GB2273782A - Eddy current flaw detection of sheet metal adjacent fasteners - Google Patents

Description

2273782 Eddy Current Induction Probe for Flaw Detection and Method of

Inspecting Sheet Metal for Flaws This invention relates to the non-destructive testing of sheet metal for flaws, such as cracks extending from an aperture, and is particularly useful for the rapid scanning of aircraft lap joints for flaws in the region of fasteners.

Corrosion and cracking may occur in one or more of the various layers of lap joints. Flaws may be initiated at a fastener and propagate outwards with time. Unless a sensitive means of flaw detection is available to scan fasteners in position, it may be necessary to remove each fastener in order to scan the aperture with a rotating probe. There may be several thousand fasteners on a single aircraft and fastener removal is a slow and expensive process.

The object of the presenit- invention is to provide a rapid and sensitive means of scanning for flaws around each fastener, utilising eddy current inspection.

A common difficulty when using eddy current inspection for the region around a fastener is that the fastener itself causes electrical changes that are large compared to the effects of small flaws and signficantly limits the sizes of flaws that can be detected.

Accordingly, the present invention provides a probe for detecting flaws in sheet metal in the region of a fastener piercing the metal sheet or sheets, comprising:

means for inducing an eddy current in the region of the fastener under inspection;. eddy current sensor coils connected to provide an electrical output indicative of the eddy currents local to the sensor coils; and means for guiding the sensor coils to move around the axis of the fastener within a common annulus in a plane parallel to the metal sheet or sheets.

The invention also provides a probe comprising a housing with a flat under surface, rotatable in use about a central axis normal to the under surface, at least two, preferably four, eddy current sensor coils with axes parallel to the central axis and angularly disposed, adjacent the under surface around that central axis all at the same radius, and an eddy current driver coil disposed on the central axis and extending adjacent the sensor is coils.

From another aspect, the invention provides a method of non-destructive, non-invasive inspection for flaws of metal sheets through which extend fasteners, comprising applying to the annular surface region of the sheet around each successive fastener a probe having electric driving and sensor coils respectively for inducing and then detecting eddy currents flowing in the sheet or sheets, rotating the probe about the central axis of the fastener so that the coils move in the same annular path over the surface of the metal sheet, simultaneously energising the driving coil, and analysing the outputs of the sensor coils provided during such rotation to determine the existence of flaws in the metal sheet or sheets around the 2 fastener.

The present invention provides a means to reduce the electrical changes in a detector circuit connected to the probe caused by the fastener as the region around it is scanned. Furthermore, the effect of neighbouring fasteners may be reduced, while good sensitivity to flaws is maintained. Electrical changes due to lift-off i.e. movement of the scanning array (the sensor coils) from the surface may be reduced.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:Figure 1 is a diagrammatic axial section through a two-part probe shown in operation centered over a fastener at a.lap joint of an aircraft wing, Figure 2 is a partial electrical circuit diagram of the probe of Figure 1 connected to a phase-plane display eddy current instrument, Figure 3 is an underneath plan view on arrow W of the probe of Figure 1 but to a scale of 2:1, and Figure 4 is a typical display from the instrument of Figure 2, with - several superimposed - scans mutually displaced.

With reference to Figure 1, a probe assembly is in two parts, 1 and 2. Part 1 is a rotationally symmetric plastics body containing an eddy current driver coil 3, f our sensor coils 4A to 4D arranged in two pairs and disposed at respective quadrants, and cylindrical ferrite 3 screens 5,6. The coil windings are connected to a connector 7. Part 2 is a rotationally symmetric plastics housing containing a bearing 8, non- slip feet 9, and a central aperture 10 for centrally locating the probe over a fastener 11. The metal fastener 11 secures three overlapping metal sheets 14,15,16 at a lap joint, each having a circular aperture for the fastener.

As shown in Figures 1 and 3, the components of the probe (except for the sensor coils) are coaxial; the four sensor coils 4A-4D and the driver coil 3 are disposed over the same annular region of the surface of the top metal sheet 16 around the fastener 11, with the sensor coils axes parallel to the axis. The sensor coils are arranged below the driver coil and in the plane of the underside of 1 the probe part 1, so as to be close to the annular region of the sheet 16 in use. The wire connector leads in the probe are flexible, to prevent mechanical forces reacting on the coils.

A copper block 12 on the axis at the lower end of the cylindrical inner ferrite screen 6, reduces electrical effects from the fastener 11.

Part 2 of the probe assembly is centred over the fastener to be inspected and held on to the surface with the frictional grip of the f eet 9. The visual centring process is facilitated by the window 10 on the axis, similar in size to the head of the fastener 11. Part 1 is then lowered into the bearing 8 and rotated, typically between 90 degrees and 180 degrees for a complete scan.

4 An alternating current (typical frequency 500 Hz-1.0kHz) is applied to the driver coil 3. The four secondary sensor coils 4A-C are spaced symmetrically around the inner ferrite screen 6. Opposite coils are connected together, as shown in Figure 2, to form two pairs of coils. A ferrite core 13 is inserted into each sensor coil in order to increase its sensitivity.

