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In what sense then has the
introduction of in-situ scanners caused a revolution in NDT?
Prior to the scanning revolution,
in-situ NDT methods could be divided into two categories: spot
measurements, and single-shot images.
In general ultrasonic, eddy-current and mechanical impedance methods were
in the spot-measurement category and X-ray and gamma radiography in the single-shot
imaging category. Recent developments such as magneto-optic (eddy-current)
imaging and Compton back-scattering profilimetry (X-ray spot measurements)
have caused a blurring of these categories somewhat. However, it is the ability
to scan a spot-measurement device over an area and build up an image that
has introduced a third category, commonly referred to as C-scanning.
C-scans have been produced
in the laboratory for many years, generally for ultrasonic imaging of flat
specimens such as carbon-fiber composite skins, but relatively little work
had been done on scanning with eddy-current or mechanical impedance devices.
The revolution that has followed the widespread introduction of in-situ scanners
has been due to the enhanced capabilities of this spot-measurement category
of techniques.
Improved
Visualization
of Defects
A very good example of the revolution is in mechanical
impedance methods. Whilst the method was capable of detecting variations in
mechanical impedance indicative of the presence of a defect, the interpretation
of the response was difficult. Inspections were often inconclusive because
structural variations could cause even larger responses. Figure 1 illustrates
the remarkable imaging resolution obtainable on a thin-skinned metal honeycomb
structure. A spot measurement on this material would be very confusing because
the signal would vary greatly within each cell of the honeycomb. However,
the ability to look at a C-scan means that the disbonded area can be clearly
identified. Effectively, the eye can filter out the honeycomb pattern variations
by looking for changes in that pattern.
Figure 1. Mechanical Impedance scan of
aluminum
honeycomb. When Mr. Trevor Liddell of NDT Squadron (RAF Swanton Morley) first
saw this scan, his comment was that “ANDSCAN has turned mechanical impedance
into a usable in-service method!”
Eddy-current and ultrasonic methods also benefit from
this use of the eye’s ability to filter out regular patterns, such as fasteners
in a scan of a metal lap-joint(1,2)
(see Figure
2 and Figure
3). Eddy-current impedance-plane instruments are notoriously difficult
to interpret for complex structures because of edge effects, probe handling
and changes in conductivity or thickness. Scanning can remove a lot of these
interpretation problems.
Figure 2. Single-frequency eddy-current scan of corrosion
in a KC135 lap joint. The number of layers varies from 1 to 5.
Figure 3. Ultrasonic depth scan of corrosion in a
KC135 lap joint.
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Figure
4 illustrates how conventional ultrasonic thickness gauge measurements,
with or without data logging, can be improved by using a scanning system and
a conventional ultrasonic flaw detector in time-of-flight mode. The software
allows the operator to move over any point on the scan and read off the measured
thickness.
Figure 4. Ultrasonic thickness scan showing thickness
remaining after blending out of corrosion.
One further example of the benefits of C-scanning
rather than spot measurements is the ability to identify patterns. For example,
ply stacking sequences can be determined in carbon-fiber composite skins by
C-scanning methods where the fiber direction can be clearly identified at
the depth of the reflected signal being monitored(2).
Other enhanced visualization
methods such as cross-sectional slices, rotating pseudo-3D images and contour
plots are shown in Figure 5 and
Figure 6.
Figure 5. Cross-section and pseudo-3D
visualization
of defects.
Figure 6. Contour plot.
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Post-processing
Capabilities
Simple image-processing techniques such as those used
for photographic or radiographic images can be of similar benefit for C-scan
images. For example, the use of scalable color palettes helps to enhance
contrast in order to extract weak defect indications (see Figure 7).
Figure 7.Ultrasonic Thickness - Simple use of
color palettes
An array of different sizing and
characterization
methods can be employed to analyze C-scan images. The standard -6 dB
defect sizing method can be employed using tools built into the software.
Figure 8 and
Figure 9 illustrate
how several different characterization techniques can be used. The data produced
can be copied into a spreadsheet using standard Windows commands.
Figure 8. Quantitative Analysis: Defect Sizing and Histograms with mean, max, min
and standard deviation for a selected area.
Figure 9. Automated -6 dB width.
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