TWI Knowledge Summary

Radiography

If a source of ionising radiation is positioned on one side of an object and a photographic film placed in close proximity to the other side, a full-size image showing internal detail may be obtained. The radiation is partly absorbed during transmission and differences in material thickness or absorption qualities are recorded on the film. Materials of higher density absorb more radiation. The technique is called radiography and the processed films are called radiographs. Industrial radiography requires X-rays or gamma ( ) rays to reveal hidden flaws in solid objects. The terms X-radiography and gamma radiography indicate the source of radiation in use. X-rays are generated electrically by means of a high voltage X-ray tube. Gamma rays are produced by the natural disintegration of nuclei in a radioactive isotope, Iridium 192 and Cobalt 60 being most commonly used.

Current status

Radiography is a well-established technique that is widely used to detect internal flaws in welds and castings and to check for mis-construction in assemblies. It is typically used to verify weld quality during the fabrication of pressure vessels, pipelines, storage tanks and other engineering structures. It can be used on all metals, from light metals such as aluminium to dense metals such as copper. It can also be used on non-metallic materials with the aid of low energy radiation sources.

Important current issues

Current research issues are the reliability of radiography for detecting planar, crack-like flaws in steel weldments.

The main development areas are real time radiography (RTR) and X-ray tomography systems that capture images digitally, as opposed to on a film that needs to be developed. These systems may also offer the possibility of automatic interpretation of images.

The main application issues are the hazards of ionising radiation, particularly for on-site radiography using portable gamma ray equipment. This often means evacuation of work areas and for this reason on-site radiography is often carried out at night to avoid disruption.

Benefits

The main benefits of radiography are that it provides a non-destructive method of detecting hidden flaws in materials and fabrications and provides a permanent record. It is particularly good at detecting volumetric flaws such as voids, gas pores and solid inclusions. It is also good at determining the nature and dimensions (length and width) of flaws. It cannot, however, be used to measure the dimensions of flaws in the through-thickness direction.

Risks

The ionising radiation regulations and transport regulations specify the legal requirements for protecting workers and others from exposure to X-rays and gamma rays. They must be used either in a shielded enclosure or room, or with appropriate barriers and warning devices to ensure there is no radiation hazard to personnel. Qualified staff must be employed. X-ray equipment does not generate radiation when the electrical supply is turned off, and therefore can be made safe to handle. Gamma ray sources emit radiation continuously and therefore require special dense metal containers to move or manipulate sources.

Further information

TWI offers training courses in radiography.

Additional information about radiography can be found in the items detailed below:

The reliability of radiography of thick section welds (July 1999)

FAQ: What are the principles of radiography in non-destructive examination?

Numerous Member reports and Bulletin articles on radiography are also available to TWI Industrial Members.

You can use the Weldasearch literature database to supplement what you find in JoinIT.