ultrasonics makes use of the correlation of continuous signals
rather than pulse-echo or pitch-catch techniques.
Spread spectrum ultrasonics is a patented new broad band spread-spectrum
ultrasonic nondestructive evaluation method. In conventional ultrasonics,
a pulse or tone burst is transmitted, then received echoes or
through-transmission signals are received and analyzed.
In spread spectrum ultrasonics, encoded sound is continuously
transmitted into the part or structure being tested. Instead of
receiving echoes, spread spectrum ultrasonics generates an acoustic
correlation signature having a one-to-one correspondence with
the acoustic state of the part or structure (in its environment)
at the instant of the measurement. In its simplest embodiment,
the acoustic correlation signature is generated by cross correlating
an encoding sequence, with suitable cross and auto correlation
properties, transmitted into a part (structure) with received
signals returning from the part (structure).
Section of biphase modulated spread spectrum ultrasonic waveform
Multiple probes may be used to ensure that acoustic
energy is propagated through all critical volumes of the structure.
Triangulation may be incorporated with multiple probes to locate
regions of detected distress. Spread spectrum ultrasonics can
achieve very high sensitivity to acoustic propagation changes
with a low level of energy.
Two significant applications
of Spread Spectrum Ultrasonics are:
- Large Structures that allow ultrasonic transducers to be "permanently"
affixed to the structures, eliminating variations in transducer
registration and couplant. Comparisons with subsequent acoustic
correlation signatures can be used to monitor critical structures
such as fracture critical bridge girders. In environments where
structures experience a great many variables such as temperature,
load, vibration, or environmental coupling, it is necessary
to filter out these effects to obtain the correct measurements
In the example below, simulated defects were created by setting
a couple of steel blocks on the top of the bridge girder.
||One block laying flat on girder
|| 12.5 sq in
||One block standing on its long side
|| 1.25 sq in
||Both blocks standing on their long sides
|| 2.50 sq in
||Both blocks laying flat on girder
|| 25.0 sq in
2. Piece-part assembly line environments where transducers and
couplant may be precisely controlled, eliminating significant
variations in transducer registration and couplant. Acoustic
correlation signatures may be statistically compared to an ensemble
of known "good" parts for sorting or accepting/rejecting
criteria in a piece-part assembly line environment.
Impurities in the incoming steel used to forge piece parts may
result in sulfite stringer inclusions. In this next example simulated
defects were created by placing a magnetized steel wire on the
surface of a small steel cylindrical piston used in hydraulic
Two discrimination technique are tested here, which are SUF-1
and SUF-2, with the latter giving the best discrimination between
defect conditions. The important point being that spread spectrum
ultrasonics can be extremely sensitive to the acoustic state of
a part or structure being tested, and therefore, is a good ultrasonic
candidate for testing and monitoring, especially where scanning
is economic unfeasible.