HK1063215B

HK1063215B - Method and apparatus for monitoring the integrity of components and structures

Info

Publication number: HK1063215B
Application number: HK04105209.2A
Authority: HK
Inventors: 肯尼思．J．戴维
Original assignee: 结构监测系统有限公司
Priority date: 2000-09-08
Filing date: 2001-07-02
Publication date: 2007-09-21

Description

Method and apparatus for monitoring the integrity of components and structures

Technical Field

The present invention is a method and apparatus for monitoring the integrity of a component or structure, particularly, but not exclusively, by monitoring the pressure conditions that can be maintained in a chamber that may be intrinsic to or specifically formed in the component or structure.

Background

The invention stems from the consideration of aircraft designers in the face of monitoring integrity, including: sandwich structures typically encountered in the vicinity of splices and cuts in the fuselage; and substantially hollow elements encountered in composite structures such as ailerons, doors, blades, and the like; and to attempt to prevent moisture from entering such structures and components. It is difficult to detect cracking, corrosion and detachment of these structures and components. But also moisture is easily introduced due to various reasons including:

capillary action and the substantially hollow nature of the structure, particularly structures made of composite materials;

exposure to temperature extremes;

exposure to large ambient pressure variations;

exposure to high humidity and rainfall conditions.

In addition to corrosion in metal structures, moisture ingress can also lead to serious structural defects such as debonding due to the periodic ingress of moisture, coupled with the progressive damage caused by expansion upon icing.

Of course the above problem is not the only area of aircraft designers. Structural integrity monitoring has a wide range of applications and can be used, for example, to monitor bonds between sound attenuating tiles on submarines or between heat resistant tiles on space shuttles.

Disclosure of Invention

It is an object of the present invention to provide a method and apparatus for monitoring the integrity of components and structures. It is another object of the invention to prevent moisture from entering the assembly or structure.

For convenience in description, including the claims, the term "structure" is used to refer to structures or elements.

According to a first aspect of the present invention there is provided a method of monitoring the integrity of a structure placed at ambient pressure in a fluid-containing environment, the structure having at least one internal cavity, the method comprising at least the steps of:

providing a first fluid source at a first pressure greater than said ambient pressure;

placing said at least one cavity in fluid communication with said fluid source; and

monitoring a change in a flow rate of the first fluid into the at least one lumen at steady state;

preferably, the first fluid source pressure is substantially constant compared to the ambient pressure.

In one embodiment, the monitoring step comprises: a high flow impedance between the at least one cavity and the fluid source to create a steady state pressure differential between the at least one cavity and the fluid source and to monitor changes in the steady state pressure differential.

Preferably, said step of providing said first fluid source at said first pressure comprises: the first pressure is set at a level sufficiently greater than the ambient pressure to overcome the effects of moisture absorption and capillary action, but insufficient to compromise the integrity of the structure.

Preferably, said step of providing said first source of fluid comprises providing a first source of gas.

Preferably, said step of providing said first gas comprises: a moisture trap is provided between the fluid source and the at least one cavity to dry the gas prior to flowing into the at least one cavity.

Preferably, when the structure comprises two or more lumens, the placing step comprises: (a) placing the lumens in fluid communication with each other; and/or (b) placing the cavity in fluid communication with the fluid source.

In another embodiment, the monitoring step comprises:

providing a quantity of fluid indicia in fluid communication with the fluid source; and

monitoring the structure to track the fluid marker.

Preferably, the fluid marking comprises a dye representing a liquid or a gas.

In another embodiment, the step of monitoring the inflow change in the steady state includes:

providing a quantity of a measurable gas in fluid communication with said fluid source;

providing a method of detecting said gas; and

monitoring a change in the gas leakage rate in the structure at steady state.

According to another aspect of the present invention there is provided a method of monitoring the integrity of a structure, the structure being placed in an environment containing a fluid at ambient pressure, the method comprising the steps of:

forming a sealed cavity in the structure;

monitoring a change in a flow rate of the first fluid into the chamber at steady state.

Preferably, the step of forming the sealed cavity comprises: forming a groove or depression in or on the structure and forming a seal across the groove or depression.

