A coaxially arranged smart susceptor conductor, comprising a smart susceptor core comprising an alloy having a first Curie temperature point and a first smart susceptor shell coaxially arranged around the smart susceptor core. The first smart susceptor shell comprising a second Curie temperature point that is different than the first Curie temperature point of the smart susceptor core. In one arrangement, the second Curie temperature point of the first smart susceptor shell is lower than the first Curie temperature point of the smart susceptor core. In another arrangement, the smart susceptor conductor further comprises a second smart susceptor shell disposed about the first smart susceptor shell. The second smart susceptor shell comprising a third Curie temperature point.

FIELD

The present disclosure relates generally to smart susceptors for use with heating blankets. More particularly, the present disclosure relates to coaxial smart susceptors for use with heating blankets and method for heating a structure to a substantially uniform temperature across the structure.

BACKGROUND

The reworking of composite structures frequently requires the localized application of heat. When installing a patch in a rework area of a composite structure, heat must typically be applied to the adhesive at the bondline between the patch and rework area in order to fully cure the adhesive. When applying heat to the patch, the temperature of the bondline must typically be maintained within a temperature range that must be held for an extended period of time until the adhesive is cured. Overheating or under heating the rework area or structure located adjacent to the rework area is generally undesirable during the rework process.

Conventional heating equipment for heating composite structures may include heating blankets comprised of electrically resistive heating elements. Variations in the construction of conventional heating blankets may result in differential heating across the rework area. In addition, conventional heating blankets may lack the ability to compensate for heat sinks located adjacent to the rework area. Such heat sinks may comprise various elements such as stiffeners, stringers, ribs, bulkheads and other structural members in thermal contact with the structure. Attempts to provide uniform heat distribution using conventional resistive heating blankets include multi-zone blanket systems, feedback loop systems, positive temperature coefficient heating elements, and temperature stabilizing plugs. Additions of such systems to conventional resistive heating blankets are generally ineffective in providing a substantially uniform temperature without substantial variation across the bondline of the rework area.

As can be seen, there exists a need for a system and method for heating a structure such as a rework area of a composite structure in a manner which maintains a substantially uniform temperature across the rework area. More specifically, there exists a need for a system and method for uniformly heating a composite structure and which accommodates heat drawn from the rework area by heat sinks and other thermal variations located adjacent to the rework area. Furthermore, there exists a need for a system and method for uniformly heating a composite structure in a manner which prevents overheating or under heating of the composite structure. Ideally, such system and method for uniformly heating the composite structure is low in cost and simple in construction. There is also a need for a system that provides for temperature regulation over a broad range of temperatures typically required for composite processing, for example, from about 70° F. to about 350° F.

SUMMARY

According to an exemplary arrangement, a coaxially arranged smart susceptor conductor is disclosed. In one arrangement, the coaxially arranged smart susceptor comprises a smart susceptor core comprising an alloy having a first Curie temperature point and a first smart susceptor shell coaxially arranged around the smart susceptor core. The first smart susceptor shell comprising a second Curie temperature point that is different than the first Curie temperature point of the smart susceptor core.

In one arrangement, the second Curie temperature point of the first smart susceptor shell is lower than the first Curie temperature point of the smart susceptor core.

In another arrangement, the smart susceptor conductor further comprises a second smart susceptor shell disposed about the first smart susceptor shell. The second smart susceptor shell comprising a third Curie temperature point.

In one arrangement, a method for heating a structure using induction heating is disclosed. The method comprising the steps of positioning a coaxial susceptor near a structure; positioning a first conductor near the coaxial susceptor; applying an alternating current to the first conductor; generating a magnetic field in response to the alternating current applied to the first conductor; generating eddy currents that travel circumferentially in the coaxial susceptor in response to the magnetic field generated by the first conductor; and heating the coaxial susceptor as a result of the generated eddy currents so as to heat the structure to a uniform temperature. The method further comprising the step of arranging the coaxial susceptor within the conductor. The method further comprising the step of arranging the coaxial susceptor within alternating conductors of the conductor. The coaxial susceptor may be arranged such that a longitudinal axis of the coaxial susceptor resides substantially perpendicular to an alternating current flowing through the conductor. The method may include the further step of positioning a second conductor near the coaxial susceptor; applying an alternating current to the second conductor; generating a magnetic field in response to the alternating current applied to the second conductor; generating eddy currents in the coaxial susceptor in response to the magnetic field generated by the second conductor; and heating the coaxial susceptor as a result of the generated eddy currents so as to heat the structure to a uniform temperature.

