GB2419187A

GB2419187A - Eddy current analysis

Info

Publication number: GB2419187A
Application number: GB0422571A
Authority: GB
Inventors: Noel Paul Hopkins; Christopher Sellars
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2004-10-12
Filing date: 2004-10-12
Publication date: 2006-04-19
Also published as: GB0422571D0

Abstract

Eddy current probes 2 have been utilised for analysing materials non destructively, but such analysis is degraded when used with materials which are temperature sensitive electrically such as abradable coatings. These coatings include a significant proportion of non metallic discontinuities and act like a semi-conductor rather than a conductive material. An arrangement 1 is provided in which heating is achieved through for example electrical coils 3 until the material to be analysed is elevated to an appropriate stable isothermal temperature whereupon the probe 2 is activated for analysis. Typically, a thermocouple 5 is provided to detect the stable elevated temperature for the material and eddy current probe analysis is performed within seconds of achievement of that stable temperature. A jacket 6 used to surround the probe 2 includes the heating means 3.

Description

Eddy Current Detection The present invention relates to electrical Eddy

current detection and more particularly to such Eddy current detection with respect to analysis of composite materials of temperature sensitive materials such as abradable coatings.

There is a clear necessity to analyse materials in order to confirm their acceptability and quality with respect to required functional and operational performance.

A number of techniques are used in order to perform these objectives including Rockwell hardness testing as well as electrical Eddy Current analysis. A number of components are of relatively high value or for other reasons it is necessary to perform non destructive in situ testing so techniques such as electrical eddy current analysis are advantageous in view of their non destructive nature.

Eddy currents are created through a process called electromagnetic induction. When an alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. This magnetic field expands as the alternating current rises to a maximum and collapses as the current is reduced to zero. If another electrical conductor is brought into close proximity to this changing magnetic field, an electrical current, Eddy current will be introduced into this second conductor. Eddy currents are induced electrical currents that flow in a circular path.

Eddy current probes induce detectable eddy currents in ferrous and nonferrous material for flaw or contaminant detection, weld inspection and other non-destructive test applications. Eddy current probes are often hand held and therefore offer the opportunity to test materials in situ.

A particular problem arises with respect to abradable coatings which are principally metal matrix composites.

Eddy current probes can be used to detect the amount of dislocation due to a dislocator or non-metallic material phase within the abradable coating. It is known that disclocator or non-metallic material phase content is linked to a number of key material properties and operational performance factors. Through properly controlled eddy current analysis it will be appreciated that within a component, analysis can be performed at production as well as throughout operational service as an investigative tool.

Previously, eddy currents have been generated within a probe using a length of electrical conductor which is formed into a coil. Alternating electrical current is allowed to flow in the coil at a pre-defined frequency.

This electrical current results in an expanding and collapsing magnetic field that forms in and around the coil. When an electrically conducting material is placed in the coils dynamic, that is to say expanding and collapsing magnetic field, an electromagnetic conduction occurs and eddy currents are induced in that electrically conducting material. Eddy currents within the material produce a secondary magnetic field opposed in the coils or primary magnetic field. The eddy currents can become disrupted by defects, that is to say dislocator or non- metallic material phase constituents within the conductive material. These disruptions in the eddy currents can be detected and analysed as changes in the conductivity of the material analysed, that is to say for example abradable coatings.

As indicated, eddy current probes are of considerable use with regard to non-destructive testing of ferrous and non-ferrous electrically conductive materials. With regard to abradable coatings containing a metal matrix and a dislocator or non metallic material phase, problems arise.

The non-metallic phase causes the abradable material to act in a similar manner to a semi conductor in that its conductivity is extremely sensitive to temperature.

Traditional eddy current probes provide a value of conductivity which is independent of material temperature.

In such circumstances, the eddy current analysis of abradable materials is very difficult to control with conventional equipment in view of the variability in temperature.

In accordance with the present invention there is provided an electrical eddy current arrangement for temperature sensitive electrically conductive materials, the arrangement comprising an electrical eddy current probe associated with heating means for heating in use material to a stable temperature and subsequent investigation by the eddy current probe.

Typically, the heating means comprises an electrical coil through which an electrical coil passes in order to provide heating. Alternatively, the heating means comprises infra red or induction or hot air flow directed in use towards a material to be investigated by the eddy current probe. Generally, the heating means in use is in thermal contact with a volume of material greater than that subject to investigation by the eddy current probe.

Typically, the arrangement includes a thermocouple to determine achievement of the stable temperature.

Preferably, a feedback control device is provided to control operation of the eddy current probe for limiting investigation by the eddy current probe to when the stable temperature is achieved.

Possibly, the arrangement incorporates a heat ballast to facilitate maintenance of the stable temperature.

In addition, also in accordance with the present invention, there is provided a heater jacket for an eddy current probe, the jacket having an opening for an eddy current probe and heating means to enable heating of a material to a stable temperature for investigation by an eddy current probe.

