The power supply and cooling unit shown in view
To determine the quality of a weld, we must
A converts 220-volt or 440-volt, three-phase, 60-cycle
examine the interior of a weld for defects. To detect
internal defects, the Navy relies heavily on an
inspection method known as radiography. In this
method, a beam of radiant energy is passed through the
metal and recorded on a radiographic film, which is
similar to photographic film. The source of the energy
is an X-ray tube of radioactive isotope (Iridium-192).
X-ray machines that are practical for use on ships are
generally of medium power; that is, they can penetrate
metal that is about 3 inches thick. The source used must
be according to the guidelines of MIL-STD-271,
Requirements for Nondestructive Testing Methods.
The action of the penetrating radiation, or "beam,"
works almost exactly as a beam of sunlight or electric
light through air. But instead of being stopped or
reflected, as light is by metal or solids, it penetrates the
solid and travels through it. Some of the radiation is
scattered or absorbed by the solid, and does not get
through. A thin section of metal will "pass" more
radiation than a thick section. A radiographic film is
placed on the side of the metal opposite the source. The
image on the film (after development) will show black
where more radiation passed through and will show
lighter shades or clear white where less radiation, or no
radiation, passed through. A hole or other defect in the
metal, of even a few thousandths of an inch, will allow
more radiation to pass through than passes through the
solid metal. The film will show a shadow that is darker
at the defect and lighter AROUND the defect.
Radiography is used where the full thickness of a
part must be inspected, or seen. For best results, use
only enough energy to get the picture on the film. The
exposure time is figured from factors such as the type
of energy, type and thickness of material, film type,
screens, distance, and required image density. A thick
piece of metal will need a longer exposure time than a
The X-ray equipment used for radiography on some
repair ships, tenders, and at shore stations is the
275-kilovolt-peak (LVP), 10-milliamperage (mA)
portable X-ray machine. Figure 11-1 shows that the
machine consists of three principal units plus electrical
Figure 11-1.--Portable X-ray unit for a radiographic
cable and water hose.
inspection of metals.