GB2256493A

GB2256493A - Testing magneto-strictive materials

Info

Publication number: GB2256493A
Application number: GB9111997A
Authority: GB
Inventors: Michael Peter Schulze; Rodney David Greenough; Mohammad Sharif
Original assignee: University of Hull
Current assignee: University of Hull
Priority date: 1991-06-04
Filing date: 1991-06-04
Publication date: 1992-12-09
Anticipated expiration: 2011-06-04
Also published as: GB2256493B; GB9111997D0

Abstract

A technique for testing a magnetostrictive material comprises applying both a magnetic field and a uniaxial pressure to the material, passing a pick-up coil along the length of the material and monitoring the output of the coil. The magnetic induction (B) is determined by applying a uniform DC field and integrating the output of the pick up coil with a long time- constant circuit; and the incremental permeability ( mu AC) is determined by applying a DC field modulated by a small AC field and integrating the output of the pick-up coil with a short time constant circuit.

Description

DESCRIT1ON A NON-DESTRUCTIVE TESTING TECHNIQUE The present invention relates to a non-destructive testing technique for determining the properties of magnetostrictive materials prior to their incorporation in devices and applications.

When a magnetic field is applied to a magnetic material the magnetic material develops a flux density or induction, B, as a result of the alignment of the magnetic domain. When a magnetic field is applied to a magnetostrictive material a change in the dimensions of the tnagnetostrictive material is manifested.

A known magneCostrictive material is Terfanol-D (terbium 0.27 dysprosium 0.73 iron 2). This is a highly magnetostrictive material and its high power capabilities and frequency response mean that it has found application in a wide variety of devices and applications, These include anti-vibrational devices, acoustic devices, and control valves to name but a few.

The properties of magnetostrictive materials which are utilised in these and other applications are: a) the ability to generate large strains (magnetostriction), typically 2000 ppm (parts per million), wlth the application of relatively modest magnetic fields, typically 400 A/m (amps/metre).

b) the efficient conversion of magnetic energy to elastic energy or vice versa - currently efficiencies of 60% can be obtained though improvements in this figure, not least as a result of the application of the testing technique of the present invention, are expected.

c) load bearing capabilities (2 to 7 MPo(Mega Pascals) is a typical pressure range for optimum performance).

However, fcr any application to succeed it is important to have consistently high quality material.

Presently, the best performance is obtained from materials in which the grains are orientated with the (112 > axes parallel to the rod axis (typically magnetostrictive materials are manufactured in the form of rods). Application of a uniaxial pressure parallel to the rod axis causes the magnetic moments to rotate perpendicular to the rod axis due to the sign of the magnetoelastic coupling. Subsequent application of a magnetic field parallel to the rod axis then causes the moments to rotate to a direction nearly parallel to the rod axis. This rotational process generates the macroscropic (bulk) strain. Misalignment of the grains comprising the magnetostrictive material means that the application of a stress is less effective and larger magnetic fields are required to generate the strains.

Detection of the misorientation of grains in the magnetostrictive material provides information on the quality of the material and on its suitability for use in a device and when related to the production technique indicates when this is not producing consistently high material.

It is an object of the present invention to provide a non-destructive testing technique which detects rnisorientation of the grains and other defects such as undersized grains, pores and fractures in a magnetostrictive material.

According to the present invention there is provided a non-destructive testing technique for magnetostrictive materials in which a uni-axial pressure is applied to the magnetostrictive material under test, a magnetic field is applied to the material, a pick-up coil (induction coil) is passed along the length of the material and the output of the coil is monitored to determine the magnetic incremental pernieability ( ac) or the magnetic Induction (B) at any position along the length of the material.

The pick up coil may consist of a few turns of fine wire (typically 50 SWG) wound close to the rod with the coil axis parallel to the rod axis. It is suitably supported by a holding rod and may be passed along the length of the magnetostrictive material under test by means of a controlling motor, e.g. a stepping motor.

In order to determine the magnetic Induction, B, a uniform DC magnetic field of variable strength is provided by, for example, a solenoid or the poles of an electromagnet and the output from the coil is fed to an integrating circuit with a very long time constant, e.g.

a DC fluxmeter. As the coil is traversed along the rod length, changes in the component of B parallel to the rod are detected.

In order to determine the incremental magnetic permeability, jl ac, the same set up is used as for determining the magnetic induction, b, exept that the uniform DC magnetic field is modulated by a small oscillatory (AC) field, typically 4 800 A/m and the output of the coil is fed to an integrating circuit with a short time constant.

The resultant small oscillation in magnetic flux through the invention coil can now be monitored at any point on the static B-H curve. By integration of the oscillatory flux (typically the AC field has a frequency of 100 Hz), the magnetic incremental permeability, ac, can be measured at any point along the rod length. This technique measures the ease with which the internal atomic moments can fluctuate about their mean position, whereas the Bsean measures deviations in their mean positions. It is a monitor of the particular magnetization processes which are dominant at any chosen point on the magnetization curve and as such is much more sensitive than the magnetic induction, B, scan method referred to above.

By monitoring the magnetic induction, B, or the magnetic incremental permeability, ac, along the length of the magnetostrictive material at different uniaxial pressures a picture can be built up of the inhomogemecties in the material. In this respect any deviation of the < 112 > axes of the crystal grains from the rod axes will result in different permeabilities in response to the uniaxial pressure.

