NDT.net • Oct 2005 • Vol. 10 No.10

Microwave Defectoscopy with Extended Eddy Current System

J. H. Hinken
FI Test- und Messtechnik GmbH, Magdeburg, Germany, www.fitm.DE and
Hochschule Magdeburg-Stendal (FH), University of Applied Sciences,
Magdeburg, Germany, www.hs-magdeburg.de

D. Beilken
FI Test- und Messtechnik GmbH, Magdeburg, Germany

Corresponding Author Contact:
Email: johann.hinken@et.hs-magdeburg.de, Internet: www.elektrotechnik.hs-magdeburg.de/...


[German]

Abstract

Microwave-based non-destructive testing has proven to be powerful for the defectoscopy of parts made from plastic or fibre-reinforced plastic. This paper describes a transverter for standard eddy current systems, which makes them applicable also for microwave-based defectoscopy. The test results represent themselves similar to those of eddy current tests. This combined instrument makes it easy for the inspector to make use of the new microwave-based non-destructive testing. As an example a polypropylene plate with flat bottom holes is inspected using this principle and results are shown: The defect signatures have features, which enable the inspector to distinguish between geometrical size of the defect and its depth below the surface.

1. Introduction

In feasibility studies the microwave-based non-destructive testing (NDT) has proven to be powerful, especially for the defectoscopy of parts made from plastic and fibre-reinforced plastic [1], [2], [3]. For weight reduction purposes such materials are more and more used in the fields of automotive, aerospace, and ship building.
Up to now, however, microwave defectoscopy represents itself more as an academic research topic than a routine procedure for practical applications. The reasons for this are probably that the microwave technique and the interpretation of the test results seem to be complicated. The present paper solves these problems by proposing an appropriate transverter for use with standard eddy current test systems. This makes it easy for the experienced inspector to use microwave-based NDT, and, furthermore, the test results represent themselves quite similar to those in the well-known eddy current testing. The following paragraphs will show these principles.

2. Principle of the test system

Figure 1 shows the principle of the test system.


Figure 1: Block diagram of test system consisting of conventional eddy current system transverter and test head. Added is the device under test.

First of all it consists of a conventional eddy current system, the example uses the PC card PC4 and the ScanAlyzer software from Rohmann GmbH. It is operated at 5 MHz. Using a 10 GHz oscillator and the single side band modulator (SSB mod.), the exciter signal is shifted to a frequency of 10.005 GHz. The test head is fed through a circulator, which transmits the signals from any of the three ports only to that one, which is adjacent in arrow direction. The test head consists of a microwave tuner and an iris at the end of a rectangular waveguide. The iris dimensions are 10.00 mm x 2.00 mm. The waveguide dimensions are 22.86 mm x 10.16 mm (X-band). The slot shaped iris is used to radiate microwave energy into the device under test. This will reflect a part of the signal, which is received by the test head and through the circulator fed to the single side band demodulator (SSB demod.). Because modulator and demodulator are fed by the same 10 GHz oscillator, the 5 MHz output of the SSB demodulator is phase coherent with the 5 MHz exciter signal. The output signal of the SSB demodulator is also the input signal of the eddy current system. Absolute value and phase of this signal correspond to those of the reflected microwave signal in the test head. Therefore, it directly depends on the geometry of the device under test and the distribution of the dielectric constant εr.  Therefore, in regions with pores (εr=1) and in regions without pores or other defects (εr>1) the reflection coefficient will be different.


It is suitable that the microwave tuner within the test head is adjusted such  that in a defect free region the microwave signal, which is reflected into the transverter is made zero. Then, the eddy current system is adjusted such that the displayed point appears in the origin of the complex plane. Then, defects represent themselves as deflections out of the origin.

3. Measuring results



a) principle distribution of FBH

b) photograph of the plate.
Figure 2: Polypropylene (PP) plate with flat bottom holes (FBH). Diameter d=4 mm, 6 mm, 10 mm. Depth or residual wall thickness of the flat bottom holes t=4 mm, 2 mm, 0 mm (through-hole). View from the open side. Plate thickness 10 mm.

Figure 2 shows the used polypropylene plate (PP) with flat bottom holes (FBH). The dielectric constant is in the region 2.2 ... 2.6 [4]

d=10mm, t=4mm, v=1 d=6mm, t=4mm, v=1 d=4mm, t=4mm, v=1
d=10mm, t=2mm, v=1 d=6mm, t=2mm, v=1 d=4mm, t=2mm, v=1
d=10mm, t=0mm, v=0.5 d=6mm, t=0mm, v=0.5 d=4mm, t=0mm, v=0.5
Figure 3: Measured signatures of flat bottom holes from the covered side. From left to right: diameter d= 10 mm, 6 mm, 4 mm. From top to bottom: residual wall thickness t= 4 mm, 2 mm, 0 mm (through-hole). Signal amplification v=0.5 in the bottom row, otherwise v=1. Plate: polypropylene, 10 mm thick.

