56
the probe is directly contact with the tested object.
8.4.5 Magnetic Field Influence
It will disturb the flaw detection seriously by the strong magnetic field generated by
the electrical equipment around.
8.4.6 Flaw Evaluation Method
In the present flaw detection, there are generally two different flaw evaluation
methods:
if the acoustic beam diameter is smaller than the flaw size, it can be used to
detect the flaw boundary and fix the area.
if the acoustic beam diameter is bigger than the flaw size, the biggest echo of
the flaw should be compared with that of the artificial flaw.
(1) Flaw Boundary Method
The smaller the acoustic beam diameter is, the more precise of the flaw boundary
and area are. But if the acoustic beam is relatively wide, the fixed flaw area may
be obviously different from the actual one. Therefore, it is cautious for you to
choose the probe which can obtain the concentrated acoustic beam as narrow as
possible in the flaw area.
(2) Echo Comparison Method
A natural flaw reflection echo is usually lower than that of an artificial flaw (e.g.
similar size disc flaw). This situation is caused by rough surface of natural flaw or
an unfavorable angle coming out when sound beam transmits onto flaw. If it is not
considered in the natural flaw evaluation, there may be risk of underestimating the
equivalent value. Regarding irregular or split flaw, for example shrinkage holes in
castings, will possibly appear strong acoustic scattering on surface of flaw
boundary, thus do not produce any echoes. In this case, different analysis method
should be taken, for example using bottom surface echo attenuation method in
analysis. Distance sensitivity of flaw echo plays an important role in huge work
piece flaw detection. When choose artificial contrast flaws, you should pay
attention that these flaws also follow the distance variation rules, same with
natural flaws.
Ultrasonic waves will attenuate when propagating in any materials, generally this
