CARACTERISTICAS GEOMECANICAS EN LA UNIDAD MINERA ARCATA

The lift-off dependence is an important practical characteristic of EMATs,

and depending on whether the transducer is operating as generator or detector of

ultrasound, the factors that contribute to the lift-off dependence are different. On

generation, there are two factors to take into account: a reduction of the bias mag-

netic field as the gap is increased, and a lower current density induction into the

sample as the coil is further from the surface. On detection, again the reduction

of the bias magnetic field has an effect, and since the coil is further away from the

eddy current in the sample surface, there is a reduction of the induced e.m.f. in the

coil, due to the decrease in magnetic field from the eddy current.

In order to demonstrate the lift-off performance of the PE-EMAT system a set of

measurements were carried out on a low carbon steel sample using the experimental

setup described in section 5.5, and presented in figure 5.15, except for the varia-

tion of the EMAT/sample separation (see figure 5.22(a)). The first part of this

experiment was performed with the transducer in direct contact with the sample

surface (lift-off equal to “zero”). The second part was carried out at four different

EMAT/sample separations: 0.5, 1.0, 1.5 and 2.0 _± 0.01 mm. For each condition, the ultrasonic signal was recorded in the oscilloscope using signal averaging (16x)

Figure 5.21: The graph above shows the effect in the ultrasonic signal amplitude when the PE-EMAT system is operating on a low carbon steel sample: free of magnetite (top), and with a magnetite layer adhered to the sample surface (bottom).

to improve signal-to-noise ratio; which was especially necessary for larger lift-offs.

Figure 5.22(b) shows the results of these experiments on the steel sample and, con-

sequently, the effect of lift-off on the ultrasonic signal output. As can be seen,

small changes in lift-off distance can greatly affect the signal amplitude. The drop

in amplitude with an increase in the EMAT/sample separation is exponential, as

theory predicts (see section 3.2). The experimental points were fitted empirically

to an exponential function of the form: y=y0+A∗exp(−b∗x), where x is the peak

amplitude, b (1.41 _± 0.07) the decay factor , y0 (0.005 ± 0.002) and A (0.101 ±

0.002) are constants. The measured decay for this PE-EMAT system is about_≈10 dB/mm.

In order to give a point of comparison and a perspective of the lift-off performance

of the PE-EMAT system presented here, a similar experiment was carried out but

using a permanent magnet (PM) EMAT of comparable characteristics concerning

the coil (number of turns and wire diameter). The PM-EMAT used for this com-

parison is shown in figure 5.23, it is contained in a brass housing, it utilises a NdFeB

magnet, producing a normal static magnetic field (512 mT when operating in the

same steel sample) and a wire wound spiral coil (diameter = 13 mm, 18 turns, poly-

mer film insulated copper wire (outer diameter = 0.36 mm)). In this configuration,

the PM-EMAT generates and detects shear waves that are polarised in the radial

direction, in the same manner as the PE-EMAT.

The results of the measurements show that, in a similar fashion as with the PE-

EMAT system, the drop in amplitude with an increase in the EMAT/sample sep-

aration is exponential. The values for the empirical exponential fitted curve (y =

y0+A∗exp(−b∗x)), are: y0 = 0.002 ± 0.001, A = 0.031 ± 0.001, andb = 0.96 ±

0.09. The measured decay in this case is about_≈ 9 dB/mm.

A comparison on the lift-off performance of both transducers can be seen in figure

(a) The distance between the PE-EMAT and the sample was varied from 0 to 2 mm, with increments of 0.5 mm 100 80 60 40 20 0 Amplitude /mV 2.0 1.5 1.0 0.5 0.0 Lift-off /mm Experimental points Exponential fit data (y=y0+A*exp^(-b*x))

(b) Lift-off performance (PE-EMAT)

Figure 5.22: Lift-off dependence of the shear wave generated in a low carbon steel sample by the PE-EMAT. Fit to exponential function: (y = y0 +A∗exp(−b∗x)),

where x is the lift-off,b (1.41_±0.07) the decay factor , andy0 (0.005±0.002) and

formance of the PE-EMAT system is clear. The signal amplitude for the range of

lift-off distances used is on average a factor of three larger for the PE-EMAT. This

is especially important for “on-line” inspections, or situations in which is preferable

not to have the transducer in direct contact with the sample, for instance, in high

temperature applications since the transducer could avoid direct contact with the

hot sample.

It should be noted that these measurements have been done maintaining the EMAT

to sample separation constant. However, this is not always possible, and the sep-

aration between the transducer and the sample may not remain constant. This

condition can be accounted for by measuring the inductance of the EMAT coil and

using it to normalise any signal amplitude measurement. This requires calibration

for the particular metal sample (inductance in the coil changes with the sample con-

ductivity), and it is advantageous because it yields a measure of the lift-off which is

independent of the acoustic measurement[12,13]_.

Figure 5.23: Photograph of the PM-EMAT used to have a point of comparison of

the lift-off performance of the PE-EMAT system. Presented here are the lateral and

100 80 60 40 20 0 Amplitude /mV 2.0 1.5 1.0 0.5 0.0 Lift-off /mm

Experimental points (PE-EMAT) Exponential fit data

Experimental points (PM-EMAT) Exponential fit data

Figure 5.24: The above graph shows the lift-off dependence of the shear wave gener- ated in a low carbon steel sample by the PE-EMAT and the PM-EMAT. The better performance of the PE-EMAT system is clear even at 2 mm away from the sample; where the signal amplitude is almost two times bigger than that obtained with the PM-EMAT. Note that the variable x in the equation represents the lift-off.