irregular shapes and rough surfaces, determination of the actual size of small discontinuities in general may not be
possible with ultrasonics. Therefore, estimating the size of small discontinuities by comparing their signal amplitude
with the signal amplitude of reference standard discontinuities is subject to errors. When making such comparisons
(only to be used for rough estimates) the transfer technique should be used (see paragraph 5.3.5). If, after applying
transfer, the test part discontinuity signal is as large or larger than the signal from the reference standard discontinuity,
it can be concluded that the test part discontinuity is at least as large as the reference standard discontinuity. The
transfer technique adjusts for differences in material attenuation, not for differences in discontinuity surface
irregularities. Estimating the size of discontinuities larger than the sound beam is done by moving the search unit over
the discontinuity and mapping the extremities of the discontinuity. The outer edges of a discontinuity can be estimated
by noting the positions of the center of the search unit when the signal amplitude from the discontinuity is reduced to
1/2 its peak value. This procedure estimates the projected area of discontinuities in a plane perpendicular to the
incident sound beam.
In evaluating discontinuities it is helpful, if possible, to evaluate the discontinuities from several different directions.
This can be accomplished by using a combination of angle and straight beam methods, and/or sound entry from
different surfaces. Inspecting in these various directions reveals more about the discontinuity. The direction where the
highest amplitude signal is obtained is most nearly perpendicular to the plane of the discontinuity for equivalent
distances. If the discontinuity signal changes very little with changing direction, the discontinuity is probably rounded.
The sound scattered from a rounded discontinuity is independent of the incident direction. A flat discontinuity gives a
maximum reflection when the incident sound beam is perpendicular to the discontinuity.
Closely spaced small discontinuities may produce multiple indications that are often accompanied by the loss of back
reflection. Figure 5-60 shows an example of how large grain size porosity can each produce multiple indications and
reduce the amplitudes of back-reflection multiples. It is necessary to change the A-scan settings to check for both the
effects, because the back surface signal probably saturates the display at the gain setting that shows the multiple
indications. By lowering the gain and lengthening the sweep range, the decreasing amplitude of multiple back
reflections is observed. The rate of decrease in the amplitudes of the back reflection signals will be greater than for an
area with no discontinuities.
Types Of Indications.
Several different types of indications will be encountered in ultrasonic inspections. Some of these indications can cause
confusion, resulting in false conclusions. It is important that the operator be familiar with paragraphs 5.1.6 through
188.8.131.52 and the additional information below. This will help the operator in evaluating inspection results and avoiding
Loss of Back Reflection And / Or Multiple Indications.
Loss of back reflection with no other indication can be caused by a number of factors such as the following:
a. Large grain size
c. Dispersion of precipitated particles in the material.
d. Overheated structure
However, these features may produce multiple indications as well (Figure 5-59). Lowering the frequency will generally
reduce the multiple indications. When either multiple indications and/or loss of back reflection is noted, the test part
should be compared with the reference standard using transfer in accordance with paragraph 5.3.5. The results should
be evaluated in accordance with the limits in paragraph 184.108.40.206.