Bundesanstalt für Materialforschung und -prüfung

International Symposium (NDT-CE 2003)

Non-Destructive Testing in Civil Engineering 2003
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Improvement and Application of NDT Methods in Civil Engineering in the Frame of a Collaborative Research Project funded by the German Research Society

Hans-Wolf Reinhardt, Christian U. Grosse
University of Stuttgart, Collaborative Research Project " Forschergruppe 384²,
Pfaffenwaldring 4, 70550 Stuttgart, Germany,
Phone: +49-711-6853323, Fax: +49-711-6857681; email: reinhardt@iwb.uni-stuttgart.de

Abstract

A collaborative research project (FOR 384) has been founded in Germany. The aim of the FOR 384 is the methods for investigating concrete structures to improve in such a way that the systematic application of NDT diagnosis is possible and the validation for many tasks is achievable. The isolated developments so far should be innovated thus that a cooperative long-lasting cooperation in working groups is possible. Acoustic and electro magnetic methods for the investigation of structures made of concrete will be checked. The members of the FOR 384 and their supporters from practical engineering and the industry try to understand the processes and mutual interdependencies in order to put the methodology of such investigations on a objective scientific basis.

Keywords:
Research cooperation, NDT on severe structures, modern NDT methods

Organisation and members of the research group FOR 384

The German Research Society (DFG) has introduced net-based collaborations in the year 2000. Such a cooperation can consist of researchers of all parts in Germany. Table 1 shows the composition of the group and the members of the so-called supporting groups from the industry.

The FOR 384 is strong because researchers with experimental background can work together with colleagues with theoretical background. Through application of modern methods and adjustment of complex theories from other areas of the NDT and a signal processing a demanding interdisciplinary cooperation is possible. Before the project was granted there were durable personal contacts between the researchers which means that there were no starting difficulties at the beginning.

I Bundesanstalt für Materialforschung und -prüfung (BAM)
Unter den Eichen 87, 12200 Berlin

IV.4 Zerstörungsfreie Schadensdiagnose und Umweltmessverfahren;
Dr. H. Wiggenhauser, Dr. C. Maierhofer,
Dr. M. Krause
VI Universität Dortmund (UDo)
Lehrstuhl für Werkstoffe des Bauwesens August-Schmidt-Str. 8, 44227 Dortmund
Prof. Dr.-Ing. J. Neisecke
Supporting group (Industrial partners) Bundesanstalt für Straßenwesen (BASt)
Deutsche Bahn AG
II Fraunhofer Gesellschaft IZFP-EADQ Dresden (FhG-EADQ)
Krügerstraße 22, 01326 Dresden

Dr. Bernd Köhler, Dr. Frank Schubert
VII Universität Kassel (UKa)
Fachbereich Elektrotechnik
Wilhelmshöher Allee 71-73, 34121 Kassel
Prof. Dr. K.J. Langenberg, Dr. Klaus Mayer
DYWIDAG Systems Int.
Hochtief AG
Agfa-NDT (Krautkrämer GmbH & Co.)
IV Materialforschungs- und -prüfanstalt an der Bauhaus-Universität Weimar (MFPA)
Amalienstr. 13, 99423 Weimar
Prof. Dr. J. Bergmann, Dipl.-Ing. M. Schickert
VIII Universität Stuttgart (USt)
Institut für Werkstoffe im Bauwesen und Otto-Graf-Institut; Pfaffenwaldring 4, 70550 Stuttgart
Prof. Dr. Hans-Wolf Reinhardt, Dr. Chr. Große
  POLYTEC GmbH
  V Technische Universität Darmstadt (UDa)
Institut für Massivbau, Alexanderstr. 5, 64283 Darmstadt
Dr. Otto Kroggel
Co-operating Institute:
Institut für zerstörungsfreie Prüfung (IZfP) der FhG
Universität, Gebäude 27, 66123 Saarbrücken
Dr. G. Dobmann, Dipl.-Phys. W. Müller,
Dipl.-Ing. W. Bähr, Dr. W. Gebhardt
 
