GB2082330A

GB2082330A - Eddy current surface probe

Info

Publication number: GB2082330A
Application number: GB8123782A
Authority: GB
Original assignee: Atomic Energy of Canada Ltd AECL
Current assignee: Atomic Energy of Canada Ltd AECL
Priority date: 1980-08-18
Filing date: 1981-08-04
Publication date: 1982-03-03
Also published as: CA1158314A; IT8168073A0; IT1144610B; FR2488693B1; FR2488693A1; DE3130685A1; JPS5772055A; GB2082330B

Abstract

The eddy current probe for detecting flaws consists of two coils (14, 15) mounted coaxially within non magnetic housing (11) on non magnetic cores (16, 17) with each coil capable of separate axial adjustment. The coils are connected to the adjacent arms of an AD bridge circuit which can be balanced at the desired test frequency. The voltage output from the AC bridge due to lift-off or defects is the difference between the voltage change of each coil. The coils are adjusted axially by means of screws (21, 23) to provide a voltage output due to lift-off which is in the order of 90 DEG with regard to the voltage output due to shallow defects making the latter simple to detect. A thin conducting shim (26) may be positioned between one of the coils (14) and the test surface in order to further rotate the lift-off signal of that coil. <IMAGE>

Description

SPECIFICATION Eddy current surface probe Background of the Invention This invention is directed to an eddy current surface probe and in particular to a probe for the detection of shallow surface defects.

Eddy current nondestructive testing techniques in use today do not readily detect shallow surface defects in the order of 0.1 mm deep. This is because the phase of the voltage change in the sensing coil due to a shallow defect is at nearly the same angle as the voltage change due to a small variation in the distance between the coil and test surface which is known as lift-off. The voltage change from deeper defects, i.e. in the order or 0.5 mm has significant phase rotation from that of lift-off signals to make detection of deeper defects much easier.

United States Patent 3,197,693 which issued to H. L. Libby on July 27, 1965 and United States Patent 3,753,096 which issued to W.C. Wiers on August 14, 1973 are examples of eddy current probes in which attempts are made to compensate for lift-off.

The Libby patent describes an eddy current probe which has two or more coils that are co-axial, co-planar and wound on separate magnetic cores. The output is taken from the inner coil, while the phase and amplitude of the outer coil is varied by electronic means to alter the phase of the lift-off signal sensed by the inner coil.

The Wiers patent describes an eddy current coil design involving an inner and outer coil wound on separate inner and outer, coaxial magnetic cores. The cores are able to move axially such that they exhibit substantially the same amount of lift-off variation in the output of each coil. The inner coil is more sensitive to defects than the outer coil. Each coil is connected to the adjacent arms of an AC bridge so that the bridge remains "substantially" balanced over normal variations in lift-off, yet is unbalanced when the coil passes over surface defects. However, this patent does not indicate what it considers to be a normal variation in lift-off or a substantially balanced bridge.

Summary of the Invention It is therefore an object of this invention to provide and eddy current probe in which the lift-off signal is rotated away from the signal due to shallow defects.

This and other objects of the invention can be achieved in an eddy current probe for connection to an AC bridge detector circuit, which includes a coil arrangement that has at least two dissimilar coils fixed to non-magnetic co-axial cores. The cores are mounted on a non-magnetic base section which controls the position of the probe with respect to the surface to be tested. The core mounting arrangement allows for the adjustment of the cores iri an axial direction within the base section. The coils in the probe may differ in radius, length and/or inductance. In addition, the eddy current probe may include a conductive shim positioned adjacent to one of the coils, which is preferably the inner coil in a two-coil probe.

Many other objects and aspects of the invention will be clear from the detailed description of the drawings.

Brief Description of the Drawings In the drawings: Figure 1 illustrates a typical AC bridge circuit used with the present invention: Figures 2,.3 and 4 include lift-off curves on the non-dimensionalized impedance planes. The slope of the lift-off curve represents the value t dZx,dZ l; Figures 5, 6 and 7 include curves showing how the rate of change in coil impedance with lift-off (dZ/dl) varies with lift-off; Figure 8 illustrates the outputs of the probe coils for various lift-off distances; Figure 9 illustrates in cross-section, a surface probe in accordance with the present invention; Figure 10 illustrates a probe's output signals for lift-offs and defects; and Figure 11 illustrates a shimmed probe's output signals for lift-offs and defects.

