Radiographic Inspection

Radiographic inspection is one of the most thorough methods of NDT. It is quite expensive to carry out, especially on items where the radiographic machinery needs to be bought to site to do an inspection.

Common uses of radiographic inspections are safety critical items such as pressure vessels, lifting equipment, and high pressure pipe works.

The two types that are used for weld inspections are X-ray and Gamma-ray.

X-Rays

X-ray is carried out in a similar manner as at a hospital or dentist. The X-ray tube is placed one side of the joint, the film on the other. Then the radiation is absorbed by the test piece at different levels, leaving an indication of the joint on the film the other side. Defects such as porosity and cracks are easily visible as the defects will not absorb the radiation as the solid material will.

The X-ray is electrically generated via an X-ray tube, operating at very high voltages (200Kva and much higher.)

There are numerous technological advances in this area now, including computer based systems which allow 3d modelling, and various software to carry out and record the process. Software that can tell the operator the defects rather that him or her learning by experience is also common now. This can also help rule out human error.

Gamma Rays

Gamma rays (Y rays) are better for inspecting thicker materials. Generally they have higher energy and can penetrate thicknesses of over 500mm. The same testing principle applies to Gamma as it does X-ray.

Unlike an X-ray though, the gamma ray is produced by a radioactive isotope, a man made one such as Cobalt60 is often used.