Table 1: Members of the collaborative research project FOR 384

Chairman: Prof. Dr.-Ing. H.-W. Reinhardt, Universität Stuttgart
Manager: Dr.-Ing. C. Grosse, Universität Stuttgart

Division into working units - overview

The scientific research work is divided into several working units in order to increase the cross linking of the projects and the synergetic profit. In the area of ultrasound there are the working units reference systems (A1), propagation (A2) and scanning and focussing systems (A3). Impact echo deals with the visual application and analysis of geometrical effects (B1) and the impact echo for investigation of hardening concrete with and without reinforcement (B2). The third working unit radar consists of the projects radar signal conditioning and data analysis (C1) and combined methods (C2). In the unit D, there are signal processing, reconstruction and modelling the themes with a division into signal processing and reconstruction (D1) and modelling (D2).

The project areas are tackled by the several researchers. Coordinators coordinate the work in the work units. The link of the researchers with institutions and industrial firms of the construction industry increase the relevance of the research activities with respect to practical application by constructive critique and feedback but also by supplying actual investigated objects.

Work and progress in detail

The aims can be summarized as follows:

  1. To identify and quantify the basic influence parameters and their effect on the performance of the methods for typical structures
  2. To develop a method for the combined application of NDT methods on structures with a link to simulation analysis and modelling
  3. To develop new methods for the structural application, for instance the air coupled ultrasound, group rays, visualization of scanning impact echo.

The total project consists of four areas. Within the areas there are the working units. The coordinating research institution has been put into brackets.

A:    ultrasound-echo
A1   reference systems (Universität Dortmund)
A2   propagation behaviour (Universität Darmstadt)
A3   scanning and focusing systems (FhG-IZFP/BAM)
B:   impact echo (Universität Stuttgart)
C:   radar
C1   radar-signal conditioning and data analysis (BAM)
C2   combined methods (BAM)
D:   signal processing, reconstruction and modelling
D1   signal processing and reconstruction (MFPA Weimar)
D2   modelling (Universität Kassel/FhG-EADQ)

Unit A (ultrasound)
The aim of working unit A1 is the development of standards for the ultrasound testing in the structural industry. This refers to the production of reference test samples for the quality assessment of measuring methods and to the development of calibrating tests.

The University of Dortmund developed a new ultrasonic-testing-method, to measure elastic properties E and m even of the components "aggregate" and "cement-matrix" of concrete. The properties are calculated from the velocities of longitudinal and Rayleigh-waves VL and VR, measured with probes of 9 mm diameter and frequencies about 2 MHz (Fig. 1). The aim of this investigation is to predict the elastic properties as well as ultrasonic-velocities of concrete from the elastic properties of its components.

Fig 1: Ultrasonic-Probes (f ~ 2MHz) and calculation of velocity VR (University of Dortmund

In the working unit A2, the propagation behaviour of ultrasound and the interaction between the local sound field with inner and outer services and the reinforcement is experimentally determined and will be compared with model analysis.

Until the beginning of the project "FOR 384" as an incorporation of scientists from several institutes S-waves were not used for investigations on concrete practically. Due to the availability of recently developed point contact S-wave transducer arrays, deviating from the original plan, the S-waves were included in the investigation of the propagation of ultrasound in concrete (UDa). The dry coupling of the transducer array allows a fast data collection also on site (Fig. 2).

Fig 2: Filled (green) and unfilled (red) part of the duct, Æ= 80mm (University of Darmstadt

Developed techniques for P-waves such as linear sweep frequency chirp generation with pulse compression also show adequate results for S-wave applications. Experimentally it is verified that the ultrasound reflectivity of filled ducts differs from that of unfilled ones. A doubtless detectability under any realistic conditions is not given yet.