DETAILED DESCRIPTION

Disclosed embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all of the disclosed embodiments are shown. Indeed, several different embodiments may be provided and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the disclosure to those skilled in the art.

Referring now to the drawings wherein the showings are for purposes of illustrating preferred and various embodiments of the disclosure only and not for purposes of limiting the same, shown inFIG. 1is a perspective illustration of a composite structure10upon which a rework process may be implemented using a heating blanket54illustrated inFIGS. 2-7. The heating blanket54illustrated inFIGS. 2-7and as disclosed herein may be installed on a patch40which may be received within a rework area20as illustrated inFIG. 1. The heating blanket54as disclosed herein may apply heat to the rework area20in order to elevate the temperature of the rework area20to a uniform temperature throughout the rework area20in order to cure adhesive bonding the patch40to the rework area20and/or to cure the composite material forming the patch40. In various embodiments, the heating blanket54as disclosed herein incorporates a combination of a plurality of coaxial smart susceptors comprising magnetic materials and high frequency alternating current in order to attain temperature uniformity to a structure10to which the heating blanket54is applied. In one preferred arrangement, and as will be described in greater detail below, the plurality of coaxial smart susceptors are positioned within a conductor comprising a Litz wire that is wound in a flattened helix (i.e., a solenoid structure). In another preferred arrangement, and as will be described in greater detail below, the plurality of coaxial smart susceptors comprise spring formed coaxial smart susceptors that are positioned around a conductor, such as a Litz wire. Alternative coaxial smart susceptor configurations are also disclosed.

Advantageously, and as will be discussed in greater detail herein, the temperature-dependent magnetic properties such as the Curie temperature of the magnetic materials used in an array of coaxial susceptor wires contained within the heating blanket54may prevent overheating or under heating of areas to which the heating blanket54may be applied. In addition, the coaxial smart susceptors comprises a core of a first magnetic material and at least one shell provided around this inner core. The at least one shell comprises a magnetic material that has a different Curie temperature than the first magnetic material of the inner core. In this manner, the coaxial smart susceptors of the heating blanket54facilitates the uniform application of heat to structures such as composite structures10(FIG. 1) during a manufacturing or rework process or any other process where uniform application of heat is required aver an enhanced temperature ranges. Importantly, the heating blanket54comprising an array of coaxial susceptor wires wherein the coaxial susceptor wire comprises two or more magnetic materials comprising two or more different Curie temperatures provide for a greater temperature regulation over a wider range of temperatures (e.g., from about 70° F. to about 350° F.).

In addition, the heating blanket54compensates for heat sinks28(FIG. 1) that may draw heat away from portions of a structure10(FIG. 1) to which the heating blanket54is applied. More specifically, the heating blanket54continues to provide heat to portions of the structure10located near such heat sinks28while areas underneath the heating blanket54that have reached or attained the Curie temperature cease to provide heat to the rework area20.

For example,FIG. 1illustrates a composite structure10which may include a skin12formed of plies14of composite material and wherein the skin12may have upper and lower surfaces16,18. The composite structure10may include a rework area20in the skin12formed by the removal of composite material. As can be seen inFIG. 2, the rework area20may be formed in the upper surface16and may extend at least partially through a thickness of the skin12although the rework area20may be formed in any configuration through the skin12. Various structures may be mounted to the lower surface18opposite the rework area20such as stringers30which may act as heat sinks28drawing heat away from certain portions of the rework area20while the remaining portions continually receive heat from the heating blanket54(FIG. 2). Advantageously, the heating blanket54(FIG. 2) facilitates the uniform application of heat to the structure10by reducing heat input to portions of the rework area20that reach approximately the Curie temperature of the magnetic materials in the heating blanket54while maintaining a relatively higher level of heat input to portions of the rework area20that are below the Curie temperature as will be described in greater detail below.

Referring still toFIGS. 2-3, the heating blanket54is illustrated as being mounted to the composite structure10over the patch40. A vacuum bag assembly100may be installed over the heating blanket54. The vacuum bag assembly100may include a bagging film116covering the heating blanket54and which may be sealed to the upper surface16of the composite structure10by means of sealant122. A vacuum probe118and vacuum gauge120may extend from the bagging film116to a vacuum generator to provide a mechanism for drawing a vacuum on the bagging film116for application of pressure and to draw out volatiles and other gasses that may be generated as a result of heating uncured composite material of the patch40.