According to another aspect of the present invention there is provided a method for monitoring the integrity of a structure disposed in an environment containing a fluid at ambient pressure, said structure being assembled from two or more elements connected to one another, said elements being disposed in juxtaposition with one another such that a surface of one element is adjacent to a surface of at least one other said element so as to form respective pairs of adjacent surfaces, said method comprising the steps of:

forming one or more cavities between one or more of said pairs of adjacent surfaces;

providing a first fluid source and a first pressure greater than said ambient pressure;

placing at least one of said cavities in fluid communication with said fluid source to create at least one fluid source pressure cavity; and

monitoring a change in a flow rate of the first fluid into the at least one fluid source pressure cavity at steady state.

Preferably the method further comprises the steps of: placing additional said cavities in fluid communication with said ambient pressure to create adjacent alternate source pressure cavities and ambient pressure cavities.

Preferably the method further comprises the steps of: and a moisture remover connected in series is arranged between the environment pressure cavity and the environment or the fluid source of the environment pressure.

Preferably, the monitoring step comprises: a high flow impedance is connected in series between the fluid source pressure chamber and the fluid source to create a steady state pressure differential between the fluid source pressure chamber and the fluid source and to monitor changes in the steady state pressure differential.

In further embodiments, the monitoring step comprises: providing an amount of fluid indicia in fluid communication with the first fluid source, and monitoring the structure to track the fluid indicia.

Preferably, if said elements of said structure are joined together by an adhesive layer or by a layer of sealant material between said adjacent pairs of surfaces, said forming step comprises: forming the cavity within the adhesive or sealant layer.

Preferably, the elements are joined by mechanical fasteners, the forming step comprising: providing a seal adjacent the pair of adjacent surfaces to form the cavity therebetween.

According to another aspect of the present invention, there is provided a method for monitoring the integrity of a structure disposed in an environment containing a fluid at ambient pressure, said structure comprising at least one lumen, said device comprising:

a first fluid source at a first pressure greater than the ambient pressure;

a communication channel for providing fluid communication between the fluid source and the at least one cavity; and

monitoring means for monitoring a change in a flow rate of the first fluid from the channel into the at least one lumen at steady state.

In one embodiment, the monitoring device comprises: a high flow resistor disposed in said communication passage in series between said at least one chamber and said fluid source, said high flow resistor creating a steady state pressure differential between said at least one chamber and said fluid source, and a measurement transducer connected across said high flow resistor to monitor changes in said steady state pressure differential.

Preferably, the first pressure is sufficient to be above the ambient pressure to overcome the forces of moisture absorption and capillarity, but insufficient to compromise the integrity of the structure.

Preferably, the first fluid is a gas.

Preferably, the apparatus further comprises: a moisture remover positioned between the fluid source and the at least one cavity to dry the gas prior to flowing into the at least one cavity.

In another embodiment, the monitoring device comprises: a fluid marker in communication with the fluid source for marking the structure where the fluid leaks from the cavity through the structure to the environment.

According to another aspect of the present invention, there is provided a method for preventing a target fluid from entering a structure disposed in an environment containing the target fluid at ambient pressure, the structure having at least one lumen, the method comprising the steps of:

providing a first fluid source at a first pressure, the first pressure being greater than the ambient pressure; and

a fluid communication path is provided between the at least one lumen and the fluid source.

Preferably, the method further comprises the step of monitoring a change in the flow rate of the first fluid into the at least one lumen at steady state, thereby facilitating monitoring of the integrity of the structure.

According to another aspect of the present invention, there is provided a device for preventing a target fluid from entering a structure disposed in an environment containing the target fluid at ambient pressure, the structure having at least one lumen, the device comprising at least:

a first fluid source at a first pressure greater than the ambient pressure; and

one or more communication channels for providing fluid communication between the fluid source and the at least one cavity.

Drawings

FIG. 1 is a schematic view of a first embodiment of the present invention;

FIG. 2 is a schematic view of an apparatus for monitoring the pressure state of a cavity in a structure, and thus monitoring the integrity of the structure, according to another embodiment of the present invention;

FIG. 3 is a schematic view of another embodiment of the present invention;

FIG. 4a is a schematic view of another embodiment of the present invention;

FIG. 4b is a schematic view of another embodiment of the present invention;

FIG. 5a is a schematic illustration of a two layer sandwich structure to which embodiments of the present invention have been applied;

FIG. 5b is a variation of the structure of the embodiment of FIG. 5 a;

FIG. 6 is a schematic illustration of a three-layer sandwich structure to which embodiments of the present invention have been applied;

FIG. 6a is a variation of the structure of the embodiment of FIG. 6;

FIG. 6b is another variation of the structure of the embodiment of FIG. 6;

FIG. 7 is a schematic illustration of a four layer sandwich structure to which embodiments of the present invention have been applied;

FIG. 8a is a schematic illustration of a three layer sandwich structure to which embodiments of the present invention have been applied;

FIG. 8b is a schematic illustration of a three layer sandwich structure to which another embodiment of the present invention has been applied;

fig. 9 shows a partial cross-sectional oblique view of another cavity configuration.