Typically, the heating means comprises an electrical coil through which an electrical current passes in order to provide heating. Alternatively, the heating means comprises infra red or induction or hot air flow directed towards material for investigation by an eddy current probe.

Typically, the heating means is thermally coupled to material for investigation by an eddy current probe over an area greater than that subject to investigation by the eddy current probe.

Typically, the heater jacket incorporates a thermocouple to determine achievement of a stable temperature.

Possibly, the heater jacket incorporates a feedback control device for controlling the eddy current probe to limit investigation by the eddy current probe to when a stable temperature is achieved.

Possibly, the feedback control comprises an automatic link between the thermocouple and the eddy current probe to allow operation of that probe only when the stable temperature is achieved.

Alternatively, the feedback control device incorporates an indicator such as an LED or audible sounder to indicate to a user that the stable temperature is achieved and therefore the eddy current probe can be utilised for investigation of material heated by the heating means.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawing, schematically illustrating an eddy current probe in accordance with the present invention.

As indicated above, there is a particular problem with respect to abradable coatings and materials in that they incorporate non metallic constituents which therefore render the electrical characteristics of the abradable material as being similar to a semi conducting material.

In such circumstances, electrical conductivity is highly dependent upon material temperature through carrier exitation. Thus, a reference for comparison is required.

The present invention provides an eddy current probe in which provision is provided to firstly heat the material under analysis to a pre- determined isothermal temperature state. Heating is typically through an electrical coil element thermocoupler to the material, although other heating techniques could be used. The temperature state is chosen for best reference and as a representative state.

The attached drawing schematically illustrates an eddy current probe in accordance with the present invention.

Thus, the probe arrangement 1 comprises an eddy current probe 2 operated in accordance with known procedure when activated. Around the probe 2 electrical heating coils 3 are provided to heat the material to be analysed below the arrangement 1. It will be appreciated for clarity this material is not depicted in the drawing. The electrical heating coils 3 are supplied with electrical power for heating through connectors 4. Thus, the electrical heating coils 3 heat the material until a thermocouple 5 determines that the material surface has reached an appropriate temperature and will then activate probe 2 measurement. It will be appreciated that the electrical heating coils 3 and electrical power connections 4 may interfere with the eddy current probe 2 operation and so may be switched off by an appropriate control arrangement just prior to such eddy current probe analysis. About the arrangement 1 appropriate shielding 6 is provided to again isolate the arrangement from environmental interference which may corrupt the validity of eddy current probe investigations.

The eddy current probe will be activated once the material bulk heated by the electrical heating coil 3 reaches temperature. Clearly, it is necessary to ensure that the material to be analysed has become thermalised in the sense that the temperature achieved by the heating coil is stable through the bulk of the material adjacent to the probe 2 even though only a small proportion of that material will actually be analysed by the probe 2. The probe 2 will be controlled by a closed loop feedback arrangement. The electrical heating coils 3 will achieve the appropriate temperature and then the probe 2 activated upon achievement of a stabilized isothermal temperature.

As indicated above, it is possible that the electrical current passing through the electrical coils 3 may interfere with the probe 2 operation so generally the electrical heating coils 3 will be switched off and immediately the probe 2 activated in order to determine the eddy current of the underlying material.

Achievement of a stable material temperature is important for validity with respect to probe 2 analysis.

Analysis immediately after switching off the electrical heating coils 3 will generally be acceptable as the material will not significantly diminish in temperature during that period. However, to provide heat stabilization, a heat well or ballast structure may be provided around the probe 2. This may comprise a ceramic or other material which is relatively bulky and in thermal contact with the material in order that it too is heated by the electrical heating coils 3 or other means and acts to stabilise temperature by acting as thermal ballast. It will also be appreciated that this structure may indirectly heat the material using electrical heating coils 3 which are sufficiently displaced from the probe 2 that the influence of the electrical current passing through the electrical heating coils 3 will not degrade operation of the probe 2.

By utilization of the probe 2, it will be appreciated that abradable materials may have been accurately analysed by eddy current techniques ensuring that small variations in non metallic dislocator content or the presence of interstitial defects within the coating material can be identified from the resultant eddy current analysis.

It will be understood that generally the isothermal temperature to which the material under test will be generally a standard value for cross analysis over a number of materials for comparison. Furthermore, the temperature will be such that the small but potentially relevant heating effects of the eddy current probe upon the subject proportion and material is irrelevant. The present invention provides a probe which will facilitate accurate eddy current analysis of available coatings as well as other temperature sensitive electrical conductor materials such as metal matrix composites (MMCs) whilst still maintaining the versatility and potential for hand held capability of conventional eddy current probes. It will be appreciated that the thermal heating effect provided by electrical heating coils 3 may be achieved through an adaptor jacket for a conventional eddy current probe. In such circumstances, a conventional eddy current probe would simply have a heater jacket surround in order to provide the necessary temperature elevation to a stabilised isothermal temperature value and then the eddy current probe arranged to facilitate analysis of the material. A thermocouple 5 will still be utilised in order to indicate achievement of the stabilised isothermal temperature. This thermocouple may be incorporated into an automatic close loop feedback control system in order to activate the probe when required or simply act by illumination of a signal, typically an LED or audio beep to indicate to a user that manual activation of the eddy current probe analysis can now be performed.

