Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Patent
1992-01-29
1994-03-29
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
324238, 324262, G01N 2787, G01N 2790, G01R 3306
Patent
active
052988589
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of non-destructive testing for the presence of flaws in electrically conductive materials and to an apparatus for the same. Both the method and apparatus are applicable to the testing of materials and structures of sites of circular symmetry and are principally though not exclusively intended for the detection of flaws under or around fasteners, such as rivets, in aircraft skins or similar structures.
The holes provided in aircraft skins and the like for the installation of rivet fasteners etc provide sites where stresses are likely to be concentrated and in consequence they are a vulnerable area for initiation of fatigue cracks. Aircraft structures typically incorporate a multitude of installed fasteners and there is need to be able to inspect these structures easily and reliably to detect small fatigue cracks arising at fastener positions before these cracks reach a size where they pose a hazard to safety or else are difficult to repair. Obviously it is desirable to undertake such inspections in situ and without removing either the surface paint (if present) or the fasteners themselves.
One established technique for the investigation of sub-surface defects in conductive materials relies upon the electromagnetic effects of eddy currents induced in the structure under test by the imposition of a time varying magnetic field. This technique can cope with surface paints and has already been applied to the investigation of flaws in aircraft structures at fastener locations. The inventor's prior UK patents GB 2028510B and GB 2078965B disclose two variants of a device which detects such flaws by detecting the change in impedance exhibited by a transducer coil by virtue of its interaction with the reaction field induced in the structure, as it is moved in steps around the circumference of a fastener. The impedance of the transducer coil is in part dependent upon the reaction field from the eddy currents in the test structure and this component may be expected to change when the normal distribution of eddy current induced in the material surrounding the fastener is disturbed by the presence of a crack. Another UK patent GB 1,113,007 discloses a device incorporating a pot core and two Hall-effect devices in a probe which is rotated around the fastener position to detect the reaction field and changes to it. All the devices disclosed in the above-mentioned patent specifications rely upon the steady state characteristics of eddy current induced by a sinusoidal excitation or the like.
One problem with prior art systems such as the inventor's own systems is that coil transducers provide a poor spatial resolution because they cover a significant area of the test structure surface at any moment. Furthermore the coils provide a response which is highly frequency dependent being proportional to the time derivative of encompassed magnetic flux. This limits the ability to use an optimum frequency from the point of view of flaw detection at depth. The worst case from the point of view of an eddy-current based system is presented by a structure comprising a non-ferrous material having ferrous fasteners. Such structures are commonplace, if not predominant, in present day aircraft and an adequate test-equipment must be able to cope with them. The ferrous fastener material dominates the magnetic field within the test structure and tends to mask eddy current signals from flaws in the surrounding material. What is more any surface feature of the fastener such as the commonplace pips dimples and identification marks present exaggerated clutter against which it is difficult to distinguish a flaw signal when the fastener is ferrous and the bonded structure is non-ferrous. No current commercial eddy-current inspection equipment known to the inventor can deal adequately with this problem, neither can the equipments disclosed in the above-mentioned prior-art patent specifications.
This invention is intended to provide a convenient and effective way of examining structures at fastener locations which can
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Edmonds Warren S.
Strecker Gerard R.
The Secretary of State for Defence in Her Britannic Majesty's Go
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