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Another Step Towards Rapid UT Inspection of Complex Composite Aircraft Components Sebastian Gripp, NUKEM Nutronik GmbH, Alzenau (D) Mario Turconi, Aermacchi SpA, Varese (I) Contact

The use of composite materials in aerospace applications stretches back to the '70s and '80s and they are now employed extensively for the manufacture of the fixed and moving surfaces of wings and tails, engine nacelles, doors and many fairings, with ever rising demands in terms of quantity and quality.

The need to monitor the airframe and engine throughout a service life that includes very severe mission profiles has forced manufacturers and operators to devote considerable efforts to finding new solutions and to refining conventional NDI methods.

Fig 1: Scanning a round part in continous rotation

Fig 2: Elliptical part in continous rotation for scanning

The most common techniques are ultrasonic, X-ray and sonic inspection methods. Ultrasonic methods are most widely used, and encounter vast improvements. The increased production of composite parts of medium and large dimensions, such as the nacelles for the CFM56-5A and CFM56-5B, and the Pratt & Whitney PW4168 engines of the Airbus A320, A321 and A330, and fairings for the EF2000 and Fairchild Dornier 328, has led Aermacchi to acquire a new UT inspection system capable of meeting current and future production inspection requirements.

Local, regional and international market research revealed that off-the-shelf ultrasonic systems did not satisfy Aermacchi's technical requirements. Finally the choice was made for Nukem Nutronik's MAUS UT inspection systems (Multi Axis Ultrasonic System), which has been commissioned in December 1998. The overall scanning ranges are 8.000 mm in X, 5.000 mm in Y, and 2.000 mm in Z direction. Both horizontal and vertical swivelling of the squirters is allowed for.

It allows the operator to programme the movement of the ultrasonic probes for the inspection of complex 3-D geometric profiles. Furthermore it allows the rapid inspection of round parts on a turn table in continuous rotation, see fig. 1. The most outstanding feature is the possibility to inspect even non rotational symmetric parts in continuous turn table motion, which gives unprecedented productivity for these parts (see fig. 2)

System description

The fully automated system is equipped with the latest digital ultrasonic instrumentation UTxx by NUKEM Nutronik, and mananaged by a computerised control station. It is designed to inspect parts using pulse-echo or transmission technique. Water jets produced by squirters provide acoustic coupling between the probes and the parts under test.

The transmission technique, generally used for composite structures, allows the simultaneous inspection of low and high ultrasonic attenuation areas without loss of system sensitivity in defect detection. Therefore, the UT electronics provides 80 dB of dynamic range and a logarithmic amplitude evaluation. Linear evaluation may be added optionally.

The special probe motion system allows both the continuous and bi-directional scanning of parts. Continuous scanning eliminates the downtime associated with decelerations, pauses and accelerations of axes, and increases inspection speed, thus providing lower machine wear and higher sensitivity.

The system is fitted with two turntables, thus eliminating the downtime associated with the loading and unloading of the parts to be inspected and the setting up of the machine.

It also offers the possibility of "batch" programming (using different sequences of programmed scanning of the whole area to be inspected). This allows the operator to programme a sequence of inspections with a single set-up and with no need to monitor the operations.

The scan plan is edited with a sophisticated geometry editor or simply imported from CAD data. Scans may be continued without loss of data, whenever they have been interrupted, be it by purpose or by accident.

Thanks to this new system, Aermacchi can carry out simultaneous ultrasonic inspections of several sub-assemblies of the same engine nacelle, and reduce data acquisition time. This performance allows the system to inspect structural surfaces in less than half the time taken by previous systems.

The systems high data processing, filing and printing capabilities reduce the analysis time required by the operators and thus make them available for the preparation of the batches to be loaded on other machines.

While it can cope with parts as big as 3 meter in diameter or more, it is accurate enough to inspect quite small items, e.g. using a fork probe holder (see fig. 3).

Fig 3a:	Fig 3b:	Fig 4a: Fig 4b:
Fig 3: High pressure CFK reinforced pipe inspected with fork, UT inspection result

In order to assure a maximum level of inspection reliability, several qualification programs using standard reference panels with artificial defects are employed. Figure 4 shows a sketch of a reference panel from the Airbus A 321 programme, together with its UT result. All defects are revealed at about 20 dB signal to noise ratio.

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