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reduces therefore the losses.
However, there are components which should have no contact with water or any other coupling
medium. This is the case e. g. for honeycomb components and sandwich parts or parts
with a foam core. For such components the air-coupled method can be applied. This technique
uses a high sound pressure to compensate losses and low frequencies (50kHz to
some 100 kHz). The scheme of air-coupled ultrasonic testing in transmission is shown in
figure 3, an example is shown in figure 4. A double shell structure with foam core inside is
investigated by impact tests. The stringer debonding is clearly visible in the C-scan.
Zeit
Amplitude
Zeit
Amplitude
Transmitter Receiver
Transmitter Receiver
Flaw
Figure 3: Air-Coupled Ultrasonic Testing
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Figure 4: Air-coupled UT Example: Double Shell Structure (top), C-scan (lower bottom)
5. Porosity Measurement
Prorosity in composites may degrade the stiffness of the structure. Porosity has to be detected
therefore in production. At Airbus Germany the requirement is that at most 2.5% volume
porosity is allowed. An ultrasonic testing method has been developed and qualified to
detect porous areas in CFRP components.
In porous areas often no intermediate echo occurs, because pores may scatter the incident
sound in all directions. The method is therefore based on measurement of backwall echo
reduction due to pores. The backwall echo amplitude decreases, if the sound wave is attenuated
by porosity. It could be shown, that there is a good correlation between backwall
echo reduction and volume porosity determined by micrographic analysis, see figure 5. Such
correlation diagrams are recorded for all different material type and thickness combinations
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used in production. The similarity of ultrasonic behaviour regarding porosity was shown by
approved statistical tests performed on a large data basis. Because of this similarity, different
material types may be merged. As a result a unique ultrasonic backwall echo reduction
threshold value for classifying porosity as greater than 2.5% by volume has been elaborated.
These threshold values depend on thickness only and are decisive for all epoxy resins reinforced
by carbon fibres in unidirectional built-up, fabric built-up and mixed built-up (based
upon statistical security by an adequate confidence level). Similar threshold values will be
available soon also for GFRP.
0
2
4
6
8
10
12
14
16
18
0 0,5 1 1,5 2 2,5 3
Porosity [% by volume]
BE reduction [dB]
Figure 5: Ultrasonic Backwall Echo Reduction versus Volume Porosity
4. Laser Ultrasonic Testing
Laser ultrasonic testing combines features of optical inspection (contactless, twodimensional)
with those of ultrasonic testing (looking inside the part), see scheme in figure 6.
A pulse laser is directed onto the surface of the component under test. It generates an ultrasonic
wave, propagating in the material. As for conventional ultrasonic testing the signal is
reflected at flaws and interfaces (backwall). The reflected signal is detected optically, e. g.
using a second (long pulse) laser and an interferometer. After filtering the detected signal is
similar to a conventional A-scan. Scanning of the focussed laser pulse over the surface generates
a C-scan.
The main advantage of laser UT is that the sound waves propagate always normally to the
part surface, independently from the incidence angle of the laser light. The method is therefore
predestinated to inspect parts with complex geometry, where conventional multi-channel
systems cannot follow the surface shape and manual testing is not economical. On the other
hand laser UT facilities are (presently) very expensive and complex and have a low scanning
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speed, because only one channel (laser spout) scans the components. Some development
work is still necessary to make this technique mature for NDT in civil aircraft production.
An example is shown in figure 7. The inspected surface is at the corner. The Laser UT Ascan
is comparable to A-scans of conventional ultrasonic testing. Normally this part is inspected
manually. An automatic inspection by conventional ultrasonic testing with transducers
is difficult due to the complex geometry.
Figure 6: Laser UT
Inspected surface A-scan
Backwall
echo
Surface
echo
C-scan artifical flaws
Figure 7: Laser UT Inspection of a Component with Curved Geometry
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7. Shearography
20 years ago the NDT community was very enthusiastic concerning the capability of the new
method Holographic Interferometry (HI). Hi is an optical two-dimensional method with very
　
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