GB2124772A

GB2124772A - Eddy current flaw detector

Info

Publication number: GB2124772A
Application number: GB08315680A
Authority: GB
Inventors: David Charles Copley
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1982-08-03
Filing date: 1983-06-08
Publication date: 1984-02-22
Also published as: IT8322195A0; IT8322195A1; JPS5952748A; IT1170175B; FR2531537A1; SE8303776L; IL68155A0; SE8303776D0; DE3327571A1; GB8315680D0

Abstract

Eddy current probe 3 is caused to scan a region 4 of a turbine engine blade containing a crack 9 and the probe output data after processing (21) and amplification (26) is plotted on a record at positions corresponding to positions on the blade scanned. The record may be produced by energising electrode pen (30) which produces heat to cause a burn mark on paper record (35). Probe (3) is located on one end of lever arm 40 to the other end of which is fastened pen (30). Since lever arm portion (40A) is greater than portion (40B) movement of pen (30) is greater than probe (3) to form a magnified mapping of the inductance changes of the probe. <IMAGE>

Description

SPECIFICATION Eddy current microscopes The invention relates to eddy current flaw detectors and, more particularly, to such detectors which provide data concerning small flaws.

It is common to scan the surface of a conductive material with a probe coil, such as an eddy current probe coil, carrying an alternating current to thereby induce an alternating electromagnetic field in the material. Measurement of the behaviour of this field during scanning can give information concerning the structural uniformity of the material. However, the scanning devices commonly used merely locate non-uniformities in the material. Such devices do not, in general, provide detailed information about the nonuniformities. Such detailed information is desirable in many applications because not all nonuniformities are deleterious: certain kinds are tolerable. Further, the commonly used devices do not identify precisely the position or orientation of a nonuniformity, but instead, such devices identify a region containing the nonuniformity.

In accordance with the present invention there is provided a method of obtaining cryptic data concerning a structural nonuniformity in an object of conducting material, the method comprising locating the general vicinity of the irregularity, scanning the general vicinity with a time-varying magnetic field generated by a probe having mutual inductance with the material, generating signals indicative of the degree of mutual inductance between the probe and the conducting material, and mapping information derived from the signals at positions corresponding to at least some of the scanned positions and at a relatively enlarged scale.A preferred apparatus for performing the invention includes probe means for generating the time-varying magnetic field within the material, processing means coupled to the probe means for sensing information concerning the mutual inductance between the probe means and the material and for generating signals indicative thereof, and mapping means coupled to the sensing means for mapping in magnified form on a record the sensed information concerning the mutual inductance.

The apparatus provides dimensional information concerning flaws of minute size compared with the size of the eddy current probe used. It may also provide information concerning the orientation of minute elongate flaws, and information concerning surface features which are as narrow as 1 /50th of the diameter of the eddy current probe used. The flaws may, for example, be approximately 0.010 inches long X 0.005 inches deep X 10 millonths of an inch wide.

By way of example only, an embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is an enlarged perspective view of an eddy current scanning a crack in a material to provide cryptic data; Figure 2 is a schematic illustration showing processing and plotting of the cryptic data on a record medium; Figure 3 is a schematic illustration similar to Fig. 2 but also showing the scanning of the probe over a blade being inspected; and Figure 4 is a plot of Inductance Signal Magnitude vs. Probe Position.

As shown in enlarged form in Fig. 1 an antenna such as an eddy current probe 3 is used to nondestructively scan a region 4 of an object 6 constructed of conducting material and containing a nonuniformity such as a crack 9. While the object 6 is schematically shown as a block, it can be in fact a gas turbine engine blade 6A shown in Figs. 2 and 3 or another engine component such as the metallic disc (not shown) that supports such blades in use in the engine. The presence and general vicinity of the crack 9 is ascertained beforehand by standard procedures which may include a form of eddy current analysis.

The scanning by probe 3 is accomplished by moving a point such as point 1 2 located on the scanning surface 1 5 of probe 3 along a path indicated by a dotted line 16. It is to be noted that the scanning surface 1 5 of probe 3 is large with respect to crack 9. The area of scanning surface 1 5 is termed the crosssectional area and a probe 3 having a crosssectional area over 9000 times the area of crack 9 has been used. There is no upper limit on the size of cracks 9 to be scanned, although it is believed that cracks in the range of .010 inches to .050 inches in length are best scanned with a probe 3 having a scanning surface of a diameter of approximately .07 inches.The area of the crack 9 herein refers to the area in a plane substantially parallel.to surface 1 5 of the surface of the object 6 which is distorted by the presence of the crack. The surface is not limited to the infinitesimally thin geometric concept of surface, but the surface has finite thickness which is determined by the depth of penetration of the electromagnetic field generated by probe 3.

