GB2187558A

GB2187558A - Determining the magnetic flux density within a specimen during magnetic particle inspection techniques

Info

Publication number: GB2187558A
Application number: GB08704940A
Authority: GB
Inventors: Kenneth Duncan Boness
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1986-03-04
Filing date: 1987-03-03
Publication date: 1987-09-09
Anticipated expiration: 2007-03-03
Also published as: GB2187558B; GB8704940D0; GB8605280D0

Abstract

In a method of determining the magnetic flux density within a specimen (2) subjected, during inspection for flaws by magnetic particle techniques, to a steady or slowly varying magnetic field, the specimen (2) is also subjected to a transient primary alternating magnetic field from an exciting coil. This latter induces eddy currents and secondary magnetic fields within the specimen. The impedance of the exciting coil is then measured and hence the incremental permeability of the specimen is derived. From the incremental permeability of the specimen, the required magnetic flux density is determined. Apparatus for use in this method comprises a microprocessor (5) for determining magnetic flux density using data, relating to the incremental permeability and magnetic flux density characteristics of the specimen material, in a data store (6). The method may be used to maintain the magnetic field in the specimen at field strengths of between 40% and 70% of saturation. <IMAGE>

Description

SPECIFICATION Magnetic particle inspection techniques The present invention relates to a method of measuring the incremental permeability of a specimen, and more particularly to the non destructive testing technique known as magnetic particle inspection.

Magnetic particle inspection is a non destructive testing technique in which a specimen is magnetised and then covered with fine magnetic particles which align themselves according to the magnetic flux patterns in the surface region of the specimen. These flux patterns are related to the properties of the specimen and defects within it. The sensitivity of the technique depends upon the degree of magnetic saturation within the specimen. In practice a broad range for magnetic saturation is acceptable-say between 40 and 70%. At present there is available no entirely satisfactory method of monitoring in real time the actual degree of magnetisation of a specimen as it is being tested by magnetic particle inspection.

According to the present invention there is provided a method of determining the magnetic flux density within a specimen subjected to a steady or only slowly varying magnetic field, comprising the operations of subjecting the specimen also to a transient primary alternating magnetic field from an exciting coil so as to induce eddy currents and secondary magnetic fields within the specimen, measuring the amplitude and phase of the resultant magnetic fields, deriving therefrom the impedance of the exciting coil and hence the incremental permeability of the specimen, and deriving from the incremental permeability of the specimen the magnetic flux density in the surface region of the specimen.

Also according to the invention there is provided an apparatus for determining the magnetic flux density within a specimen of known constitution subjected to a steady or only slowly varying magnetic field, comprising an exciting coil for generating eddy currents in the specimen, means for energising the exciting coil and measuring the impedance of the exciting coil when it is energised, a data store for storing data relating to the incremental permeability and magnetic flux density characteristics of the material from which the specimen is made, and a microprocessor having access to the data store and arranged to determine the magnetic flux density within the specimen from the measured value of the impedance of the exciting coil.

If the apparatus is to be used to measure the magnetic flux density within a relatively few known types of specimen materials, then the data store can be in the form of a readonly memory.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which; Figure 1 is a block circuit diagram of an embodiment of the invention; Figure 2 illustrates the principle of the invention, and Figure 3 illustrates graphically factors which are relevant to the carrying out of the invention.

Referring to Fig. 1, an apparatus for determining th magnetic flux density within a specimen undergoing magnetic particle inspection consists of a probe 1, which may be either an inductance coil such as is used with a conventional impedance measuring eddy current testing instrument or a field coil plus an induced field detector, such as a secondary coil, a Hall device or a magnetoresistor, which gives a complex representation of the magnetic field induced in a specimen 2 by the field coil; a probe energising and impedance measuring device 3; a digitiser 4 arranged to digitise the output from the probe energising and impedance measuring device 3 and apply it to a microprocessor 5 to which also are connected a data store 6, a display device 7, an interface 8 and a mode selector and data input circuit 9.The microprocessor 5 is arranged to produce control signals which are used to control the measurement sequence within the probe energising and impedance measuring device 3. The data store 6 is filled with predetermined (or learnt in-situ) magnetic flux and incremental permeability characteristics for the materials to test which the apparatus is to be used. The display device 7 is arranged to be used in three modes; as a "go, no-go'' gauge, that is Bmin < B < Bmax; to display the interpreted value of B; or to display the measured value of the incremental permeability.The interface 8 enables the operation of the apparatus to be synchronised with external events such as the magnetic cycling which is part of a test using magnetic particles for crack detection, and the mode selector and data input circuit 9 enables the operator to indentify to the apparatus the material type that is being inspected, and to determine the mode of operation of the apparatus.

Fig. 2 illustrates the principle of the present invention. Graph 2(a) shows how the magnetic flux density (B) in a specimen varies with the strength of the magnetising field (H). Graph 2(b) shows how the relative permeability lir varies with the magnetising field (H), and graph 2(c) shows the incremental permeability (/lr=X) of the specimen, which arises when a small amplitude magnetic field is superimposed on a larger continuous magnetic field, varies with the magnetising field (H). It can be seen that the relative permeability, the variation of which satisfies the equation jur=B/OH has a broad maximum which corresponds to a flux density of between 40% and 70% of the saturation value in the specimen.It can be seen also that the incremental permeability which satisfies the equation

has a value equal to ,uO when the flux density in the specimen reaches its saturation value.