Figure 2 illustrates the electrical connections used in this form of the invention. Each pair of sensor coils is connected so that the electrical effect of each coil in the pair is additive. The two pairs of coils are then arranged to be in opposition to each other, by means of a differential amplifier 17. The probe is energised by alternating current in the driver coil 3. During the is scanning operation, the sensor coils will move around their annular path over the sheet 16. Any flaw present will generally extend radially from the hole. As one sensor coil is rotated over such a flaw, an electrical signal is detected. Further rotation causes an opposite signal as the next coil, which must necessarily be from the other pair of coils, is moved over the flaw.

As shown in - the example of Figure 3, wires 21 connecting the sensor coil windings are led across the undersurface of the probe part 1 and through bores 20 parallel to the axis. They are then led through further bores to connect to pins, and on to connector 7 as illustrated in Figure 1.

The probe is used typically with a phase-plane display eddy current instrument 18 and the differential amplifier 17 is typically the input amplifier of the instrument set in its differential mode. A probe cable 19 couples from the probe connector 7 to the instrument 18.

A typical phase plane display eddy current instrument accepts the electrical signal from its input amplifier and splits this into two quadrature components by means of two phase sensitive detectors which are in quadrature with each other. These two component signals are filtered to remove frequency components of the driver coil alternating current, and then amplified and filtered further.

Balancing circuits are used to offset the D.C. signals present when the probe is resting.

Rotation of the probe around a fastener causes small is changes in the electrical impedance of the probe. Typically, both resistive and reactive changes occur, i.e. in-phase and quadrature phase changes in impedance.

The two quadrature signals of the instrument are combined in a phase rotation circuit and then displayed as vertical and horizontal signals. By this means, the small resistive and reactive changes in impedance as the probe rotates, are displayed as signals at right angles to each other on a cathode ray tube face (or liquid crystal display etc) to create a phase-plane display.

Flaws occurring in the metal close to the fastener cause patterns of impedance change as the probe rotates.

A number of factors contribute to the effectiveness of the probe in minimising unwanted electrical effects and thereby enabling small flaws to be detected. Referring again to Figure 1, factors contributing to minimising unwanted effects from the fastener being inspected include:- 1. The central aperture 10, enabling the probe to be rapidly and accurately centred over each successive fastener 11. A number of different probes, each with an aperture size 10 suited to a corresponding fastener head diameter, would be provided.

2. The centre ferrite shield 6, which, together wLth copper block 12, reduces the induction of eddy currents in the fastener head and therefore reduces the effect of the fastener on the sensor (pickup) coils.

3. The driver coil 3 and ferrite shields 5,6 are is rotationally symmetrical about the fastener and, therefore, cause minimum electrical change as the probe rotates around the fastener.

4. Eddy currents will be mainly circular, circulating round the fastener in the metal sheets that are being examined for flaws. Typical flaws would be cracks lying radially, hence directly across the eddy current's tangential path, for maximum effect.

5. Each sensor coil of each pair is symmetrically spaced from the central axis. This reduces errors arising from inexact registration of probe and fastener head. As the probe is rotated, a slight increase in distance between the fastener head and one coil of a pair is compensated by the decreased distance for the other coil of the pair.

6. The two pairs of sensor coils are in electrical opposition, minimising the effect of the presence of the fastener and of electrical changes resulting from slight asymmetry as the probe is rotated.

7. The use of dual or multi-frequency alternating current to the driver coil together with a dual or multi-frequency phase plane eddy current instrument provides further opportunities for reducing the electrical effect of fasteners. For example, two different frequencies may be chosen so that the lower will penetrate deeper and be responsive to flaws in lower layers 14,15 while the higher frequency will be affected mainly by the fastener head. Subtraction of the two vector signals after gain and phase adjustment following well known eddy current techniques, enables electrical effects from a fastener to be further reduced.

-8. The driver coil is large in diameter relative to the fastener, enabling signficant eddy currents to be generated at a depth of several layers of metal when low frequency alternating current is used.

9. Any cracks that extend in opposite directions from a fastener will cause additive effect in the pick up coils.

10. The direct pick-up of the secondary coils from the flux of the primary coil is reduced by the positioning of the secondary coils and the presence of the flux conducting ferrite shields. The flux from the driver coil is diverted mainly into the two ferrite shields, bypassing the sensor coils. Further, of the flux that does pass 8 through the sensor coils, the flux near the inner ferrite is in opposition to the flux near the outer ferrite, and results in significant cancellation of the induced voltage in each sensor coil. This results in lower electrical noise because less noise from the alternating current generated is picked up.

11. Variation of electrical output with lift off (i.e.

the separation of probe from the test region) is minimised by the connection in opposition of the two pairs of coils.

12. Rotation in a smooth, accurate bearing 8 optimises the clarity of small electrical changes.

13. The 'non-slip' i.e. high friction feet 9 reduce the chance of displacement as the probe is rotated.

14. The outer ferrite shield 5 protects the pick-up is (sensor) coils from the effects of neighbouring fasteners.

Typical fastener sizes, as measured across the head, are from 6-18mm. In Figure 1, part 2 of the probe may typcially have an aperture of 8mm to deal with fasteners of 6-8mm diameter. Fasteners of 9-12mm are inspected by means of a probe with a 12mm aperture. 13-18mm fasteners are inspected by means of one with an 18mm aperture. The larger fasteners require probes with larger diameter internal and external ferrite shields.

