Waygate Technologies Krautkramer USM 100, Руководство по эксплуатации, Страница: 30 / 330

1 Introduction Important information on ultrasonic testing
30 Rev. 9 (09/2023) Krautkrämer USM 100
Echo display comparison method
The echo from a small, natural flaw is usually smaller
than the echo from an artificial comparison flaw, e.g. cir-
cular disc flaw of the same size. This is due, for in-
stance, to the roughness of the surface of a natural flaw,
or to the fact that the sound beam does not impinge on
it at right angles.
If this fact is not taken into account when evaluating nat-
ural flaws, there is a risk of false evaluation.
In the case of very jagged or fissured flaws, e.g. shrink
holes in castings, it may be that the sound scattering oc-
curring at the boundary surface of the flaw is so strong
that no echo at all is produced. In such cases, a different
evaluation method should be chosen, e.g. use of the
backwall echo attenuation in the evaluation.
The distance sensitivity of the flaw echo plays an import-
ant part when testing large components. Pay close at-
tention to choosing artificial comparison flaws which are
as far as possible governed by the same "distance laws"
as the natural flaws to be evaluated.
The ultrasonic wave is attenuated in any material. This
sound attenuation is very low, e.g. in parts made of fine-
grained steel, likewise in many small parts made of oth-
er materials. However, if the sound wave travels larger
distances through the material, a high cumulative sound
attenuation can result, even with small attenuation coef-
ficients. There is then a danger that echoes from natural
flaws appear too small. For this reason, an estimate
must always be made of the effects of attenuation on the
evaluation result and taken into account if applicable.
If the test object has a rough surface, part of the incident
sound energy will be scattered at its surface and is not
available for the test. The larger this initial scattering, the
smaller the flaw echoes appear, and the more errors oc-
cur in the evaluation result.
It is therefore important to take the effect of the test ob-
ject's surfaces on the height of the echo into account
(transfer correction).