Thus, nonuniformities located below the surface but within the electromagnetic field are located on the surface. For example, the area, as defined above, of a subsurface spherical nonuniformity would be the area of a circle.

The mutual inductance between the probe 3 (also called inductance of probe 3) and the conducting material 6 is determined by factors such as (1) the orientation of probe 3 with respect to the object 6, (2) the distance between probe 3 and the object 6, and (3) the composition and configuration of the object 6. If all factors which determine the mutual inductance are held substantially constant during scanning except those of (3), then changes in the mutual inductance will indicate changes in the factors of (3).

Mapping of symbols on a record, such as a paper sheet, indicative of the mutual inductances at positions corresponding to the actual positions of those inductances in the region scanned can yield information about the material scanned. In the case of the crack 9 in Fig. 1, the mapping should be done on an enlarged scale because the crack 9 may very weli be invisible to the unaided eye, in which case a 1-to-1 size mapping would be similarly invisible. It should be noted that this mapping of the mutual inductances does not give information which can be interpreted in an obvious way to yield dimensional information about the crack scanned. Consequently, the information or data obtained is herein termed "cryptic data." Symbols representing the cryptic data are plotted on a record in the following manner.

In Fig. 2, a conduit 1 8 carries information concerning the mutual inductance from probe 3 to processing circuitry 21. The processing circuit 21 is connected by a conduit 24 (which is interruptable by a switch 24B) to an amplifier 26. The processing circuitry 21 generates an inductance signal 24A indicative of the change in the inductance of probe 3. The inductance signal 24A ranges from zero to ten volts. This signal 24A is modified by amplifier 26 having an approximate voltage gain of six to provide a current on a conduit 28 which corresponds to the inductance of probe 3. The current is fed by conduit 28 to an electrode pen 30 at a voltage approximately between zero and 60 volts.The current, which is of the order of 0.8 milliamps, flows through the electrode pen 30 to its point of contact 33 with a paper record 35 and through the paper record 35 to conduits (indicated as electrical ground 38) which complete a current circuit with the amplifier 26. The electrode pen 30 produces heat, roughly in proportion to the current passing through it, and thus, roughly according to the degree of mutual inductance of eddy current probe 3. The heat produced causes a burn mark to occur in the paper record 35, with greater heat producing a darker mark. Therefore, a small inductance change in eddy current probe 3 produces a light mark in paper record 35; and a large inductance change in eddy current probe 3 produces a dark burn mark in paper record 35.

Fig. 3 illustrates a perspective schematic view of an apparatus 36 for scanning the eddy current probe 3 with respect to the blade 6A. Eddy current probe 3 is fastened to one end of a lever arm 40, to the other end of which is fastened the electrode pen 30. Lever arm 40 is supported by a fulcrum or pivot 43.

Mechanical actuators (not shown) move lever arm 40 so that probe 3 moves uniformly along the surface of blade 6A in the direction shown by arrows 46 and 48. At the same time, the electrode pen 30 moves in the directions respectively shown by arrows 50 and 52. Since lever arm portion 40A is longer than lever arm portion 40B, the relative movement of electrode pen 30, with respect to eddy current probe 3, is greater. Thus, the burn spots in the paper record 35 form a magnified mapping of the inductance changes of probe 3. Following one scan, the blade 6A is moved in the direction of arrow 55 so that a new region is scanned. Also, the paper record 35 is moved in the direction of arrow 56, and to a greater degree than the motion of blade 6A to maintain the magnification of mapping.The cycle of scanning and burning followed by movement of the blade 6A and paper record 35 is repeated until a desired region of the blade 6A has been scanned in the desired pattern.

Fig. 3 shows both probe 3 and burning pencil 30 as following arcuate paths. A pantograph-type mechanism may replace lever arm 40 so that the paths traveled by both probe 3 and burning pencil 30 are straight, parallel, raster-type lines such as those indicated by the dotted line 1 6 in Fig. 1. The scanning pattern need not be raster-type; spiral scanning may be used. Many apparatus are available to duplicate and magnify the scanning motion of the probe 3 in burning pencil 30.

For example, a scanner designated as one in the US 450 series manufactured by Automation Industries, Danbury, Connecticut, can be used.