Detailed Description

FIG. 1 schematically illustrates one embodiment of the method and apparatus of the present invention for preventing the ingress of fluid F into a structure 10. The structure 10 is a hypothetical structure made from three composite structures and is provided herein for the purpose of illustrating the principles of embodiments of the present invention. The structure 10 has a housing 12 and a plurality of internal cavities 14a, 14b and 14c (hereinafter referred to as "cavities 14"). The actual shape of the cavity 14 is a function of the type of structure 10. Cavity 14a illustrates a structure 10 having an inner cavity of arbitrary configuration; cavity 14b illustrates a structure 10 having a honeycomb-type or cell-type core; and cavity 14c illustrate structure 10 having a foam-type core.

The structure 10 is disposed in an environment 16 containing a fluid F at ambient pressure, which acts on the structure 10. For example, the environment 16 may be the atmosphere at an altitude of 4000 meters, while the fluid F is air; or environment 16 may be 100 meters below the sea surface, in which case fluid F is seawater.

The device 18 according to embodiments of the present invention serves to prevent or at least minimize the ingress of fluid F into the structure 10. The device 18 comprises: a source of pressurized fluid 20 for providing a first fluid, such as air or an inert gas, at a pressure greater than the pressure of the fluid F. A communication channel in the form of a conduit 22 provides fluid communication between the fluid source 20 and one or more of the lumens 14 of the structure 10. If the cavities 14 of the structure 10 are in direct or indirect fluid communication with each other, then the conduit 22 need only extend through the housing 12 to a point in the structure 10 in order for the gas of the fluid source 20 to be in fluid communication with the cavities 14. Also, although not shown, a plurality of conduits 22 may be disposed between the fluid source 20 and the structure 10. However, if the chambers 14 are not in fluid communication with each other, or are configured in a sealed layer or set, the communication path of the device 18 includes one or more channels or conduits 24 contained within the housing 12 in communication with the conduit 22, thereby providing fluid communication between the gas of the fluid source 20 and the chambers 14. Alternatively, smaller holes may be made between the lumens 14 to allow fluid communication therebetween. This can be achieved by using, for example, a laser.

The fluid source 20 is set at a pressure greater than the pressure of the fluid F (which may be static or dynamic) to prevent the fluid F from entering the cavity 14. More specifically, the pressure of the fluid source 20 is set to be sufficient to overcome the hygroscopic forces and capillary action, thereby preventing moisture from entering the structure 10, but not sufficient to compromise the integrity of the structure 10.

It should be appreciated that if the housing 12 is to be substantially impermeable to the fluid F. And which does not contain any defects or which does not create any defects during the life cycle of the structure 10, the fluid F of the environment 16 cannot enter the structure 10. In practice, however, the outer shell 12 will, or sooner or later, become permeable to the fluid F for a variety of reasons including the effect of material permeability, dynamic loading, localized impact damage, actual imperfections in the fabrication of the structure 10, or the assembly of the structure with fasteners.

Fig. 2 shows a device 18a that can monitor the pressure state of the cavity 14, and thus the integrity of the structure 10 a. The device 18a includes a fluid source 20 and a conduit 22a, the conduit 22a functioning similarly to the conduit 22 in the embodiment shown in fig. 1 and interconnected with a monitoring device 26 for monitoring the flow of fluid from the fluid source 20 into the cavity 14. The monitoring device 26 is based on that disclosed in international application No. pct/AU94/00325 (WO 94/27130), the contents of which are incorporated herein by reference. The essential differences are: in an embodiment of the invention, a source of constant (positive) pressure fluid is used, whereas in international application No. pct/AU94/00325 (WO 94/27130) a constant vacuum source is used. The monitoring device 26 monitors changes in the rate of fluid flow from the fluid source 20 into the cavity 14 at steady state. In the present embodiment, the monitoring device 26 includes: a high flow resistor 28 is disposed in series in conduit 22a between fluid source 20 and chamber 14. The high flow resistor 28 preferably comprises a small bore tube of substantial length that allows a small amount of fluid to flow therethrough. Alternatively, the high-flow resistor 28 may comprise a permeable material such as porous glass.

