Patent Publication Number: US-2009218040-A1

Title: Method of Manufacturing a Sensor for Detecting Surface Cracks in a Structure

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of manufacturing a sensor for detecting surface cracks in a structure, and a sensor that is manufactured in accordance with the method. 
     BACKGROUND OF THE INVENTION 
     It is known to use differential pressure monitoring techniques (also known as “comparative pressure monitoring”) to monitor for the presence of a surface flaw, such as a crack, in a structure or component. Furthermore, it is known to use a sensor pad, which engages the surface of the structure or component to be monitored, together with a monitoring apparatus to establish differential pressure in regions adjacent the surface of the structure or component. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method of manufacturing a sensor for use in a differential pressure monitoring system, the method comprising the steps of:
         forming a body portion of the sensor by delivering molten material to a mould, the body portion having a first surface that, in use, is affixed to the surface of a component to be monitored; and   forming one or more channels in the body portion, the channels opening onto the first surface.       

     The channels can be formed concurrently with forming the body portion. 
     The method may additionally comprise the step of forming one or more connectors that each define a throughway, and bringing each throughway into fluid communication with one of the channels. 
     The body portion and the connectors can be formed concurrently, such that the body portion and connectors are contiguous. 
     In one embodiment, the method further comprises delivering an adhesive to the first surface of the body portion, the adhesive, adapted to affix the first surface to the surface of the component. 
     The method may further comprise providing an adhesive layer comprising a substrate having opposed first and second substrate surface with adhesive applied to both of said first and second substrate surfaces, and affixing the first substrate surface to the first surface with said adhesive. 
     The method may further comprise forming one or more apertures in the adhesive layer, each of the apertures registering with a respective one of the channels in the body portion. 
     The aperture may further comprise forming the or each aperture comprises forming the or each aperture of a configuration so that when in registration with a corresponding channel a footprint of channel lies wholly within a footprint of its corresponding aperture. 
     One embodiment of the method further comprises providing a release liner, and applying the release liner to the adhesive such that the adhesive is covered prior to affixing the sensor to the component. 
     The release liner may be provided with one or more apertures. 
     In one embodiment, the apertures in the release liner are formed concurrently with the forming of the apertures in the adhesive layer. 
     The release liner may be applied to the adhesive prior to forming the apertures in the release liner. 
     According to a second aspect of the present invention there is provided a sensor that is manufactured in accordance with the method of the first aspect. 
    
    
     
       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 drawings, 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 exploded view of the sensor of  FIG. 1 ; 
         FIG. 3  is a side cross sectional view of the sensor of  FIG. 1 , as seen along the line A-A of  FIG. 1 ; 
         FIG. 4  is an enlarged bottom view of the sensor of  FIG. 1 ; 
         FIG. 5  is an axonometric view of a sensor in accordance with a second embodiment of the present invention; 
         FIG. 6  is a side cross sectional view of the sensor of  FIG. 5 , as seen along the line B-B of  FIG. 5 ; 
         FIG. 7  is a bottom view of the sensor of  FIG. 5 ; 
         FIG. 8  is an axonometric 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 , as seen along the line C-C of  FIG. 8 ; 
         FIG. 10  is an axonometric view of a sensor in accordance with a fourth embodiment of the present invention; 
         FIG. 11  is a side cross sectional view of the sensor of  FIG. 10 , as seen along the line D-D of  FIG. 10 ; and 
         FIG. 12  is a flow chart of a method in accordance with a fifth embodiment of the present invention, the method being for manufacturing a sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 to 4  show a sensor  10 , in accordance with a first embodiment, for use in a differential pressure monitoring system (not shown). The sensor  10  has a body portion  12  that has a first surface  14 , which, in use, is affixed to the surface of a component (not shown) that is to be monitored. In this embodiment, the body portion  12  is generally elongate. 
     Throughout this specification including the claims, except where the context requires otherwise due to express language or necessary implication the word “affixed” or variations such as “affix” or “affixing” are used to indicate fixing or attaching in a manner the creates or forms a substantially hermetic seal. 
     As shown in  FIG. 3 , a channel  16  is formed within the body portion  12 . The channel  16  is open to the first surface  14  such that, when the sensor  10  is affixed to a component, the channel  16  faces the surface of the component. The width of the channel  16  can be 5 mm or less. In some embodiments, the width of the channel  16  can be 0.5 mm or less. 
