|NDT.net - October 2002, Vol. 7 No.10|
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.
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:
|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 mm˛ 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.
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 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:
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 (Mł-UT-System) Saphir plus including the optimized modular software package supporting the full-scale examination of components like the Airbus 380 vertical stabilizer.
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.|
M3-UT System Saphir plus
The main system features of the multiple-multichannel and multipurpose UT equipment (Mł-UT-System) Saphir plus are:
|Fig 4: Saphir plus M3-UT System.|
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.
|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 systemscapabilities, 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.
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.
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