Detailed Description of the Embodiments The eddy current surface probe includes two co-axial coils C1 and C2 mounted in a housing such that when the probe is being utilized, the coils may be positioned substantially parallel to the surface being tested whether that surface is flat or curved such as the interior of a large tube. The coils C1 and C2 are connected as arms of an AC bridge circuit 1 as shown in figure 1. The AC bridge 1 is energized by an AC source 2 which normally would have an adjustable frequency. The other two arms of the bridge consists of variable impedances 3 and 4 so that the bridge 2 can be balanced at any desired test frequency. Detector 5 is used to balance the bridge 5 and to detect the signals due to lift-off and/or defects in the test surface.

The impedance of a coil is denoted by the symbol Z. It is composed of a reactive component (Zx) and a resistive component (ZR). When the distance between the coil and test surface, commonly called lift-off (I), changes by a small amount, there is a corresponding change in reactive impedance (AZx) and in resistive impedance (AZR). It has been determined that the value I dZx,dZ I for a small change in lift off increases with coil diameter, with increasing test frequency, with increasing lift-off and to a lesser extent with increasing coil length. In figures 2 to 4, lift-off curves are shown which illustrate this behaviour. The slope of the lift-off curve represents the value i dZX/dZR 1.The vertical axis Zx has been normalized by dividing by 27rfLo where f is the operating frequency (Hertz) and L0 is the inductance of the coil in air (Henry). The horizontal axis has been normalized by first subtracting the coils DC resistance (Rcoii) and then dividing by 27rfLo.

Figure 2 which shows lift-off curves A, B and C for three coils with different diameters, illustrates that I dZX/dZR I increases with increasing diameter of the coil.

Figure 3 which shows lift-off curves A, B, C and D for a coil operating at four different frequencies, illustrates that I dZX/dZR I increases with an increasing source frequency.

Figure 4 which shows lift-off curves A, B, and C for the coils having three different lengths, illustrates that I dZX/dZR ) increases with an increasing length of the coil.

It has also been determined that the coupling of the magnetic field created by the electric current in the coil to the test material increases as coil diameter increases, as coil length decreases and as liftoff (I) decreases. With increasing test frequency, the induced eddy current density near the test surface increases. Therefore, the coupling of the magnetic field generated by eddy currents to the coil increases as test frequency increases. The rate of change of coil impedance with lift-off I dZ/dl t is a complicated function of these parameters -- coil length, coil diameter, lift-off and test frequency. Figures 5 to 7 illustrate these affects.

Figure 5 which shows Lnl dZ/dl I vs. lift-off curves A, B and C for three coils with different diameters, illustrates that with increasing coil diameter, i dZ/dl / increases.

Figure 6 which shows Ln[ dZ/dl t vs. lift-off curves A, B, C and D for a coil operating at four different frequencies, illustrates that with increasing operating source frequency, t dZldl / increases.

Figure 7 which shows Lnl dZ/dl g vs. lift-off curves A, B and C for three coils having different lengths illustrates that with increasing coil length, ss dZ/dl j decreases.

The information contained in figures 2 to 7 shows that the lift-off signals from coils of different shapes and sizes can be very different, and it can be inferred that the curves resulting from the signal of either coil taken separately can also be very different from a curve resulting from the differences in the lift-off signals from two coils. To confirm this, the series of lift-off curves illustrated in figure 8, where obtained using a probe having two coaxial coils connected to an AC bridge operating at 500 kHz with a sample surface made of Zircaloy-2 (trademark).The AC bridge was balanced with inner and outer coils set at the distances from the surface listed in the table below: TABLE 1 Inner Coil Outer Coil Dist. (mm) Dist. (mm) a) 0.32 0.0 b) 0.32 0.32 c) 0.32 0.64 d) 0.32 0.80 e) 0.32 0.95 t) 0.32 1.11 With the bridge in balance, a state represented by the X in each set of the series of curves a-f, each set of lift-off curves were obtained by: I) raising only the outer coil giving the horizontal line to the right, i.e. curves la--lf; II) raising only the inner coil giving the line dipping to the left, i.e. curves Ila--llf; and finally III) raising both coils simultaneously in intervals of 0.086 mm producing the dotted differential lift off curves Illa--lllf.

It can be seen from figure 8 that the initial direction of the differential lift-off curve Ill can be rotated relative to the lift-off curves of coil I or II by simply adjusting the initial axial distance of the outer coil from the test surface. This effect may be enhanced by selecting different coil diameters and coil lengths, by having differing coil inductances and by operating at different frequencies.

