Method and apparatus for short term inspection or long term structural health monitoring

• US 20010022514A1
• Filed: 05/15/2001
• Published: 09/20/2001
• Est. Priority Date: 03/17/1999
• Status: Active Grant

• Current Assignee: Southwest Research Institute
• Original Assignee: Southwest Research Institute
• Inventors: Kwun, Hegeon, Kim, Sang-Young, Light, Glenn M., Spinks, Robert L. Jr.
• Granted Patent: US 6,396,262 B2
• Time in Patent Office: Days
• US Class Current: 324/240
• CPC Class Codes: G01N 17/006 of metals
• CPC Class Codes: G01N 2291/0421 Longitudinal waves
• CPC Class Codes: G01N 2291/0422 Shear waves, transverse waves, horizontally polarised waves
• CPC Class Codes: G01N 2291/0428 Mode conversion
• CPC Class Codes: G01N 2291/044 Internal reflections (echoes), e.g. on walls or defects
• CPC Class Codes: G01N 2291/2632 flat
• CPC Class Codes: G01N 2291/2634 cylindrical from outside
• CPC Class Codes: G01N 29/11 by measuring attenuation of acoustic waves
• CPC Class Codes: G01N 29/2412 using the magnetostrictive properties of the material to be examined, e.g. electromagnetic acoustic transducers [EMAT]; investigating the presence of flaws using eddy currents G01N27/90, magnetostrictive transducers B06B1/08, measuring magnetostrictive properties G01R33/18

Abstract

A method and apparatus is shown for implementing magnetostrictive sensor techniques for the nondestructive short term inspection or long term monitoring of a structure. A plurality of magnetostrictive sensors are arranged in parallel on the structure and includes (a) a thin ferromagnetic strip that has residual magnetization, (b) that is coupled to the structure with a couplant, and (c) a coil located adjacent the thin ferromagnetic strip. By a transmitting coil, guided waves are generated in a transmitting strip and coupled to the structure and propagate along the length of the structure. For detection, the reflected guided waves in the structure are coupled to a receiving strip and are detected by a receiving magnetostrictive coil. Reflected guided waves may represent defects in the structure.

Claims

1. A method of nondestructive, short term inspection or long term monitoring of a structure to determine if said structure (a) has a defect such as a crack, corrosion or erosion or (b) has a transient stress signal due to vibrations, cracking or mechanical impacts, said method comprising the following steps:

preparing a plurality of thin strips of ferromagnetic material of appropriate width and length;

inducing residual magnetization along said length of said thin strips by applying an external magnetic field and thereafter removing said external magnetic field;

coupling said thin strips in parallel to said structure;

installing a magnetostrictive probe on each of said thin strips;

generating a pulse signal in a transmitter control circuit and delivering said pulse signal to a first of said magnetostrictive probes to create guided waves in a first of said thin strips, which guided waves are coupled to said structure for propagation therein;

magnetostrictively detecting any reflected waves by a second of said magnetostrictive probes in combination with a second of said thin strips, said reflected waves being coupled from said structure to said second of said thin strips; and

determining if said reflected waves are due to said defect or said transient stress signal.

2. The method of nondestructive, short term inspection or long term monitoring of said structure as recited in

claim 1

wherein said coupling step includes bonding said thin strips to said structure.

3. The method of nondestructive, short term inspection or long term monitoring of said structure as recited in

claim 2

wherein said guided waves are shear waves.

4. The method of nondestructive, short term inspection or long term monitoring of said structure as recited in

claim 2

wherein said structure is a pipe and said guided waves are torsional waves.

5. The method of nondestructive, short term inspection or long term monitoring of said structure as recited in

claim 1

wherein said coupling step includes using a thick, viscous material as a couplant.

6. The method of nondestructive, short term inspection or long term monitoring of said structure as recited in

claim 4

wherein said determining step includes storing a reference reflected wave and, after an appropriate period of time, repeating said generating step and said magnetostrictively detecting step, comparing a second reflected wave with said reference reflected wave to determine if defects have occurred during said appropriate period of time.

