Abstract:
A sensor for detecting surface cracks in a component or structure. A preferred embodiment of the device comprises a flat body portion with a central hole through which a main structural bolt passes. The body portion has a throughway providing fluid communication between an exterior port and a substantially hermetically-sealed area on the structural surface being monitored. A crack which develops in the monitored area surrounding the bolt hole will cause venting of the hermetically-sealed area, in turn causing a change in fluid pressure that can be detected and/or measured to warn of the presence of the crack.

Description:
PRIORITY CLAIM 
     The present application is a United States national phase application filed pursuant to 35 USC §371 of International Patent Application Serial No. PCT/AU2007/000603, entitled SENSOR FOR DETECTING SURFACE CRACKS IN A COMPONENT OR STRUCTURE, filed May 4, 2007; which application claims priority to Australian Patent Application Serial No. 2006902360, filed May 5, 2006; all of the foregoing applications are incorporated by reference herein in their entireties. 
     FIELD OF THE INVENTION 
     The present invention relates to a sensor for detecting surface cracks in a component or structure. 
     BACKGROUND OF THE INVENTION 
     It is known that structural discontinuities, such as holes and notches, can cause localized stress concentration in a component or structure. When the applied stress is sufficiently high the stress concentration results in cracks forming in the component or structure, which propagate from the structural discontinuity. Excessive cracking can ultimately lead to failure of the component or structure. 
     In many structures, such as aircrafts and bridges, holes and fastener assemblies are used to connect components within the structure. A hole site is a structural discontinuity at which failure can first be observed. Early detection of component failure can prevent catastrophic failure. Planning for early detection can be used during the design phase to minimize redundancy within a structure, and thus the overall weight of the structure. 
     Visual detection of surface cracking that propagates from a hole in a component or structure through which a fastener assembly is disposed can be performed by removing the fastener assembly from the hole. This can be a labor intensive task. Further, the actual removal of a fastener assembly may damage the component or structure and provide the source of a crack. Alternatively, cracks can be observed when they extend beyond the outer edge of the fastener assembly. However, it is to be appreciated that by such a time the crack may already be several millimeters long. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a sensor for detecting surface cracks in a component or structure, the sensor comprising:
         a body portion having: a first surface; a cavity that opens onto the first surface; and a throughway that extends through the body portion and provides fluid communication between the cavity and a second surface of the body portion; and   a sealing system configured to establish a substantially hermetic seal between the first surface and an outer surface of the component or structure on opposite sides of the cavity in response to a compressive load that is exerted on the body portion.       

     According to a second aspect of the invention there is provided a sensor for detecting surface cracks in a component or structure having an outer surface, the sensor comprising:
         a body portion having first surface;   a sealing system cooperating with the body portion to form a cavity that opens onto the first surface, the sealing system configured to form a substantially hermetic seal between the first surface and the outer surface of the component or structure on opposite sides of the cavity;   wherein the body portion is provided with a conduit that provides fluid communication between the cavity and a port accessible on the body portion.       

     According to a third aspect of the present invention, there is provided a fastener assembly for fastening a component or structure, the fastener assembly comprising at least one fastener element having a sensor in accordance with the first aspect. 