     The sensor  10  further has a connector  18  that extends from the body portion  12 . The connector  18  defines a throughway (i.e. a passage)  20  that extends between an opening  22  (which is remote from the channel  16 ) and one end of the channel  16 . In this embodiment, an end portion of the throughway  20  is conical and widens towards the opening  22 . Accordingly, tubing (not shown) that is used to plumb the sensor  10  into a differential pressure monitoring system can be connected to the sensor by inserting a free end of the tubing into the throughway  20  to establish an interference fit. 
     In this embodiment, the body portion  12  and connector  18  are contiguous. 
     An adhesive layer  24  is affixed to the first surface  14  of the sensor  10 . In this embodiment, the adhesive layer  24  is in the form of a substrate that has pressure sensitive adhesive (PSA) on two opposing surfaces, a first of which is affixed to the first surface  14  of the body portion  12 . Hence, when this embodiment is affixed to the surface of a component, the second surface of the adhesive layer  24  is affixed to, and in contact with, the surface of the component. 
     The adhesive layer  24  has a peripheral shape that corresponds with the peripheral shape of the first surface of the body portion  12 . In addition, the adhesive layer  24  has an aperture  26  that registers with the channel  16  in the body portion  12 . The aperture  26  has the same overall shape as the channel  16 . In the embodiment shown in  FIGS. 1 to 4 , the aperture  26  is oversize with respect to the channel  16 , in that the aperture  26  is larger in the width and length dimensions when compared to the channel  16 . 
     A release liner  28  is provided to cover the PSA on the second surface of the adhesive layer  24  prior to affixing to the surface of a component. If desired, the release liner  28  may have an aperture  30  that registers with the aperture  26  in the adhesive layer  24 . 
     In use, the sensor  10  is applied to the surface of a component. The pressure sensitive adhesive on the second surface of the adhesive layer  24  affixes the sensor  10  to the surface and forms a seal between the body portion  12  and the surface. The channel  16  and the surface of the component together form a conduit that can be substantially in fluid isolation with respect to atmospheric air. The sensor  10  may be plumbed via the connector  18  to, for example, the instrumentation of a vacuum monitoring system. 
     A pressure differential can be created in the conduit. A crack in the component that opens onto the surface and intersects the channel  16  will allow fluid to flow through the crack and into the first channel  16 . Where a pressure differential exists between two regions of the crack, such a fluid flow will occur. Accordingly, a change in fluid flow (and/or a change in pressure state of the channel  16 ) can be indicative of the presence of a crack. 
     The pressure differential maybe relative negative or relative positive differential. That is the pressure in the conduit may be less than ambient pressure (i.e. relative negative) or higher than ambient pressure (i.e. relative positive). 
     A crack may extend from a region beyond one of the peripheral edges of the sensor  10  and intersect the channel  16 . In an embodiment in which there is a pressure differential between the atmosphere surrounding the sensor  10  and the conduit, fluid flow through the crack may occur. 
     Clearly, the distance between the channel  16  and the peripheral edges of the body portion  12  is a factor that influences the minimum crack length that can be detected by the sensor  10 . 
     The body portion  12  and connector  18  can be made of plastics materials such as thermosets, thermoplastics or elastomers. The body portion  12  and connector  18  can be formed simultaneously by delivering a raw material in a molten state into a female mould having the form the body portion  12  and connector  18 . The molten material is then allowed to cool and solidify to form the body portion  12  and connector  18  of the sensor  10 . For example, injection moulding may be used. 
     The adhesive layer  24  may be formed by cutting the peripheral shape of the adhesive layer  24  from a larger sheet of adhesive layer material. Simultaneously or subsequently, the aperture  26  can be created by cutting or otherwise removing material from the adhesive layer  24  to form the aperture  26 . 
     In some embodiments of the sensor, the width of the aperture  26  is to be approximately equal to the width of the channel  16 . Accordingly, the width of the aperture  26  may be 0.5 mm or less. It is to be appreciated that for best performance of the sensor  10 , the channel  16  should not be obstructed by the adhesive layer  24 . Accordingly, in such embodiments of the sensor, a high degree of accuracy in forming the aperture  26  is desirable. 
     The adhesive layer  24  can then be affixed to the first surface of the body portion  12 . 
     The release liner  28  may also be formed by cutting the peripheral shape of the release liner  28  from a larger sheet of release liner material. The aperture  28  can be created simultaneously or subsequently by cutting or otherwise removing material from the release liner  28  to form the aperture  28 . The adhesive layer  24  and release liner  28  can be provided together in a larger sheet such that the peripheral shape and respective peripheral shapes are formed together. Alternatively, the release liner  28  can be applied to the surface of the adhesive layer  24  after the release liner  28  has been formed. 
