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WO2005066601A1 - Method and apparatus for monitoring the integrity of a structure having a surface in contact with a liquid - Google Patents

Method and apparatus for monitoring the integrity of a structure having a surface in contact with a liquid Download PDF

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Publication number
WO2005066601A1
WO2005066601A1 PCT/AU2005/000023 AU2005000023W WO2005066601A1 WO 2005066601 A1 WO2005066601 A1 WO 2005066601A1 AU 2005000023 W AU2005000023 W AU 2005000023W WO 2005066601 A1 WO2005066601 A1 WO 2005066601A1
Authority
WO
WIPO (PCT)
Prior art keywords
cavity
fluid communication
fluid
liquid
pressure
Prior art date
Application number
PCT/AU2005/000023
Other languages
French (fr)
Inventor
Kenneth John Davey
Original Assignee
Structural Monitoring Systems Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from AU2004900114A external-priority patent/AU2004900114A0/en
Application filed by Structural Monitoring Systems Ltd filed Critical Structural Monitoring Systems Ltd
Publication of WO2005066601A1 publication Critical patent/WO2005066601A1/en

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Classifications

    • GPHYSICS
    • G01MEASURING; TESTING
    • G01MTESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M3/00Investigating fluid-tightness of structures
    • G01M3/02Investigating fluid-tightness of structures by using fluid or vacuum
    • G01M3/04Investigating fluid-tightness of structures by using fluid or vacuum by detecting the presence of fluid at the leakage point
    • G01M3/20Investigating fluid-tightness of structures by using fluid or vacuum by detecting the presence of fluid at the leakage point using special tracer materials, e.g. dye, fluorescent material, radioactive material
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01MTESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M3/00Investigating fluid-tightness of structures
    • G01M3/02Investigating fluid-tightness of structures by using fluid or vacuum
    • G01M3/04Investigating fluid-tightness of structures by using fluid or vacuum by detecting the presence of fluid at the leakage point
    • G01M3/20Investigating fluid-tightness of structures by using fluid or vacuum by detecting the presence of fluid at the leakage point using special tracer materials, e.g. dye, fluorescent material, radioactive material
    • G01M3/22Investigating fluid-tightness of structures by using fluid or vacuum by detecting the presence of fluid at the leakage point using special tracer materials, e.g. dye, fluorescent material, radioactive material for pipes, cables or tubes; for pipe joints or seals; for valves; for welds; for containers, e.g. radiators
    • G01M3/226Investigating fluid-tightness of structures by using fluid or vacuum by detecting the presence of fluid at the leakage point using special tracer materials, e.g. dye, fluorescent material, radioactive material for pipes, cables or tubes; for pipe joints or seals; for valves; for welds; for containers, e.g. radiators for containers, e.g. radiators