7. The method of nondestructive, short term inspection or long term monitoring of said structure as recited in

claim 6

wherein said determining step includes subtracting said reference reflected wave from said second reflected wave.

8. The method of nondestructive, short term inspection or long term monitoring of said structure as recited in

claim 1

wherein said determining step includes storing a reference reflected wave, thereafter repeating said generating step and said magnetostrictively detecting step and comparing subsequent reflected waves with said reference reflected wave to determine if said transient stress signal has occurred.

9. The method of nondestructive, short term inspection or long term monitoring of said structure as recited in

claim 8

wherein said reference reflected wave is continually updated.

10. The method of nondestructive, short term inspection or long term monitoring of said structure as recited in

claim 1

wherein said thin strip may be selected from the group of ferromagnetic materials having appropriate magnetostrictive coefficients consisting of nickel, grain-oriented silicon steel or TERFENDOL-D®

.

11. The method of nondestructive, short term inspection or long term monitoring of said structure as recited in

claim 5

wherein said couplant is honey.

12. An apparatus for nondestructive, short term inspection or long term monitoring of a structure to determine if said structure (a) has a defect, such as a crack, corrosion or erosion, or (b) has a transient stress signal due to vibrations, cracking or mechanical impact, said apparatus comprising:

a plurality of thin ferromagnetic strips that have residual magnetization therein, said thin ferromagnetic strips being coupled in parallel to said structure;

a transmitter coil being located adjacent to a first of said thin ferromagnetic strips;

a receiver coil being located adjacent to a second of said thin ferromagnetic strips;

a transmitter control circuit connected to said transmitter coil for generating a pulse signal and delivering said pulse signal to said transmitter coil, said transmitter coil creating magnetostrictively a guided wave that is coupled from said first thin ferromagnetic strip to said structure to propagate along said structure;

said receiver coil magnetostrictively detecting said guided wave and any reflected signals, including any caused by defect or transient stress signals in said structure;

said transmitter coil and said receiver coil being wound adjacent said first and second thin ferromagnetic strips, respectively, said guided waves moving perpendicular to said first and second thin ferromagnetic strips.

13. An apparatus for nondestructive, short term inspection or long term monitoring of a structure as recited in

claim 12

wherein said residual magnetization is in a lengthwise direction of said thin ferromagnetic strips and said guided wave is a shear wave.

14. An apparatus for nondestructive, short term inspection or long term monitoring of a structure as recited in

claim 13

further including a computer for storing a first of said reflected signals and, after appropriate periods of time, comparing new reflected signals with said stored reflected signal to determine if changes have occurred.

15. An apparatus for nondestructive, short term inspection or long term monitoring of a structure as recited in

claim 14

wherein a couplant for said coupling of said plurality of said thin ferromagnetic strips is a bonding material, such as epoxy.

16. An apparatus for nondestructive, short term inspection or long term monitoring of a structure as recited in

claim 12

wherein a couplant for said coupling of said plurality of said thin ferromagnetic strips is a thick, viscous material, such as honey.

17. An apparatus for nondestructive, short term inspection or long term monitoring of a structure as recited in

claim 12

wherein said plurality of thin ferromagnetic strips retain said residual magnetization for a long period of time, such as nickel, grain-oriented silicon steel, or TERFENDOL®

.

18. An apparatus for nondestructive, short term inspection or long term monitoring of a structure as recited in

claim 15

wherein at least four of said thin ferromagnetic strips are used, two for transmitting and two for receiving, so that direction of travel of said guided wave can be determined, therefore only said reflected signals in a given direction being stored in said computer.

19. An apparatus for nondestructive, short term inspection or long term monitoring of a structure recited in

claim 15

wherein said transmitter coil and said receiver coil are coils on a flexible printed circuit board which generates or receives said guided wave in said ferromagnetic strips.