     According to a fourth aspect of the present invention, there is provided a method for detecting surface cracks in components joined by a fastener assembly having a fastener element that passes through holes in the components, the method comprising:
         providing a sensor comprising a body portion having a first surface; a cavity that opens onto the first surface, and a throughway that extends through the body portion and provides fluid communication between the cavity and a second surface of the body portion;   providing a sealing system that is operable between the first surface and the outer surface;   locating the sensor within the fastener assembly such that the first surface is adjacent the outer surface with the cavity adjacent the outer surface;   tensioning the fastener assembly to exert a compressive load on the sensor wherein the sealing system establishes the substantially hermetic seal between the first surface and the outer surface on opposite sides of the cavity; and   monitoring fluid flow through the cavity.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be more easily understood, embodiments will now be described, by way of example only, with reference to the accompanying figures, in which: 
         FIG. 1 : is an axonometric view of a sensor in accordance with a first embodiment of the present invention; 
         FIG. 2 : is an axonometric bottom view of the sensor of  FIG. 1 ; 
         FIG. 3 : is a schematic transparent view of the sensor as shown in  FIG. 2 ; 
         FIG. 4 : is a side elevation view of fastener assembly that incorporates a sensor in accordance with a second embodiment of the present invention; 
         FIG. 5 : is a bottom view of the sensor of  FIG. 4 ; 
         FIG. 6 : is a side cross sectional view of the sensor of  FIG. 4 , as viewed along the line A-A in  FIG. 5 ; 
         FIG. 7 : is a side cross sectional view of fastener assembly of  FIG. 4 ; 
         FIG. 8 : is an axonometric cross sectional view of a sensor in accordance with a third embodiment of the present invention; 
         FIG. 9 : is a side cross sectional view of the sensor of  FIG. 8 ; 
         FIG. 10 : is a side cross sectional view of a fastener assembly that incorporates the sensor of  FIG. 8 ; 
         FIG. 11 : is a view of detail B of  FIG. 10 ; 
         FIG. 12 : is a bottom axonometric view of a sensor in accordance with a fourth embodiment of the present invention; 
         FIG. 13 : is a side cross sectional view of the sensor of  FIG. 12 ; 
         FIG. 14 : is an axonometric cross sectional view of the sensor of  FIG. 12 ; 
         FIG. 15 : is a side cross sectional view of a fastener assembly that incorporates the sensor of  FIG. 12 ; and 
         FIG. 16 : is a view of detail D of  FIG. 10 . 
         FIG. 17 : is a side cross section view of a fifth embodiment of the sensor; and 
         FIG. 18 : is a side cross section view of a sixth embodiment of the sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 to 3  show a sensor  10  in accordance with a first embodiment. The sensor  10  has a body portion  12  that defines a hole  14  through which a fastener element (such as the shank of a bolt) can pass. The body portion  12  has a generally annular shape. The body portion  12  also has a first surface  16  and a second opposing surface  18 . 
     As shown in  FIG. 2 , the body portion  12  has a cavity, which in this embodiment is in the form of a channel  20  that extends partially through the thickness of the body portion  12  and opens onto the first surface  16 . The channel  20  extends about the hole  14 . In this embodiment, the channel  20  has a generally C-shape when viewed perpendicularly from the first surface  16 . 
     The sensor  10  is further provided with a sealing system, which in this embodiment is in the form of a compliant stratum  22 . The compliant stratum  22  is affixed to the first surface  16  of the body portion  12 . A first aperture  24  extends through the stratum  22  and is the same configuration as the hole  14  in the body portion  12 . A second aperture  25  extends through the stratum and is substantially the same configuration as the channel  20 . The aperture  24  registers with hole  14 . Similarly, the aperture  25  registers with the channel  20 . 
     The sensor  10  further has a tail piece  26  extending from the body portion  12 . As shown in  FIG. 3 , two conduits or throughways  28  extend through the tail piece  26 . Each throughway  28  has an opening  30  at the end of the tail piece  26  that is remote from the body portion  12 . In addition, each throughway  28  opens onto the channel  20  at an end of the C-shape. Each throughway  28  can be considered, and in effect act, as a conduit; and each opening  30  can be considered, and in effect act, as a port which is located on an accessible part of the body portion. Thus the throughways (i.e., conduits) provide fluid communication between the cavity/channel  20  and an opening (i.e., port)  30 . 
     Two secondary holes  32  extend through the tail piece  26 . Each of the holes  32  is transverse to, and is in fluid communication with, one of the throughways  28 . 
