Intrumented patch for repair of fatigue damaged or sensitive structure

• US 5,553,504 A
• Filed: 10/19/1995
• Issued: 09/10/1996
• Est. Priority Date: 11/23/1993
• Status: Expired due to Fees

• Current Assignee: Vought Aircraft Industries, Inc. (Triumph Group, Inc.)
• Original Assignee: Grumman Aerospace Corporation (Northrop Grumman Corporation)
• Inventors: Lyons, Donald R., Shyprykevich, Peter, Reich, Stanley M.
• Primary Examiner(s): Chilcot, Richard
• Assistant Examiner(s): PATEL, HARSHAD R
• Application Number:
  US08/545,167
• Time in Patent Office: 327 Days
• Field of Search: 73/775, 73/766, 73/800, 73/799, 73/763, 356/35.5, 52/514
• US Class Current: 73/799
• CPC Class Codes: B64F 5/60 Testing or inspecting aircraft components or systems
• CPC Class Codes: G01B 11/18 using photoelastic elements
• CPC Class Codes: G01B 5/30 for measuring the deformation in a solid, e.g. mechanical strain gauge
• CPC Class Codes: G01M 5/0033 by determining damage, crack or wear
• CPC Class Codes: G01M 5/0083 by measuring variation of impedance, e.g. resistance, capacitance, induction
• CPC Class Codes: G01M 5/0091 by using electromagnetic excitation or detection

Abstract

An apparatus for preventing, detecting, and predicting the formation and propagation of cracks in structural members. The apparatus includes a plurality of strain sensors integrally formed with a patch to be fixed to an area on the surface of a structural member. The patch operates to reduce stress levels in the patch-covered area for deterring the formation or propagation of a crack therein. The sensors monitor changes in the strain field in the patch-covered area for detecting crack formation and detection. The apparatus may also include a temperature-compensating strain sensor and a temperature sensor both integrally formed with the patch for detecting and predicting crack formation and propagation, respectively.

Claims

1. An apparatus for preventing and detecting formation or propagation of a crack in a structural member, said apparatus comprising:

a patch, adapted to be fixed to a surface of a structural member, which forms a means for reducing stress levels in a patch-covered area on the surface of the member by bearing a substantial portion of a load on said surface; and

strain-sensing means integrally formed with said patch for sensing a strain field in a fatigue sensitive region within the patch-covered area, said strain-sensing means providing a signal indicative of changes in the strain field in said fatigue sensitive region.

2. The apparatus of claim 1, further comprising:

temperature-compensating strain-sensing means integrally formed with said patch for providing an external strain field signal indicative of the strain field in an area which lies outside of said fatigue sensitive area within the patch-covered area.

3. The apparatus of claim 2, further comprising:

first processing means for processing the signal from said strain-sensing means to provide a first strain signal indicative of a stress distribution in said fatigue sensitive region;

first transmitting means for transmitting the strain signals from said strain-sensing means to said processing means;

second processing means for processing the external strain field signal to provide a second strain signal indicative of temperature-induced stress in said area which lies outside of said fatigue sensitive area within the patch-covered area; and

second transmitting means for transmitting the signal from said temperature-compensating strain-sensing means to said second processing means; and

comparison means for comparing said first and said second strain signals to obtain a temperature-independent strain signal indicative of a contribution to the strain field in the fatigue-sensitive region caused by the formation or propagation of a crack in said region.

4. The apparatus of claim 2, wherein said strain-sensing means and said temperature-compensating strain-sensing means are constructed from one of the group of strain-sensing devices consisting of wire foil strain-sensing devices, semiconductor strain-sensing devices, or optical fiber strain-sensing devices.

5. The apparatus of claim 2, wherein said strain-sensing means and said temperature-compensating strain-sensing means are Bragg reflection strain sensors.

6. The apparatus of claim 2, wherein said strain-sensing means is an extrinsic Fabry-Perot interferometer sensor and wherein said fatigue sensitive region is a location remote from an area where a crack has formed or is expected to form.

7. The apparatus of claim 2, wherein said strain-sensing means is a Bragg reflection strain sensor and wherein said fatigue sensitive region is a location remote from an area where a crack has formed or is expected to form.

8. The apparatus of claim 2, wherein said temperature-compensating strain-sensing means is an extrinsic Fabry-Perot interferometer sensor.

9. The apparatus of claim 1, further comprising extreme temperature-sensing means formed integrally with said patch for providing a signal proportional to the temperature of said structural element in said patch-covered area in order to predict possible formation or propagation of a crack in said fatigue sensitive area resulting from extreme temperature conditions.

10. The apparatus of claim 1, wherein said structural member is constructed from the group of materials consisting of metal, wood, plastic, concrete or a composite structural material.

11. The apparatus of claim 1, wherein said patch and structural member are made from like material.

12. The apparatus of claim 1, wherein said strain sensing means comprises at least two strain sensors dispose beyond respective ends of an existing crack for detecting propagation of the crack.

13. A method for detecting formation or propagation of a crack in a structural member, comprising the steps of:

fixing a repair patch to a surface of a structural member such that said repair patch covers an area on said surface containing at least a portion of a crack and reduces a level of stress in said area by bearing a portion of a load on said surface;

sensing changes in the strain field in said area and deriving therefrom a first strain signal indicative of the stress distribution in said patch-covered area;

sensing the strain field in an area under the patch lying outside of the area occupied by the crack and deriving therefrom a second strain signal indicative of temperature-induced stress in an area within said patch-covered area which lies outside of the area occupied by the crack; and

comparing said first and said second strain signals to determine an amount of said first strain signal which results from formation or propagation of the crack in said patch-covered area.

14. The method of claim 13, further comprising the steps of:

sensing the temperature of the member in the patch-covered region and providing a signal proportional thereto; and

processing said temperature signal to predict temperature-induced formation or propagation of a crack in said patch-covered area.

15. The method of claim 13, wherein the step of fixing the repair patch to the surface comprises the step of fixing the repair patch over two ends of the crack such that strain sensors integral with the patch are positioned beyond respective said ends of the crack.