     In some embodiments, it may be desired to provide the adhesive layer material with a release liner material covering the PSA on both surfaces of the adhesive layer material. In such an embodiment, it may also be convenient to cut or otherwise form the adhesive layer  24  and with two like release liners  28 , a first of which is removed to affix the adhesive layer  24  to the body portion  12 , and a second of which may be removed immediately prior to affixing the sensor  10  to the surface of a component. Alternative sensor shapes and/or structures may be manufactured as described above in connection with the sensor  10 . Three such alternative embodiments of the sensor are described below in reference to  FIG. 5 to 11 . 
       FIGS. 5 to 7  show a sensor  110 , in accordance with a second embodiment, for use in a differential pressure monitoring system (not shown). The sensor  10  has a body portion  112  that has a first surface  114 , which, in use, is affixed to the surface of a component (not shown) that is to be monitored. In this embodiment, the body portion  112  is generally elongate. 
     As shown in  FIG. 7 , two channels  116   a,    116   b  (hereinafter referred to collectively as “channels  116 ”) are formed within the body portion  112 . The channels  116  open onto the first surface  114  such that, when the sensor  110  is affixed to a component, the channels  116  face the surface of the component. 
     The sensor  110  further has four connectors  118  that extend from the body portion  112 . The connectors  118  each define a throughway  120  that extends between an opening  122  (which is remote from the first surface  114 ) and one end of a respective one of the channels  116 . In this embodiment, an end portion of each throughway  120  is conical and widens towards the opening  122 . Accordingly, tubing (not shown) that is used to plumb the sensor  110  into a differential pressure monitoring system can be connected to the sensor by inserting a free end of the tubing into the throughway  120  to establish an interference fit. 
     The quality of the interference fit between the tubing and the connector  118  can influence the reliability of the sensor  110 . Factors that influence the quality of the interference fit include the opening angle of the conical end portion of the throughway  128 , the relative dimensions of the tubing and the end portion of the throughway  128 , the material properties (such as relative stiffness) of the connector  118  and the tubing, and the tubing and the presence of surface imperfections in the throughway  128  and tubing. 
     An adhesive layer  124  is affixed to the first surface  114  of the sensor  110 . In this embodiment, the adhesive layer  124  is in the form of a substrate that has pressure sensitive adhesive (PSA) on two opposing surfaces, a first of which is affixed to the first surface  114  of the body portion  112 . Hence, when this embodiment is affixed to the surface of a component, the second surface of the adhesive layer  124  is affixed to, and in contact with, the surface of the component. 
     The adhesive layer  124  has a peripheral shape that corresponds with the peripheral shape of the first surface of the body portion  112 . In addition, the adhesive layer  124  has two apertures  126  that each register with one of the channels  116  in the body portion  112 . Each aperture  126  has the same overall shape as the respective channel  116 . 
     A release liner (not shown) may be provided to cover the PSA on the second surface of the adhesive layer  124  prior to attachment to the surface of a component. If desired, the release liner may also have an apertures that register with the apertures  126  in the adhesive layer  124 . 
     In use, the sensor  110  is applied to the surface of a component. The pressure sensitive adhesive on the second surface of the adhesive layer  124  affixes the sensor  110  to the surface and forms a seal between the body portion  112  and the surface. Each of the channels  116   a,    116   b  and the surface of the component together form respective conduits which can be insubstantial fluid isolation with respect to atmospheric air. Accordingly, in this embodiment there are two such conduits. The sensor  110  may be plumbed via the connectors  118  to, for example, the instrumentation of a vacuum monitoring system. 
     A pressure differential can be created in one or both of the channels  116 . A crack in the component that opens onto the surface and intersects one or both of the channels  116  will allow fluid flow between the crack and the respective channels  116 . Where a pressure differential exists between two regions of the crack, such a fluid flow will occur. Accordingly, a change in fluid flow (and/or a change in pressure state of the respective channels  116 ) can be indicative of the presence of a crack. 
     A crack may extend from a region beyond one of the peripheral edges of the sensor  110  and intersect one or both of the channels  116 . In an embodiment in which there is a pressure differential between the atmosphere surrounding the sensor  110  and the conduits, fluid flow through the crack may occur. 
     Alternatively or additionally, a crack may intersect the channels  116 . In an embodiment in which there is a pressure differential between the conduits, fluid flow through the crack may occur. 
