Optical fiber strain-to-failure sensor

• US 5,639,968 A
• Filed: 10/23/1995
• Issued: 06/17/1997
• Est. Priority Date: 10/23/1995
• Status: Expired due to Fees

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Bobb, Lloyd C., Krumboltz, Howard D.
• Primary Examiner(s): Chilcot, Richard
• Assistant Examiner(s): Noori, Max H.
• Application Number:
  US08/546,974
• Time in Patent Office: 603 Days
• Field of Search: 73/800, 73/760, 385/28
• US Class Current: 73/800
• CPC Class Codes: G01M 11/083 by using an optical fiber in contact with the device under test [DUT]

Abstract

A sensor and method are disclosed for determining if a region, defined by two end points, in a structure has exceeded a predetermined amount of strain. The sensor has an optical waveguide which has two ends for receiving and emitting light and which is fixable at two locations thereon to respective ones of the end points to define a sensing region therebetween. The sensing region has a first portion with a first length and a first cross-sectional area, and a second portion with a second length and a second cross-sectional area which is smaller than the first cross-sectional area. The lengths and cross-sectional areas are sized so that the optical waveguide has a strain failure point equal to the predetermined amount of strain. The lengths and cross-sectional areas are approximately sized according to the formula R=α+1/ (α/β+1), wherein R=the ratio of the amount of strain in the second portion which will cause the optical waveguide to fail to the predetermined amount of strain in the structure, α=the ratio of the length of the first portion to the length of the second portion, and β=the ratio of the cross-sectional area of the first portion to the cross-sectional area of the second portion.

Claims

1. A sensor for determining if a region, defined by two end points, in a structure has exceeded a predetermined amount of strain along the line through the two end points, comprising:

an optical waveguide having two ends for receiving and emitting light, fixable at two locations along the length thereof to respective ones of the end points to define a sensing region therebetween, the locations on said optical waveguide being fixed to the two end points on the structure so that said optical waveguide will be pulled in tension as the strain in the region increases the distance between the two end points, the sensing region having a first portion with a first length and a first cross-sectional area, and a second portion with a second length and a second cross-sectional area which is smaller than the first cross-sectional area, said lengths and cross-sectional areas being sized so that said sensing region of said optical waveguide has a strain failure point equal to the predetermined amount of strain.

2. The sensor according to claim 1, wherein said first portion comprises two segments and said second portion is between said two segments of said first portion.

3. The sensor according to claim 2, wherein said two segments of said first portion are of equal length.

4. The sensor according to claim 1, wherein said lengths and cross-sectional areas are approximately sized according to the formula R=α

+1/(α

/β

+1), wherein R=the ratio of the amount of strain in the second portion which will cause said optical waveguide to fail to the predetermined amount of strain in the structure, α

=the ratio of the length of the first portion to the length of the second portion, and β

=the ratio of the cross-sectional area of the first portion to the cross-sectional area of the second portion.

5. A method of determining if a region, defined by two end points, in a structure has exceeded a predetermined amount of strain, comprising the steps of:

providing an optical waveguide capable of transmitting light therethrough and having two ends for receiving and emitting light therethrough;

selecting two locations on the optical waveguide whereupon the waveguide will be fixed to respective ones of the two end points on the structure, which two locations define a sensing region of the optical waveguide therebetween;

providing the sensing region of the optical waveguide with a first portion having a first length and a first cross-sectional area, and a second portion having a second length and a second cross-sectional area which is smaller than that of the first portion, the lengths and cross-sectional areas being sized so that when the optical waveguide is fixed to the structure at the two end points thereon the strain in the optical waveguide will reach the known strain failure point of the second portion of the optical waveguide when the structure reaches the predetermined strain; and

fixing the locations on the optical waveguide to the two end points on the structure so that the optical waveguide will be pulled in tension as the strain in the region increases the distance between the two end points.

6. The method according to claim 5, further including the step of observing whether or not the optical waveguide has failed, as an indication as to whether the region in the structure has exceeded the predetermined amount of strain.

7. The method of claim 6, wherein said step of observing whether or not the optical waveguide has failed includes the step of launching light into one end of the waveguide and observing that it does not emerge from the other end thereof.

8. The method according to claim 5, wherein the lengths and cross-sectional areas are approximately sized according to the formula R=α

+1/(α

/β

+1), wherein R=the ratio of the known strain failure point of the second portion of the optical waveguide to the predetermined amount of strain in the structure, α

=the ratio of the length of the first portion to the length of the second portion, and β

=the ratio of the cross-sectional area of the first portion to the cross-sectional area of the second portion.

9. The method of claim 5, wherein said step of fixing the locations on the optical waveguide to the two end points on the structure further includes the step of pretensioning the optical waveguide so that it is taut when fixed to the structure.

10. The method of claim 5, wherein said step of fixing the locations on the optical waveguide to the two end points on the structure further includes the step of applying a UV-cured epoxy between locations and their respective end points.

11. The method of claim 5, wherein said step of providing the sensing region of the optical waveguide with a first portion having a first length and a first cross-sectional area, and a second portion having a second length and a second cross-sectional area which is smaller than that of the first portion includes the step of reducing the cross-sectional area of the provided optical waveguide in the area thereof which is the second portion.

12. The method of claim 11, wherein said step of reducing the cross-sectional area of the provided optical waveguide in the area thereof which is the second portion includes the step of etching away the outer surface of the optical waveguide until the desired cross-sectional area of the second portion is achieved.

13. The method of claim 12, wherein said step of etching includes chemical etching.

14. The method of claim 12, wherein said step of etching includes plasma etching.