Applied and residual stress measurements using magnetic field sensors

• US 20030173958A1
• Filed: 01/24/2003
• Published: 09/18/2003
• Est. Priority Date: 01/25/2002
• Status: Active Grant

• Current Assignee: Jentek Sensors, Inc.
• Original Assignee: Jentek Sensors, Inc.
• Inventors: Goldfine, Neil J., Grundy, David C., Schlicker, Darrell E., Washabaugh, Andrew P., Zilberstein, Vladimir A., Lyons, Robert J., Shay, Ian C., Tsukernik, Vladimir, Fisher, James M.
• Granted Patent: US 7,526,964 B2
• Time in Patent Office: Days
• US Class Current: 324/209
• CPC Class Codes: G01L 5/0047 measuring forces due to residual stresses
• CPC Class Codes: G01N 27/904 with two or more sensors

Abstract

Methods are described for the use of conformable eddy-current sensors and sensor arrays for characterizing residual stresses and applied loads in materials. In addition, for magnetizable materials such as steels, these methods can be used to determine carbide content and to inspect for grinding burn damage. The sensor arrays can be mounted inside or scanned across the inner surface of test articles and hollow fasteners to monitor stress distributions. A technique for placing eddy-current coils around magnetizable fasteners for load distribution monitoring is also disclosed.

Claims

1. A method for monitoring load on a fastener in a structural article, said method comprising:

mounting a hollow fastener in the structural article, wherein the fastener has an electrical property that varies with stress;

measuring said electrical property inside the hollow fastener with at least one sensor; and

relating said electrical property to the stress and load on the fastener.

2. The method as claimed in claim 1, wherein the fastener is steel with permeability greater than one.

3. The method as claimed in claim 1, wherein the electrical property is measured with an eddy-current sensor.

4. The method as claimed in claim 3, wherein the sensor is an eddy-current sensor array.

5. The method as claimed in claim 4, wherein the sensor array is mounted to the inner surface of the fastener.

6. The method as claimed in claim 4, wherein the sensor array is scanned over the inner surface of the fastener.

7. The method as claimed in claim 1, wherein the electrical property is the magnetic permeability.

8. The method as claimed in claim 1, wherein the electrical property is the electrical conductivity.

9. The method as claimed in claim 1, wherein the sensor contains a giant magnetoresistive sensor.

10. A method for monitoring the stress on a structural article, said method comprising:

affixing a stress-sensitive material to the structural article, wherein the material has an electrical property that varies with stress;

measuring said electrical property with at least one sensor; and

relating said electrical property to the stress on the article.

11. The method as claimed in claim 10, wherein the electrical property is measured with at least one eddy-current sensor.

12. The method as claimed in claim 11, wherein the sensor is an eddy-current sensor array.

13. The method as claimed in claim 12, wherein the sensor array is mounted to the surface of the article.

14. The method as claimed in claim 12, wherein the sensor array is scanned over the surface of the article.

15. The method as claimed in claim 10, wherein the stress-sensitive material is steel with a magnetic relative permeability greater than one.

16. The method as claimed in claim 10, wherein the stress-sensitive material is nonmagnetic metal.

17. The method as claimed in claim 10, wherein the electrical property is the magnetic permeability.

18. The method as claimed in claim 10, wherein the electrical property is the electrical conductivity.

19. The method as claimed in claim 10, wherein the stress-sensitive material is mounted behind the article, on the opposite surface from the sensor.

20. The method as claimed in claim 10, wherein the stress-sensitive non-magnetic material is mounted on top of the article, between the article and the sensor.

21. The method as claimed in claim 10 wherein the sensor is not in contact with surface of the article.

22. The method as claimed in claim 10 wherein the sensor contains a giant magnetoresistive sensor.

23. A method for inspecting magnetic material comprising:

disposing at least two parallel drive conductors with at least one sense element on a substrate proximate to a surface of a test material;

passing a time-varying electric current through the drive conductors to create a magnetic field;

measuring the response from each sense element to measure an electrical property of the test material; and

correlating the electrical property with state of the material.

24. The method as claimed in claim 23, wherein the substrate is flexible

25. The method as claimed in claim 23, wherein there is a plurality of sense elements.

26. The method as claimed in claim 25, wherein the sense elements are aligned in a direction parallel to the drive conductors.

27. The method as claimed in claim 25, wherein the sense elements are scanned over the surface.

28. The method as claimed in claim 23, wherein the response is measured in two orientations.

29. The method as claimed in claim 23, wherein the response is measured at multiple frequencies.

30. The method as claimed in claim 23, wherein the electrical property is magnetic permeability.

31. The method as claimed in claim 23, wherein the state of the material relates to stress on the material.

32. The method as claimed in claim 23, wherein the state of the material relates to carbide content in the material.

33. The method as claimed in claim 23, wherein the state of the material relates to presence of grinding burn.

34. A method for monitoring load on a fastener made out of magnetic material having properties that vary with stress, said method comprising:

mounting an eddy-current sensor around a fastener in a structural article;

measuring the sensor response to the fastener; and

relating the response to the load on the fastener.

35. The method as claimed in claim 34, wherein the fastener material is steel.

36. The method as claimed in claim 34, further comprising placing a second eddy-current sensor around the fastener on the opposing side from the first eddy-current sensor.

37. A method for inspecting test material, said method comprising:

disposing a sensor having a direction of higher sensitivity to test material properties near the test material;

measuring an effective property response image for the sensor in at least two sensor orientations; and

differentiating between the response images to separate a directional material property from a non-directional material property.

38. The method as claimed in claim 37, wherein the sensor is flexible.

39. The method as claimed in claim 37, wherein the sensor is an eddy current sensor.

40. The method as claimed in claim 37, wherein the sensor has a plurality of sense elements.

41. The method as claimed in claim 40, wherein the sense elements are aligned in a direction perpendicular to the direction of sensitivity of the sensor.

42. The method as claimed in claim 40, wherein the sensor is scanned over the surface of the material.

43. The method as claimed in claim 37, wherein the non-directional property is grinding burn.

44. The method as claimed in claim 37, wherein the directional property is residual stress.

45. The method as claimed in claim 37, wherein one of the sensor orientations is in the direction of the maximum for the effective property.

46. The method as claimed in claim 37, wherein one of the sensor orientations is in the direction for the minimum for the effective property.

47. The method as claimed in claim 37, wherein the response is measured at multiple frequencies.

48. The method as claimed in claim 37, wherein the effective property is magnetic permeability.

49. The method as claimed in claim 37, wherein the effective property is electrical conductivity.

50. The method as claimed in claim 37, wherein the effective property is lift-off.