In this paper, the eddy current method using a transducer with orthogonal coils, is proposed. The honeycomb composite materials are made of thin and rigid ...
The 10th International Conference of the Slovenian Society for Non-Destructive Testing »Application of Contemporary Non-Destructive Testing in Engineering« September 1-3, 2009, Ljubljana, Slovenia, 367-373
ELECTROMAGNETIC NONDESTRUCTIVE EVALUATION OF HONEYCOMB STRUCTURES WITH CONDUCTIVE SKINS AND CORE Adriana Savin1, Paul Barsanescu2, Rozina Steigmann1, Nicoleta Ifitimie1, Mariana Stanciu3, Raimond Grimberg 1 1
National Institute of R&D for Technical Physics,47 D.Mangeron Blvd 700050, Iasi, Romania 2
Technical University Gh.Asachi, 67 D.Mangeron Blvd 700050, Iasi, Romania 3
Transilvania University, 29 Eroilor St., 500036, Brasov, Romania
ABSTRACT
In this paper, the eddy current method using a transducer with orthogonal coils, is proposed. The honeycomb composite materials are made of thin and rigid skins bonded on a thick and light core. The core is made of aluminum and skins made of carbon fiber reinforced plate. Sandwich materials may induce some debonding defects between the skins and core due to lack of pressure or adhesive during curing or due to the presence of an inclusion. Using this method, debonded zone with at least 8x8mm2 surface can be emphasized, the method is proved to be superior than the classical ones. Keywords: honeycomb composite, conductive skins and core, desbonding, electromagnetic transducer with orthogonal coils
1.
Introduction
Aluminum Honeycomb panels are used to form lightweight, high strength sandwich units that are extremely rigid. The aluminum core is manufactured in several grades from 3003 general applications to 5052 and 5056 for the use in military applications. Aluminum honeycomb panels core material wafer block is stretched into a sheet of hexagonal structure in purpose designed computer-controlled equipment to give a consistent cell dimensions. The walls of the cells can have micro perforations to allow the movement of air between adjacent cells equalizing pressure in environments with rapid temperature variation. The expanded core is bonded to the outer facings with a two pack adhesive; its properties allow it to "wick" on to the core cell walls to ensure large area of contact. The adhesive is applied on a dedicated computer controlled spray booth facility, which allows close monitoring of coat weights and accurate application to all substrates. During this stage any inserts or conduits can be included 367
within the core to allow site fixing or wiring of ancillary fittings to concealed points. Panels can have structural frames incorporated in the same manner to agreed configuration. The final bonding process is carried out in a heated hydraulically controlled press to ensure uniformity of each panel. The panel remains in the press during the adhesive curing process, this is closely controlled taking account of press and ambient temperatures to ensure reliability of bond. After the curing process has been completed the panels are removed from the press, and inspected [1]. Aluminum 3003 as a core material has the following attributes: temperature usage up to 175 ° C, high thermal conductivity, flame resistance, moisture and corrosion resistance, fungi resistant, high strength and low weight. To evaluate the quality of honeycomb panel, series of nondestructive methods have been developed as: infrared thermography [2], electromagnetic infrared method [3], ultrasonic pulse-echo C-scan [4], [5], speckle interferometry [6], radiographic methods [7]. In this paper we propose the use of eddy current holography [8], [9] to emphasize the deformations of aluminum core from the sandwich composites carbon-epoxy laminate plate/aluminum honeycomb/ carbon-epoxy laminate plate type.
2.