     The sensor  10  can be installed in a fastener assembly (not shown), which fastens two or more components (also not shown) together. A through hole extends through each of the components, and the components are arranged such that the respective holes are in alignment. A shank, or shank-like member, of one of the fastener elements within the fastener assembly extends through the aperture  14  in the sensor and through the aligned holes in the components. The sensor  10  is arranged such that the compliant stratum  22  is disposed in contact with the outer surface of the component or structure to be monitored. 
     In response to a compressive load applied to the body portion, which is generated by tension in the shank of the fastener element, the compliant stratum  22  is partially deformed. In this embodiment, the compliant stratum  22  is formed of material that is partially elastic. Accordingly, the deformation is also elastic, such that a substantially hermetic seal is established between the body portion and the outer surface of the component. 
     A compressive load can be applied to the first and second surfaces  16 ,  18  by tension in the shank of the fastener element; that is, a distributed load can be applied to the first surface  16  and an equal and opposite force can be applied to the second surface  18 . The body portion  12  is capable of supporting the compressive load. 
     However, it is to be appreciated that the elasticity of the material of the compliant stratum  22 , and the dimensions of the aperture  25  should be selected such that when a compressive load (such as that applied by a fastener element) is applied to the compliant stratum  22 , the aperture  25  is not pinched or otherwise closed. 
     When the sensor is located on the outer surface of the component, the channel  20  and the surface of the component together form a conduit that extends between the two throughways  28 . Furthermore, when the component is intact (that is, no surface cracks are present) a substantially hermetic seal can be formed between the body portion  12  and the surface of the component. Thus, the conduit is in fluid communication with the throughways  28 , but in fluid isolation from the atmosphere surrounding the sensor  10 . 
     Tubing, such as flexible piping (not shown), can be connected to the throughways  28  of the sensor  10  via either the opening  30  or the secondary holes  32 . The other of the secondary holes  32  or throughways  28  can be blocked off by a plug or a sealant. The tubing plumbs the sensor  10  into, for example, a differential pressure monitoring system. 
     In use, fluid (such as air) within the conduit can be either (a) evacuated to establish low pressure state within the conduit; or (b) pressurized to establish a high pressure state within the conduit, relative to ambient pressure. A gap, which exists between the component (or components) and the shank of the fastener assembly, can be arranged to be at a differential pressure to the conduit, such as, for example, atmospheric pressure. When a crack of sufficient size extends through the component, a fluid flow path is formed through the crack, and between the gap and the channel  20 . Where a pressure differential exists between two regions of the crack, fluid may flow through the crack. Accordingly, a change in fluid flow through the conduit (which may be observed as a change in pressure state of the conduit) can be indicative of the presence of the crack. 
     As shown in  FIG. 1 , in some embodiments the second surface  18  of the body portion  12  can be provided with channels or notches  34  that extend in a radial direction between the hole  14  and an outer edge of the body portion  12 . The channels  34  can assist establishing fluid communication between the atmosphere surrounding the fastener assembly, and the gap that exists between the component(s) and the shank of the fastener element. 
     As the conduit is continuous between the throughways  28 , it is possible to test for a blockage in the conduit. A blockage indicates that continuity does not exist through the conduit and that portions of the sensor  10  are inactive. Clearly, a crack that intercepts an inactive portion of the conduit will not be detected. For example, a continuity test may be achieved by introducing fluid into the conduit via one of the throughways  28  and monitoring the steady state flow of fluid exhausted via the corresponding other throughway  28 . 
       FIGS. 4 and 7  show a fastener assembly  150  that fastens a first component  152  to a second component  154 . The fastener assembly  150  has a bolt  155  with a head  162 , and a shank  156  that extends through holes  158 ,  160  in each of the first and second components  152 ,  154 , respectively. A sensor  110  (as also shown in  FIGS. 5 and 6 ) according to a second embodiment is disposed between the head  162  and the first component  152 . The fastener assembly  150  further has a washer element  164  and a nut  166 . The shank  156  is provided with an external thread (not shown) and the nut  166  is provided with a complementary internal thread. Both the washer  164  and the nut  166  are disposed about the shank  156  with the washer  164  in contact with the second component  154 . 
