Dead zone, Near zone, and Far zone in Ultrasonic testing
We have used three terms connected with
the beam of sound in the test material,
namely
2. ‘Near Field’,
3. ‘Far Field’.
Dead zone: The dead zone is that part of the time base occupied by the
initial pulse when using a single-crystal contact probe.
Near zone: The near field is the distance in the material that suffers from
interference effects.
The intensity of the sound beam will not
be uniform due to interference of the sound waves at the exit point of probe.
Far zone: The far field is the rest of the beam beyond the near field.
Variations in intensity due to interference effects occur for some
distance in front of the transducer, as we have just seen. This region is known
as the ‘Near Field’ and the extent of the near field, known as the near
field distance, can be calculated from:
Where: NF = Near field distance
D = Crystal diameter
λ (lambda)= Wavelength
Example 1
Calculate the Near field distance for a 10 mm-diameter 5 MHz crystal
transmitting into steel (Velocity 5960 m/s therefore “lambda” λ = 1.192 mm).
NF=100 /4.x768
NF
= 21 mm (approx.)
This means that for this probe, in
steel, we can expect fluctuations in intensity of sound for the first 21 mm of
steel depth due to interference effects. As a result, it is unwise to rely
solely on amplitude as the criterion for acceptance or rejection of the part
for discontinuities that are in the near field region.