The principle of eddy current holography
Let’s consider a small dimension eddy current sensor placed on the surface of an electrical conductive material. If the tested material has the electrical conductivity V and magnetic permeability P, the electric field generated by the sensor supplied with an electrical current with angular frequency Z will propagate inside the material as wave, having the wavelength O O
2SG
(1)
2
(2)
where G
ZPV
Modifying the lift-off it can be reached the situation in which, starting from a certain distance in material, the wave front might be considered as spherical. This distance, f, can be considered as the focal distance of the eddy current sensor. Let U x, y, z const be the complex signal obtained by scanning the inspected surface and V u, v, z const
its 2D Fourier transform. The holographic image is given by [8]: U(x, y, z ')
1 (2S) 2
f f
³ ³ V(u, v, f ) P(u, v, z ') exp > 2Sj(ux vy)@ dudv
(3)
f f
where P(u, v, z ') is the complex propagator of the hologram 2 2 1/2 º ª 4Sm § § Ou · § Ov · · » « P(u, v, z ') exp j z ' ¨1 ¨ ¨ © 2m ¹¸ ©¨ 2m ¹¸ ¸¸ » O « © ¹ ¼ ¬
where z’=z-f, m=1,2,.... . 368
(4)
The displaying intensity is computed as
I ( x, y )
3.
U ( x, y , z ')
2
(6)
Eddy current holographic transducer
The eddy current transducer used at nondestructive examination of composite materials sandwich type is a send-receive absolute type [10] having orthogonal coils. The emission part is wound on a small ferrite cup core. The reception part of the transducer is orthogonally disposed on the reception one. In Figure 1 is presented the principle scheme of the transducer with orthogonal coils, its geometrical dimensions and the physical realization.
(a )
(b)
(c)
Fig. 1:.Eddy current transducer with orthogonal coils: (a) geometrical dimensions; (b) the principle scheme; (c) physical realization.
The emission part of the transducer has 6 turns wound with 0.2 mm Cu wire. The ferrite cup core is made by FERRINOX, having the relative magnetic permeability 10 at 6 MHz. The reception part has also 6 turns wound with 0.2 mm Cu wire. To determine the focal distance of the transducer, its emission part functioning has been simulated using the CIVA 9.1 software, considering the transducer placed over a conductive plate having electrical conductivity 1000 S/m and relative magnetic permeability 1 and varying the lift-off. These features are the same with those of carbon epoxy laminate plates bonded on the faces of aluminum honeycomb core. The simulations have been made at 6MHz frequency and the results are presented in figure 2a, b and c. Examining the data from Figure 2, it is observed that the wave front of electric field can be considered as being spherical for 4mm lift-off. 369
(a )
(b)
(c) Fig. 2: The phase of electric filed in material for variable lift-off: (a) 0.5 mm; (b) 2 mm; (c) 4mm
4.
The studied sampled
Composite materials sandwich type made from two carbon epoxy laminate plates with 4.1mm thickness, reinforced with 16 plies of prepreg carbon woven and volume ratio 0.42 have been taken into study. Between the two plates aluminum honeycomb core, made from Al3005 having 3.175mm cell dimension and high 15mm has been inserted. The structure has been bond with epoxy resin Hexcel 600 and pressed with 1000 N force. During the pressing the temperature was maintained constant at 80r10C.
(a )
(b)
(c) Fig. 3. Studied samples: (a) honeycomb core with good cells; (b) honeycomb with damaged cells; (c) composite sandwich structure 370
The pressing time was 4h, the optimal time in which resin polymerizes. In a region of honeycomb core, the cells were intentional damaged. In Figure 3 we present the honeycomb core and the structure taken into study.
5.
The experimental set-up
The sandwich composite structure has been placed on the table of a XY motorized stage –Newmark type, commanded by PC using a code written in Matlab 6.5. The transducer was coupled to Network/Spectrum/Impedance Analyzer 4395A Agilent. The equipment is commanded by the same PC. Surfaces of 6x6mm2 from a good region and 10x10mm2 from a bad region have been scanned. The amplitude and the phase of the electromotive force at the terminals of reception coil were measured. The measurements were effectuated at 2MHz, 4MHz and 8MHz. The lift-off was set-up at 4mm, so that the wave front of the electric field in the upper part of aluminum honeycomb core shall be considered spherical. The experimental set-up is presented in Figure 4.
