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Eddy Current Array based Imaging |
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Eddy current array technology is an extension of the conventional eddy current technique. The technique uses an array of small coils assembled in an organized fashion to produce mappings of different profiles. It can electronically drive and read several eddy current coils placed side-by-side in the same probe assembly. The eddy current array is composed of three different elements: the driver unit, the probe itself and a multiplexing unit. The use of the Eddy current array probe can dramatically reduce the inspection time. An array probe can scan a surface with just one linear movement hence there is no need for a raster scan with a complex mechanical device.
A model of two dimensional ax symmetric coils with air core or ferrite core in the presence of a conducting half space is developed. The half space is accounted for by computing the appropriate Green’s functions. The problem consists of a cylindrical ferrite core, excited by a coaxial coil and in the presence of conducting half-space (Figure). Since most commercial probes use ferrite cores, the model calculates the induced magnetization in the ferrite core which acts as secondary source for the electromagnetic field where the primary source is the externally applied excitation current to the coil.
Hence the induced magnetization in the core depends on the excitation current applied to the coil. The magnetization or induced magnetic field of the ferrite core is calculated by introducing equivalent Amperian currents within the core which is taken into account while deriving the volume integral equation. The volume integral equation is transformed by method of moments into a vector matrix equation which is then solved using biconjugate gradient methods
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Fig 3(a) Air cored coil Impedance display for various
frequencies and lift off |
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Fig 3(b) Ferrite cored coil Impedance display for various frequencies and lift off |
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Fig 3(c) Impedance display for Ferrite cored (pink) and air cored coil (blue)for various freq
and lift off of 1mm |
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Fig 1 (a) EC image of Circular
EDM notch with varying depth |
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Fig 1 (b) EC image
of three surface cracks |
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(a) ECA array probe
(b) Raster scan of pencil probe
Fig 2 Results Comparison
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Pulsed Eddy Current Imaging |
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The pulsed eddy-current instrument records the transient current induced in an absolute air cored coil placed next to a layered sample, where the coil is excited with a step function change in voltage. The step function voltage excitation implies a spectrum that is inversely proportional to the frequency and thus strongly emphasizes the low frequency components of the signal. The time response signals are digitized with 16 bit resolution at a sampling rate of 1 mega sample per second, and the excitation is repeated at a rate of 1 kHz. The three features of the differential transient signal are the peak height, the time of occurrence of the first peak, and a characteristic zero crossing time. The PEC instrument has better discrimination at low frequencies than frequency domain instrumentation which is one of the motivations for the development of PEC system.
The transient response of the PEC system is computed by applying the inverse Fourier transform (IFFT) to the product of voltage frequency spectrum and frequency spectrum of the excitation current pulse. The frequency spectrum of the excitation current pulse is obtained by fast Fourier transform (FFT). The voltage response at each frequency of the excitation current pulse is calculated to obtain the voltage frequency spectrum.
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Microwave NDT for under paint Corrosion Estimation
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Microwave NDE is emerging as an attractive option for assessment of such hidden corrosion as microwave inspection requires no couplant, can “see” through paint, and can also detect the moisture that is usually associated with corrosion. For realistic structures, it also facilitates the imaging of the extent of corrosion in larges area structures through the non-contact scanning mode.
The objective of this is to investigation is to determine the feasibility of using near field microwave non destructive testing(NDT) techniques for the detection of corrosion precursor pitting, and to investigate the minimum detectable size of pitting for a given range of frequencies. Near field microwave NDT techniques, utilizing open-ended rectangular waveguide probes, have been shown to detect the presence of corrosion under paint and primer in aluminium substrate, including the spatial extent of the corrosion. The samples are thin aluminum sheet, 1.14 mm thick, subject to various degrees of corrosion over specific areas leading to loss of thickness ranging from under 0.1 mm to about 0.65 mm. Measurements were obtained on plate, before and after painting, using three different microwave based techniques and the results were quantitatively compared. The microwave methods of measurement used here includes (a) simple reflection method, (b) comparator method using the magic tee, and (c) slotted line method.
There are several advantages that make the use of microwave NDT techniques suitable for the detection of corrosion precursor pitting under paint and primer. Microwave signals are capable of penetrating dielectric materials, such as paint and primer, and measurements can be made in a one-sided and non – contact fashion. Further more, high spatial resolution can be obtained, and the spatial extent of damage can be determined with no or minimal use of signal processing. Finally, since microwave signals are totally reflected of the metallic substrate, the type of metal on which corrosion is being detected is of no consequence.
We report experiments performed in X-Band frequencies at 9.4 GHz using the “simple-reflection” method and comparator method using the magic tee. All experiments were performed to design a simple, low cost, standalone microwave NDT technique with an effort to replace the more expensive alternate techniques that use the network analyser. In this paper, the measurements were performed using open-ended waveguide, and a coaxial probe, for the detection of wall thickness loss in metal that is painted.
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