     A tensile load is established in the shank  156  such that the sensor  110 , the first and second components  152 ,  154 , and the washer  164  are all held in compression between the head  162  and the nut  166 . 
     The sensor  110  has a body portion  112  that defines a hole  114  through which a fastener element (such as the shank of a bolt) can pass. In this embodiment, the body portion  112  has the form of a generally annular ring. The body portion  112  also has a first surface  116  and a second opposing surface  118 . Accordingly, in this embodiment, the sensor  110  has an overall shape that is similar to that of a washer or disc. 
     The body portion  112  has a cavity, which in this embodiment is in the form of a channel  120  that extends partially through the thickness of the body portion  112  and opens onto the first surface  116 . In this embodiment, the channel  120  extends concentrically about the hole  114 . Furthermore, in this embodiment, the channel  120  is in the form of an annular ring. 
     Two annular grooves  122   a ,  122   b  are formed in the first surface  116 . The first groove  122   a  is concentric with the channel  120  and has an outer diameter that is less than the inner diameter of the channel  120 . The second groove  122   b  is concentric with the channel  120  and has an inner diameter that is less than the outer diameter of the channel  120 . The sensor  110  is further provided with a sealing system, which in this embodiment is in the form of two compressible elements. Furthermore, the compressible elements in this embodiment are o-rings  124   a ,  124   b . Each o-ring  124   a ,  124   b  is disposed within a respective one of the grooves  122   a ,  122   b . In their relaxed state, each o-ring  124   a ,  124   b  has a thickness that is greater than the depth of the respective grooves  122   a ,  122   b.    
     As shown in  FIG. 7 , in response to a compressive load applied to the body portion, which is generated by tension in the shank of the fastener element, the o-rings  124   a ,  124   b  can both be deformed. In this embodiment, the o-rings  124   a ,  124   b  can be made of an elastomeric material. Accordingly, the deformation is largely elastic. Thus, substantially hermetic seals can be formed about the channel  120 , between the body portion  112  and the first component  152 . 
     A benefit of the use of the o-rings is that irrespective of the tension in the fastener and thus the compressive load, the load on the o-rings and thus the sealing will remain substantially constant as the compressive load is transmitted through metal to metal contact throughout the assembly. In general, the compressive load on the o-rings will be determined by the hardness of the material from which they are made and the shape of the grooves  122  in which they sit. 
     The body portion  112  is further provided with a throughway  128  that extends through the body portion  112 , and between an opening  130  on a circumferential edge of the body portion  112  and the channel  120 . Accordingly, the channel  120  is in fluid communication with the throughway  128 . 
     Tubing (not shown) to plumb the sensor  110  into a differential pressure monitoring system (also not shown) can be connected to the throughway  128  such that the tubing is in isolated fluid communication with the throughway  128 . 
     The channel  120  and the surface of the first component  152  together form a conduit. When the first component  152  is intact, the channel  120  (and thus the conduit) can be in fluid isolation from the surrounding environment. 
       FIG. 7  shows cracks C in each of the first and second components  152 ,  154 , which extend from the holes  158 ,  160 , respectively. The crack C in the first component  152  opens onto the surface adjacent the sensor  110 , and intersects the channel  120 . 
     In use, fluid (such as air) within the conduit can be evacuated or pressurized to establish a pressure differential between the conduit and ambient pressure. A gap, which exists between the first and second components  152 ,  154  and the shank  156  of the bolt  155 , can be arranged to be at a differential pressure to the conduit, such as, for example, atmospheric pressure. When the crack C is of sufficient size a fluid flow path is formed through the crack C, and between the gap and the channel  120 . Where a pressure differential exists between two regions of the crack, fluid may flow through the crack C. Accordingly, a change in fluid flow through the conduit (which may be observed as a change in pressure state of the conduit) can be indicative of the presence of the crack C. 