Fig.4: The experimental set-up
6.
Experimental results
According to the simulations, the focal distance of the transducer can be considered as being approximate 8.2mm. During the measurements, the lift-off z was 4mm, resulting that z’=-4.2mm (according to eq. (5)). The phase multiplier m was chosen by try-outs, the optimal value being m=6. In Figure 5 we present the holographic image of a region in the examined composite material in which the aluminum honey comb core is good. It can be noticed the fact that the structure of cells is almost visible, even if above them it is placed a carbon epoxy plate. In Figure 6 is presented the holographic image of a region in the structure of the composite material of type sandwich in which the aluminum honey comb core is determined. It can be noticed the fact that there is a plain difference between the aspect of the damaged region and the one in which the aluminum honey comb core is righteous. Thus, a non destructive examination through electromagnetic procedures for the composite materials, sandwich type, having aluminum honey comb core, is possible.
371
Fig. 5: Holography of a good region of honeycomb core
Fig. 6: Holography of a damaged region of honeycomb core
7. Conclusions The eddy current transducer with orthogonal coils permit the obtaining of the holographic eddy current images for the carbon epoxy laminate plates/ aluminum honey comb/carbon epoxy laminate plate. The focal distance of the transducer that has been used was determined by means of numerical simulations, permitting – this way- the fastening of the adequate lift – off. The method permits the emphasizing of the flaws in the aluminum honey comb core, even if this is bonded between two carbon epoxy composite plates.
8. Acknowledgments This paper is partially supported by Romanian Ministry of Education and Research under CNMP Contract no. 71-016/2007 MODIS and Nucleus 372
9. References [1]
T.N Bitzer, Honeycomb Technology: Materials, design, manufacturing, applications and testing, Tom Holyer, NY, 2002 [2] W. Swiderski – Nondestructive testing of honeycomb type composites by an inforced thermography method – IV Conferencia Panamericana de END Buenos Aires – Octobre 2007 – http://www.ndt.net/article/panndt2007/papers [3] D. Balageas, S. Bourasseau, M. Dupont, E. Bocherens, V. Dewwynter – Marty, P. Ferdinand – Comparison between Non Destructive Evaluation Techniques and Integrated Fiber Optic Health Monitoring Systems for Composite Sandwich Structures – J of Intelligent Material Systems and Structures, 11, (2000) pp 426 – 437 [4] D. K. Hsu; D. J. Barnard; J. J. Peters – Nondestructive evaluation of repairs of aircraft composite structures – Proceedings, SPIE, 4336, (2001), pp 100 – 107 [5] J. Blitz, O. Simpson – Ultrasonic Methods of Non Destructive Testing , Chapman & Hall, N. Y, 2000 [6] P. Famitchov, L.S. Wang and S. Krishnaswamy , Advanced image-processing techniques for automatic nondestructive evaluation of adhesively bonded structures using Spackle Interferometry – Journal of Nondestructive Evaluation, 16, (1997) pp 215 – 227 [7] C.M. Siwek and J. N. Gray, "Real Time X-Ray Microfocus Inspection of Honey Comb", Review of Progress in Quantitative Nondestructive Evaluation, D. O. Thompson, D. E. Chimenti eds, 9B, (1990), pp 1549 – 1555 [8] M. Prince, B.P. Hildebrand and G.L. Hower, The eddy current probe as a holographic transducer. Journal of Nondestructive Evaluation 12 (1993), pp. 209 – 217 [9] R. Grimberg, A. Savin, E. Radu, S. Chifan – Eddy current sensor for holographic visualization of material discontinuities, Sensors&Actuators, A, 81, (2000), 251-253 [10] R. Grimberg, A.Savin, E.Radu, O.Mihalache, Nondestructive Evaluation of the Severity of Discontinuities in Flat Conductive Materials Using the Eddy Current Transducer with Orthogonal Coils, IEEE Trans on Mag. 36, 1, (2000), 299-307
373
