should have enough intensity to allow you to view film
structure. Steel bulkheads, decks, overheads, and
that ranges in density from gray to black. The light
machinery also provide good shielding provided that
intensity source should have a variable control, a
they form a solid partition between the source and the
cooling fan, and masking facilities. All radiographs are
operator.
required to be within given density ranges. A
densitometer is used to measure the density ranges of a
RADIOGRAPHIC LIMITATIONS
film to ensure compliance with the requirements.
Shadow Formation
are pointed out in the information that follows. The
greatest limitation is the shortage of qualified film
interpreters. The accessibility of the item to be
Remember, a radiograph is a shadow picture. Like
any shadow cast by an object in a visible light source,
radiographed is an important limitation. Accessibility
the radiograph of a specimen containing a defect is
controls the placement of both the radiation source and
the film, which, in turn, controls the amount of
subject to distortion. The shadow cast by your body
distortion involved. The distortion factor will be
depends on the relative position of your body to both
discussed later with the interpretation of radiographs.
the light source and the surface upon which the shadow
is cast. In the same way, the different positions of the
Other limitations are the thickness and type of
source of radiant energy, the defect, and the film
material being radiographed. A 300 kVP X-ray
determine the extent to which the shadow cast by the
machine that can penetrate 3 inches of steel in a
defect is distorted.
reasonable length of time can only penetrate
approximately 2 inches of various copper-base alloys
The principles of shadow formation are shown in
figure 11-6. Note that enlargement and distortion arise
using the same kVP rating. The thickness range for
in several different ways. First, view A shows that
steel, using Iridium-192 is 1/4 inch to 2 1/2 inches. The
thickness range for copper-base alloys is approximately
enlargement occurs unless the surface upon which the
the same as steel.
shadow is cast is flush with the object itself. The greater
the distance between the object and the surface upon
which the shadow is cast, the greater the enlargement.
INTERPRETATION OF RADIOGRAPHS
Second, views E and F show that if the angle of the
radiating beam or the surface upon which the shadow is
The interpretation of radiographs is guided by
cast is other than at a right angle (90") to the object or
radiographic standards set by the Naval Sea Systems
film, the shadow is distorted as well as enlarged. A third
Command and published in MIL-STD-2035,
kind of distortion occurs when the radiating source is
Nondestructive Testing Acceptance Criteria. Copies of
not an ideal point source, as shown in views A, E, and
these standards are available aboard each ship or station
F, but radiates from a beam area containing innumerable
that have radiographic equipment. A decision on
points, as shown in B, C, and D. As a consequence, a
whether or not a part is acceptable is based on a
halo effect results, as shown in B, C, and D, which adds
comparison of the radiograph of the part with the
to the distortion stemming from other causes. You can
appropriate radiographic standards. The standards
minimize this halo effect by keeping the proper focal
provide specific guidance on the size, number, and
distance between the test specimen, the radiating
dispersion of various defects, and on what decisions for
source, and the surface upon which the shadow is cast.
either acceptance or rejection should be made by a
qualified radiographic inspector.
Keep the principles of shadow formation in mind.
Remember that the size of the shadow cast by any defect
You must be familiar with what is and what is not
other than a spherical defect is influenced by the way
acceptable, and with the design of the part. Unless all
the beam is aimed at the defect. For example, if the
sections of the metal radiographed are of uniform
defect is cylindrical and the beam is parallel to the long
thickness, a radiograph of a perfectly sound section of
axis of the cylinder, the shadow cast is that of a circle.
metal will vary in density. Thin sections and cored
On the other hand, if the beam is parallel to the diameter
internal cavities appear as darker areas on the
of the cylindrical defect, the shadow has a rectangular
radiograph. Density differences caused by the design
shape. In other instances, what appears to be a hairline
must not be confused with density differences caused
by defects. Film should be viewed only in an enclosure
crack may in reality be a cold shut. It is sometimes
possible to overcome this difficulty by radiographing
where all background light is prevented from causing
the specimen twice, with the direction of the radiating
11-6