Definitions

  • the present invention is for a method and apparatus for monitoring the integrity of a structure having a surface in contact with a liquid.
  • the present invention has its genesis from consideration of the problems faced by aircraft engineers in monitoring the integrity of load carrying structure that may be located within, or that partly defines, a liquid storage area such a fuel containment "wet" areas in wings of aircraft.
  • This monitoring method is complicated by the use of tank sealants and can be subordinate to statutory inspection requirements of aviation regulators .
  • the inspection process is very expensive due to the time required to dry out the inspection area and the difficulty in accessing some areas.
  • a method for monitoring the integrity of a structure having a surface in contact with a liquid at a first pressure comprising:
  • Preferably forming the first cavity comprises sealing a fluid impervious material along the surface wherein the cavity is defined between the fluid impervious material and an underlying portion of the surface .
  • providing the fluid comprises providing a gas and the monitoring comprises drawing the gas through the cavity and passing the gas through a detection device for detecting traces of the liquid entrained in the gas.
  • drawing the gas through the first cavity comprises placing one end of the cavity in fluid communication with a supply of the gas, placing an opposite end of the first cavity in fluid communication with the detection device and placing the detection device in fluid communication with a relative vacuum.
  • an apparatus for monitoring the integrity of a structure having a surface in contact with a liquid at a first pressure comprising:
  • a fluid impervious cavity defining structure sealed along the surface forming a cavity between the surface and the cavity defining structure, the cavity having a first end and a second end;
  • a first conduit having one end sealed to the first end arid in fluid communication with the cavity
  • a pressure equaliser in fluid communication with an other end of the first conduct, the pressure equaliser having an opening at substantially the same level as the cavity;
  • the apparatus further comprises a monitoring device in fluid communication with the second end of the cavity for monitoring the traces of the liquid in the fluid.
  • the monitoring device comprises a liquid trap.
  • the monitoring device further comprises a relative vacuum in fluid communication with the liquid trap.
  • the monitoring device comprises a manometer coupled across the cavity.
  • the apparatus further comprises a high fluid flow impedance in fluid communication with the second end of the cavity, and wherein the manometer comprises a manometer leg having one end in fluid communication with the first conduit and a second end in fluid communication with the high fluid flow impedance.
  • the manometer further comprises a storage vessel containing a liquid in fluid communication with the one end of the manometer leg, and wherein the storage vessel is in fluid communication with the first conduit.
  • the monitoring device further comprises a vacuum source in fluid communication with the high fluid flow impedance and the first conduit.
  • the pressure equaliser comprises a bladder in the opening provided and a housing defining a volume in which the bladder is disposed, the volume being in fluid communication with the cavity.
  • the pressure equaliser comprises a vessel in which the opening is formed, the vessel in fluid communication with the cavity.
  • the pressure equaliser further comprises a pipe in fluid communication between the vessel and the cavity.
  • the pipe extends substantially vertically.
  • the pressure equaliser further comprises a gas permeable filter in fluid communication between the vessel and the cavity.
  • Figure 1 is an oblique view of a portion of an aircraft fuel tank sectioned at a location of a crack to which an embodiment of the present method may be applied;
  • Figure 2 is a schematic representation of an apparatus relating to Figure 1 with apparatus for maintaining a pressure of the fluid to be substantially the same as the first pressure, apparatus for testing and periodic monitoring, and additional apparatus for testing;
  • Figure 2a is a plan view of a terminal device incorporated in the apparatus shown in Figure 2 with an upper shell half removed;
  • Figure 2b is a schematic representation of a prior art monitoring system that can be utilised in embodiments of the present invention
  • Figure 3 is a schematic representation of a further apparatus for use in an alternative method of maintaining pressure of the fluid to be substantially the same as the first pressure.
  • Figure 4 is a schematic representation of the function of the further apparatus of Figure 3.
  • Figure 5 is a schematic representation of periodic monitoring of the apparatus of Figure 3.
  • Figure 6 is a variation of the embodiment of Figure 1.
  • Figure 7 is a schematic representation of apparatus relating to Figure 6 and apparatus for maintaining pressure of the fluid to be substantially the same as the first pressure.
  • Figure 8 is a schematic representation of periodic monitoring of the apparatus of Figure 6.
  • Figure 9 is a schematic representation of additional apparatus for testing and periodic monitoring of the apparatus of Figure 6.
  • Figure 10 is a schematic representation of further additional apparatus for testing and periodic monitoring of the apparatus of Figure 1. Detailed Description of Preferred Embodiments
  • Figure 1 illustrates an oblique view of a structure 10 in the form of a portion of an aircraft wing with an integral fuel tank 12 sectioned through a plane of a fatigue crack 14 (see the magnified view) in a lower spar cap 16.
  • a surface 18 of the spar cap 16 which is exposed to the fuel in the interior of tank 12 will be in contact with (ie immersed in) the liquid fuel.
  • the spar cap 16 is fastened to other components of the tank 12 and structure 10 by rivets 20. It is common for fatigue cracks 14 to form in a metallic structure or member such as the spar cap 16 about the rivets 20 and moreover about holes 22 through which the rivets 20 pass.
  • An embodiment of the present method for monitoring the integrity of structures can detect a flaw such as a crack 14 in the structure 10 and more particularly the spar cap 16, which has its surface 18 in contact with a liquid, comprises forming a cavity 24 on and along the surface 18 of the spar 16. This is achieved by sealing a fluid impervious material
  • the cavity 24 is filled with a fluid.
  • the fluid is a gas such as air.
  • Respective ends of the cavity 24 are coupled to ducts 30 and 31, (see Figure 2) .
  • the cavity 24 can be formed by directly laying a bead of fluid impervious sealant material on the surface, where the bead is shaped to self define a recess that together with the surface 18 forms the cavity. For example this may be achieved by an extrusion process . Alternately the material can be laid over a former (such as a fine wire) that is subsequently withdrawn. In a further alternative the manifold- can be made separately and sealed onto the surface.
  • the equaliser 100 accommodates for expansion of the entrapped fluid due to variations in altitude and assists in preventing the ingress of fluid into the cavity 24 directly from permeation into manifold 26. It is considered advantageous to make provision for a nine fold expansion in the volume of the entrapped fluid at ground level for aircraft cruising up to 40,000ft.
  • the pressure equaliser 100 comprises a bladder or sachet 101 located within a protective shell 97 comprising an upper half 102U and lower half 102L.
  • the sachet 101 is in fluid communication with a vent 99 formed in the shell 97 and can be fluidly connected to duct 30 by connector Cl .
  • the sachet 101 is shown in an expanded state which would occur at altitude. At sea level the sachet 101 would normally be in a collapsed state.
  • the sachet 101 is made of a material that is solvent resistant, flexible and has low permeability at the temperature extremes experienced in the tank 12.
  • One such material is Mylar.
  • a vent 103 is provided in the shell 97 between shell halves 102U and 102L and allows some hydrostatic pressure influence together with reference to variance of surrounding atmospheric pressure.
  • the duct 30 which is attached to an opposite end of the manifold 26 and hence cavity 24 facilitates the periodic ground testing and monitoring by allowing induction of a fluid to confirm continuity of the cavity 24 and allow monitoring for the presence of liquid (fuel) . Thorough pre- service testing of the seal of the manifold 26 (defining
  • Figure 2b illustrates an apparatus 150 for the initial dry testing of the sealing integrity of cavity 24 by connecting to either duct connector Cl or C2, and sealing the other duct with a terminal cap 33.
  • the apparatus 150 is described fully in US 5,770,794 the contents of which are incorporated herein by way of reference and in brief comprises a high fluid impedance Z in series connection in a duct D, one end of which is in communication with a vacuum source V, the other end of duct D being connected to connector Cl .
