Echo Display Comparison Method - Epson USN 52R Operating Manual

Important notes

Echo display comparison method

The echo from a small natural flaw is usually smaller
than the echo from an artificial comparison flaw (e.g.
circular disc reflector) of the same size. This is due, for
instance, to the roughness of the surface of a natural
flaw, or to the fact that the beam does not impinge on it
at right angles.
If this fact is not taken into account when assessing
natural flaws, there is a danger of underestimating their
magnitude.
In the case of very jagged or fissured flaws (e.g. shrink
holes in castings) it may be that so much scattering of
the sound occurs at the surface of the flaw that no flaw
echo is produced. In such cases, a different assess-
ment method should be chosen, e.g. backwall echo
attenuation method.
When testing large components, the distance sensitive-
ness of the flaw echoes plays an important role. Care
should be taken here to choose artificial comparison
flaws whose distance sensitivity laws come as close
as possible to those of the natural flaws being as-
sessed.
Krautkramer USN 52R/USN 52L
Ultrasound is subject to attenuation as it passes
through any material. This attenuation is very small in,
for instance, fine grained steel, and also in many small
components made of other materials.
If, however, the sound travels a large distance into the
medium, then even at low attenuation coefficients a
large cumulative attenuation can result. There is then a
danger that echoes from natural flaws appear too small.
For this reason, the effects of sound attenuation on the
evaluation results must always be estimated and, if
necessary, taken into account.
If the test object has a rough surface, part of the inci-
dent sound energy will be scattered at its surface and
is lost to the test instrument. The larger this initial scat-
tering, the smaller the echoes appear, and the more
incorrect also the results of the evaluation.
It is therefore important to make an allowance for the
surface roughness of the test object and apply a cor-
rection to the observed height of the echoes (transfer
correction).
Issue 05, 02/00
Introduction
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