Material condition assessment with spatially periodic field sensors

• US 20040066188A1
• Filed: 08/04/2003
• Published: 04/08/2004
• Est. Priority Date: 09/07/1999
• Status: Active Grant

• Current Assignee: Jentek Sensors, Inc.
• Original Assignee: Jentek Sensors, Inc.
• Inventors: Goldfine, Neil J., Schlicker, Darrell E., Washabaugh, Andrew P., Zilberstein, Vladimir A., Tsukernik, Vladimir, Walrath, Karen E.
• Granted Patent: US 7,161,350 B2
• Time in Patent Office: Days
• US Class Current: 324/228
• CPC Class Codes: G01B 7/24 using change in magnetic properties
• CPC Class Codes: G01B 7/34 for measuring roughness or irregularity of surfaces
• CPC Class Codes: G01N 27/20 Investigating the presence of flaws
• CPC Class Codes: G01N 27/72 by investigating magnetic variables
• CPC Class Codes: G01N 27/902 by moving the sensors
• CPC Class Codes: G01R 33/12 Measuring magnetic properties of articles or specimens of solids or fluids involving magnetic resonance G01R33/20

Abstract

Inductive sensors measure the near surface properties of conducting magnetic materials. The sensors generally include parallel winding segments to induce a spatially periodic magnetic field in a material under test. The sensors may provide a directionally dependent measure with measurements made in varying orientations of the sensor with respect to the material property variation directions. The sensors may be thin, conformable sensors that can be mounted on a test material and, for example, monitor crack initiation under the sensor. A second sensor may be left in air to provide a reference measurement, or the temperature of the material under test can be varied to verify the response of the individual sensing elements. Sensors can be mounted to materials under test in order to not modify the environment that is causing the stress being monitored. A sensor may be flexible to conform to the shape of the surface of the material under test and may be mounted in difficult to access locations such as around fasteners of an aircraft. Spatially periodic field eddy current sensors may be scanned across a material to create images of the absolute material property beneath the sensor.

Claims

1. A method for monitoring material properties comprising:

mounting an electromagnetic sensor on a test material surface; and

periodically measuring at least one electrical property of the material under the sensor footprint.

2. A method as claimed in claim 1 where the sensor is an eddy-current sensor.

3. A method as claimed in claim 1 where the sensor is a conformable spatially periodic field eddy-current sensor.

4. A method as claimed in claim 3 where the sensor has a periodic primary winding to excite a periodic magnetic field and at least one sensing element to detect variations in the magnetic field related to electrical properties of the material under test.

5. A method as claimed in claim 4 where multiple sensing elements are used to measure absolute electrical conductivity at each sensing element location with at least one primary current excitation frequency.

6. A method as claimed in claim 5 where at least one sensing element is located in a region expected to see less damage than at other elements.

7. A method as claimed in claim 6 where the sensing element at the location of less damage is used as a reference for measurements at other elements.

8. A method as claimed in claim 1 further comprising intentionally varying the temperature of the material under test to verify the proper performance of the sensor.

9. A method as claimed in claim 1 further comprising intentionally varying the temperature of the material under test, recording the temperature variations with a temperature measurement sensor, and recording the sensor data at a plurality of temperature levels.

10. A method as claimed in claim 9 further comprising calibrating the sensor using the sensor data at different temperatures.

11. A method as claimed in claim 9 further comprising assessing the test material condition using sensor data at multiple temperatures.

12. A method as claimed in claim 1 further comprising measuring the electrical property at specified times.

13. A method as claimed in claim 12 further comprising using a change in the electrical property above the noise level to indicate a significant change in the test material condition.

14. A method as claimed in claim 1 further comprising mounting the sensor with a gap between the sensor and the test material to permit the environment under the sensor to substantially match the environment if the sensor was not present.

15. A method as claimed in claim 1 further comprising perforations in the sensor to allow corrosion to occur at the test material surface.

16. A method as claimed in claim 1 further comprising providing guides to permit scanning eddy-current sensors to inspect with the permanently mounted eddy-current sensor remaining in place.

17. A method as claimed in claim 1 further comprising a mechanism to permit relative motion of the sensor windings relative to the test material using a remote actuation.

18. A method as claimed in claim 3 further comprising shaping the test material to create a stress distribution so that fatigue damage initiates under the sensor.

19. A method as claimed in claim 18 where the test material is formed into a dogbone shape and the center section is thinned to localize fatigue damage.

20. A method as claimed in claim 19 where the test material further comprises reinforcement ribs on the edges.

21. A method as claimed in claim 20 where the test material further comprises radius cutouts on both sides of the thinned section.

22. A method as claimed in claim 18 where the test material further comprises radius cutouts on both sides of the thinned section.

23. A method as claimed in claim 1 where the sensor is conformable.

24. A method as claimed in claim 1 where the sensor is a dielectrometer.

25. A method as claimed in claim 1 where a second sensor is incorporated with the electromagnetic sensor.

26. A method as claimed in claim 25 where the second sensor is a strain gauge.

27. A method as claimed in claim 25 where the second sensor is a temperature gauge.

28. A method as claimed in claim 1 where the sensor comprises a drive conductor to carry an applied electric current and at least one sensing element to measure the electrical properties of the material under test.

29. A method as claimed in claim 28 where the sensing elements are absolute inductive coils.

30. A method as claimed in claim 28 where the sensing elements are magnetoresistive elements.

31. A method as claimed in claim 28 where the sensing elements are SQUIDS.

32. A method as claimed in claim 28 where the sensing elements are differential inductive coils.

33. A method as claimed in claim 1 where measurement grids are used to convert the sensor response to one or more properties of interest.