In order to obtain a satisfactory flow rate through the PCT/AU94/00325 (WO 94/27130) device, the maximum flow rate is the minimum that can be measured using existing digital flow meters. For example, a tube having an orifice diameter of less than 0.3mm and a length of more than 3 meters, with an air pressure differential of 20kPa over the length, will have a flow rate of about 2-3 microliters/minute. It is contemplated that the sensitivity of the device will increase exponentially closer to zero, and very little flow may be measured if the size of the high flow resistor envisioned is infinitely extendable.

Typically the size of the high flow resistor should be large enough to create a significant pressure drop over the length of the high flow resistor corresponding to the minute flow through the high flow resistor.

Measuring means in the form of a differential pressure transducer 30 are connected at both ends of the resistor 28. The transducer 30 is connected at both ends of the resistor 28 by fluid connections 32 and electrical conductors 36 are connected to an enlarged display 34. Additionally, where current is not required, the differential pressure transducer 30 connected across the resistor 28 may be a non-electrical indicator.

Given the degree of permeability of the housing 12 of the structure 10a itself, a characteristic steady state rate of fluid leakage into and out of the structure 10a occurs after activation of the device 18 a. If the permeability of the housing 12/structure 10a is changed, the flow rate of fluid from the fluid source 20 into the structure 10a is correspondingly increased. This can be monitored and detected by the monitoring device 26. Typical applications of this embodiment are aircraft doors, ailerons or the like, to which nitrogen gas above ambient pressure is applied.

Fig. 3 depicts the apparatus and method as applied to an embodiment of a structure 10b comprising acoustical panels 38 adhered to the hull 40 of a submarine.

Fillets of resilient grout 48 are provided around each tile, with cavities 50 created under grout 48 between adjacent panels 38, or between the edges of panels 38 and the adjacent surfaces of hull 40. The chamber 50 is connected to the monitoring device 18 b. This device 18b is similar to the device 18a described in fig. 2, and comprises: a monitoring device 26b and a fluid source 20b (a gas source in this embodiment). Monitoring device 26b includes a conduit 22b providing a fluid communication path between chambers 50, and fluid source 20b is connected in series to high flow resistor 28. A differential pressure transducer 30 is connected by a tube 32 to both ends of the resistor 28. An enlarged display 34 connected by an electrical conductor 36 to the transducer 30 provides an indication of the steady state pressure differential across the resistive element 28. Fluid from supply 20 is measured by a pressure regulator 52 disposed in conduit 22b between stop 28 and source 20. The regulator 52 is also referenced to the pressure of the surrounding atmosphere, in this case sea water, via a pipe 54, here indicated by the reference F^SAnd the associated white pressure indication arrows. Regulator 52 maintains the pressure of the gas from source 20 at a substantially constant level above the water pressure. Because the external water pressure varies with the depth of 18b, regulator 52, in particular, is able to dynamically vary the pressure of the gas dispensed into cavity 50 from fluid source 20. In operation, the monitoring device 26b stabilizes at a relatively constant pressure value, which is different from the pressure at the high flow ends, regardless of the external water pressure.

Monitoring the integrity of the viscous coupling of the tiles 38 may be facilitated by monitoring the pressure differential across the high flow resistor 28. An increase in pressure differential due to a small amount of air leaking from any of the cavities 50 can give a warning of sufficient debonding of any of the tiles 38 or damage to the grout 48. Since the pressure difference across the resistance 28 rises and is detected by the transducer, a danger of disconnection and water ingress can be immediately detected. The reduction of tiles 38 results in a dramatic rise in pressure differential. An adjustable branch of the resistor 28 may also be provided for the monitoring device 26b to provide a high flow rate of air from the fluid source 20 while allowing for some crack tolerance and maintaining positive pressure protection for the cavity 50.

Because of the sea water F around the hull 40 of the submarine^SThe ambient pressure of (a) from the commander tower fins to the belly of the hull is significantly different, so it may be necessary to group the tiles 38 into several vertically layered layers which are separately monitored to ensure that the air pressure supplied to a particular set of tiles remains slightly higher than the ambient pressure acting on those panels, thus preventing excessive positive pressure of the upper set of tiles. This can be achieved by: a manifold is provided in a portion of conduit 22b between source 20 and regulator 52, with a plurality of regulators 52 fed from the manifold and connected to an equivalent arrangement of high flow resistor 28, converter 30 and ambient pressure reference point 54.