     Each of the conduits formed by the channels  116  and the surface of the component is continuous between its two respective connectors  118 . Thus, it is possible to test for a blockage in the conduits. A blockage indicates that continuity does not exist through the conduit, and that portions of the sensor  110  are inactive. Clearly, a crack that intercepts an inactive portion of the conduit will not be detected. For example, a continuity test of a conduit may be achieved by introducing fluid into a first of the connectors  118  and monitoring the steady state flow of fluid exhausted via the corresponding other connector  118 . 
       FIGS. 8 and 9  show a sensor  210 , in accordance with a third embodiment, for use in a differential pressure monitoring system (not shown). The sensor  210  has a body portion  212  that has a first surface  214 , which, in use, is affixed to the surface of a component (not shown) that is to be monitored. In this embodiment, the body portion  212  is generally elongate. 
     The sensor  210  is provided with two channels  216  that are formed within the body portion  212 . The channels  216  open onto the first surface  214  such that, when the sensor  210  is affixed to a component, the channels  216  face the surface of the component. 
     The sensor  210  further has two connectors  218  that are contiguous with the body portion  212 . The connectors  218  each define a pair of throughways  220  that each extends between an opening  222  (which is remote from the first surface  214 ) and one end of a respective one of the channels  216 . In this embodiment, an end portion of each throughway  220  is conical and widens towards the opening  222 . Accordingly, tubing (not shown) that is used to plumb the sensor  210  into a differential pressure monitoring system can be connected to the sensor  210  by inserting a free end of the tubing into the throughway  220  to establish an interference fit. 
     The connectors portions  218  allow the tubing to extend from the sensor  210  at an acute angle to the first surface  214 . Thus, the “take-off” angle of the tubing is also at an acute angle to the surface of the component to be monitored. 
     An adhesive layer  224  is affixed to the first surface  214  of the sensor  210 . In this embodiment, the adhesive layer  224  is in the form of a substrate that has pressure sensitive adhesive (PSA) on two opposing surfaces, a first of which is affixed to the first surface  214  of the body portion  212 . Hence, when this embodiment is affixed to the surface of a component, the second surface of the adhesive layer  224  is affixed to, and in contact with, the surface of the component. 
     The adhesive layer  224  has a peripheral shape that corresponds with the peripheral shape of the first surface  214  of the body portion  212 . In addition, the adhesive layer  224  has two apertures that each register with one of the channels  216  in the body portion  212 . Each aperture has the same overall shape as the respective channel  216 . 
     A release liner (not shown) may be provided to cover the PSA on the second surface of the adhesive layer  224  prior to attachment to the surface of a component. If desired, the release liner may also have an apertures that register with the apertures  226  in the adhesive layer  224 . 
       FIGS. 10 and 11  show a sensor  310 , in accordance with a third embodiment, for use in a differential pressure monitoring system (not shown). The sensor  310  has a body portion  312  that has a first surface  314 , which, in use, is affixed to the surface of a component (not shown) that is to be monitored. In this embodiment, the body portion  312  is generally elongate. 
     The sensor  310  is provided with two channels  316  that are formed within the body portion  312 . The channels  316  open onto the first surface  314  such that, when the sensor  310  is affixed to a component, the channels  316  face the surface of the component. 
     The sensor  310  further has two connectors  318  that are contiguous with the body portion  312 . The connectors  318  each define a pair of throughways  320  that each extends between an opening  322  (which is remote from the first surface  314 ) and one end of a respective one of the channels  316 . In this embodiment, an end portion of each throughway  320  is conical and widens towards the opening  322 . Accordingly, tubing (not shown) that is used to plumb the sensor  310  into a differential pressure monitoring system can be connected to the sensor  310  by inserting a free end of the tubing into the throughway  320  to establish an interference fit. 
     The connectors portions  318  allow the tubing to extend from the sensor  310  in a direction that is generally parallel with the first surface  314 . Thus, the “take-off” angle of the tubing is also generally parallel to the surface of the component to be monitored. 
     An adhesive layer  324  is affixed to the first surface  314  of the sensor  310 . In this embodiment, the adhesive layer  324  is in the form of a substrate that has pressure sensitive adhesive (PSA) on two opposing surfaces, a first of which is affixed to the first surface  314  of the body portion  312 . Hence, when this embodiment is affixed to the surface of a component, the second surface of the adhesive layer  324  is affixed to, and in contact with, the surface of the component. 
