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Manual and automatic ultrasonic testing with single element transducers and linear phased
array probes in pulse echo and through transmission mode is applied mainly. But also resonance
methods, shearography, thermography and special methods are used. Laser ultrasound
is a candidate for future contactless automatic inspection.
1. Introduction
Composite materials are used more and more in aircraft production. Main composite types
are Carbon Fibre Reinforced Plastics (CFRP), Glass Fibre Reinforced Plastics (GFRP) and
metal-aluminium laminates (e. g. Glass Fibre Reinforced Aluminium GLAREÓ). Typical parts
made of CFRP are flaps, vertical and horizontal tail planes, center wing boxes, rear pressure
bulkheads, ribs and stringers. For the Airbus A380 GLARE is used even for some shells of
the upper fuselage. The weight percentage of composites in modern civil aircrafts like the
A380 is in the order of 25%. It may be expected that this percentage will further increase for
the next generation of civil aircrafts and that main structure parts like fuselage and wings will
be composed of composites, too.
These composite parts require adequate Non-Destructive Testing (NDT) methods. NDT is
applied in production as well as during maintenance (in-service). Flaws to be detected are
delaminations and debondings, porosity and foreign body inclusion. The most applied NDT
method is ultrasonic testing (manual as well as automatic, in pulse echo and through transmission,
with single element transducers and linear phased array probes). But also resonance
methods, shearography and thermography and special methods are used. This papers
reviews the present NDT methods for composite inspection in aircraft industry and reports
about future demands.
2
2. Manual and Automatic Ultrasonic Testing
Manual ultrasonic testing with single element transducers is still the most applied method for
composite parts with small and medium size. The inspectors are evaluating the A-scans for
flaw detection. Flaw sizes are measured by manual scanning of the probe around the maximum
echo, e. g. using the 6dB method.
In the last years the first linear ultrasonic phased array (PA) inspection systems were applied
for aircraft inspection. With PA’s any desired wavefront can be generated by pure electronic
means. The individual transducers are triggered with a certain delay. The superposition of
individual wavefronts results to a new wavefront with the desired properties. The PA techniques
can be used to tilt and focus a sound beam by electronic means and for electronic
scanning of a sound beam. Another advantage with respect to single element transducers is
the availability of images (B-scan, C-scan, S-scan) instead of only A-scans, which allows
better interpretation of signals inside complex structures. An example of PA inspection of a
bonded stringer is shown in figure 1. The signals of various reflecting surfaces and interfaces
can be clearly identified in the B-scan (right part of the figure).
CFK - stringer
CFK – outer skin (thickn.: 2mm - 8mm)
PHASED ARRAY – Probe / 5MHz
128 elements (Array: 96mm x 10mm)
+ water path
1
128
Wasservorlaufstrecke
Backwall echo outer skin (BE)
Intermediate echo glue line (IE)
Backwall echo stringer (BE)
No backwall echo on stringer web
z.B.: A – scan, element 65
5L128E96-
10
Surface echo outer skin (FSE)
FSE
IE
B
E
Figure 1: Phased Array Inspection of Stringer Skin Bonding
Big CFRP parts are inspected automatically or semi-automatically with multi-channel facilities.
Such machines consist of a portal, which moves the inspection probe over the surface
of the part. An example from an Airbus multi-channel facility working in pulse-echo mode is
shown in figure 2. The probe consists of 96 individual transducers (channels). The maximum
testing area is 13m ´ 7m, with a speed of 100mm/s. The part in the figure is a rear pressure
bulkhead. At Airbus multi-channel facilities are used to inspect vertical and horizontal tail
planes (shells, up to 10 meter long), rear pressure bulkheads, flaps and other long or big
CFRP parts. Evaluation of data is assisted by special software tools.
3
Figure 2: left: Airbus Stade Multi-channel Facility No. 2; right: D-scan
3. Air-Coupled Ultrasound
In ultrasonic testing a coupling medium (usually water) is used to couple the sound field in
the component with minimum losses. Losses due to reflection occur at interfaces, where the
sound impedance changes drastically. This is the case e. g. at the interface between air and
part. This impedance change is much lower between water and part. The coupling medium
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