The inductance signal 24A produced by processing circuitry 24 will in general be a continuous analog signal of varying magnitude, such as the one shown in Fig. 4 in which the inductance signal magnitude is plotted as a function of position of probe 3 as the probe 3 moves across a surface under investigation. It has been found that the activation of the burning pencil 30 with a current representing an amplified version of this signal as described above produces a continuous variation in the color of burn marks on paper record 35. That is, the color ranges from light grey to nearly black. For some purposes, this causes difficulty in visual comparison of the degree of burning on the paper record 35 at two places. That is, it can be difficult to distinguish between two similar shades of grey which are very close in space on the paper record 35. Such a distinction is sought in the attempt to determine whether one shade corresponds to a larger change in inductance than the other shade. Therefore, in one embodiment, analog-to-digital (A/D) conversion circuitry is utilized to alleviate this problem.

That is, as shown in Fig. 3, the A/D circuitry 60 can be spliced into conduit 24 by means of opening switch 24B and closing switches 62 and 63. The A/D circuitry 60 receives as an input the inductance signal 24A from the processing circuitry 21. The A/D circuitry 60 has several preestablished signal thresholds and it compares the inductance signal therewith. The A/D circuitry 60 produces as an output a digitized signal 63A which represents the highest threshold crossed by the inductance signal 24A. For example, if the A/D circuitry 60 has thresholds of 1, 2, 3 and 4 volts, all input signals equal to or greater than one volt but less than two volts will result in a digitized signal of one volt. Inductance signals equal to or greater than two volts but less than three volts will result in a digitized signal of two volts and so on.The digitized signal is then fed to amplifier 26 and thence to electrode pen 30. A suitable A/D converter is more fully described and separately claimed in our copending application Serial No. , entitled "Signal Quantizer" filed concurrently herewith.

One embodiment has been implemented in which an eddy current probe of approximate diameter of .070 inches (1.78mm) having a coil diameter approximately .05 inches (1.27mm) was used to scan four nonuniformities in metallic objects. The probe used was a Nortec 2MHz single-coil absolute probe obtained from Nortec Corporation, Kennewick, Washington. The probe signal was processed by a Nortec NDTl6 Eddy Current Instrument and a Gould Strip chart recorder was used to move the paper record. The paper used was Timefax NDK electrosensitive paper which comprises a bond paper backing which supports three layers of material: an innermost low resistivity carbon fill layer, a nextmost carbon fill layer of greater resistivity, and an outermost surface layer of zinc oxide.This embodiment produced four distinguishable cryptic data maps in response to four different nonuniformities scanned. The first nonuniformity comprises a rectangular notch which was approximately .010 inches (.25mm) long x .010 inches (.25mm) deep x .003 inches (.075mm) wide. The second irregularity comprised a cylindrical pit which was approximately .010 inches (.25mm) in diameter x .010 (.25mm) deep. The third irregularity comprises a subsurface crack which was approximately .04 inches (1.05mm) long X .04 inches (1.05mm) deep X 10 X 10-6 inches (.00025mm) wide.

The fourth irregularity comprised a subsurface iron oxide sphere approximately .010 inches (.25mm) in diameter. Further, orientations could be ascribed to the maps of the first and third nonuniformities which corresponded to the actual orientations of the nonuniformities.

It is to be noted that the cross-sectional area of the probe is approximately .0038 square inches (2.48mm2). The area of the third irregularity is approximately 4 x 10-7 square inches (.00026mm2). Thus, to accuracy of one significant digit, the ratio of the crosssectional area of the probe to the area of the third irregularity is greater than 9000.

A detector has been described in which an eddy current probe of relatively large diameter with respect to a flaw dimension can be used to nondestructively scan the flaw in a rastertype pattern to produce inductance signals. In the embodiment described, the signals produced during the scanning are mapped onto a magnified record as cryptic data. The data is so termed because it only indirectly yields information about the dimensions of the flaw.

However, the mapping produces a record having a pattern which is believed to be uniquely determined by the shape and dimensions of the particular flaw. Thus, scanning unknown cracks and producing enlarged maps in response followed by examination of the cracks by optical microscopy, electron microscopy, or other methods to determine the exact shape and size of the cracks will allow definite size and shape information to be associated with the cryptic pattern as mapped. Therefore, a reference library of cryptic patterns can be generated corresponding to cracks and flaws of known sizes and shapes. Then a cryptic pattern can be generated by scanning an unknown flaw and this pattern can be compared with the patterns in the library to infer the size and shape of the unknown flaw.

General Considerations It has been stated that the probe 3 is to be scanned uniformly over the object 6. The purpose of this uniformity is to scan the electrode pen 30 at a correspondingly uniform rate because the degree of burning of electrode pen 30 is affected by rate of travel. If other mapping methods are used, uniform scanning may not be necessary. Mapping onto an enlarged television video display can be done.

