Abstract

The inspection of large aerospace CFRP components with ultrasonic techniques is subject to very challenging requirements in terms of ensuring a reliable and time efficient non- destructive examination. The different approaches such as squirter, immersion and contact techniques are discussed under these aspects, which seem to be contradictory at first view.

Assessing the advantages and the weak points of each of these methodologies, the conclusion resulted in a contact technique combining high frequency conventional and low frequency phased array probes. This system facilitates pulse echo technique and backwall monitoring - even in the nonparallel sections - as two complementary sources of information about the structural integrity of the component.

The development of this technique and its optimization performed have helped to determine the optimized parameters of such techniques and the optimum number of channels between "very high" associated with high cost fast examination and "low" as a low-cost version, which in turn requires a considerable time for the examination.

Most of this work leading to the implementation of such system was performed in the framework of the German research project MaTech, partly sponsored by the Federal Ministry for Education and Research. Project partners are Airbus, Bremen, Ingenieurbüro Dr. Hillger, Braunschweig and intelligeNDT Systems & Services, Erlangen.

The experience achieved with this system has proven its compliance with the performance as planned under the aspect of reliability and time-efficiency.

Introduction

The inspection of large aerospace CFRP components poses some considerable challenges to the application of NDE to be integrated into the fabrication process in an optimum way. The criteria for an optimum solution seem to be almost contradictory:

• The integration into the fabrication sequence leaves a short time interval for the examination performance relative to the large dimensions of the components to be scanned
• For the purpose of safety, the pulse echo technique and through transmission or pitch and catch technique via backwall signal monitoring should be combined. For the latter case, the method has to cope with nonparallel contours
• The relatively high sound attenuation would call for lower frequencies
• The resolution of defects even in immediate subsurface location would call for higher frequencies
• The demands for the positioning accuracy are very stringent.

Components to be inspected

Fig 1: Vertical Stabilizer on the Test Bench.

The system has been designed to examine large CFRP components like the shell of the vertical stabilizer. The dimensions of such component are typically in the range of up to 20 m in height, and from 2 to 4 m in width.

It is obvious that, in the interest of an at least reasonable time frame for the examination of such component, inspection systems need to have a large index pattern realized by a wide footprint of the system with sufficient beam overlap in the index direction. The size of defect to be detected is a 6 x 6 mm² square reflector in the orientation of a delamination, i.e. parallel to the layers in the structure. This criterion means that a defect should be hit at least twice, resulting in a 3 mm index rastering pattern.

Comparison of different approaches

In the conceptual phase, the advantages and disadvantages of the three categories of available methodology have been thoroughly studied. They can be summarized as follows:

• The squirter technique works predominantly in the through transmission mode. However, recent development facilitates the use of pulse-echo technique additionally. Still, questions such as the signal-to-noise-ratio and the near surface resolution need to be answered to complete satisfaction.
  In a necessary multiple sensor arrangement, the water jets tend to interfere with each other, if they are too close to each other, disturbing the laminar water flow in the jet for an impracticably large width of the system if more squirters should be used. This makes the system bulky with a complex rastering scheme, if the geometry of the component is not ideally rectangular.
• The immersion technique requires a precise adjustment of the angular positioning of each probe due to the relatively large influence on the refraction angle in the case of even slight deviations of the plain component geometry such as some curvature systematically occurring at the peripheral parts of the component.
  This would mean an unreasonable effort of 3-d-controlled positioning of each of the multiple probes.
• The contact technique allows for the arrangement of a multiple probe system with relatively simple probe holder system allowing the probes to individually adapt to the geometry they are riding on. If probes with different parameters such as frequency are to be used, they can easily be accommodated whereas the other two techniques may require different jet or immersion distance conditions, which would even more complicate the complex system.

Conceptual Considerations

The conclusion of the study comparing the capabilities and the potential of the three techniques clearly spoke in favour of the contact technique approach. The probe system combines high frequency conventional probes for probe-near surface-near examination in pulse-echo technique as well as the continuos monitoring of the through transmission by the back-wall reflection in the case of smaller wall thickness. Pulse-echo-technique and the monitoring of the backwall reflection for larger wall thickness as well as backwall reflection in the case of non-parallel contour is the task for the low frequency phased array probes.

Fig 2: Basic Methodology Concept.

The intent of the use of phased array probes for the monitoring of the backwall echo is the following. As soon as the backwall contour is non-parallel, the beam of a conventional straight beam probe is deflected at the backwall and no or a minimal backwall signal is obtained. In this case the lack of backwall reflection does no more allow the interpretation of backwall loss as a delamination. The phased array probe allows to vary the beam angle and therefore to obtain a backwall signal even in non-parallel sections. The backwall signal would only be missing if a delamination is interrupting the beam propagation.

In a study jointly performed by Airbus and intelligeNDT Systems and Services, the basic parameters of such system have been determined and the UT-system in its basic features has been designed.

This design of the probe system was performed in terms of optimizing the number of conventional and the number of phased array probes. The following criteria were taken into account:

• the required indexing and
• data sampling,
• rastering pattern optimization,
• probe holder design,
• manipulator design,
• inspection time and
• cost-time benefit considerations

Subsequently, a research program, partly sponsored by the Federal Ministry for Education and Research has been launched.

The first purpose was to lead the multisensoric approach through another phase of optimization in terms of parameters like frequency, damping (spectrum), transducer sizes, beam shape in the interest of beam overlap. The target was to achieve the maximum resolution in order to be able to detect delamination of the first layers counting from both surfaces.

The second purpose was to establish the design principles for a multiple-multichannel and multipurpose UT equipment (M³-UT-System) Saphir ^plus including the optimized modular software package supporting the full-scale examination of components like the Airbus 380 vertical stabilizer.

