de Lisboa, Portugal
Abstract
In this paper we use tunnel magnetoresistance sensors to inspect an aluminum plate, under
eddy current method [1] with sinusoidal excitation at 5 kHz. For the purpose of this work we
considered a thin plate with 1 mm of thickness that was inspected using a planar probe with
two tunnel magneto-resistive (TMR) sensors. The aluminum plate contained a machined
linear through defect with a length L=10 mm and width W=0.5 mm. The planar probe
generates a uniform magnetic field inside a surface with area approximately equal to 44 cm2.
The probe generates a uniform eddy current at the aluminum surface that is directed across the
linear effect. From now on we shall consider the defect oriented along the x-direction, and the
applied excitation field is also oriented on the x-direction. The eddy currents are launched
across the defect on the y-direction. Thus, the material surface is oriented parallel to the xyplane.
Two single-axis TMR sensors were used to measure the magnetic field along the ydirection,
perpendicular to the defect and along the z-direction perpendicular to the xy-plane.
The general idea of the experimental side of this work was described in the last paragraph.
In the next section we shall present and explain the two data field maps obtained directly from
the experiments. In section 3 we preview the configuration of the current density on the
material surface. The maps were inverted, by considering suitable transformation kernels.
Fig. 1a represents the amplitude of the field measured along the y-direction. Note that the
excitation field is mainly directed along Ox, but a small part of the excitation also exists along
Oy, and must be removed using adequate signal processing. Note that the real direction of the
field By is positive on the first and third quadrants and negative on the second and fourth
quadrants. Fig. 1b represents the field measured on the direction perpendicular to the plate
surface. Two amplitude picks appear close to the defect tips, due to the higher current density.
The eddy currents were launched in the positive Oy direction. In the presence of the defect the
eddy currents curl anticlockwise around the right tip and clockwise around the left tip.