Fig. 4a shows another embodiment of the invention applied to a structure 10c, which structure 10c comprises three elements 56, 58 and 60 connected in a sandwich structure. In particular, the structure 10c is part of a pressurized fuselage of an aircraft. The members 56, 58 and 60 are secured together by rivets 62 that pass through holes 71 formed in the members 56, 58 and 60. Each rivet 62 has a head 64 flush with member 56 and a flat tail 66 at the other end that bears against member 60. Flat head rivets are shown by way of example, round head rivets or possibly bolt fasteners being optional.

In general, the sandwich component will have a layer of sealing material between each fastening layer, which partly prevents corrosion and wear. To facilitate the manufacture of the chamber, this arrangement is modified in the present invention so that only a sealing layer 68 is provided between the elements 56 and 58, the seal between the elements 58 and 60 being typically at least partially removed, forming a gas-permeable gap 70 therebetween. According to this embodiment, the gap 70 may be formed into the cavity 72 by providing a peripheral seal 74 around the gap 70. A small amount of sealant 75 is applied around the flat end 66 of the rivet 62, adjacent the surface of the element 60. The apparatus 18c, including the monitoring device 26c, is connected to the cavity 72 to monitor the integrity of the structure 10 c. The monitoring device 26c includes: resulting in a conduit 22c connected in parallel between the high flow resistor 28 and the pressure transducer 30. The transducer 30 is connected to the magnifying display 34 by an electrical lead 36. Additionally, the differential pressure transducer 30, which is connected across the resistive member 28, may be a non-electrical indicator.

The pressure fluid source 20c of the present embodiment is the cabin pressure of the aircraft, which enters the resistor 28 and the transducer 30. Cabin pressure is indicated by "CP" and associated pressures are shown by black arrows.

If a crack 100 forms in the intermediate member 58 around the rivet 62, an atmosphere F to the exterior high altitude will be created around the rivet 62 and head 64^A(associated with the white pressure indication arrows) fluid leak flow paths (shown with small black flow indication arrows) due to the cracks 100 and subsequent loosening of the fixings. The result of the increased air flow into the cavity 72 via the obstruction 28 may be detected by the transducer 30 as a change in pressure differential, thereby providing an indication of a crack 100 in the element 58.

In another embodiment, shown in fig. 4b, an apparatus 18' for monitoring the integrity of a structure 10c comprises: a flexible container 76 containing a fluid marker, such as a liquid or gaseous dye, or a measurable gas, is connected to chamber 72 by conduit 22' instead of conduit 22 c. If a crack 100 develops in the element 58 as in the previous embodiment, extending to the rivet 62, there will be a mark leaking from the container 76 through the conduit 22', the cavity 72, the crack 100 and around the rivet 62 to the outside atmosphere. This is so because the flexible container 76 is also subjected to the cabin pressure CP. Detection of a dye or gas around the rivet 62 provides an indication of the presence of a crack in the element 58.

If the fluid is labeled as a liquid, it can be detected by visual inspection of the structure. The presence of the dye around the head of the rivet 62 can potentially be attributed to the presence of cracks. If a measurable gas is used as a marker, such as helium, gas monitoring and inspection equipment needs to detect the escape of gas from the structure. In the case of structures that are permeable themselves, there will of course be steady state air flow through the structure, in which case changes in steady state conditions need to be monitored. On the other hand, if the structure is absolutely impermeable after initial manufacture, the presence of any gas marks needs to be detected. This is of course the same as monitoring for the occurrence of a significant pressure differential across the monitoring device 26c, indicating fluid at a fluid location that was not previously present. However, because of the sensitivity of the flow resistance device 26c, the fluid marking method is more likely to be used as a defect location indicator.

Fig. 5 to 7 depict various "sandwich" structures to which embodiments of the present invention may be applied.

In FIG. 5a, a portion of structure 10d includes two elements 56 and 60 secured together by a rivet 62. Between two opposite adjacent faces of the elements 56 and 60 there is a gas permeable gap 70. A cavity 72 is formed by sealing gap 70 with a perimeter seal 74. Similar to fig. 4a, the structure 10d is part of the aircraft fuselage containing the cabin pressure CP and is arranged in the fluid F^A(i.e., high ambient air pressure). The integrity of the structure 10d may be monitored by connecting the cavity 72 to a monitoring device 26d similar to 26c in fig. 4a of the type described above, via the conduit 22 d.

