Remote optical crack sensing system including fiberoptics

• US 4,636,638 A
• Filed: 10/12/1984
• Issued: 01/13/1987
• Est. Priority Date: 10/12/1984
• 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: Gause, Lee W., Libeskind, Mark, Scotese, Arthur, Huang, Shih L.
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): Oen, William L.
• Application Number:
  US06/660,349
• Time in Patent Office: 823 Days
• Field of Search: 250/227, 250/231 R, 356/32, 356/33, 356/34, 73/800
• US Class Current: 250/231.1
• CPC Class Codes: G01B 11/18 using photoelastic elements
• CPC Class Codes: G01M 11/085 the optical fiber being on or near the surface of the DUT

Abstract

An apparatus is disclosed for remotely detecting cracks of sub-critical lth which originate from stress raisers in structural materials, wherein the apparatus typically comprises a source of light, a continuous length of optical fiber to conduct the light, strain responsive devices coupled to the fiber and bonded to the structural member in close proximity to the stress raisers, and a light responsive detector which is optically coupled to the fiber in order to detect breaks in the fiber induced by either tensile or shear forces eminating from cracks around the stress raisers.

Claims

1. Apparatus for detecting cracks which originate from a stress raiser in a structural material, comprising in combination:

a length of optical fiber having two ends;

means for injecting light into one of said ends;

strain responsive means coupled to said fiber and attched to the structural material in close proximity to the stress raiser such that cracks originating therefrom cause tensile forces which will break said fiber in tension, said strain responsive means consisting of two non-contiguous rigid members bonded by their lower surfaces to the structural material to form a gap between said members and a graphite epoxy composite having lower strain than said fiber by which said fiber is attached to said members, said composite spanning said gap; and

light responsive means optically coupled to the other of said ends for detecting breakage of said fiber.

2. Apparatus for detecting a crack in a structure comprising, in combination:

two spaced connectors formed to be rigidly attached to the structure;

an optical fiber of finite length having two ends and extending between said connectors;

a composite bonded to said fiber along the entire length thereof and to said connectors, said composite having a lower deformation per unit of length than said fiber;

a light source at one of said ends of the fiber positioned to transmit light along the length of said fiber; and

a light detector at the other of said ends of said fiber for detecting a discontinuity in the transmitted light;

whereby a crack in the structure between the connectors is quickly detected.