Between these extremes it varies in a manner dictated by the magnetisation curve 2(a).

Thus, by deriving boundaries for the value of ,UrA which correspond to values of B between 40% and 70% of the saturation value, and measuring llr, one can determine the value of B in the specimen and, by controlling the value of the magnetising field H, ensure that the relative permeability ,u is in the range most suitable for magnetic particle inspection.

Prior to use with a specimen of known constitution the apparatus described with reference to Fig. 1 has the appropriate curves 2(a), 2(b) and 2(c) stored in the data store 6. Measurement of the incremental permeability are then made by means of eddy currents generated by the probe 1, and the corresponding flux density in the specimen 2 is determined by the microprocessor 5.

The eddy current testing procedure is sensitive to the separation of the probe 1 from the surface of the specimen 2, the lift-off, and the electrical conductivity of the specimen 2.

These effects can be allowed for by interpretation of the way in which the measured value of the impedance of the probe coil changes.

For any given size of the exciting/measuring coil within the probe 1, three distinct regimes occur. With a low excitation frequency (of the order of 1 kHz) the effects of the electrical restivity and magnetic permeability of the specimen 2 in the measured impedance plane of the probe coil are nearly mutually orthogonal, and vary with the lift-off of the probe 1 from the specimen 2. At higher excitation frequencies (50-100 kHz), the effects of the permeability of the specimen 2 and the lift-off of the probe from the surface of the specimen 2 are truly orthogonal in the impedance plane of the probe coil. However, the effects of the resistivity and the permeability of the specimen 2 cause the impedance loci to be indistinguishable (apart from their length).At higher frequencies (about 400 kHz) the sensitivity to the effects of the resistivity and permeability of the specimen 2 is reduced, but the sensitivity to the lift-off of the probe 1 from the specimen is maintained, thus providing a means for compensating for the effects of the lift-off of the probe 1 from the specimen 2 in the first situation. The three operating regimes are il iustrated in Figs. (a), (b) and (c), respectively.

In the method of testing known as magnetic particle inspection, in which a specimen is subjected to a steady or slowly varying magnetic field and dusted with ferromagnetic particles, the degree of magnetisation, that is the flux density within the specimen, needs to be kept between 40 and 70% of the saturation value. Using the above technique, it is necessary only to monitor the reactance of the probe 1 and vary the resultant magnetic field so as to maintain the measured incremental impedance in the region which corresponds to the desired degree of magnetisation of the specimen.

Claims

1. A method of determining the magnetic flux density within a specimen subjected to a steady or only slowly varying magnetic field, comprising the operations of subjecting the specimen also to a transient primary alternating magnetic field from an exciting coil so as to induce eddy currents and secondary magnetic fields within the specimen, measuring the amplitude and phase of the resultant magnetic fields, deriving therefrom the impedance of the exciting coil and hence the incremental permeability of the specimen, and deriving from the incremental permeability of the specimen the magnetic flux density in the surface region of the specimen.

2. A method according to claim 1 including the further operation of monitoring the impedance of the exciting coil and varying the value of the steady or only slowly varying magnetic field so as to maintain the resultant magnetic field within predetermined limits and hence the magnetic flux density in the surface region of the specimen also within predetermined limits.

3. An apparatus for determining the magnetic flux density within a specimen of known constitution subjected to a steady or only slowly varying magnetic field, comprising an excitinhg coil for generating eddy currents in the specimen, means for energising the exciting coil and measuring the impedance of the exciting coil when it is energised, a data store for storing data relating to the incremental permeability and magnetic flux density characteristics of the material from which the specimen is made, and a microprocessor having access to the data store and arranged to determine the magnetic flux density within the specimen from the measured value of the impedance of the exciting coil.

4. Apparatus according to claim 3 wherein the exciting coil comprises an inductance coil.

5. Apparatus according to claim 3 wherein the exciting coil comprises a field coil together with an induced field detector.

6. Apparatus according to claim 5 wherein the induced field detector comprises a secondary coil, a Hall effect device, or a magnetic resistor.

7. Apparatus according to any of claims 3 to 6 including means for producing signals indicative of the measured value of the impedance of the exciting coil, which together with the data from the data stored are applied to thedata processor which is adapted to produce control signals to control the action of the means for energising and measuring the impedance of the probe coil, a display device adapted to display output data signals from the microprocessor, and a mode selection device adapted to control the action of the output display device.

8. An apparatus according to claim 7 wherein the output display is adapted to provide an indication of the value of the magnetic flux within the specimen in relation to predetermined limiting values.

9. An apparatus according to claim 7 wherein the output display device is adapted to provide an indication of the interpreted value of the magnetic flux within the specimen.

10. An apparatus according to claim 7 wherein the output display device is adapted to provide an indication of the measured value of the incremental permeability of the specimen.

11. Apparatus according to any of claims 3 to 10 including an interface adapted to enable the action of the apparatus to be synchronised with a cyclic magnetic field applied to the specimen.

12. A method of determining the magnetic flux density within a specimen substantially as hereinbefore described with reference to the accompanying drawings.

13. An apparatus for determining the magnetic flux density within a specimen substantially as hereinbefore described with reference to the accompanying drawings.