Working unit A3 contains the investigation and further development of scanning and focusing systems in the area of ultrasound. Laservibrometer measurements, arrays with 2D- and 1D-aperture and air coupled ultrasound and phased array.

Generally speaking, the task of BAM is to elaborate the scientific bases of ultrasonic echo, impact-echo and radar in order to extend the field of application for these methods. In this particular unit A, different scanning ultrasonic methods were used and compared. For the quantitative comparison of the ultrasonic methods quantitative parameters were defined in close collaboration with the project partners. An innovation for the investigation of concrete is the possibility to focus and/or deflect the front of ultrasonic wave-fronts (phased array), which has been developed in the project.

Unit B (Impact-Echo)
The B area applies the impact-echo methods in scanning and automatic way. Commercial and also own measuring systems will be used in order to determine the thickness of structural elements and floors in the element. As a sight application, fresh concrete will also be investigated.

For the analysis of area-measured impact-echo datasets a 3-dimensional data visualisation was developed by BAM, which resulted in an analysing model of the influence of the tested structure geometry on the measurements.

To give another example of the research activities in unit B, investigations about the effects on Impact-Echo signals caused by geometrical boundary conditions are presented (Fig. 3). Several specimens were prepared to evaluate the resolution of impact-echo measurements. To be able to detect the thickness of a geometrical complex specimen, it is necessary to illustrate all the measured points as a Frequency-B-Scan.

Fig 3: Left: Frequency-B-Scan of a shallow delamination in ddel = 9cm; right: sketch of step-shaped specimen tested (University of Stuttgart).

Shallow delaminations determine flexural modes of vibration. The frequency of these modes has higher amplitude and is typically lower than the frequency of the thickness resonance. In addition, horn liked frequency peaks appear from the middle of the maximum peak. In the surrounding area of a delamination, these frequencies interfere with the resonance frequency, so that the thickness can not be located exactly.

Unit C (Radar)
The activities in the C area refer to the application of radar on NDT structural analysis. Additionally, in the area C1 will investigate the super-position of combined analysis (data fusion) and acoustic data.

The radar investigations (BAM) were concentrated on the influence of boundary layers on the impulse characteristics.

Furthermore the requirements of the combinations of radar, ultrasonic and impact-echo were designed (BAM). The features resulting from fusing different 3-dimensional datasets have been demonstrated. Fig. 4 shows C-Scans from superposition of different radar measurements and ultrasonic results. In the different depth slices the reinforcement and an artificially grouting fault are clearly imaged.

Fig 4: Four C-scans (1,8 x 1,3 m2) representing different depth slices of the data set calculated by data fusion of radar and ultrasonic data sets (BAM, Berlin).

Unit D (Signal processing, reconstruction and modelling)
The aim of the area D1 is the further development of signal analysis and reconstruction methods. SAFT and polarimetric methods will be used together with Wavelet, moving average and coded signals. Ultrasonic SAFT reconstruction (Synthetic Aperture Focusing Technique) as implemented at the MFPA Weimar delivers two-dimensional sectional images through a concrete element. One of the main objectives of the project is to characterise the image quality by objective means, and to enhance object detection while dealing with various parameter influences. As an example, Fig. 4 shows the SAFT reconstruction of eight small, line-shaped targets in a test specimen with 8mm maximum aggregate size. The targets are diverse hollow and steel cylinders of circular and quadratic shape, all having diameters of 8mm and being located at depths of 255 and 195mm, respectively. The back wall appears at a depth of 346mm. A rebar indication is marked, imaged with a lateral resolution of 26mm and a signal-to-noise ratio of 13dB relative to the noise in the rectangle. It follows that it is possible to image an 8mm rebar covered by 250mm concrete with sufficient contrast.

In D2 modelling methods will be applied and further developed in order to simulate ultrasound 2D and 3D data and impact-echo results - modelling examples of both are presented here. Additionally the validating of modelling tools through experiments and the investigation of the influence of realistic diffusers (i. e. prestressing ducts, reinforcement) and of phase boundaries on modelling.