     The compressible elements may be formed by dispensing an elastomeric material into the grooves  122   a ,  122   b . The elastomeric may be delivered in an uncured state such that it flows readily during dispensing, but subsequently cures to form a stiffer material. 
       FIGS. 8 and 9  show a sensor  210  in accordance with a third embodiment. The sensor  210  has a body portion  212  that defines a hole  214  through which a fastener element (such as the shank of a bolt) can pass. The body portion  212  has the form of a generally annular ring. The body portion  212  also has a first surface  216  and a second opposing surface  218 . 
     The body portion  212  has a cavity, which in this embodiment is in the form of a channel  220  that extends partially through the thickness of the body portion  212  and opens onto the first surface  216 . In this embodiment, the channel  220  extends concentrically about the hole  214 . Furthermore, in this embodiment, the channel  220  is in the form of an annular ring. 
     The sensor  210  is further provided with a sealing system, which in this embodiment is in the form of first and second compliant annular rings  222 ,  224 . The first compliant annular ring  222  has an inner diameter that is substantially the same as the diameter of the aperture  214 , and an outer diameter that is substantially the same as the inner diameter of the channel  220 . The second compliant annular ring  224  has an inner diameter that is substantially the same as the outer diameter of the channel  220 , and an outer diameter that is substantially the same as the outer diameter of the body portion  212 . Both the first and second compliant annular rings  222 ,  224  are affixed to the first surface  216  of the body portion  212 . 
     The body portion  212  is further provided with a throughway  228  that extends through the body portion  212 , and between an opening  230  on a circumferential edge of the body portion  212  and the channel  220 . Accordingly, the channel  220  is in fluid communication with the throughway  228 . 
     In response to a compressive load, such as loads generated by the tensioned fastener assembly, the first and second compliant annular rings  222 ,  224  can be deformed. In this embodiment, the first and second compliant annular rings  222 ,  224  are formed of material that is at least partially elastic. Accordingly, the deformation is largely elastic. Thus, in use, substantially hermetic seals can be formed about the channel  120 , between the first surface  216  of the body portion  212  and the surface of the component in contact first and second compliant annular rings  222 ,  224 . 
       FIGS. 10 and 11  show a fastener assembly  250  that fastens a first component  252  to a second component  254 . The fastener assembly  250  has bolt  255  with a shank  256  that extends through holes  258 ,  260  in each of the first and second components  252 ,  254  respectively. The shank  256  extends from, and is integral with, the head  262  of the bolt  255 . Sensor  210  is disposed between the head  262  and the first component  252 . The fastener assembly  250  further has a washer element  264  and a nut  266 . The shank  256  is provided with an external thread (not shown) and the nut  266  is provided with a complementary internal thread. Both the washer  264  and the nut  266  are disposed about the shank  256  with the washer  264  in contact with the second component  254 . 
     A tensile load is established in the shank  256  such that the sensor  210 , the first and second components  252 ,  254 , and the washer  264  are all held in compression between the head  262  and the nut  266 . It is to be appreciated that the material of the first and second compliant annular rings  222 ,  224  should be sufficiently rigid that when a compressive load (such as that applied by a fastener assembly in normal operating conditions) is applied, the material is not excessively deformed such that the gap between the first and second compliant annular rings  222 ,  224  is not pinched or otherwise constricted. 
     Tubing (not shown) to plumb the sensor  210  into a differential pressure monitoring system (also not shown) can be connected to the throughway  228  such that the tubing is in isolated fluid communication with the throughway  228 . 
     The channel  220  and the surface of the first component  252  together form a conduit. When the sensor  210  is applied to a first component  252  that is intact, the conduit can be in fluid communication with the throughway  228 , but in fluid isolation from the atmosphere surrounding the sensor  210 . 