In Figure 1, probe part 2 can be quickly located over each fastener head. Part 1 fits into Part 2 easily and a complete fastener inspection can be accomplished in a few seconds. It can then easly be moved over the surface of the sheet 16 to inspect the next fastener.

Where fasteners are raised above the general surface level, they can be allowed to enter partially into the aperture 10 area. Higher fastener profiles require the Part 1 probe assembly to be spaced further from the surface.

Figure 4 illustrates typical results printed out from a multi-frequency phase plane display unit, using the form of probe described in Figure 1. The zero point has been moved for each scan, for clarity. A flaw in the second layer of 1.5mm is clearly seen. The change of phase of approximately 180 degrees from surface flaw, through second layer to third layer flaw, shows up clearly.

The invention has been illustrated with four sensor coils, but a different number would also give useful is results due to those features which isolate the electrical effect of the fastener without requiring exactly four sensors.

An arrangement with eight sensors, of which four are connected in one group and four are connected in another group in electrical opposition to the first group, with the members of the groups alternating in sequence around the probe, would be feasible; so would arrangements of e.g. 6, 10 or 12 sensors with similar electrical connections. Alternate sensors should ideally make contributions electrically in opposition.

With the four sensors, cracks extending in opposite directions from the fastener both contribute to the strength of the signal displayed on the display unit. With eight sensors arranged equi-angularly, cracks at 900 to one another would also give rise to mutually reinforcing signals.

The probe illustrated is rotated a quarter to half a turn manually, but clearly other rotational arrangements are feasible, some involving power drive or geared manual drive with a return spring, for example.

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Claims (14)

1. A probe for detecting flaws in sheet metal in the region of a fastener piercing the metal sheet or sheets, comprising: means for inducing an eddy current in the region of the fastener under inspection; eddy current sensor coils connected to provide an electrical output indicative of the eddy current level local to the sensor coils; and means for guiding the sensor coils to move around the axis of-the fastener within a common annulus in a plane parallel to the metal sheet or sheets.

2. A probe according to Claim 1, in which the sensor coils are housed in the same rotatable housing, and the guiding means is a rotary platform for receiving and guiding the rotatable housing, having an under surface is with means f or gripping the surf ace of the metal sheet around the fastener to prevent relative movement.

3. A probe according to Claim 2, in which the platform has a window centred on the axis of rotation of the housing to facilitate positioning of the platform in register with the head of the fastener in use.

4. A probe according to Claim 1, 2 or 3, having an electromagnetic shield between the sensor coils and the axis of rotation, such as to isolate the electrical effects of the fastener on the sensor coils.

5. A probe according to any preceding claim, having an electromagnetic shield outside the sensor coils, such as to isolate them from the effects of external influences including other fasteners.

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6. A probe comprising a housing with a flat under surface, rotatable in use about a central axis normal to the under surface, at least two eddy current sensor coils with axes parallel to the central axis and angularly disposed, adjacent the under surface around that central axis all at the same radius, and an eddy current driver coil disposed on the central axis and extending adjacent the sensor coils.

7. A probe according to Claim 6, comprising a cylindrical electromagnetic screen on the central axis, within the sensor coils and extending to the under surface, for isolating them in use from a metallic object on the axis.

8. A probe according to Claim 6 or 7, comprising a cylindrical electromagnetic screen on the central axis, outside the sensor coils and extending to the under surface, for isolating them in use from external metallic objects.

9. A probe according to any preceding claim, comprising apparatus responsive to the variation of signals output from the sensor coils with the time-varying angular position of the probe about its axis, to provide an indication of flaws in a metal sheet adjacent or underlying the annular path, of movement of the sensor coils.

10. A probe according to Claim 6, 7 or 8, in which there are at least four sensor coils connected electrically as two groups in mutual phase opposition, with the sensor 13 - coils alternating - from group to group with angular position around the probe.

11. A probe according to Claim 10, comprising two groups of two sensor coils disposed at mutual angular separations of 900, the coiled windings being interconnected such that the electrical outputs of sensor coils at relative positions 00 and 1800 are added, those of the sensor coils at relative positions 900 and 2701 are added, and the said added outputs are subtracted from one another to provide 10 said phase opposition.

12. A method of non-destructive, non-invasive inspection for flaws of metal sheets through which extend fasteners, comprising applying to the annular surface region of the sheet around each successive fastener a probe having is electric driving and sensor coils respectively for inducing and then detecting eddy currents flowing in the sheet or sheets, rotating the probe about the central axis of the fastener so that the coils move in the same annular path over the surface of the metal sheet, simultaneously energising the driving coil, and analysing the outputs of the sensor coils provided during such rotation to determine the existence of flaws in the metal sheet or sheets around the fastener.

13. A probe substantially as described herein with reference to the accompanying drawings.

14. A method of detecting flaws, substantially as described herein with reference to the accompanying drawings.

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