  • a transducer T and associate gauge G is coupled in parallel across the impedance Z to monitor changes in vacuum condition in duct D.
  • the equaliser 100 and terminal 33 are raised free of liquid and dried.
  • the equaliser 100 is removed from connector Cl and terminal 33 removed from connector C2.
  • a relative vacuum 1 with a liquid detector such as for example a transparent liquid trap 90 is connected to the duct 30 (or 31) at connector Cl (or C2) to draw the gas from the cavity 24. Any liquid that has seeped into the cavity 24 through the opening of a developing fatigue crack 14 under loading in flight will be entrained in the air and can be detected in the liquid detector 90.
  • the liquid is in this instance aviation fuel.
  • the connected combination of the cavity 24, pressure equaliser 100 and liquid detector 90 comprise broadly an embodiment of an apparatus 200 for monitoring the integrity of a structure having a surface in contact with a liquid.
  • FIG 3 shows a method and apparatus 200' for monitoring the structure 10 which differs from the apparatus 200 by replacement of the equaliser 100 with an alternate pressure equaliser 100 ' and corresponding method of maintaining the pressure of the fluid (i.e. air) within the cavity 24 of manifold 26 substantially the same as the pressure of the liquid (i.e. fuel) within the tank 12.
  • This equaliser 100' comprises a three-way valve 40, a gas permeable filter 42, and a standpipe 44 fluidly connected to a vessel 48, with the vessel having a hole in its base 50 to admit or expel liquid (fuel) .
  • the base 50 is arranged to be at approximately the same level as the manifold 26, and the three-way valve 40 and gas permeable filter 42 are arranged for access close to an inspection hatch (not shown) in the top of the tank Figure 1.
  • Figure 4 features a progressive series of views of the vessel 48 labelled A, B, C, D, E, F, G, H, I, J, 'A', and ⁇ BA
  • the vessel 48 accommodates for expansion of gas within the ducts 30 and 31 and cavity 24 arising from variations in altitude and changes in the hydrostatic pressure of the surrounding fuel.
  • the liquid level relative to containment within vessel 48 is shown as 45.
  • it is preferable to make provision for the expansion of the entrapped gas by at least nine times the volume of the entrapped gas at ground level for aircraft cruising up to 40,000ft and the volume of vessel 48 should be designed accordingly.
  • the vessel 48 will accommodate varying contained levels of liquid 45 during flying operations as follows: A) Aircraft on ground after refuelling. Vessel 48 is immersed in a full tank of fuel. Some fuel enters the vessel to level 45 due to hydrostatic pressure.
  • Figure 5 illustrates a method of monitoring the integrity of the fuel tank using the apparatus 200' shown in Figure 3.
  • terminals 33 of Figure 3 are removed, valve 40 positioned to allow fluid communication with cavity 24; and the relative vacuum 1 with transparent liquid trap (or other suitable liquid detector 90) connected to connector Cl.
  • Small white arrows at terminal C2 represent air induced into the apparatus 200' and connecting ducts 30 and 31 during such monitoring. Air is inducted through valve 40 by the action of the vacuum 1. This air passes through the cavity 24 and entrains any liquid that has seeped into the cavity 24 during cyclic loading of the fuel tank 12 as would occur in flight.
  • any cracks that propagate along the surface 18 to the cavity 24 forms a channel via which fuel can flow into the cavity 24.
  • the air inducted into the channel 24 by the vacuum 1 together with any entrained fuel is carried to the liquid trap 90.
  • the presence of liquid within the trap 90 is indicative of the formation of a crack in the spar cap 16 that extend into the cavity 24.
  • Figures 6-9 illustrate a further embodiment of the method and corresponding apparatus 200'' for monitoring the integrity of a structure 10, which is based on the first embodiment shown in Figures 1 and 2.
  • the single cavity 24 In this embodiment the single cavity 24
  • valve 40 is positioned to provide fluid communication between the duct 31b (and hence cavities 24a and 24b) and the pressure equaliser 100 A This maintains substantially equal fluid pressure between the liquid (ie fuel) in which the manifold 26 is immersed and the fluid (i.e. air) within the cavities 24a and 24b.
  • valve 40 is positioned to allow fluid communication with cavities 24a and 24b, and the relative vacuum 1 with transparent liquid trap (or other suitable liquid detector means) 90 is connected to connector Cl as shown in Figure 8.
  • Small white arrows at connector C2 represent air induced through the system 30a, 31a, 30b 31b and cavities 24a and 24b and valve 40. Any fuel that has seeped into the cavities 24a and/or 24b will be carried by the induced air to, and be visible at, the detector 90.
  • a benefit of apparatus 200'' is that by the provision of an isolation device 60 (depicted in Figure 9) between ducts 30a and 31a, the cavities 24a and 24b can be isolated from each other and thereby enable a backup test of the integrity of the cavities and ducts .
  • This is achieved by moving the valve 40 to a position where the conduit 31b and cavity 24b are in communication with the surrounding atmosphere and hence at ambient pressure (isolating the pressure equaliser 100' from conduit 31b) and attaching the apparatus 150 to connector Cl .
  • the cavity 24b is at ambient pressure and the cavity 24a is subjected to a relative vacuum via vacuum V and the impedance Z.
  • the apparatus 150 can be replaced with a mechanical, (ie non-electrical) equivalent apparatus 150' illustrated in Figure 10 and modified to include means of detecting the ingress of fuel into a cavity 24 or 24a as an alternative to the apparatus comprising relative vacuum 1 with transparent liquid trap 90, to produce a further embodiment 200'"' of the apparatus for monitoring the integrity of a structure having a surface in contact with a liquid.
  • the apparatus 200''' comprises manifold 26 and duct 30 similar to manifold 26 and duct 30 of Figures 1 and 2. However, duct 31 is replaced with duct 31' that performs the function of the high fluid flow impedance Z of the apparatus 150'. Although not shown in Figure 10 the apparatus 200''' further comprises a pressure equaliser 100, 100' coupled to duct 31' during operational service.
  • the high fluid flow impedance would have dimensions of a bore of 0.2 mm and a length in excess of 3 metres determined by the desired sensitivity.
  • the ducts 30 and 31' may be terminated in similar manner to ducts 30, and 31 of Figure 2 and reside within the tank 12 of Figure 1 (that is, if no feed through of ducting from integral fuel tank 12 is permitted during the initial installation) .
  • additional apparatus 150' is connected to the free ends at connections Cl and C2 of the ducts 30 and 31' .
  • the electrical transducer T of the apparatus 150 is replaced with a manometer 155 comprising valve 81, transparent vessel 83, transparent tubing column 85, valve 87, vessel 89, liquid 91 and valve 93.
  • the vacuum V of apparatus 150 is replaced with a container 97 and ducts 95 which together form a vacuum storage that may be charged mechanically by a simple air vacuum pump (not shown) to provide a vacuum in the order of -20kPa in preparation for periodic testing and monitoring.
  • the small diameter transparent column 85 provides a single manometer leg.
  • Reservoir 89 is a squat transparent vessel containing the indicating liquid 91.
  • the surface area of the reservoir 89 is large in comparison to that of column 85 so that its contained liquid level will not vary significantly in response to liquid 91 being drawn up the column 85.
  • the use of jet fuel is satisfactory as an indicating liquid 91.
  • the liquid should substantially fill the vessel 89 (as indicated by liquid surface 92) to minimise the amount of gas above the liquid. This reduces response time in achieving a stable differential pressure across column 85.
  • the purpose of the transparent vessel 83 on top of column 85 is to trap unintentional liquid egress from the top of column 85 due to exposure to excessive differential pressure.
  • the valve 93 is provided to prevent exposure to excessive pressure differential across column 85 when first carrying out a test.
  • Valves 81 and 87 provide containment of liquid 91 during transport and storage and additionally, the apparatus should remain upright.
  • any other form or type of differential pressure measurement device may be used such as pneumatically powered or driven devices .
  • the apparatus is tested for self integrity using for example the apparatus 150 or 150 ' .
  • the fuel tanks are filled and aircraft operated in a normal manner.
  • Testing for the integrity of the spar cap 16 can be incorporated as a part of scheduled maintenance. The testing need not be performed on each landing. Rather it is envisaged that testing would occur on the basis of flying time. It should also be understood that the present method is performed with the aircraft on the ground.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Examining Or Testing Airtightness (AREA)

