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There are four main formats for displaying ultrasonic data; A, B, C and D-scans.
A-Scan: This format provides quantitative information concerning signal amplitudes and time-of-flight data obtained at a single point on the surface of the specimen. The amplitude of the received signal, and its position relative to the signals corresponding to the upper and lower surfaces of the target, indicates the degree of severity and location of the damage or defect. The A-scan display is used to analyse the type, size and relative depth of discontinuities.
B-Scan: This format provides a quantitative display of time-of-flight data obtained along a line of the test specimen. The B-scan is essentially a linear collection of A-scans and can be considered as equivalent to taking a through-thickness slice of the specimen. B-scan displays show the relative depths of discontinuities and are used mainly to determine size (length in one direction), location (both planar position and depth) and, to a limited degree the geometry and orientation of damage or defects.
C-scan: This format provides a two-dimensional scanning pattern of ultrasonic attenuation, with threshold discrimination in the form of either a grey scale or a range of colours. For this type of presentation the transducer is scanned, in a plane that is essentially parallel to the specimen surface, in a rectilinear raster pattern. The C-scan format can also be used to display time-of-flight data.
D-scan, Time-of-Flight Scan or Depth Scan: This is essentially a C-scan format where a two-dimensional map of time-of-flight data is recorded rather than amplitude data. The time-of-flight mode is more effective for inspecting the integrity of an adhesive bondline between composite skins of uniform thickness and non-metallic substructures.
Scanning Acoustic Microscopy (SAM) utilizes very high frequency (5 to 75 MHz) ultrasound to view internal material integrity. Frequencies in this range are highly attenuated in air, and therefore the part is immersed in coupling medium, usually deionised water. The system is generally operated in pulse-echo mode with a single ultrasonic transducer emitting and receiving the ultrasonic beam. The ultrasonic beam is focused using lenses to a specific range of depth in the test sample. Since ultrasonic transducers have depth of field, the entire sample may be in focus on thinner parts. The ultrasonic signal is a short pulse that travels through the material being tested. It is reflected back to the transducer when it strikes interfaces within the material. The transmitted sound waves that are reflected back arrive back at the transducer at different times, which correspond to different depths in the material.
The ultrasound data is digitised (A-Scan) and amplitude, phase and time-of-flight (depth) data is processed and imaged (C-Scan). When the sound wave strikes and air gap, a complete reflection of the ultrasonic energy is returned and the resultant amplitude C-Scan image shows the defect as bright areas. It is also possible to generate an image that is a cross-sectional view of the depth and amplitude data (B-Scan). The system can also operate in single through-transmission mode using two transducers, one as a transmitter and one as a receiver. This enables the display of all internal defects in one low-resolution C-Scan image. Special software can be used to generate a tomographic scan of a sample, thus enabling all of the internal interfaces of the sample to be displayed in a single scan.
Ultrasonic Spectroscopy can be used to detect the damage caused to fibre-reinforced polymer materials due to hygrothermal ageing. Acoustic parameters, such as velocity and attenuation, are linked to the viscoelasticity and microstructure of the propagation medium. The acoustic parameters can be measured by means of pulsed ultrasonic spectroscopy. It is possible to relate changes in these properties to moisture content and the level of material degradation, and also identify damage mechanisms. Changes in frequency dependency of velocity and attenuation have been related to matrix cracking.
Coin Tap Test: This involves using a coin (10 p) to tap a bonded structure in different spots, listening for a change in sound, which would indicate the presence of a defect (i.e. debond). In the hands of experienced personnel this technique has proved successful in accurately detecting debonds and delaminations. Electronic hand-held devices are commercially available.
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