     The adhesive layer  324  has a peripheral shape that corresponds with the peripheral shape of the first surface  314  of the body portion  312 . In addition, the adhesive layer  324  has two apertures that each register with one of the channels  316  in the body portion  312 . Each aperture has the same overall shape as the respective channel  316 . 
     A release liner (not shown) may be provided to cover the PSA on the second surface of the adhesive layer  324  prior to attachment to the surface of a component. If desired, the release liner may also have an apertures that register with the apertures  326  in the adhesive layer  324 . 
     It is to be appreciated that the volume of the conduit(s) formed by the channel(s) will influence the sensitivity of sensor. Accordingly, the dimensions of the channel(s) may require matching to the desired sensitivity of the sensor and measurement system. In some embodiments the channel(s) may have a width of 5 mm or less. In some embodiments, the width of the channel(s) can be 1 mm or less. 
       FIG. 12  shows a flow chart  410  in accordance with a fifth embodiment of the invention. The flow chart  410  illustrates a method for manufacturing a sensor for use in a differential pressure monitoring system. The sensor may be, for example, the sensors illustrated in  FIGS. 1 to 11 . 
     The method includes the step  412  of forming a body portion of the sensor by delivering molten material to a mould. The body portion has a first surface that, in use, is affixed to the surface of a component to be monitored. 
     The method also includes the step  414  of forming one or more channels in the body portion. The channels open onto the first surface. Accordingly, when the sensor is affixed to the surface of a component, channels open onto the surface of the component. 
     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. 
     In the embodiments described in reference to  FIGS. 1 to 11 , the pressure sensitive adhesive is provided on a substrate such that the adhesive is transferred to the body portion by the substrate. Alternatively, pressure sensitive adhesive may be applied to the first surface of the body portion. This may be achieved by spraying the PSA directly onto the first surface. It will be appreciated that spraying adhesive onto the first surface may result in adhesive being directed into the channel(s). In some embodiments this may be problematic as the PSA may cause blockages in the channel(s). A mask may be provided to minimise the amount of PSA directed into the channel(s). 
     In a further alternative, the pressure sensitive adhesive may be dispersed within a formulation containing a curable adhesive, such as structural adhesive, which is in the form of a thin stratum. The stratum is applied to the first surface of the body portion. The PSA within the curable adhesive allows the sensor to be removed and repositioned prior to the curable adhesive being cured. 
     It is to be appreciated that the connector(s) and the throughway(s) may be of any desired shape and structure, provided that the connectors fulfill the function of bringing the channel(s) of the sensor in fluid communication with the tubing that plumbs the sensor into the monitoring system. Furthermore, the connection(s) should also form a substantial hermetic seal. 
     In one alternative embodiment, the connector(s) may be in the form of a rigid tube that registers with a respective throughway. The rigid tube(s) can be inserted into the mould cavity prior to delivery of the body portion/connector material. Accordingly, during the moulding step the rigid tube is affixed within the throughway. In a further alternative embodiment, the throughway(s) may be provided with an internal thread that engages a complementary thread on a secondary connector. The internal thread may be formed during the moulding step, or alternatively subsequent to the body portion 
     The channel(s) in the body portion may be formed during the step of moulding the body portion by providing female channel elements within the die. Alternatively, the channels may be formed subsequent to the forming of the body portion. The channel(s) may be formed by removing and/or cutting or otherwise ablating material from the body portion following the moulding step. 
     As described previously, in embodiments of the sensor in which the PSA is transferred to the first surface of the body portion, the apertures in the adhesive layer and/or release liner may be formed by cutting material from the adhesive layer and/or release liner. Alternatively, the apertures may be formed by ablating material from the adhesive layer and/or release liner using, for example, laser ablation. This may be of advantage for achieving appropriate dimensional tolerancing in narrow apertures. Alternatively, an adhesive layer and/or release liner, without apertures formed therein, may be applied to the first surface of the body portion. Subsequently, the apertures in the adhesive layer and/or release liner may be formed by ablating material. In some embodiments, the channel(s) in the body portion may be formed simultaneously with the forming of the apertures in the adhesive layer and/or release liner by ablating material from the adhesive layer and/or release liner, and the body portion. 
     In one alternative embodiment, the body portion may be formed using transfer moulding, in which the female mould is at least partially heated as the molten material is delivered into the mould. 
     It is to be appreciated that the connector(s) of the sensor may be formed separately of the body portion and subsequently joined to the body portion. 
     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.