Specifications

Important Notice
Page 2 USN52 L Operating Manual
In testing extended areas beyond the ex-
pected sound beam path, extrapolations or
other assumptions are sometimes based
on statistical and other modifications of actu-
al data obtained. Such results and their use
and interpretation are the sole responsibil-
ity of the user.
Flaw Size Evaluation
In present test practice there are basically
two methods of assessing flaws. If the di-
ameter of the sound beam is smaller than
the extent of the flaw, then the beam can
be used to explore the boundaries of the
flaw to determine its area.
If, however, the diameter of the sound beam
is greater than the size of the flaw, the maxi-
mum echo response from the flaw must be
compared with the maximum echo re-
sponse from an artificial flaw provided for
comparison purposes.
Flaw Boundary Method: The smaller the
diameter of the sound beam, the more ac-
curately the boundaries can be determined.
If, however, the sound beam is relatively
broad, the flaw area determination can dif-
fer from the actual. Care should be taken
to select a probe with sufficiently narrow
beam at the position of the flaw.
Echo Comparison Method: The echo from
a small natural flaw is usually smaller than
the echo from an artificial comparison flaw
of the same size. This is often due to irreg-
ularity or oblique orientation of the flaw sur-
face. This fact should be considered when
evaluating flaw size to avoid underestimat-
ing size.
In cases of very jagged or fissured flaws,
e.g. shrink holes in castings, an echo may
not be detected. In such cases a different
method may be required, such as measur-
ing loss of transmission through the piece.
When testing large parts, distance of the
flaw from the probe is significant. It is im-
portant to choose an artificial comparison
flaw that is as near as possible the same
distance as the flaw being assessed.
Ultrasound is subject to attenuation as it
passes through any material. Some mate-
rials attenuate less than others. However,
if the sound travels a long distance through
the medium, then even at low attenuation,
a large effect can result.
The danger here is that natural flaws may
be underassessed. Therefore, an estimate
should be made of the effect of attenuation
on test results and appropriate corrections
applied.
If the test part has a rough surface, part
of the incident sound energy will be scat-
tered at the surface and lost to the test
instrument. The larger this scattering,
the smaller the echoes, and greater the
underestimation of detected flaws. It is im-
portant to make allowance for surface
roughness and apply a correction to ob-
served echo heights.