Limits Of Testing; Wall Thickness Measurements With Ultrasonics; Effect Of The Material Of The Test Object - Epson USN 52R Operating Manual

Important notes

Limits of testing

Ultrasonic tests can only provide information about
those parts of the test object which the sound beam
from the probe used has actually passed through.
Extreme caution is advised in making any conclusions
from the tested areas of the test object as to the condi-
tion of those parts of the test object which have not
actually been tested.
Such conclusions are usually only possible where a
large amount of previous data is available and proven
methods of statistical evaluation are used.
Boundary surfaces within the test object can com-
pletely reflect the sound beam so that flaws or reflec-
tion points lying deeper remain undetected. Care must
therefore be taken to ensure that the sound beam does
in fact penetrate all those parts of the test object which
are to be tested.

Wall thickness measurements with ultrasonics

All wall thickness measurements with ultrasonics are
based on a time-of-flight measurement. Accurate mea-
surement results require a constant sound velocity
within the material. In test objects of steel, even with
Krautkramer USN 52R/USN 52L
varying alloying constituents, this condition is usually
fulfilled: the variation of the sound velocity within the
material is so slight that it is only of importance for
high-precision measurements. In other materials, how-
ever, e.g. nonferrous metals and plastics, the sound
velocity can vary substantially within the material and
cause inaccurate measurement results.

Effect of the material of the test object

If the test object is not of a single, homogeneous mate-
rial, then the sound may propagate at different veloci-
ties in different parts of the object. In this case, the test
range adjustment should be chosen for the average of
the different velocities in the different parts of the ob-
ject. This is done by using a calibration block in which
the velocity of sound is the same as the average within
the test object.
If substantial variations in the velocity of sound within
the objects are anticipated, then the instrument should
be readjusted at frequent short intervals to the actual
sound velocity values present. If this is not done, false
results may be obtained for the wall thickness.
Issue 05, 02/00
Introduction
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