To give an example of the work in unit D2 first results for ultrasound wave propagation models are presented. Elastic wave propagation in heterogeneous concrete is rather complicated, therefore, the design and optimization of imaging algorithms requires a thorough physical understanding of the underlying phenomena, which can be obtained via modeling. Fig. 5 shows the geometry of a proposed test specimen, which was two-dimensionally modeled with the EFIT-code, i.e. synthetic pulse-echo ultrasonic data were computed within a finite aperture along the surface.

Fig 5: Ultrasonic SAFT reconstruction of a test specimen containing eight small, line-shaped targets (MFPA Weimar).

These data were processed with the FT-SAFT algorithm, and the result is displayed in Fig. 6a. It turned out that the originally proposed 35 cm height of the specimen was way too large to ensure a proper image of even the smallest backwall notch. Hence, for the realization of the specimen a height of only 25 cm was chosen, and, indeed, Fig. 6b confirms a very good agreement between experimental and synthetic results, which, on the other hand, underlines the usefulness of pre-experimental modelling.

Fig 6: Geometry of the modelled test specimen (reality - left, simulated - right) - University of Kassel.

The second example for the work in unit D2 is addressing the impact-echo modelling. The elasto-dynamic finite integration technique (EFIT) has proven to be a very effective, powerful, and flexible method for time-domain simulation of impact-echo testing. A good agreement between numerical calculations and experimental measurements has been obtained as demonstrated in Fig. 7. Therefore, EFIT represents an important tool for a better interpretation of the received signals, for determination of the physical possibilities and limits of the impact-echo method as well as for the development and optimisation of imaging algorithms.

Fig 7: FT-SAFT reconstruction of simulated (200 kHz) (left) and measured data (150 kHz) (right) - University of Kassel.
Fig 8: Comparison of impact-echo B-Scans (left column) and impact-echograms (right column) obtained by numerical time-domain simulation (bottom line) and experimental measurement (top line) along a linear scanning line at the top surface of the concrete specimen under investigation. The geometrical patterns are caused by reflections at the outer boundaries of the specimen and partly affect the accuracy of thickness measurements and flaw detection. (EADQ, Dresden)

Electronic exchange of research results

The increasing use of digital media in the scientific communication and publication has severely changed the former information infrastructure and traditional publication processes. For the effective utilisation of the new communication and publication nets effective instruments and infrastructures are essential.

The framework of the DFG sponsoring various instruments is used for the project independent, durable and transregional information infrastructure. The presentation of research activities of the working group is performed via a web homepage. The managing coordination has installed a www-server (Fig.9) which is permanently to be reached under the address http://www.for384.uni-stuttgart.de/. There one can find information on the various projects, but also vacancies and general information about the research group and the contact addresses of the members. Additionally there is an internal web-server which can reached through the general internet address but the access is only allow via a password. On this platform information of the scientific exchange is available also on the status of the internal project of the cooperation and also other organisatorial items.

Fig 9: Homepage of the FOR384: http://www.for384.uni-stuttgart.de.

There is a FTP-server installed in order to improve the data and document exchange between the members. The access code is only known to the members. Here, for instance, meeting minutes, application text and measuring data are implemented. The last item, the measuring data, is important for the common evaluation and interpretation of data but also for the comparison of measuring results and simulation. The server is always accessible and allows a simple and efficient access to relevant research results during the daily work.

A well functioning communication system is essential for the cooperation of the members of the research group.

Besides the modern communication possibilities other types of information tools like fax, letter, e-mail, and telephone are used. According to our experiences the modern media have an increasing use and communication with scientists in the own country and abroad. During the research project, the research group has regular meetings and once per year a workshop is organised to which the representatives of the supporting groups and the DFG jury members are invited. Research results are published mainly in traditional way, i. e. as papers in journals and lectures on symposia.