Abstract

A method and apparatus for monitoring the integrity of a surface (18) of a structure (10) immersed in a liquid comprises a fluid impervious cavity (24) formed along the surface (18) of the structure (10). The cavity (24) is in fluid communication via a duct (30) to a pressure equaliser (100) the pressure equaliser has an opening at substantially the same level as the cavity. A fluid is disposed within the cavity (24) and in fluid communication with the pressure equaliser. In the event of a crack developing along the surface (18) that extends into the cavity (24) liquid within which the surface (18) is immersed will flow into the cavity (24) and can be detected by various detection devices such as a liquid trap (90).

Description

METHOD AND APPARATUS FOR MONITORING THE INTEGRITY OF A STRUCTURE HAVING A SURFACE IN CONTACT WITH A LIQUID
Field of the Invention
The present invention is for a method and apparatus for monitoring the integrity of a structure having a surface in contact with a liquid.
Background of the Invention
The present invention has its genesis from consideration of the problems faced by aircraft engineers in monitoring the integrity of load carrying structure that may be located within, or that partly defines, a liquid storage area such a fuel containment "wet" areas in wings of aircraft.
Typically, engineers have relied on the leakage of fuel as a warning of the development of cracks and to initiate the inspection of the structure for the development of cracks.
This monitoring method is complicated by the use of tank sealants and can be subordinate to statutory inspection requirements of aviation regulators . The inspection process is very expensive due to the time required to dry out the inspection area and the difficulty in accessing some areas.
Notwithstanding the above background, embodiments of the present invention may be applied in non-aviation fields and to any structure in contact with, containing or immersed in a liquid, and has particular application where the structure is subject to dynamic loading. Summary of the Invention
It is an object of the present invention to provide a method and apparatus for monitoring of the integrity of a structure having a surface that is in contact with, contains or is immersed in a liquid.
According to the present invention there is provided a method for monitoring the integrity of a structure having a surface in contact with a liquid at a first pressure, the method comprising:
forming a first cavity on and along the surface of the structure;
providing a fluid in the cavity;
maintaining pressure of the fluid to be substantially the same as the first pressure; and,
periodically monitoring the fluid for traces of the liquid.
Preferably forming the first cavity comprises sealing a fluid impervious material along the surface wherein the cavity is defined between the fluid impervious material and an underlying portion of the surface .
In one embodiment, providing the fluid comprises providing a gas and the monitoring comprises drawing the gas through the cavity and passing the gas through a detection device for detecting traces of the liquid entrained in the gas. Preferably, drawing the gas through the first cavity comprises placing one end of the cavity in fluid communication with a supply of the gas, placing an opposite end of the first cavity in fluid communication with the detection device and placing the detection device in fluid communication with a relative vacuum.
According to another aspect of this invention there is provided an apparatus for monitoring the integrity of a structure having a surface in contact with a liquid at a first pressure comprising:
a fluid impervious cavity defining structure sealed along the surface forming a cavity between the surface and the cavity defining structure, the cavity having a first end and a second end;
a first conduit having one end sealed to the first end arid in fluid communication with the cavity;
a pressure equaliser in fluid communication with an other end of the first conduct, the pressure equaliser having an opening at substantially the same level as the cavity; and,
a fluid contained in the cavity and in fluid communication with the pressure equaliser.
Preferably the apparatus further comprises a monitoring device in fluid communication with the second end of the cavity for monitoring the traces of the liquid in the fluid. Preferably the monitoring device comprises a liquid trap.
Preferably the monitoring device further comprises a relative vacuum in fluid communication with the liquid trap.
Preferably the monitoring device comprises a manometer coupled across the cavity.
Preferably the apparatus further comprises a high fluid flow impedance in fluid communication with the second end of the cavity, and wherein the manometer comprises a manometer leg having one end in fluid communication with the first conduit and a second end in fluid communication with the high fluid flow impedance.
Preferably the manometer further comprises a storage vessel containing a liquid in fluid communication with the one end of the manometer leg, and wherein the storage vessel is in fluid communication with the first conduit.
Preferably the monitoring device further comprises a vacuum source in fluid communication with the high fluid flow impedance and the first conduit.
Preferably the pressure equaliser comprises a bladder in the opening provided and a housing defining a volume in which the bladder is disposed, the volume being in fluid communication with the cavity.
Preferably the pressure equaliser comprises a vessel in which the opening is formed, the vessel in fluid communication with the cavity.
Preferably the pressure equaliser further comprises a pipe in fluid communication between the vessel and the cavity.
Preferably the pipe extends substantially vertically.
Preferably the pressure equaliser further comprises a gas permeable filter in fluid communication between the vessel and the cavity.
Brief Description of the Drawings
Figure 1 is an oblique view of a portion of an aircraft fuel tank sectioned at a location of a crack to which an embodiment of the present method may be applied;
Figure 2 is a schematic representation of an apparatus relating to Figure 1 with apparatus for maintaining a pressure of the fluid to be substantially the same as the first pressure, apparatus for testing and periodic monitoring, and additional apparatus for testing; Figure 2a is a plan view of a terminal device incorporated in the apparatus shown in Figure 2 with an upper shell half removed;