Figure 3 shows the measured signatures of the FBH within the 10 mm thick PP plate, measured from the covered side. The nine single plots are the signatures of the defects, displayed in the complex plane of the reflection coefficient. This is very similar to the eddy current testing, where the impedance is displayed in its complex plane. In each single plot of figure 3 the origin r=0 is located in the centre of the mesh.

The signatures are generated by moving the device under test under the test head from one defect free position across a defect to the next defect free position. The long side of the iris was perpendicular to the moving direction. As described in paragraph 2 the microwave tuner was adjusted such that r = 0 over defect free positions. Therefore, all signatures begin and end at r ≃ 0. Slight deviations from the origin r = 0 are caused by liftoff changes. When crossing the defects a curve is generated, which under a certain angle reaches a point with maximum distance from the origin. Comparing the single plots of the horizontal rows in figure 3 shows that the maximum distance is a measure for the geometrical size of the defects. Comparing the single plots of the vertical columns shows that the angle of this point is a measure for the depth of the defect, which is equal to the residual wall thickness of the FBH.


Figure 4: Maximum deflection and angle of the signatures of figure 3, displayed in the complex plane of the reflection coefficient r. d - diameter of FBH,  t -  residual wall thickness

Figure 4 summarises the results. Although only nine defects are tested, the systematic behaviour can clearly be recognised. Therefore, the dependencies are supplemented by a mesh. So, figure 4 has lines of constant diameter d and lines of constant residual wall thickness t. These lines are almost perpendicular to each other. On principle they are spirals around the origin and straight lines through the origin, respectively. Using this diagram the inspector can evaluate the depth of the defect and its geometrical size from the signature. This type of procedure is similar to that in eddy current testing, when a distinction is to be made between cracks and magnetic inclusions [5].

4. Conclusions

It has been shown how a standard eddy current test system can be extended such that it is useful for microwave based defectoscopy, performed on parts from plastic or fibre-reinforced plastic. Especially, it was shown that the defect signatures in the complex plane are similar to those known from eddy  current testing of metal devices. Therefore, the interpretation of the microwave test results will be easy for an experienced eddy current inspector.  
The principle was shown using a polypropylene plate with flat bottom holes having various diameters and residual wall thicknesses. Other systematic features  of the signatures are to be expected for other dependencies, for example caused by changes of liftoff and metallic inclusions. For a quantitative evaluation, as common in NDT, a calibration of the test method will be necessary.
By using a standard eddy current system, also its additional functions are useful for the microwave testing: amplification adjustment, filtering, threshold function, C-Scans of absolute value, phase, real part or imaginary part, statistical evaluations etc.
This work was to present a new test and evaluation principle, which is filed for a patent [6]. For the detectability limits of microwave based testing we refer for example to [2].
Technical support by Christian Ziep is thankfully acknowledged .
The systematic was shown using a polypropylene plate with flat bottom holes having various diameters and residual wall thicknesses. Other systematic features  of the signatures are to be expected for other dependencies, for example cause by changes of liftoff and metallic inclusions. For a quantitative evaluation, as common in NDT, a calibration of the test method will be necessary.
By using a standard eddy current system, also its additional functions are useful for the microwave testing: amplification adjustment, filtering, threshold function, C-Scans of absolute value, phase, real part or imaginary part, statistical evaluations etc.
This work was to present a new test and evaluation principle, which is filed for a patent [6]. For the detectability limits of microwave based testing we refer for example to [2].
Technical support by Christian Ziep is thankfully acknowledged .

References

    1. R. Zoughi: Microwave Non-Destructive Testing and Evaluation, A Graduate Textbook, Kluwer Academic Publishers, The Netherlands, 2000
    2. G. A. Green et al.: An Investigation into the Potential of Microwave NDE for Maritime Application, 16th World Conference of Non-Destructive Testing, Montreal, Canada, 30.08-03.09.2004
    3. D. Beilken and J. H. Hinken: Fibre Reinforced Plastic: A Feasibility Study of Microwave Based Non-Destructive Testing, Magdeburg, 2005, http://www.elektrotechnik.hs-magdeburg.de/Mitarbeiter/hinken/news/n15.html
    4. Ch. A. Harper: Handbook of Plastics and Elastomers, McGraw-Hill Book Company, 1975
    5. J.H. Hinken, G. Mook, W.-D. Feist, H. Wrobel, J. Simonin: Detektion und Charakterisierung ferromagnetischer Einschlüsse in nicht-ferromagnetischen Legierungen, DACH-Jahrestagung 2004, Salzburg, published on CD-ROM, P 26
    6. DE 10 2005 040 743.9 Verfahren und Anordnung zur zerstörungsfreien Prüfung, filed as a patent on 26 August 2005

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