       FIGS. 10 and 11  show cracks C in each of the first and second components  252 ,  254 , which extend from the holes  258 ,  260 , respectively. The crack C in the first component  252  opens onto the surface adjacent the sensor  210 , and intersects the channel  220 . 
     In use, fluid (such as air) within the conduit can be evacuated or pressurized to establish pressure differential between the conduit and ambient pressure. A gap, which exists between the first and second components  252 ,  254  and the shank  256  of the bolt  255 , can be arranged to be at a differential pressure to the conduit, such as, for example, ambient atmospheric pressure. When the crack C is of sufficient size a fluid flow path is formed through the crack C, and between the gap and the channel  220 . Where a pressure differential exists between two regions of the crack, fluid may flow through the crack C. Accordingly, a change in fluid flow through the conduit (which may be observed as a change in pressure state of the conduit) can be indicative of the presence of the crack C. 
       FIGS. 12 to 14  show a sensor  310  in accordance with a fourth embodiment. The sensor  310  has a body portion  312  that defines a hole  314  through which a fastener element (such as the shank of a bolt) can pass. The body portion  312  has the form of a generally annular ring. The body portion  312  also has a first surface  316  and a second opposing surface  318 . 
     The body portion  312  has a cavity, which in this embodiment is in the form of a channel  320  that extends partially through the thickness of the body portion  312  and opens onto the first surface  316 . In this embodiment, the channel  320  extends concentrically about the hole  314 . Furthermore, in this embodiment, the channel  320  is in the form of an annular ring. 
     The sensor  310  is further provided with a sealing system, which in this embodiment is in the form of a flowable sealant  322  that is applied to the first surface  316 , such that the sealant  322  surrounds the channel  320 . 
     The body portion  312  is further provided with a throughway  328  that extends through the body portion  312 , and between an opening  330  on a circumferential edge of the body portion  312  and the channel  320 . Accordingly, the channel  320  is in fluid communication with the throughway  328 . 
       FIGS. 15 and 16  show a fastener assembly  350  that fastens a first component  352  to a second component  354 . The fastener assembly  350  has bolt  355  with a head  362 , and a shank  356  that extends through holes  358 ,  360  in each of the first and second components  352 ,  354  respectively. Sensor  310  is disposed between the head  362  and the first component  352 . The fastener assembly  350  further has a washer element  364  and a nut  366 . The shank  356  is provided with an external thread (not shown) and the nut  366  is provided with a complementary internal thread. Both the washer  364  and the nut  366  are disposed about the shank  356  with the washer  364  in contact with the second component  354 . 
     A tensile load is established in the shank  356  such that the sensor  310 , the first and second components  352 ,  354 , and the washer  364  are all held in compression between the head  362  and the nut  366 . 
     The thickness of the sealant  322  can be selected according to the surface roughness of the first component  352  and the first surface  316  of the body portion  312 . Moreover, the thickness of the sealant  322  can be selected to be equal to the greater surface roughness of the first component  352  and the first surface  316 . Accordingly, when a compressive load is applied to the sensor  310  and the first component  352  and the body portion  312  brought into direct physical contact, the sealant  322  flows into the gaps and cavities that exist between the first component  352  and the body portion  312 . Thus, a substantially hermetic seal can be formed between the first surface  316  of the sensor  310  and the first component  352 . The sealant  322  can be viewed as being activated by compression (that is, a compressive load applied to the sealant) because the sealant  322  flows in response to a compressive load to establish the substantially hermetic seal. 
     This method also in substance provides metal contact between the sensor  310  and the first component  352  to transmit compressive load. Maintaining the metal to metal contact has benefits in terms of maintaining the tension in the fastener over long periods of time. In contrast, sealing systems that have elements which are sandwiched between metal components can in time creep or deform thereby changing the fastener tension and thus adversely affect the strength of a join. 