Optimization of the multisensoric Approach

These activities were mainly relying on an iterative process including modelling, experimental investigations on testblock fully representative for original components with real or realistic defects and the design of new probes as prototypes.

The original concept of combining the advantages of two different probe types for different wall thickness layers was kept.

Subsystem with Conventional Probes
After the optimization of the probe parameters including wedge design for optimum

resolution and commensurate sound attenuation proven by experimental investigations, these probes had to be combined into an array in order to provide the required beam overlap.

For this reason, the probes were arranged within a lateral distance of ~ 3 mm which means a slightly distorted triangular pitch of the probe positions. Each probe array with such arrangement consists of 8 probes. In the interest of an even larger system footprint, a total of 12 arrays were combined which means a total lateral coverage of the system of 280 mm.

From this probe arrangement, the main requirement for the UT system is the number of channels for the conventional probes, which has to be 96.

Subsystem with Phased Array Probes
The use of the phased array system is restricted to the lower section of the vertical stabilizer, which - with the larger wall thickness - constitutes a smaller part of the entire component.

For this reason, the reduction of scanning time due to a very large lateral coverage is of less importance.

Therefore and in view of the larger number of channels required for each phased array probe, the number of 10 phased array probes was selected.

The required lateral beam overlap could be maintained by a indexing of the activation of a subset of elements within each phased array probe.

The total lateral coverage amounts to 80 mm.

Combination into Multisensor Arrangement


Fig 3: Multisensor Examination Module.

These subsystems had to be combined into one system, which operates the conventional probes in the section with smaller wall thickness. In these areas, the conventional probes have two tasks: pulse echo testing and backwall monitoring. In the section with the larger wall thickness, the conventional system is operating in pulse echo mode only, the phased array system works in pulse echo mode in the larger wall thickness range as well and is monitoring the backwall echo continuously. In this section, both subsystems are operating simultaneously.

M³-UT System Saphir ^plus
The main system features of the multiple-multichannel and multipurpose UT equipment (M³-UT-System) Saphir ^plus are:

Fig 4: Saphir plus M3-UT System.

• Modular architecture up to 448 channels
• Combination and interaction of phased array and conventional probes in one system
• Various data reduction modes: RF, Pixeling, ALOK, Gates
• Precise beam pattern control (vertical, horizontal or combined skew)
• Equipment parameters for most challenging requirements
• Extensive self-check
• Automated fast self calibration of all equipment functions in accordance with national standards
• Extensive data base of individual probe and probe system data for easy set-up, system planning and operation
• Real-time scanning for only one scanning movement without rastering (no meander)
• Different access levels to the system according to the individual qualification both for data acquisition and analysis

Specific Software Features

Within the large range of the modular Saphir ^plus software package some modules which are of specific assistance for the fast and reliable assessment of the CFRP-components integrity shall be named:

The so-called D-scan is principally a C-scan with the measured wall thickness represented by colour code.

The routine combined top, side and end view display with zoom function and A-scan as identified by cursor positioning was slightly adapted for the specific conditions of CFRP- examination, like separated backwall display (otherwise the C-scan would be completely blotched). The capability of the software to combine a large number of channels into one display helps to generate a clear uninterrupted overview over a larger section of the component as if it had been raster-scanned.

Experience Highlights

Fig 5: Results of SquirterSystem: Porosities remain undetected.

Among the large experience achieved with the system since the start of its operation in 2001, the most significant results were obtained in an internal Round Robin Test using a section of a CFRP component. Aside from intentionally inserted defects representing delamination in all kinds of occurrence, porosities were inserted additionally. The reliable detection of these porosities was the decisive criterion for the level of performance of the different systems, specially with regard to minor composite porosities and even more or less singular porosities.

The test piece was examined by squirter system and by the multisensor multichannel system.

The results, as displayed in Fig. 5 and Fig.6, clearly demonstrate, that the detection of the delaminations did not pose any difficulty for both systems. However, as is self-evident, the through transmission technique as used by the squirter technique, cannot give any information about the through wall position of the defects. This information can easily be obtained by the pulse echo technique integrated into the contact multichannel system.

	Fig 6a: Results of Multisensor Multichannel System in same Area: With switched off TGC the near- surface delaminations and the delaminations in the volume can be distinguished because of the different amplitude/colour.
	Fig 6b: Results of Multisensor Multichannel System: The puls echo technique enables the detection of porosities in the upper area of the CFRP testpiece between the 1st and 2nd layer.

The contours of the defects are definitely more clear depicted by the contact technique, which can easily be derived from the finer index used. A defect contouring could be useful, if defect characterization is desirable in the interest of optimizing the fabrication process.

Finally and very important, the porosities do not show up in the display of the results of the squirter technique, whereas thy can easily be recognized in the display of the multichannel contact technique. This is a very relevant difference in the systems’capabilities, as porosities, even minor composite ones have gained considerable interest in the structural integrity assessment of aeroplane CFRP components.

The high signal to noise ratio as obtained with the multichannel contact technique has been confirmed by the large and growing experience gained in the in-process examination of aeroplane CFRP components.

Conclusion

Based on the stringent requirements for an ultrasonic system in terms of timely performance and high sensitivity and defect detection reliability a system has been generated, which is seemingly complex and bulky. However, this system helped to reduce the inspection time by a factor of up to 10 related to systems used up till now. Therefore, the higher cost of such system is compensated by its time-saving performance and reliability within a very short period.

References

1. G. Engl, F. Mohr, Siemens NDT, Erlangen, Germany and A. Erhard, BAM, Berlin, Germany; “The Impact of Implementation of Phased Array Technology into the Industrial NDE Market”, 2^nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, New Orleans, May 2000

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