Because the cavity 72 in the structure 10d is completely surrounded by the fluid F, any leakage of gas from the cavity 72 past the element 56 to the external environment F^A。

In a variation of the above-described configuration of FIG. 50, as shown in FIG. 5b, the device 26c may be connected between the chamber 72 and the external atmospheric pressure F^AIn the meantime. In this example, with a penetration formed in the element 60, the cavity 72 becomes a conduit for a source of pressure fluid in the form of Cabin Pressure (CP). This configuration is further described in subsequent fig. 6a and 6 b.

In fig. 6, structure 10e is very similar to structure 10c in fig. 4 a. However, in this case, the structure 10e is formed with two cavities 72s and 72 n. The cavity 72s is formed between two adjacent surfaces of the elements 56 and 58 of the structure 10 e. The cavity 72s is in fluid communication with a source of pressurized fluid 20e (cp) via a conduit 22s and a monitoring device 26e similar in structure and function to that described above. However, the cavity 72n existing between the elements 58 and 60 is in fluid communication with the fluid pressure of the reference external environment FA. This now facilitates the detection of a crack 101 in the intermediate element 58, which extends between the cavities 72s and 72n, instead of directly to the external environment F via the element 56^A. The dotted trace shows the flow path: starting from the pressure fluid source 20e, passing through the tube 22s and the device 26e, the cavity 72s, the crack 101, the cavity 72n, the conduit 22n, and then to the external atmospheric "ambient" pressure F^A。

As another approach, tube 22n may be used to connect chamber 72n to a device like 26e (26x) and then to atmospheric pressure reference F^A. This is shown in fig. 6a, using a monitoring device 26x substantially identical to the configuration shown in fig. 5 a. The purpose of this configuration is to monitor the integrity of the element 60. The formation of a crack or defect 102 in the element 60 that communicates the cavity 72n with the cabin pressure CP (fluid source 20e) will cause a pressure drop due to the flow resistance to the external environment F through the device 18x^AThe leakage flow (small arrows and dots). If this feature is desired, the connection of the device 26x may be performed generally by-pass or at intervals to prevent a continuous series connection of the two monitoring devices 26e and 26x (as shown in FIG. 6 b) that would halve the sensitivity of the devices 26e and 26x to defects 101 generated in the component 58. Leakage flow from cavity 72s, through crack (101) in element 58, to cavity 72n would have to pass through these two high flow resistors: with monitoring devices 26e and 26x in series, the two associated differential pressure sensors will share the pressure drop that results in halving the sensitivity. The problem of series flow is illustrated by small arrows and dots. Of course, if the cracks formed in the element 56 extend directly to the external environment F^AThis problem does not occur.

FIG. 7 shows another sandwich structure 10f having rivets 62 attachedFour layers 56, 57, 58 and 60 joined together. Layer 57 includes two contiguous faces. The structure 10f is also part of the aircraft fuselage containing the cabin pressure CP and is disposed in a fluid environment that is high external air pressure. The method of monitoring the integrity of the structure 10f includes: forming a cavity 72s in fluid communication with a source of pressurized fluid CP of the above-described typical 26c and 26x type monitoring devices via conduit 22 s; forming a pressure F with the ambient environment via the conduit 22n^AAnd cavities 72n in fluid communication, with cavity 72n between cavities 72 s. The configuration of monitoring the integrity of the element 60 of fig. 5a and 6a can be similarly applied to fig. 7.

Fig. 8a shows another embodiment of the present invention. This embodiment is applied to a structure 10g comprising three layers 56, 58 and 60, the three layers 56, 58 and 60 being joined together by rivets 62 to form a sandwich structure. As explained in connection with the embodiment shown in fig. 4a, it is common for such a configuration to incorporate a sealing material 68 between adjacent layers. Layer 68 is specifically provided to prevent corrosion and wear of layers 56, 58, 60 with respect to rivet 62. In the present embodiment, the step of providing the cavity 72 in the structure 10g includes: eliminating a portion of the sealant 68 between adjacent layers. However, a region of sealant 68 remains around rivet 62 to maintain the function of minimizing wear of layers 56, 58, and 60 and to form a boundary seal for cavity 72. The removed sealant 68 creates a sealed cavity 72 that can be in fluid communication with a source of pressurized fluid. Indeed, the alternate chamber 72, as described in connection with the embodiment of fig. 6 and 7, can be in fluid communication with the atmosphere and a fluid source. The removal of encapsulant 68 to create cavities 72 is preferably accomplished during the fabrication of structure 101g by placing a barrier over layers 56, 58, and 60 to prevent deposition of encapsulant 68 in selected areas. After the sealant layer 68 is made and the barrier is removed, the structure 10g is fastened together with rivets 62.