As can be seen inFIG. 3, the vacuum bag assembly100may include a caul plate102positioned above a porous or non-porous parting film110,108. The caul plate102may facilitate the application of uniform pressure to the patch40. The porous or non-porous parting film110,108may prevent contact between the caul plate102and the patch40. The vacuum bag assembly100may include additional layers such as a bleeder layer112and/or a breather layer114. The patch40may be received within the rework area20such that a scarf44formed on the patch edge42substantially matches a scarf24formed at the boundary22of the rework area20. In this regard, the interface between the patch40and rework area20comprises the bondline46wherein adhesive is installed for permanently bonding the patch40to the rework area20and includes adhesive located at the bottom center26portion of the rework area20.

As shown inFIG. 2, thermal sensors70such as thermocouples72may be strategically located on upper and lower surfaces16,18of the composite structure10such as adjacent to the rework area20in order to monitor the temperature such areas during the application of heat using the heating blanket54. In this regard, thermocouples72may be placed on heat sinks28such as the stringer30body and stringer flanges32illustrated inFIG. 3in order to monitor the temperature of such heat sinks28relative to other areas of the composite structure10.

FIG. 4is a perspective illustration of a heating blanket54in an embodiment as may be used for heating the rework area of the composite structure. The heating blanket54comprising a flattened helical wire conductor80and an array of coaxial susceptor wires82. Preferably, the array of coaxial susceptor wires82are arranged within alternating conductors of the helical wire conductor80of the heating blanket. More preferably, the array of coaxial susceptor wires are arranged perpendicular to the plurality of conductor portions making up the helical wire conductor80. In one preferred arrangement, the flattened helical wire conductor80comprises a Litz wire that is wound in a flattened helical like structure (e.g., a solenoid) so as to define a plurality of alternating conductors. For example,FIG. 5is a schematic illustration of the heating blanket54illustrated inFIG. 4(with the heating blanket housing58and matrix78removed) so as to illustrate the helical wire conductor80connected to a power supply90, a controller92, and a sensor94. As illustrated, the helical wire conductor80comprises a unitary wire that winds back and forth between a first side S1of the heating blanket54and a second side S2of the heating blanket in a flattened helical structure, along a length LHBof the heating blanket54. Importantly, in this illustrated arrangement of the heating blanket54, the coaxial susceptor wires82are positioned between the alternating conductors or wires making up the helical wire conductor80for inductive heating of the array of coaxial susceptor wires82in the presence of an alternating current provided by the power source90. The inductively heated array of coaxial susceptor wires82thermally conducts heat to a matrix78(FIG. 4). The matrix78may thermally conduct heat to a structure10to which the heating blanket54is mounted (See, e.g.,FIGS. 1-3).

Referring toFIGS. 4 and 5, the heating blanket54may include a housing defining an interior60. This interior may be formed of a suitable material which is preferably thermally conductive and which may also be flexible and/or resilient such that the heating blanket54may conform to curved areas to which it may be applied. In this regard, the housing58is preferably formed of a pliable and/or conformable material having a relatively high thermal conductivity and relatively low electrical conductivity. The housing58may comprise upper and lower face sheets62,64formed of silicone, rubber, polyurethane or other suitable elastomeric or flexible material that provides dimensional stability to the housing58while maintaining flexibility for conforming the heating blanket54to curved surfaces. Although shown as having a generally hollow interior60bounded by the upper and lower face sheets62,64, the housing58may comprise an arrangement wherein the conductor80and the associated magnetic material integrated or embedded within the housing58such that the conductor80is encapsulated within the housing58to form a unitary structure50that is preferably flexible for conforming to curved surfaces.

FIG. 5illustrates a perspective view of certain components of the heating blanket54showing the flattened helical structure of the conductor80and the array of coaxial susceptor wires82residing within this helical structure in greater detail. In one preferred arrangement, and as illustrated inFIG. 5, the coaxial susceptor wires82are arranged within the helical conductor80such that a longitudinal axis of the array of coaxial susceptor wires82resides substantially perpendicular to an electrical current flowing through the helical conductor80. In this manner, the varying magnetic fields generated by the helical conductor80induce eddy currents in the array of coaxial susceptor wires82as will be discussed in greater detail herein.

A power supply90providing alternating current electric power may be connected to the heating blanket54by means of the heating blanket wiring56. The power supply90may be configured as a portable or fixed power supply90which may be connected to a conventional 60 Hz, 110 volt or 220 volt outlet. Although the power supply90may be connected to a conventional 60 Hz outlet, the frequency of the alternating current that is provided to the conductor80may preferably range from approximately 1,000 Hz to approximately 400,000 The voltage provided to the conductor80may range from approximately 10 volts to approximately 300 volts but is preferably less than approximately 60 volts. Likewise, the alternating current provided to the conductor80by the power supply is preferably between approximately 10 amps and approximately 1000 amps.

