Spread Spectrum Ultrasonics

Spread spectrum 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 ultrasonicwaveform. The signal looks like portions of sign waves cut and pasted together.
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.

A complicated circuit of devices is used to produce spread spectrum signals for ultrasonic inspection.

Two significant applications of Spread Spectrum Ultrasonics are:

1. 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 of defects.

In the example below, simulated defects were created by setting a couple of steel blocks on the top of the bridge girder.

Contact Area
Baseline No Flaw --
Flaw 1 One block laying flat on girder 12.5 sq in
Flaw 2 One block standing on its long side 1.25 sq in
Flaw 3 Both blocks standing on their long sides 2.50 sq in
Flaw 4 Both blocks laying flat on girder 25.0 sq in

Large aluminum girders often have transducers afixed to them to be used for future inspections.

The flaw indications in the aluminum beam can be read similarly to other a-scan ultrasonic data.

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 transmissions.

Two flaws were placed inside structures for ultrasonic inspection.

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.

SUF-1 shows two ultrasonic pulses that overlap each other. THe resolution of the pulses is, therefore, somewhat poor, but the flaws have very different amplitudes.

SUF-2 testing has very good resolution as shown by the two distinct pulses indicating the two flaws. Unlike SUF-1 test results, the flaws of the SUF-2 tests yield the same pulse amplitude.