·Table of Contents
·Materials Characterization and testing
Evaluation of delamination in Carbon Fibre Composites Using the Eddy Current MethodAdriana Savin, R. Grimberg, S. Chifan,
National Institute of R&D for Technical Physics, 47 Mangeron Blvd, 6600, Iasi, ROMANIA
D. Premel, Y. Le Bihan,
LESIR, ENS Cachan, 61, Av. du Président Wilson 94235 Cachan Cedex, FRANCE
|Fig 1: Eddy current transducer with orthogonal coils|
The emission coil (3) is placed inside of a H30 FERRINOX ferrite pot (1), has 75 turns and is parallel to the inspected surface. The reception coil (2) has also 75 turns and is placed perpendicularly to the emission coil. The transducer is connected to the SFT 6000N-control equipment produced by SOFRATEST, France.
The transducer is scanning a rectangular synthetic aperture; a displacement robot being used for this, the positioning accuracy being 2.5mm. The lift-off was kept at 0.1mm all along the measurements.
The installation output signal is quantified on 12 bits, the sampling step being 1mm on both directions. Making an adequate phase adjustment the noises were projected mainly on one of the measuring channels, the useful information being obtained on other channel. The signals were stored and holographically post-processed using Matlab 5.3.
|Fig 2: Transducer - inspected object system for holographic reconstruction of discontinuities|
Based on this principle the real field source is replaced by a point-like source located at the distance f0. When a volumetric non-homogeneity is present, a scattered field appears resulting in the generation of an induced electromagnetic voltage u(x,y,z) at the reception coil terminals. The hologram is generated by recording the response u of the eddy current transducer which scans over a rectangular aperture in xy plane at z = f0, when the non-homogeneity is placed at a distance p from the inspected surface. In order to visualize the recorded hologram, this has to propagate backward to the source, i.e. to the non-homogeneity to be imaged.
Let V(u,v,f0) be the 2D Fourier transform of the signal U(x,y,f0) delivered by the control equipment.
The holographic image is obtained in the plane z = - p and is expressed by
where z'=z- f0 represents the coordinate of the point belonging to the non-homogeneity, on which the scattering occurs
and represents the wavelength w is the angular frequency, m0= 4p10-7H/m, s is the transversal conductivity of the composite material (s»102S/m) and m=1,2,31/4 is a natural number representing the phase multiplication constant.
The measurements have been carried out at the frequencies 100kHz, 200kHz, 400kHz and 600kHz.
The transducer focal distance has been calculated for each frequency apart, using the finite element method.
The light intensity of a point of the hologram is given by
|Fig 3: a - signal delivered by the installation,b - holography of the impact zones|
In figure 3b the hologram of the impact zones is presented for a phase multiplicity m=16 and in figure 4 the location of delamination into the depth of material for impact energies of 2.5J and 3.0J is plotted.
For impact energies lower than 2.5J the impacted zones could not be detected starting from measurements on the impacted surface, these being rendered evident on the opposite surface for energy of 1J. For impacts with energies of 0.75J no delamination has been noticed.
|Fig 4: Location of delamination into the depth of material||Fig 5: Dependence between the impact energy and the area of delamination|
The same dependence has been noticed between the depths of delamination location and the impact energy.
This work was supported by the National Agency for Science Technology and Innovation of Romania under Grant no. 5218/99-C38/B2
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