Fig. 8b shows a different structure 10h from that shown in fig. 8a, except that: recesses 80 are formed in the surface of layers 56, 58 and 60 in the areas where encapsulant 68 is eliminated. This provides a larger and more distinct cavity 72. The recess 80 may be formed by any known method, including but not limited toAnd (4) chemically etching. The cavity 72 may be brought into fluid communication with a source of constant pressure fluid by means of conduits 22c to 22x as specified in the method described above in relation to figures 4a to 7. Of course, in another variation, additional chamber 72 may be connected to fluid source 20(CP) and ambient pressure F^AAnd (4) communicating.

Figure 9 shows a partially sectioned oblique view of another structure in the form of: the lap joint 10j with the preformed, elastic self-adhesive film backing 110 is sandwiched between layers 56 and 60 and secured by rivets 62. The liner 110 includes a shape that is cut and assembled to define a plurality of cavities 72 when sandwiched between the plies 56 and 60. To facilitate drawing, the cavity 72 is drawn with a thick line. By alternating the connection of the chamber 72 with the atmospheric pressure reference value and the cabin pressure in connection with the monitoring devices of 26c to 26e of the previous examples, cracks that would penetrate the surface 56 or 60 of the layer can be detected earlier before a rupture of any of the layers occurs. This is important because in the past, a rapidly failing chain reaction has occurred on the fuselage of an aircraft due to the invisibility of the crack. By sequentially reversing the cabin pressure/ambient pressure relationship, determination of crack growth can be achieved to assist in eliminating ineffective positive pressure by recording the second interruptions for the three cavities shown on either side of each rivet 62. For another reference, the examiner may refer to the specification of International application No. PCT/AU94/00325 (WO 94/27130), which discloses the contents of crack detection.

Now that embodiments of the present invention have been described in detail, it will be apparent to those skilled in the relevant art that numerous modifications and variations can be made without departing from the broad inventive concept thereof. For example, when the fluid source 20 is a gas source, a moisture trap may be disposed between the fluid source 20 and the resistor 28 to dry the gas before the gas flows into the structure 10. Additionally, the fluid source 20 may be an inert gas source. Further, the defined cavity may include an anti-corrosion agent. If the structure 10 is a composite material having a plurality of lumens sealed to one another, embodiments of the present invention include forming communication paths in the composite material between the lumens.

All such modifications and variations are within the scope of the invention, as characterized by the foregoing description and the appended claims.

Claims

1. A method of monitoring the integrity of a structure placed in an environment containing a fluid at ambient pressure, the structure having at least one lumen and being permeable, the method comprising at least the steps of:

providing a trace amount of fluid to the at least one cavity by connecting a high flow resistor in series between the at least one cavity and the fluid source, placing the at least one cavity in fluid communication with the fluid source; and

monitoring a change in a flow rate of the trace amount of the first fluid into the at least one lumen at steady state.

2. The method of claim 1, wherein the first fluid source pressure is substantially constant relative to the ambient pressure.

3. A method according to claim 1 or claim 2, wherein the monitoring step comprises monitoring a change in pressure differential at steady state between the at least one chamber and the fluid source.

4. The method of claim 1 or 2, wherein the step of providing the first fluid source at the first pressure comprises: the first pressure is set at a level sufficiently greater than the ambient pressure to overcome the hygroscopic forces and capillary action, but not sufficiently to compromise the integrity of the structure.

5. The method of claim 1 or 2, wherein the step of providing the first fluid source comprises providing a first gas source.

6. The method of claim 5, wherein the step of providing the first gas comprises providing a moisture barrier between the fluid source and the at least one cavity to dry the gas prior to flowing into the at least one cavity.

7. The method of claim 1 or 2, wherein if the structure comprises two or more lumens, the placing step comprises: (a) placing the lumens in fluid communication with each other; and/or (b) placing the cavity in fluid communication with the fluid source.

8. The method of claim 1 or 2, wherein the monitoring step comprises:

monitoring the structure to track the fluid marker.

9. The method of claim 8, wherein the fluid marker comprises a dye marking a liquid or a gas.

10. The method of claim 1 or 2, wherein the step of monitoring steady state inflow changes comprises:

providing a method of detecting said gas; and

monitoring a change in said gas leakage rate from said structure at steady state.

