray sources may be used where long-term movement caused by changes in stress or temperature are required to be measured.

Fast moving images may be recorded at very high frame rates using suitable cine cameras or high-speed video systems allowing slow-motion replay, or ‘frozen’ on X-ray film using flash pulser X-ray sources with pulse durations of 20 to 50 nanoseconds, when suitable synchronising systems are used.

Repetitively pulsed sources may be synchronised to cyclic rotational events, yielding techniques of strobo-radiography/radioscopy. Variable precession of the synch pulses enables examination of whirl and vibration amplitudes of rotating shafts or assemblies from differing angles, and slight variations in the frequency of the X-ray pulses can yield slow-motion presentations of high-speed events.

See also: Real-time radiography, radioscopy.