Acknowledgement

The authors acknowledge gratefully the support by the German Research Society (DFG) under the project no. FOR 384, 1-1.

References

The following list of references gives a survey on the activities of the members of the research group. More publications and information can be asked from the project leaders.

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  2. Proceedings: Bauwerksdiagnose - Praktische Anwendungen Zerstörungsfreier Prüfungen, Deutsche Ges. für Zerstörungsfreie Prüfung Berlin, BB 60 (1999), contributions published on CD
  3. Proceedings: Bauwerksdiagnose - Praktische Anwendungen Zerstörungsfreier Prüfungen, Deutsche Ges. für Zerstörungsfreie Prüfung Berlin, BB 76 (2001), contributions published on CD
  4. Deutsche Gesellschaft für Zerstörungsfreie Prüfung: Merkblatt B4 für Ultraschallverfahren zur Zerstörungsfreien Prüfung mineralischer Baustoffe und Bauteile, September 1998
  5. Krause, M., Bärmann, R., Frielinghaus, R., Kretzschmar, F., Kroggel, O., Langenberg, K., Maierhofer, C., Müller, W., Neisecke, J., Schickert, M., Schmitz, V., Wiggenhauser, H. und Wollbold, F.: Comparsion of pulse-echo methods for testing concrete in: NDT & E International Sonderheft Vol. 30 (1997) 4, pp. 195-204
  6. Krieger, J., Krause, M., Wiggenhauser, H.: Erprobung und Bewertung zerstörungsfreier Prüfmethoden für Betonbrücken BAST-Bericht Heft B 18, Bremerhaven: Wirtschaftsverlag NW (1998) 143 Seiten
  7. Krieger, J., Krause, M., Wiggenhauser, H.: Tests and assessments of NDT methods for concrete bridges in: Medlock, R. D. and Laffrey, D. C. (Eds.); Structural Materials Technology III, Bellingham: Proceedings of SPIE, Vol. 3400 (1998) pp. 258-269
  8. Krause, M., Müller, W., Wiggenhauser, H.: Ultrasonic Inspection of Tendon Ducts in Concrete Slabs using 3D-SAFT in: Lees, S. and Ferrari , L. (Eds.); Proceedings of the XXIII. Symposium on Acoustical Imaging, Boston, April 1997, New York and London: Plenum Press Vol. 23 (1997) pp. 433-439
  9. Krause, M., Wiggenhauser, H., Müller, W., Kostka, J. und Langenberg, K. J.: Ultrasonic bridge inspection using 3D-SAFT in: Fahlstedt, K. (Ed.); Proceedings of the CIB World Building Congress, Symposium A, Gävle, Schweden, 7.-12. June 1998, Gävle: KTH Built Environment (1998) pp. A 87
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  12. Grosse, C.; Motz, M.; Reinhardt, H.-W.; Kröplin, B.: Signal conditioning in acoustic emission analysis using wavelets. Internet publication: http://www.ndt.net/article/v07n09/08/08.htm, NDT.net 7 Nr. 9 (2002)
  13. Ruck, H.-J.; Beutel, R.: A new method to analyse impact-echo signals, Otto Graf Journal 12 (2001), pp 81-92
  14. Motz, M.; Krüger, M.; Grosse, C.U.; Haller, P.; Beutel, R.: Impact-echo: New developments regarding hard- and software. Proc. NDT-CE, Berlin Sept. 2003, DGZfP (2003), this proceeding
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  19. Schubert, F., Lausch, R., and Wiggenhauser, H., "Geometrical Effects on Impact-Echo Testing of Finite Concrete Specimens", International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), September 16-19, 2003, Berlin, Germany.
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  25. Marklein, R., Bärmann, R., Langenberg, K. J.: Die AFIT- und EFIT Codes zur Modellierung von Ultraschallwellen in dissipativen oder dämpfenden Materialien. Proceedings der DGZfP-Jahrestagung 1994, DGZfP, Berlin (1995) pp. 409-421
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