Pulsed eddy current method for detection of corrosion in multilayer structures using the lift-off point of intersection

• US 6,344,741 B1
• Filed: 06/20/2000
• Issued: 02/05/2002
• Est. Priority Date: 06/20/2000
• Status: Expired due to Fees

• Current Assignee: Her Majesty The Queen In Right of Canada As Represented By The Minister of National Defence
• Original Assignee: Her Majesty The Queen As Represented By The Minister of National Defence of Her Majesty's Canadian Government
• Inventors: Dubois, Stéphane J. M., Giguere, Sylvain
• Primary Examiner(s): Metjahic, Safet
• Assistant Examiner(s): Zaveri, Subhash
• Application Number:
  US09/597,221
• Time in Patent Office: 595 Days
• Field of Search: 324/240, 324/225, 324/228, 324/229, 324/242, 324/243, 324/232, 324/233, 324/260, 324/238, 324/639
• US Class Current: 324/240
• CPC Class Codes: G01N 17/006 of metals
• CPC Class Codes: G01N 27/9046 by analysing electrical signals

Abstract

A method for the detection and the characterization of corrosion in multi-layer metallic structures using a pulsed eddy current technique. For this technique, a coil (or coils) is used both as field source (driven by a square wave voltage-controlled excitation), and/or as field sensor (measuring a transient response). The field sensor allows the capture of information about the condition of the area of the structure under inspection. The ability of this technique to detect corrosion hinges on the use of a transient response feature (i.e., Lift-off Point of Intersection) to infer the presence of material loss. With the help of a calibration standard, the Lift-off Point of Intersection provides the ability to quantify material loss in multi-layered structures. The results obtained with this method are independent of lift-off variations inherent to field inspections (i.e., changes in distance between the transducer and test object).

Claims

1. A method for detecting defects in a metallic structure, the method comprising locating a transducer at a first distance from said metal structure at one area that lacks any significant defects in the structure, activating said transducer with a square wave voltage-controlled excitation to generate eddy currents in the structure and then sensing, with at least one magnetic flux sensor, time-varying magnetic fields generated by the transducer and said eddy currents, signals obtained from said at least one sensor being recorded, this process being repeated to obtain at least one other recorded signal that is obtained with the transducer being located in the same location but at a different lift-off distance from said one area, determining where the recorded signals cross to establish a Lift-off Point of Intersection at a point in time, placing said transducer at other areas of said structure which are to be tested for defects, activating said transducer with similar voltage-controlled excitation as applied at said one area, then obtaining and recording signals sensed from the time-varying magnetic fields generated by the transducer and eddy currents in a similar manner as at said one area, comparing the recorded signals amplitudes which are obtained at said other areas at said point in time with those of signals obtained at said one area wherein differences in signal amplitude provides indications of any defects present and a quantitative evaluation of such defects at areas being tested.

2. A method as defined in claim 1, wherein at least three recorded signals are obtained with the transducer being located at different distances from said one area.

3. A method as defined in claim 2, wherein one recorded signal is obtained at each of said other areas of said structure.

4. A method as defined in claim 1, wherein said transducer and said at least one magnetic flux sensor is a single coil.

5. A method as defined in claim 2, wherein said transducer and said at least one magnetic flux sensor is a single coil.

6. A method as defined in claim 3, wherein said transducer and said at least one magnetic flux sensor is a single coil.

7. A method as defined in claim 1, wherein said transducer comprises separate coils, one being used as a field source and another coil being said at least one magnetic flux sensor used exclusively to sense the rate of change of the total magnetic flux.

8. A method as defined in claim 2, wherein said transducer comprises separate coils, one being used as a field source and another coil being said at least one magnetic flux sensor used exclusively to sense the rate of change of the total magnetic flux.

9. A method as defined in claim 3, wherein said transducer comprises separate coils, one being used as a field source and another coil being said at least one magnetic flux sensor used exclusively to sense the rate of change of the total magnetic flux.

10. A method for detecting the material loss due to corrosion in a multi-layer metal structure comprising placing a transducer near said metal structure at one area that lacks any significant defects, activating said transducer with a square wave voltage-controlled excitation to generate eddy currents in said structure and then sensing the magnetic flux generated by the transducer and said eddy currents with transient signals thereby obtained being recorded, placing said transducer at said one area but at a different distance from said structure and activating the transducer with a similar voltage-controlled excitation to again generate eddy currents whose magnetic flux are sensed together with the transducer produced flux, signals obtained from the sensed flux being recorded, determining where the recorded signals cross to establish a Lift-off Point of Intersection at a point in time, placing said transducer at other areas of said structure which are to be tested for corrosion, activating said transducer with similar voltage-controlled excitation and obtaining and recording signals sensed from the magnetic flux generated by eddy currents and the transducer in a similar manner as at said one area, comparing the recorded signals amplitude which was obtained at each of said other areas at said point in time with those of signals obtained at said one area whereby differences in recorded signal amplitude provide an indication of any corrosion being present at an area being tested and a quantitative evaluation of the material loss due to corrosion.

11. A method as defined in claim 10, wherein at least three recorded signals are obtained with the transducer being located at different distances from said one area.

12. A method as defined in claim 11, wherein one recorded signal is obtained at each of said other areas of said structure.

13. A method as defined in claim 10, wherein said transducer and said at least one magnetic flux sensor is a single coil.

14. A method as defined in claim 11, wherein said transducer and said at least one magnetic flux sensor is a single coil.

15. A method as defined in claim 12, wherein said transducer and said at least one magnetic flux sensor is a single coil.

16. A method as defined in claim 10, wherein said transducer comprises separate coils, one being used as a field source and another coil being said at least one magnetic flux sensor used exclusively to sense the rate of change of the total magnetic flux.

17. A method as defined in claim 11, wherein said transducer comprises separate coils, one being used as a field source and another coil being said at least one magnetic flux sensor used exclusively to sense the rate of change of the total magnetic flux.

18. A method as defined in claim 12, wherein said transducer comprises separate coils, one being used as a field source and another coil being said at least one magnetic flux sensor used exclusively to sense the rate of change of the total magnetic flux.