The uniaxial pressure may be applied to the magnetostrictive material by means of compressed springs, hydraulic rams or the like. As a consequence of the application of a uniaxial pressure to the magnetostrictive material the magnetic moments are caused to reorientate. In the absence of an applied magnetic field the majority of magnetic moments are caused to lie more perpendicular to the rod axis of the magnetostrictive material. However, the response of the magnetic moments to the application of pressure depends very much on the grain orientation. It follows from this that the respcnse of the material to the subsequent application of a magnetic field is changed when there is grain misorientation. According to the alignments of the grains the responses of the magnetic moments are changed upon the application of an AC modulated DC magnetic field and thus variations in P ac are greatly increased.

Using the testing technique of the present invention defects even in the best of samples can be detected especially when the applied DC magnetic field is chosen to give maximum permeability for a given applied stress for a particular grain orientation.

If the frequency of the AC modulating field is changed then the corresponding depth of penetration of this field into the magnetostrictive material under test is changed in a radial direction It follows from this that by repeating the testing technique of the present invention at different AC frequencies it is possible to scan the material at different depths and thereby obtain information for the computation of a three dimensional plot of internal defects within the material.

Appropriate computing means may, of course, be employed to provide a plot of magnetic incremental permeability along the length of the magnetostrictive material and at different depths within the magnetostrictive material.

The improved sensitivity in measuring both the magnetic induction, B, and the magnetic incremental permeability, peat, using the measuring technique of the present invention will now be illustrated with reference to the accompanying plots.

Plot 1 shows a scan of magnetic induction, B, along the length of a sample of magnetostrictive material (Tb 0.27 Dy 0.7;3 c 1.g5 ) with the non-uniform de-magnetizing effects at the ends of the rod compensated for using a polynomial fit method.

No uni-axial pressure is applied to the sample.

Plots 2a to 2h show a number of scans of magnetic induction, B, along the length of a sample of magnetostrictive material (Tb 0.3 Dy 0.7 Fc 1.90) at different uniaxial pressures (d ) and with the bias field (Hb) adjusted in each case to give maximum permeability (frLr) at each pre-stress (). The variations between the plots can be related directly to inhomogeneities in the sample, although the sensitivity of this measurement to these inhomogeneities is acknowledged to be relatively low.

Plot 3 shcws a scan of magnetic incremental permeability, p-ac, along the length of a sample of magnetostrictive material (Tb 0.27 Dy 0.73 Fc with the non-uniform de-magnetflsing effects at the ends of the rod compensated for using a polynomial fit method.

Comparison of Plots 1 and 3 illustrate that the magnetic incremental permeability ac is a more sensitive indicator of inhomogeneities than magnetic induction, B.

However, by now comparing Plot 3 with Plots 4a to h, which show a number of scans of magnetic incremental permeability pL ac at different uni-axial pressures (?) and with the bias field ( Hb ) adjusted to give maximum permeability ( r) at each pre-stress ( # ), it can be seen that inhomogeneities indicated by the arrows, in the sample are readily discernable.

Claims

1. A non-destructive testing technique for magnetostrictive materials in which a uniaxial pressure is applied to the magnetostrictive material under test, a magnetic field is applied to the material, a pick-up coil (induction coil) is passed along the length of the material and the output of the coil is monitored to determine the magnetic incremental permeability (* < ac) or the magnetic induction (B) at any position along the length of the material.

2. A non-destructive testing technique according to Claim 1, wherein the said magnetic field is a uniform DC magnetic field and the output of the coil is fed to an integrating circuit with a very long time constant, so that as the coil is traversed along the material under test, changes in the magnetic conduction, B, parallel to the material can be determined.

3. A non-destructive testing technique according to Claim 1, wherein the said magnetic field is a uniform DC magnetic field which is modulated by a small oscillatory (AC) field, and the output of the coil is fed to an integrating circuit with a short time constant so that as the coil is traversed along the material under test changes in the incremental magnetic permeability,~ ac, along the length of the material can be determined.

4. A non-destructive testing technique according to Claim 3, wherein the magnetic incremental permeability, P ac, of the material under test is determined by repeating the testing technique at different AC modulating frequencies in order to scan the material at different depths and thereby compile information for the computation of a three dimensional plot of internal defects within the material.

5. A non-destructive testing technique according to Claim 4 wherein computing means are employed to provide a plot of magnetic incremental permeability r. ac,along the length of the magnetostrictive material under test and at different depths within the material.

6. A non-destructive testing technique according to any of Claims 2 to 5, wherein the DC magnetic field is chosen to give maximum permeability for a given applied stress for a particular grain orientation.

7. A non-destructive testing technique according to any preceding Claim, wherein the magnetic induction, B, or the magnetic incremental permeability, < ac, along the length of the magnetostrictive material under order to compile a picture of the inhomogeneties in the material.

8. A non-destructive testing technique according to any preceding Claim, wherein the uniaxial pressure is applied to the magnetostrictive material under test by means of compressed springs, hydraulic rams or the like.

9. A non-destructive testing technique according to any preceding Claim, wherein the pick-up coil is supported by a holding rod and is passed along the length of the magnetostrictive material under test by means of a stepping motor.

10. A non-destructive testing technique substantially as hereinabove described.