Figure 2b is a schematic representation of a prior art monitoring system that can be utilised in embodiments of the present invention;
Figure 3 is a schematic representation of a further apparatus for use in an alternative method of maintaining pressure of the fluid to be substantially the same as the first pressure. Figure 4 is a schematic representation of the function of the further apparatus of Figure 3.
Figure 5 is a schematic representation of periodic monitoring of the apparatus of Figure 3.
Figure 6 is a variation of the embodiment of Figure 1.
Figure 7 is a schematic representation of apparatus relating to Figure 6 and apparatus for maintaining pressure of the fluid to be substantially the same as the first pressure.
Figure 8 is a schematic representation of periodic monitoring of the apparatus of Figure 6.
Figure 9 is a schematic representation of additional apparatus for testing and periodic monitoring of the apparatus of Figure 6.
Figure 10 is a schematic representation of further additional apparatus for testing and periodic monitoring of the apparatus of Figure 1. Detailed Description of Preferred Embodiments
Figure 1 illustrates an oblique view of a structure 10 in the form of a portion of an aircraft wing with an integral fuel tank 12 sectioned through a plane of a fatigue crack 14 (see the magnified view) in a lower spar cap 16. When the fuel tank 12 contains liquid fuel to a sufficient level, a surface 18 of the spar cap 16 which is exposed to the fuel in the interior of tank 12 will be in contact with (ie immersed in) the liquid fuel. The spar cap 16 is fastened to other components of the tank 12 and structure 10 by rivets 20. It is common for fatigue cracks 14 to form in a metallic structure or member such as the spar cap 16 about the rivets 20 and moreover about holes 22 through which the rivets 20 pass.
Hitherto the generation of fatigue cracks 14 which extend through the thickness or width of the spar cap 16 would be eventually detected by the existence of fuel on the outside of the tank 12. However, due to the location of the fuel tank 12, this may only become apparent when a partial dismantling of the wing occurs during certain scheduled maintenance.
An embodiment of the present method for monitoring the integrity of structures can detect a flaw such as a crack 14 in the structure 10 and more particularly the spar cap 16, which has its surface 18 in contact with a liquid, comprises forming a cavity 24 on and along the surface 18 of the spar 16. This is achieved by sealing a fluid impervious material
along the surface 18 effectively forming a manifold 26, with the cavity 24 defined between an inside surface of the manifold 26 and a portion of the surface 18 overlaid by the manifold 26. The cavity 24 runs adjacent the rivets 20, which form the most likely area in which cracks 14 will be formed, however cavities can be formed at other locations . The cavity 24 is filled with a fluid. In a first embodiment the fluid is a gas such as air. Respective ends of the cavity 24 are coupled to ducts 30 and 31, (see Figure 2) . The cavity 24 can be formed by directly laying a bead of fluid impervious sealant material on the surface, where the bead is shaped to self define a recess that together with the surface 18 forms the cavity. For example this may be achieved by an extrusion process . Alternately the material can be laid over a former (such as a fine wire) that is subsequently withdrawn. In a further alternative the manifold- can be made separately and sealed onto the surface.
If modification of the structure 10 is permissible, free ends of the ducts 30 and 31 are passed through the walls of fuel tank 12 so as to be located outside of the tank 12 with protection from moisture ingress for easier access for periodic monitoring. If structural modification is not permissible, the free end of duct 31 can be temporarily sealed with a terminal 33 via a connector C2 (the connector is represented by an associated double ended black arrow) (see Figure 2), and immersed in the liquid. Additionally, an immersed pressure equaliser 100 is attached via connector Cl to the free end of duct 30 for maintaining the pressure of
the fluid (i.e. gas) within the cavity 24 substantially the same as the pressure of the liquid (i.e. fuel) within the tank 12. The equaliser 100 accommodates for expansion of the entrapped fluid due to variations in altitude and assists in preventing the ingress of fluid into the cavity 24 directly from permeation into manifold 26. It is considered advantageous to make provision for a nine fold expansion in the volume of the entrapped fluid at ground level for aircraft cruising up to 40,000ft. With reference to Figures 2 and 2a the pressure equaliser 100 comprises a bladder or sachet 101 located within a protective shell 97 comprising an upper half 102U and lower half 102L. The sachet 101 is in fluid communication with a vent 99 formed in the shell 97 and can be fluidly connected to duct 30 by connector Cl . The sachet 101 is shown in an expanded state which would occur at altitude. At sea level the sachet 101 would normally be in a collapsed state. Advantageously, the sachet 101 is made of a material that is solvent resistant, flexible and has low permeability at the temperature extremes experienced in the tank 12. One such material is Mylar. A vent 103 is provided in the shell 97 between shell halves 102U and 102L and allows some hydrostatic pressure influence together with reference to variance of surrounding atmospheric pressure.
The duct 30 which is attached to an opposite end of the manifold 26 and hence cavity 24 facilitates the periodic ground testing and monitoring by allowing induction of a fluid to confirm continuity of the cavity 24 and allow monitoring for the presence of liquid (fuel) . Thorough pre- service testing of the seal of the manifold 26 (defining
cavity 24) and ducts 30 and 31 is required to confirm no liquid seepage into cavity 24 when immersed in liquid.
Figure 2b illustrates an apparatus 150 for the initial dry testing of the sealing integrity of cavity 24 by connecting to either duct connector Cl or C2, and sealing the other duct with a terminal cap 33. The apparatus 150 is described fully in US 5,770,794 the contents of which are incorporated herein by way of reference and in brief comprises a high fluid impedance Z in series connection in a duct D, one end of which is in communication with a vacuum source V, the other end of duct D being connected to connector Cl . A transducer T and associate gauge G is coupled in parallel across the impedance Z to monitor changes in vacuum condition in duct D.