11. A method of monitoring the integrity of a structure disposed in an environment containing a fluid at ambient pressure, the structure being permeable, the method comprising the steps of:

forming a sealed cavity in the structure;

monitoring a change in a flow rate of the trace amount of the first fluid into the cavity at a steady state.

12. The method of claim 11, wherein the step of forming the sealed cavity comprises: forming a groove or depression in or on the structure and forming a seal across the groove or depression.

13. A method of monitoring the integrity of a structure disposed in an environment containing a fluid at ambient pressure, said structure being assembled from two or more interconnected elements and being permeable, said elements being juxtaposed with each other such that a surface of one element is adjacent to a surface of at least one other of said elements to form respective adjacent pairs of surfaces, said method comprising the steps of:

providing a first fluid source, and a first pressure greater than the ambient pressure;

providing a trace amount of fluid to said at least one cavity by connecting a high flow resistor in series between said at least one cavity and said fluid source, placing at least one of said cavities in fluid communication with said fluid source to create at least one fluid source pressure cavity; and

monitoring a change in a flow rate of the trace amount of the first fluid into the at least one fluid source pressure cavity at steady state.

14. The method of claim 13, further comprising the steps of: placing additional said cavities in fluid communication with said ambient pressure to create adjacent alternate source pressure cavities and ambient pressure cavities.

15. The method of claim 14, further comprising the steps of: a moisture trap is provided in series between the ambient pressure chamber and the source of ambient or ambient pressure fluid.

16. The method of any one of claims 13 to 15, wherein the monitoring step comprises: monitoring a change in pressure differential between the at least one cavity and the at least one fluid source at steady state.

17. The method of any one of claims 13 to 15, wherein the monitoring step comprises: providing an amount of fluid indicia in fluid communication with the first fluid source, and monitoring the structure to track the fluid indicia.

18. A method according to any one of claims 13 to 15, wherein, if the elements of the structure are joined together by an adhesive layer, or a layer of sealing material is bonded between the adjacent pairs of surfaces, the forming step comprises forming the cavity in the adhesive or sealing layer.

19. A method according to any one of claims 13 to 15, wherein the elements are joined by mechanical fasteners, the forming step comprising providing a seal adjacent the pair of adjacent surfaces so as to form the cavity therebetween.

20. There is provided a method for monitoring the integrity of a structure disposed in an environment containing a fluid at ambient pressure, said structure having at least one lumen and being permeable, said device comprising at least:

a first fluid source at a first pressure greater than the ambient pressure;

a communication channel for providing fluid communication between the fluid source and the at least one cavity, the channel comprising a high flow resistor connected in series between the at least one cavity and the fluid source to provide a trace amount of fluid to the at least one cavity; and

and a monitoring device for monitoring the change of the flow rate of the trace first fluid flowing into the at least one inner cavity through the channel in a steady state.

21. The apparatus of claim 20 wherein said monitoring means comprises transducer means connected across said high flow resistor for monitoring changes in differential pressure between said at least one chamber and said fluid source during steady state conditions.

22. A device according to claim 20 or 21, wherein the first pressure is sufficiently greater than the ambient pressure to overcome hygroscopic forces and capillary action, but insufficient to cause damage to the structure.

23. Apparatus according to claim 20 or 21, wherein the first fluid is a gas.

24. The apparatus of claim 20 or 21, further comprising a moisture separator between the fluid source and at least one chamber to dry the gas before the gas flows into the at least one chamber.

25. The device of claim 20, wherein the monitoring device comprises a fluid marker in communication with the fluid source for marking the structure where the fluid leaks from the cavity through the structure to the environment.

26. A method for preventing a target fluid from entering a structure disposed in an environment containing the target fluid at ambient pressure, the structure having at least one lumen and being permeable, the method comprising the steps of:

providing a trace amount of fluid flow to the at least one lumen through a series-connected high-flow resistor provides a fluid communication path between the at least one lumen and the fluid source.

27. The method of claim 26, further comprising the step of: monitoring a change in a flow rate of the trace amount of the first fluid into the at least one lumen at steady state, thereby facilitating monitoring of the structural integrity.

28. An apparatus for preventing a target fluid from entering a structure disposed in an environment containing the target fluid at ambient pressure, the structure having at least one lumen and being permeable, the apparatus comprising at least:

a first fluid source at a first pressure greater than the ambient pressure; and

one or more communication channels for providing fluid communication between the fluid source and the at least one cavity, the channels comprising a high flow resistor connected in series between the at least one cavity and the fluid source to provide a trace amount of fluid flow to the at least one cavity.