The scanning by probe 3 has been described with respect to a conducting material.

The term conducting material is used to include all materials sufficiently conductive of electric current to be amenable to eddy current analysis, including, of course, most metals. Further, the signal produced by processing circuitry 21 has been termed a mutual inductance signal 24A. The term mutual inductance signal is intended as a generic term covering the signals produced by electromagnetic scanning techniques such as eddy current scanning. The following references provide information concerning eddy current scanning and are herein incorporated by reference: (1) M.V.K. Chari, "Finite Element Solution of the Eddy Current Problem in Magnetic Structures." IEEE Trans. Vol. PAS-93, Number 1, 1973.

(2) R. Palanisamy and W. Lord, "Finite Ele ment Analysis of Eddy Current Pheno mena." Materials Evaluation, October 1980.

(3) T. G. Kincaid, "A Theory of Eddy Current NDE for Cracks in Non-magnetic Ma terials." Proceedings of the ARPA/AFML Review of Progress in Quantitative NDE.

Boulder, Colorado, 1981.

(4) B. A. Auld, F. Muennemann, and D. K.

Winslow, "Eddy Current Probe Response to Open and Closed Surface Flaws." Jour nal of Nondestructive Evaluation, Vol. 2, No. 1, 1981.

(5) C. V. Dodd and W. E. Deeds, "In-service Inspection of Steam Generator Tubing Us ing Multiple Frequency Eddy Current Techniques." Eddy Current Characteriza tion of Materials and Structures. ASTM STP 722, 1981. pp 229-239.

(6) P.G. Doctor, T.P. Harrington, T.J. Davis, C.J. Morris, D.W. Fraley, "Pattern Recog nition Methods for Classifying and Sizing Flaws using Eddy Current Data." Eddy Current Characterization of Materials and Structures. ASTM STP 722, 1981. pp 464-483.

(7) C.L.. Brown, D.C. Defibaugh, E.B. Morgan, A.N. Mucciardi, "Automatic Detection, Classification and Sizing of Steam Genera tor Tubing Defects by Signal Processing." Eddy Current Characterization of Materials and Structures. STP 722, 1981. pp 484-493.

Claims

1. A method of obtaining cryptic data concerning a structural nonuniformity in an object of conducting material, the method comprising: (a) locating the general vicinity of the irreqularity, (b) scanning the general vicinity with a timevarying magnetic field generated by a probe having mutual inductance with the material, (c) generating signals indicative of the degree of mutual inductance between the probe and the conducting material, and (d) mapping information derived from the signals at positions corresponding to at least some of the scanned positions and at a relatively enlarged scale.

2. A method in accordance with claim 1 in which the probe has a cross-sectional area relatively larger than the area of the irregularity scanned.

3. A method in accordance with claim 1 in which the cross-sectional area of the probe is at least 4000 times the area of the irregularity scanned.

4. A method according to claim 1 in which the object is a gas turbine engine component.

5. A method according to claim 1 in which the area of the nonuniformity is less than 4 X 10-7 square inches.

6. Apparatus for obtaining cryptic data concerning a structural nonuniformity in an object of conducting material, the apparatus comprising: (a) probe means for generating a time-varying magnetic field within the material, (b) processing means coupled to the probe means for sensing information concerning the mutual inductance between the probe means and the material and for generating signals indicative thereof, and (c) mapping means coupled to the sensing means for mapping in magnified form on a record the sensed information concerning the mutual inductance.

7. Apparatus according to claim 6 and further comprising means for scanning the probe means in substantially parallel paths across the surface of the object.

8. A map of cryptic data symbols generated by the steps of: (a) scanning an object of conducting material with a time-varying magnetic field generated by a probe; and (b) mapping symbols indicative of the inductance of the probe onto a record at positions corresponding to at least some of the scanned positions and at a relatively enlarged scale.

9. Apparatus according to claim 8 in which the probe follows parallel paths in scanning the object.

10. Apparatus according to claim 7 or 8 in which the object is a gas turbine engine component.

11. Apparatus according to claim 7 or 8 in which the area of the nonuniformity is less than 4 x 10-7 square inches.

1 2. A method of obtaining cryptic data concerning a structural non-uniformity in an object of conductivity material, the method being substantially as herein described with reference to the accompanying drawings.

1 3. Apparatus for obtaining cryptic data concerning a structural non-uniformity in an object of conducting material, the apparatus being substantially as herein described with reference to the accompanying drawings.