For in-service periodic monitoring on the ground, the equaliser 100 and terminal 33 are raised free of liquid and dried. The equaliser 100 is removed from connector Cl and terminal 33 removed from connector C2. A relative vacuum 1 with a liquid detector such as for example a transparent liquid trap 90 is connected to the duct 30 (or 31) at connector Cl (or C2) to draw the gas from the cavity 24. Any liquid that has seeped into the cavity 24 through the opening of a developing fatigue crack 14 under loading in flight will be entrained in the air and can be detected in the liquid detector 90. The liquid is in this instance aviation fuel.
The connected combination of the cavity 24, pressure equaliser 100 and liquid detector 90 comprise broadly an embodiment of an apparatus 200 for monitoring the integrity of a structure having a surface in contact with a liquid.
Figure 3 shows a method and apparatus 200' for monitoring the structure 10 which differs from the apparatus 200 by replacement of the equaliser 100 with an alternate pressure equaliser 100 ' and corresponding method of maintaining the pressure of the fluid (i.e. air) within the cavity 24 of manifold 26 substantially the same as the pressure of the liquid (i.e. fuel) within the tank 12. This equaliser 100' comprises a three-way valve 40, a gas permeable filter 42, and a standpipe 44 fluidly connected to a vessel 48, with the vessel having a hole in its base 50 to admit or expel liquid (fuel) . The base 50 is arranged to be at approximately the same level as the manifold 26, and the three-way valve 40 and gas permeable filter 42 are arranged for access close to an inspection hatch (not shown) in the top of the tank Figure 1.
Figure 4 features a progressive series of views of the vessel 48 labelled A, B, C, D, E, F, G, H, I, J, 'A', and λBA The vessel 48 accommodates for expansion of gas within the ducts 30 and 31 and cavity 24 arising from variations in altitude and changes in the hydrostatic pressure of the surrounding fuel. The liquid level relative to containment within vessel 48 is shown as 45. As stated previously, it is preferable to make provision for the expansion of the entrapped gas by at least nine times the volume of the entrapped gas at ground level for aircraft cruising up to 40,000ft and the volume of vessel 48 should be designed accordingly.
The vessel 48 will accommodate varying contained levels of liquid 45 during flying operations as follows: A) Aircraft on ground after refuelling. Vessel 48 is immersed in a full tank of fuel. Some fuel enters the vessel to level 45 due to hydrostatic pressure.
B) Aircraft climbing and due to decreasing atmospheric pressure, gas escapes in bubbles from vessel 48 (The bubbles are illustrated by small circles at B, I and λB' .
C) Aircraft at cruising height, liquid level 45 just within vessel 48.
D) Aircraft descending and due to increasing atmospheric pressure, fuel is entering vessel 48 as indicated by a ascending vertical black arrow and level 45. E) Aircraft on ground with no refuelling. Vessel 48 contains fuel that has entered to level 45 due to loss of trapped air within ducts 30 and 31 and cavity 24 during climb (illustrated stage B above) .
F) Aircraft climbing and due to decreasing atmospheric pressure, trapped air within ducts 30 and 31 and cavity 24 gas expands as indicated by downward facing black arrow and level 45.
G) Aircraft at cruising height, level 45 within vessel 48.
H) Aircraft descending and due to increasing atmospheric pressure, fuel is entering vessel 48 as indicated by a ascending vertical black arrow and level 45.
I) Level of fuel within aircraft tank 52 falling just below bottom of vessel 48 allowing loss of contained fuel shown by bubbles entering vessel 48.
J) All liquid (fuel) is lost from vessel 48 as fuel level 52 continues to fall. 'A' and 'B' indicate reoccurrence of the cycle A, B, etc.
Figure 5 illustrates a method of monitoring the integrity of the fuel tank using the apparatus 200' shown in Figure 3. To facilitate assurance of continuity of ducts 30 and 31 and the monitoring of the cavity 24 for the ingress of fuel, terminals 33 of Figure 3 are removed, valve 40 positioned to allow fluid communication with cavity 24; and the relative vacuum 1 with transparent liquid trap (or other suitable liquid detector 90) connected to connector Cl. Small white arrows at terminal C2 represent air induced into the apparatus 200' and connecting ducts 30 and 31 during such monitoring. Air is inducted through valve 40 by the action of the vacuum 1. This air passes through the cavity 24 and entrains any liquid that has seeped into the cavity 24 during cyclic loading of the fuel tank 12 as would occur in flight. Any cracks that propagate along the surface 18 to the cavity 24 forms a channel via which fuel can flow into the cavity 24. The air inducted into the channel 24 by the vacuum 1 together with any entrained fuel is carried to the liquid trap 90. Thus the presence of liquid within the trap 90 is indicative of the formation of a crack in the spar cap 16 that extend into the cavity 24.
Figures 6-9 illustrate a further embodiment of the method and corresponding apparatus 200'' for monitoring the integrity of a structure 10, which is based on the first embodiment shown in Figures 1 and 2. In this embodiment the single cavity 24
and ducts 30 and 31 of the first embodiment are replaced with dual cavities 24a and 24b and dual connecting ducts 30a and 30b and 31a and 31b. With particular reference to Figures 6 and 7, it can be seen that the dual cavities 24a and 24b are formed in the manifold 26a and extend parallel to each other. Starting from the connector Cl, there is a series connection of duct 30b, cavity 24a, duct 31a, duct 30a, cavity 24b, duct 31b and equaliser 100' . In Figure 7 the valve 40 is positioned to provide fluid communication between the duct 31b (and hence cavities 24a and 24b) and the pressure equaliser 100 A This maintains substantially equal fluid pressure between the liquid (ie fuel) in which the manifold 26 is immersed and the fluid (i.e. air) within the cavities 24a and 24b.
When it is desired to use the apparatus 200'' to monitor the integrity of the spar cap (not shown) on which the manifold 26 is laid the terminals 33 are removed, valve 40 is positioned to allow fluid communication with cavities 24a and 24b, and the relative vacuum 1 with transparent liquid trap (or other suitable liquid detector means) 90 is connected to connector Cl as shown in Figure 8. Small white arrows at connector C2 represent air induced through the system 30a, 31a, 30b 31b and cavities 24a and 24b and valve 40. Any fuel that has seeped into the cavities 24a and/or 24b will be carried by the induced air to, and be visible at, the detector 90.