Figure 9 illustrates an eddy current probe in accordance with the present invention. The probe 10 shown in cross-section, includes a base section 11 which, as shown, is cylindrically shaped with an open end 12 and a closed end 13. The closed end 13 is very thin and contacts the surface to be tested.

Two coaxial coils, the inner coil 14 and the outer coil 15 are mounted on cores 1 6 and 1 7 respectively such that they may be positioned within the base section 11 at a predetermined desired distance from end 1 3 of the base section 11. The outer core 1 7 has a hollow cylindrical body 1 8 with a flange 1 9, the body 18 slides into base section 11. A cap 20 fits over and is secured to flange 19. Screw 21 fixes the core 1 7 to the body section 11 and springs 22 over the screws 21 maintain the outer coil 1 5 at the desired distance from end 13. Core 1 6 is cylindrical in shape and moves freely within core body 1 8. It is secured within the body 1 8 by a bolt 23.The position of core 1 6 relative to body 1 8 may be adjusted by a nut 24 and is maintained by a spring 25. All of the elements in the probe 10, including the screws, are made from non-magnetic material such as Delrin (Registered Trade Mark). The parameters of the copper wire coils 14 and 1 5 for one particular probe are listed below in Table II.

TABLE II I.D. O.D, Length L (mm) (mm) (mm) yH Tums Coil 14 0.5 2.5 0.5 13.5 128 Coil 15 4.6 5.1 1.3 16 42 In operation, it is desirable to adjust the two coils 14 and 1 5 relative to one another in the probe as well as to the end 13 of the probe and thus the surface to be tested such that the defect signal from a shallow defect is in the order of 900 out of phase with the differential lift-off curve. In a probe of the type described above, the inner coil 14 was adjusted to be 0.32 mm from a test surface and the outer coil 1 5 was adjusted to be 1.0' mm from the test surface.The AC bridge operating at a frequency of 500 kHz was then balanced, represented by the point X in figure 1 0. The differences between the resulting output signals due to lift-off and defects are clearly shown. The lift-off curve is substantially perpendicular to the defect signals produced by the three notches in the test surface which were 0.13 mm, 0.25 mm and 0.50 mm deep; 0.13 mm wide; and 6.3 mm long.

In order to increase the ratio of defect signal to lift-off signal, a thin conducting shim 26 may be inserted between the inner coil 14 and test surface as shown in figure 9. The shim 26 changes the liftoff characteristic of coil 14 by increasing the value of the slope of the lift-off curve | dZx/dZ t and decreasing the value t dZ/dl I. In selecting the conductivity and thickness of the shim 26, it is possible to have nearly identical lift-off curves of the inner and outer coils 14 and 1 5, respectively. over a significant lift-off range and at a specific test frequency. The effect of a 0.1 mm Zircaloy-2 shim inserted between the inner coil 14 and a test surface is shown in figure 11. The lift-off signal is rotated about 1800 and compared to the 0.13 mm deep defect signal, there is very little lift-off signal for about 0.1 7 mm of lift-off. As seen in figure 10 for a no-shim probe, a 0.17 mm change in lift-off produces a large signal compared to the 0.13 mm deep defect signal.

Many modifications in the above described embodiments of the invention can be carried out without departing from the scope thereof, and therefore, the scope of the present invention is intended to be limited only by the appended claims.

Claims

1. An eddy current probe for connection to an AC bridge detector circuit to detect shallow defects in surfaces, comprising: -coil means having at least two dissimilar coils fixed to non-magnetic co-axial cores; - a non-magnetic base section for positioning the probe near the surface to be tested; and - adjusting means for mounting the cores to the base section such that each coil may be adjusted in an axial direction within the base section.

2. An eddy current probe as claimed in claim 1 wherein the coils differ in radius.

3. An eddy current probe as claimed in claim 1 wherein the coils differ in length.

4. An eddy current probe as claimed in claim 1 wherein the coils differ in inductance.

5. An eddy current probe as claimed in claim 1,2 or 3, which further includes conductive shim means positioned adjacent one of the coils.

6. An eddy current probe as claimed in claim 1 wherein the coil means consists of an inner coil and an outer coil, the outer coil having an inner diameter greater than the outer diameter of the inner coil.

7. An eddy current probe as claimed in claim 6 wherein the inner and outer coils differ in length.

8. An eddy current probe as claimed in claim 6 wherein the inner and outer coils differ in inductance.

9. An eddy current probe as claimed in claim 6, 7 or 8, which further includes conductive shim means positioned adjacent the inner coil.