Ultrasonic Evaluation Of Flaws; Flaw Boundary Method; Echo Display Comparison Method - GE USM 36 Series Technical Reference And Operating Manual

Important information on ultrasonic testing

Ultrasonic evaluation of flaws

In present-day test practice, there are basically two dif-
ferent methods of flaw evaluation:
If the diameter of the sound beam is smaller than the ex-
tent of the flaw, then the sound beam can be used to ex-
plore the boundaries of the flaw and thus determine its
area.
If, however, the diameter of the sound beam is larger
that the extent of the flaw, the maximum echo indication
from the flaw must be compared with the maximum
echo indication from an artificial flaw provided for com-
parison purposes.

Flaw boundary method

The smaller the diameter of the probe's sound beam,
the more accurately the boundaries, i.e. the actual flaw
area, can be determined by the flaw boundary method.
If, however, the sound beam is relatively broad, the flaw
area determined can substantially differ from the actual
flaw area. Care should therefore be taken to select a
probe which will give a sufficiently narrow sound beam
at the position of the flaw.
USM 36

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.
Issue 2 (12/2013)
1 Introduction
1-7