A benefit of apparatus 200'' is that by the provision of an isolation device 60 (depicted in Figure 9) between ducts 30a and 31a, the cavities 24a and 24b can be isolated from each other and thereby enable a backup test of the integrity of the cavities and ducts . This is achieved by moving the valve 40 to a position where the conduit 31b and cavity 24b are in communication with the surrounding atmosphere and hence at ambient pressure (isolating the pressure equaliser 100' from conduit 31b) and attaching the apparatus 150 to connector Cl . Now the cavity 24b is at ambient pressure and the cavity 24a is subjected to a relative vacuum via vacuum V and the impedance Z. In the event that the integrity of the ducts and cavities is sound, a particular pressure reading will appear on the gauge G associated with transducer T. However in the event of a fault such as a separation between the manifold 26 and spar cap, or a leakage in the connection between say duct 31a and cavity 24a, or a crack occurs directy under the sensor, a different reading will appear on display D. Further, the atmospheric duct provides an air reference as fuel drawn directly into the crack, instead of air, may mask the presence of a tightly closed crack.
In the further interests of safety, the apparatus 150 can be replaced with a mechanical, (ie non-electrical) equivalent apparatus 150' illustrated in Figure 10 and modified to include means of detecting the ingress of fuel into a cavity 24 or 24a as an alternative to the apparatus comprising relative vacuum 1 with transparent liquid trap 90, to produce a further embodiment 200'"' of the apparatus for monitoring the integrity of a structure having a surface in contact with a liquid.
The apparatus 200''' comprises manifold 26 and duct 30 similar to manifold 26 and duct 30 of Figures 1 and 2. However, duct 31 is replaced with duct 31' that performs the function of the high fluid flow impedance Z of the apparatus 150'. Although not shown in Figure 10 the apparatus 200''' further comprises a pressure equaliser 100, 100' coupled to duct 31' during operational service.
Typically, the high fluid flow impedance would have dimensions of a bore of 0.2 mm and a length in excess of 3 metres determined by the desired sensitivity.
During flight and ground operations, the ducts 30 and 31' may be terminated in similar manner to ducts 30, and 31 of Figure 2 and reside within the tank 12 of Figure 1 (that is, if no feed through of ducting from integral fuel tank 12 is permitted during the initial installation) .
During ground periodic testing and monitoring, additional apparatus 150' is connected to the free ends at connections Cl and C2 of the ducts 30 and 31' . The electrical transducer T of the apparatus 150 is replaced with a manometer 155 comprising valve 81, transparent vessel 83, transparent tubing column 85, valve 87, vessel 89, liquid 91 and valve 93.
The vacuum V of apparatus 150 is replaced with a container 97 and ducts 95 which together form a vacuum storage that may be charged mechanically by a simple air vacuum pump (not shown) to provide a vacuum in the order of -20kPa in preparation for periodic testing and monitoring.
In the manometer 155 the small diameter transparent column 85 provides a single manometer leg. Reservoir 89 is a squat transparent vessel containing the indicating liquid 91. The surface area of the reservoir 89 is large in comparison to that of column 85 so that its contained liquid level will not vary significantly in response to liquid 91 being drawn up the column 85. The use of jet fuel is satisfactory as an indicating liquid 91. Further, the liquid should substantially fill the vessel 89 (as indicated by liquid surface 92) to minimise the amount of gas above the liquid. This reduces response time in achieving a stable differential pressure across column 85. The purpose of the transparent vessel 83 on top of column 85 is to trap unintentional liquid egress from the top of column 85 due to exposure to excessive differential pressure. The valve 93 is provided to prevent exposure to excessive pressure differential across column 85 when first carrying out a test.
Valves 81 and 87 provide containment of liquid 91 during transport and storage and additionally, the apparatus should remain upright.
The purpose of the location of the high fluid flow impedance fluid flow device 31' will now be explained.
In the event of a crack 14 occurring during flying operations, fuel will seep into the cavity 24, during cyclic flexing of spar cap 16. A portion of this fuel would enter the impedance duct 30', due to capillary action. The connection of apparatus 150' during a subsequent ground test, with associated applied vacuum, would induce further liquid to be drawn into the high impedance fluid flow device 31' assisted by gas trapped in duct 30.
The effect of liquid entering the high impedance fluid flow device 31' effectively chokes it resulting in a very substantial increase in differential pressure relative indication to that normally associated with gas (air) . Hence a sensitive method of liquid ingress into a test cavity is provided. Further, the back up feature of flaw detection in the integrity of a sealed cavity is provided in accordance with US 5,770,794.
Instead of the manometer any other form or type of differential pressure measurement device may be used such as pneumatically powered or driven devices . After installation of the apparatus 200, 200', 200'', or 200''', the apparatus is tested for self integrity using for example the apparatus 150 or 150 ' . Thereafter the fuel tanks are filled and aircraft operated in a normal manner. Testing for the integrity of the spar cap 16 can be incorporated as a part of scheduled maintenance. The testing need not be performed on each landing. Rather it is envisaged that testing would occur on the basis of flying time. It should also be understood that the present method is performed with the aircraft on the ground.
Modifications and variations and embodiments to the present invention that would be obvious to a person of ordinary skill in the art are deemed to be within the scope of the present invention the nature of which is to be determined from the above description and the appended claims .