     Tubing (not shown) to plumb the sensor  310  into a differential pressure monitoring system (also not shown) can be connected to the throughway  328  such that the tubing is in isolated fluid communication with the throughway  328 . 
     The channel  320  and the surface of the first component  352  together form a conduit. When the sensor  310  is applied to a first component  352  that is intact, the conduit can be in fluid communication with the throughway  328 , but in fluid isolation from the atmosphere surrounding the sensor  310 . 
       FIGS. 15 and 16  show cracks C in each of the first and second components  352 ,  354 , which extend from the holes  358 ,  360 , respectively. The crack C in the first component  352  opens onto the surface adjacent the sensor  310 , and intersects the channel  320 . 
     In use, fluid (such as air) within the conduit can be evacuated or pressurized to establish a pressure differential between the conduit and ambient pressure. A gap, which exists between the first and second components  352 ,  354  and the shank  356  of the bolt  355 , can be arranged to be at a differential pressure to the conduit, such as, for example, ambient atmospheric pressure. When the crack C is of sufficient size a fluid flow path is formed through the crack C, and between the gap and the channel  320 . Where a pressure differential exists between two regions of the crack, fluid may flow through the crack C. Accordingly, a change in fluid flow through the conduit (which may be observed as a change in pressure state of the conduit) can be indicative of the presence of the crack C. 
     It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the scope of the invention. 
     Embodiments of the sensor described in reference to the figures have overall shape of a plate or washer. It is to be appreciated that embodiments of the sensor may be incorporated into alternative fastener elements, such as a nut, or the head of a bolt or rivet. Indeed, in one embodiment in which the sensor is provided in the head of a bolt, the bolt head may be counter sunk, such that the first surface is a conical frustum. However, it is to be appreciated that in order to monitor for the presence of surface cracking in the component the channel in the fastener element should be disposed adjacent the surface of the component. 
     Furthermore, embodiments of the sensor described in reference to the figures have a single cavity (i.e., channel) in the body portion. It is also to be appreciated that alternative embodiments may be provided that have two or more spaced apart cavities that open onto the first surface. For example, as shown in  FIG. 17  one embodiment of the sensor  410  is provided a body portion  412  having a first surface  416  onto which open two concentric, annular cavities  420  and  421  that are spaced apart and substantially in fluid isolation from one another. In this embodiment the sealing system comprises three o-rings  424   a ,  424   b  and  424   c . O-rings  424   a  and  424   b  will form a substantially hermetic seal on opposite sides of channel  420 , while o-rings  424   b  and  424   c  will form a substantially hermetic seal on opposite sides of channel  421 . When such an embodiment is provided in a fastener assembly, a surface crack that propagates from the hole (through which a portion of the fastener assembly extends) may first intersect a first (inner) channel  421 . Following further crack growth, the surface crack may intersect a second (outer) channel  420 . Throughways  428   a  and  428   b  provide fluid communication between channels  420  and  421  respectively and corresponding openings or ports  430   a  and  430   b . In this embodiment the channel  421  may be vented to atmosphere via throughway  428   b  while channel  420  is placed in a higher or lower pressure state and coupled to a pressure monitoring system via the throughway  428   a  and port  430   a . Accordingly, the rate of crack growth can be determined. 
     In the embodiments illustrated in the figures, the channels are either circular or C-shaped in their arrangement on the first surface of the body portion. However, it is to be appreciated that the channel (or channels) of the sensor may be of alternative shapes. For example, the channel(s) may have vertices. Furthermore, the channel(s) may have segments of varied radii, or even straight segments. 
     It is to be appreciated that, in some embodiments, relative rotation of the body portion with respect to the seal may cause interference between the seal and the channel(s) in the body portion. Relative rotation may occur when the fastener assembly is being tightened to the desired torque. Such interference may be detrimental to the performance of the sensor or, in the worst case, cause the channels to be completely obstructed. The person skilled in the art will appreciate that the effects of relative rotation between the seal and the body portion may be more detrimental in embodiments in which the channels are not annular and concentric with the likely axis of rotation of the body portion during application of the fastener assembly to a structure. 