By ensuring that the material to be analysed, in the example case, an abradable coating, is at a pre-defined stabilized temperature during analysis by the probe, it will be understood that in situ testing of abradable coatings can be performed. In situ testing of abradable coatings as well as other temperature sensitive electrically conducting materials has significant advantages. For example, within an aircraft engine, in situ testing avoids the necessity for removal of the engine from its aircraft mountings for inspection significantly reducing maintenance and servicing costs. By the present eddy current probe arrangement a simple approach to producing an immediate estimation of sub surface structure of an abradable coating is provided. It will also be understood that non disruptive techniques such as eddy current analysis can be carried out many times on the same engine component without causing damage to the abradable coating or a shroud material in comparison with more destructive Rockwell hardness tests, etc. With regard to eddy current analysis, it would be appreciated there are two principal modes of analysis, namely absolute and differential. With absolute detection techniques a single eddy current probe pick up coil is utilised for flaw and crack detection. This single pick up coil can detect both gradual and sharp changes in eddy current and provide an appropriate response for analysis.

With differential eddy current detection, two pick up coils are compared for greater resolution of sharp discontinuities or flaws. The differential nature makes this type of probe less suitable for detection and gradual changes in material. The present approach with respect to providing heating to an isothermal and substantially stable temperature for the material can be utilised with either absolute or differential eddy current analysis.

Modifications and alterations to the embodiment of the present invention described above will be readily appreciated by those skilled in the art. Thus, as indicated, a conventional eddy current probe may be associated with an adaptor jacket which can surround the probe in order to provide heating of the material to be analysed as required with the jacket removed when the eddy current probe is used in situations other than those in relation to abradable coatings or other temperature sensitive electrically conducting materials. The temperature to which the electrical heating coils heat the material to be analysed may be altered as required by particular material types. It will also be understood that the heating means of an electrical coil could be replaced with infra red or blown hot air or another appropriate method provided the criteria of uniform heating of the material to be analysed as well as feedback control is achievable.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. An electrical eddy current arrangement for temperature sensitive electrically conductive materials, the arrangement comprising an electrical eddy current probe associated with heating means for heating in use material to a stable temperature and subsequent investigation by the eddy current probe.

2. An arrangement as claimed in claim 1 wherein the heating means comprises an electrical coil through which an electrical current passes in order to provide heating.

3. An arrangement as claimed in claim 1 wherein the heating means comprises infra red or induction or hot air flow directed in use towards a material to be investigated by the eddy current probe.

4. An arrangement as claimed in any of claims 1, 2 or 3 wherein the heating means in use is in thermal contact with a volume of material greater than that subject to investigation by the eddy current probe.

5. An arrangement as claimed in any preceding claim wherein the arrangement includes a thermocouple to determine achievement of the stable temperature.

6. An arrangement as claimed in any preceding claim wherein a feedback control device is provided to control operation of the eddy current probe for limiting investigation by the eddy current probe to when the stable temperature is achieved.

7. An arrangement as claimed in any preceding claim wherein the arrangement incorporates a thermal ballast to facilitate maintenance of the stable temperature.

8. An electrical eddy current arrangement for temperature sensitive electrically conductive materials substantially as hereinbefore described with reference to the accompanying drawing.

9. A heater jacket for an eddy current probe, the jacket having an opening for an eddy current probe and heating means to enable in use heating of a material to a stable temperature for investigation by an eddy current probe.

10. A jacket as claimed in claim 9 wherein the heating means comprises an electrical coil through which an electrical current passes in order to provide heating.

11. A jacket as claimed in claim 9 wherein the heating means comprises infra red or induction or hot air flow directed towards material for investigation by an eddy current probe.

12. A jacket as claimed in any of claims 9 to 11 wherein the heating means is thermally coupled to material for investigation by an eddy current probe over an area greater than that subject to investigation by the eddy current probe.

13. A jacket as claimed in any of claims 9 to 12 wherein the heater jacket incorporates a thermocouple to determine achievement of a stable temperature.

14. A jacket as claimed in any of claims 9 to 13 wherein the heater jacket incorporates a feedback control device for controlling the eddy current probe to limit investigation by the eddy current probe to when a stable temperature is achieved.

15. A jacket as claimed in claim 14 when dependent upon claim 13 wherein the feedback control comprises an automatic link between the thermocouple and the eddy current probe to allow operation of that probe only when the stable temperature is achieved.

16. A jacket as claimed in claim 14 wherein the feedback control device incorporates an indicator such as an LED or audible sounder to indicate to a user that the stable temperature is achieved and therefore the eddy current probe can be utilised for investigation of material heated in use by the heating means.

17. A heater jacket for an eddy current probe substantially hereinbefore described with reference to the accompanying drawing.

18. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.