follows, therefore, that the radiation per square inch on the surface at (C2) is only one quarter that at the
level (C1). Thus the exposure that would be adequate at (C1) must be increased 4 times in order to produce at
(C2) a radiograph of equal density. In practice this can be done by increasing the time or increasing the
milliamperage. Mathematically the inverse square law is expressed as follows:
where I1 and I2 are the intensities at distances D1 and D2 respectively.
Example: An intensity of 2 mR was measured at 40 from the source. What would be the intensity reading at
30 inches, and at 20 inches? When determining unknown distances do not forget to take the square of the
predetermined value for D2.
Radiography can be used quite reliably to detect cracks provided certain stringent criteria are met. It is very
easy to produce an apparently high quality radiograph that does not show an existing crack or with a crack
indication so faint it can barely be seen. The resolution of cracks depends upon total density change and film/
subject contrast. The human eye can detect density changes of 0.02 H & D units, however to detect cracks a
density change of 0.05 H & D units is more reasonable. There are several factors that produce density
changes on X-ray film. The primary factor in the case of crack detection is a change in thickness or mass
between the crack and part being inspected. A general rule is the crack must be at least 2 percent of the part
thickness if it is to produce a readable indication. This rule has variables that inf luence film density
changes, and in some cases as little as a 1 percent thickness change will produce a visible indication. In other
instances, a crack exceeding 5 percent of the part thickness may not produce a readable density change.
Regardless of total density change across an indication, if contrast is not high, crack indications can be
missed. Example: A change in density of 0.05 H & D units can be easily seen if it is an abrupt change.
Conversely, a change or 0.25 H & D units (5 times 0.05) is difficult to see if it is a gradual change over an area
(i.e., gradual increase, increase over 1/2 inch width as opposed to a 1/8 inch width).
When an X-ray tube focal spot is centered directly over a crack whose depth is parallel to the beam (X-ray
beam and crack plane coincide), the film density change will be a function of the ratio of crack depth to metal
thickness. Indications of narrow cracks with parallel sides will appear as fine dark lines with high contrast.
Wide cracks with sloping sides will result in broader indications of lower contrast. Figure 6-27 is a sketch
illustrating the film density changes between two different width cracks when the X-ray tube is centered
over the crack origin. The stress on a part will effect crack width. Example: compressive stress in the lower
wing surface of an aircraft on the ground tends to reduce, crack width. The compressive stress is due to the
weight of the structure, engines, ordnance pylons, etc. Jacking the aircraft to place the lower surface in
neutral stress or in tension is frequently done to enhance detection of small cracks. One general
characteristic of cracks and their indication is the tendency for them to curve or deviate from a straight line.
An apparent exception is a very short crack or a crack between two adjacent fasteners, but even here, when
the indication is examined under magnification there will be some edge jaggedness or change in edge