     Accordingly, in some embodiments the body portion may be provided with a structure to restrain the body portion from being rotated during assembly and/or removal of the fastener assembly. For example, side portions of the body portion may be provided with flats to receive a tool. 
     It is to be appreciated that in embodiments in which the sealing system is in the form of a sealant that is applied to the first surface of the body portion the relative depth of the channel(s) and thickness of the sealant on application should be carefully selected to minimize the likelihood of the channel(s) being blocked by the sealant. 
     Alternative embodiments may be provided in which the channel (or channels) and/or throughway (or throughways) are formed by affixing of two or more strata to one another. The stratum may be a single material, or two or more different materials. For example, strata of a titanium alloy may be used. Alternatively or additionally, plastics (such as, for example, polyimide) may be used. 
     In one such embodiment, a channel may be formed by an aperture that extends through the thickness of a first stratum that, in use, contacts the component/structure. A second stratum may be provided that extends across, and is affixed to, the first stratum to cover the aperture and form the channel. One or more throughways may be formed in the sensor in the first and/or second strata. Such a structure is described in Applicants&#39; co-pending Australian Provisional Patent Application Number 2006901823, the contents of which are incorporated herein by way of reference. 
     It is to be appreciated that the connection between the body portion and the tubing may be of any desired type, provided that a connection is formed between the throughway and the tubing that plumbs the sensor into the monitoring system, with the two in fluid communication. Furthermore, the connection should also form a substantially hermetic seal. The connection may be established by interference of the tubing with the body portion defining the throughway. Alternatively, the tubing can be affixed to the body portion about the opening of the throughway. In a further alternative, a connector may be provided to which tubing can be attached. Such a connector may be of any suitable shape and/or structure, as will be apparent to the person skilled in the art. 
     In one alternative embodiment exemplified in  FIG. 18  the sensor  510  has a body portion with a first surface that, in use, is located adjacent the outer surface of a component or structure to be monitored. The first surface of the body portion has a pair of spaced apart ridges  524   a  and  524   b  that define a cavity  520  (such as, for example, an elongate channel) therebetween. The cavity  520  extends about a central axis of the sensor  510 . For example, the body portion  512  may have a hole  514  through which the shank of a fastener element can be inserted. 
     Each of the ridges is raised relative to the surrounding area first surface of the body portion  512 . The ridges  524   a  and  524   b  may form a knife-edge like protrusion. In use, the ridges are brought into direct physical contact with the outer surface of a component or structure to be monitored. When a compressive load is applied to the body portion the ridges are at least partially elastically deformed. As the contact surface area between the sensor and the outer surface of the component is small, the contact pressure is high. Accordingly, a substantially hermetic seal is established in response to the compressive load applied to the body portion, and thus the ridges function as an alternate form of sealing system. In this embodiment the cavity  520  is formed by and between the ridges and thus by the sealing system. The cavity  520  is not recessed into the first surface  516 . However, in a variation of this embodiment the cavity can include a channel formed in the first surface  516  between the ridges  524   a ,  524   b.    
     When the sensor is located on the surface of a component, the channel, the ridges and the outer surface of the component together form a conduit. The sensor further has a throughway  528  that extends through the body portion  512 , and between an opening or port  530  on the body portion and the cavity  520 . Thus, the conduit is in fluid communication with the throughway, but in fluid isolation from the atmosphere surrounding the sensor. 
     Tubing, such as flexible piping (not shown), can be connected to the throughway at the opening on the body portion. The tubing plumbs the sensor into, for example, a differential pressure monitoring system. 
     In the claims of this application and in the description of the invention, except where the context requires otherwise due to express language or necessary implication, the words “comprise” or variations such as “comprises” or “comprising” are used in an inclusive sense, i.e., to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.