Claims

The Claims Defining the Invention are as Follows :
1. A method for monitoring the integrity of a structure having a surface in contact with a liquid at a first pressure, the method comprising: forming a first cavity on and along the surface of the structure; providing a fluid in the cavity; maintaining pressure of the fluid to be substantially the same as the first pressure; and, periodically monitoring the fluid for traces of the liquid.
2. The method according to claim 1 wherein forming the first cavity comprises sealing a fluid impervious material along the surface wherein the cavity is defined between the fluid impervious material and an underlying portion of the surface.
3. The method according to claim 1 or 2 wherein providing the fluid comprises providing a gas and the monitoring comprises drawing the gas through the cavity and passing the gas through a detection device for detecting traces of the liquid entrained in the gas.
4. The method according to any one of claims 1 - 3 wherein drawing the gas through the first cavity comprises placing one end of the cavity in fluid communication with a supply of the gas, placing an opposite end of the first cavity in fluid communication with the detection device and placing the detection device in fluid communication with a relative vacuum.
5. The method according to any one of claims 1 - 4 wherein maintaining pressure of the fluid comprises placing one end of the first cavity in fluid communication with a pressure equalizer.
6. An apparatus for monitoring the integrity of a structure having a surface in contact with a liquid at a first pressure comprising: a fluid impervious cavity defining structure sealed along the surface forming a cavity between the surface and the cavity defining structure, the cavity having a first end and a second end; a first conduit having one end sealed to the first end and in fluid communication with the cavity; a pressure equaliser in fluid communication with an other end of the first conduct, the pressure equaliser having an opening at substantially the same level as the cavity; and, a fluid contained in the cavity and in fluid communication with the pressure equaliser.
7. The apparatus according to claim 7 further comprising a monitoring device in fluid communication with the second end of the cavity for monitoring the traces of the liquid in the fluid.
8. The apparatus according to claim 7 wherein the monitoring device comprises a liquid trap.
9. The apparatus according to claim 8 wherein the monitoring device further comprises a relative vacuum in fluid communication with the liquid trap.
10. The apparatus according to claim 7 wherein the monitoring device comprises a manometer coupled- across the cavity.
11. The apparatus according to claim 10 further comprising a high fluid flow impedance in fluid communication with the second end of the cavity, and wherein the manometer comprises a manometer leg having one end in fluid communication with the first conduit and a second end in fluid communication with the high fluid flow impedance.
12. The apparatus according to claim 11 wherein the manometer further comprises a storage vessel containing a liquid in fluid communication with the one end of the manometer leg, and wherein the storage vessel is in fluid communication with the first conduit.
13. The apparatus according to claim 12 wherein the monitoring device further comprises a vacuum source in fluid communication with the high fluid flow impedance and the first conduit.
14. The apparatus according to any one of claims 6 - 13 wherein the pressure equaliser comprises a bladder in the opening provided and a housing defining a volume in which the bladder is disposed, the volume being in fluid communication with the cavity.
15. The apparatus according to any one of claims 6 - 13 wherein the pressure equaliser comprises a vessel in which the opening is formed, the vessel in fluid communication with the cavity.
16. The apparatus according to claim 15 wherein the pressure equaliser further comprises a pipe in fluid communication between the vessel and the cavity.
17. The apparatus according to claim 16 wherein the pipe extends substantially vertically.
18. The apparatus according to claim 16 or 17 wherein the pressure equaliser further comprises a gas permeable filter in fluid communication between the vessel and the cavity.
PCT/AU2005/000023 2004-01-12 2005-01-12 Method and apparatus for monitoring the integrity of a structure having a surface in contact with a liquid WO2005066601A1 (en)

Applications Claiming Priority (2)

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AU2004900114A AU2004900114A0 (en) 2004-01-12 Method and apparatus for monitoring the integrity of a structure having a surface in contact with a liquid
AU2004900114 2004-01-12

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5770794A (en) * 1993-05-06 1998-06-23 Tulip Bay Pty Ltd Monitoring apparatus for monitoring impending faults in the integrity of a component or structure
WO2002001175A1 (en) * 2000-06-28 2002-01-03 Bae Systems Plc Detection of fluid leak sites in fluid containers
WO2002021096A1 (en) * 2000-09-08 2002-03-14 Structural Monitoring Systems Ltd Method and apparatus for monitoring the integrity of components and structures
WO2002057733A1 (en) * 2001-01-18 2002-07-25 Structural Monitoring Systems Ltd Self-monitoring method and apparatus for condition monitoring of a structure
US6539776B2 (en) * 2000-06-19 2003-04-01 Structural Monitoring Systems, Ltd. Apparatus for condition monitoring of the integrity of fasteners and fastened joints

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5770794A (en) * 1993-05-06 1998-06-23 Tulip Bay Pty Ltd Monitoring apparatus for monitoring impending faults in the integrity of a component or structure
US6539776B2 (en) * 2000-06-19 2003-04-01 Structural Monitoring Systems, Ltd. Apparatus for condition monitoring of the integrity of fasteners and fastened joints
WO2002001175A1 (en) * 2000-06-28 2002-01-03 Bae Systems Plc Detection of fluid leak sites in fluid containers
WO2002021096A1 (en) * 2000-09-08 2002-03-14 Structural Monitoring Systems Ltd Method and apparatus for monitoring the integrity of components and structures
WO2002057733A1 (en) * 2001-01-18 2002-07-25 Structural Monitoring Systems Ltd Self-monitoring method and apparatus for condition monitoring of a structure

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