FIG. 6illustrates a cross sectional view of the array of coaxial susceptor wires82that may be used with the heating blanket54illustrated inFIGS. 2-5taken along line5-5ofFIG. 5. As illustrated, the array of coaxial susceptor wires82comprise a plurality of coaxial susceptor wires88that may be loosely bundled together. In one preferred arrangement, at least one of the coaxial susceptor wires88within the bundled array of coaxial susceptor wires82comprise a susceptor core and at least one susceptor shell that surrounds the susceptor core. In such a bundled configuration, the coaxial susceptor wires82are preferably spaced about on the order of 1.2 diameters apart from an adjacent susceptor wire.

For example,FIG. 7illustrates a cross sectional view of one of the coaxial susceptor wires88of the array of coaxial susceptor wires82illustrated inFIG. 6. In one arrangement, the coaxial susceptor wire88comprises a susceptor core84and a susceptor shell86surrounding this core84. Preferably, in one arrangement, the susceptor core84comprises a first Curie temperature alloy124and the susceptor shell86comprises a second Curie temperature alloy128that is different from the first Curie temperature alloy of the core124.

More preferably, the susceptor core84comprises a first Curie temperature alloy124and the susceptor shell86comprises a second Curie temperature alloy128wherein the second Curie temperature of the shell86is a lower temperature than the first Curie temperature alloy of the core84. For example, in one preferred arrangement, the first Curie temperature allay comprises Alloy34having 34% Ni and 66% Fe having a Curie temperature point about 450° F. and comprises a negligible magnetic properties above 400° F. In this same arrangement, the second Curie temperature alloy comprises Alloy32having 32% Ni and 68% Fe having a Curie temperature of about 392° F. and comprises a negligible magnetic properties above 250° F. In such an arrangement, the lower Curie temperature alloy shell will act to shield the inner higher Curie temperature core so that only the shell alloy generates heat at lower temperatures.

Then, at higher temperatures, the permeability of the coaxial susceptor shell86having the tower Curie temperature will decrease to unity. At this lower permeability, the coaxial susceptor shell86becomes substantially transparent to the magnetic field generated by the conductor80. At this point, the alloy of the susceptor core84then generates heat with an enhanced temperature control aver the higher temperatures. As such, the coaxial susceptor82comprising such a core and shell configuration provides an enhanced level of temperature regulation at the lower temperatures.

In one arrangement, more than one susceptor shell may be utilized. For example, a second shell104as illustrated inFIG. 7may be provided to surround the first susceptor shell86. Similarly, the second shell104may comprise a Curie temperature alloy that is different than (i.e., lower than) the Curie temperature alloy of the first shell. Again, this second shell104will therefore act to shield the inner lower Curie temperature shell86and the higher Curie temperature core84so that only the second shell alloy106generates heat at the lowest of desired temperatures. Increasing the number of susceptor layers or susceptor shells provided around or surrounding the susceptor core84is therefore beneficial to obtaining an enhanced temperature regulation over an even wider range of operating temperatures.

The magnetic fields generated by the alternating current flowing through the helical conductor80wound in a Litz wire flattened helix (or solenoid) and inducing eddy currents within the array of coaxial susceptor wires82will now be described with reference toFIG. 6. As those of ordinary skill in the art recognize, a Litz wire is typically used to carry alternating current and may consist of many thin wire strands, individually insulated and twisted or woven together.

As can be seen as an example inFIG. 6, seven (7) coaxial susceptor wires or conductors82are illustrated and these coaxial conductors82reside between two alternating conductors of a helical conductor80, such as the helical conductor80illustrated inFIG. 5. In one preferred helical conductor arrangement, the helical conductor is of unitary construction and comprises a single conductor that is wound from one end of the heating blanket to the other in a continuous, flattened helix shape. As just one example, if the helical conductor comprises a single conductor such as helical conductor80illustrated inFIG. 5, this single conductor80may make ten (10) turns per inch in the helix.

In an alternative helical conductor arrangement, the helical conductor may comprise two or more conductors forming two or more parallel circuits. Utilizing two or more conductors does not materially affect the generated magnetic field as long as each conductor carriers the same amount of current as the single conductor. With such a multiple conductor helical configuration, the controller92and sensor94may be operated to adjust and maintain this type of desired current control. One advantage of such a multiple conductor helical configuration is that it acts to reduce the voltage need to provide current from one end of the blanket to the other end of the blanket. For example, instead of having one conductor making ten (10) turns per inch in the helix, the multiple conductor configuration may have, for example, ten (10) conductors making one (1) turn per inch.

As illustratedFIG. 6, at least one of the coaxial susceptor wires88comprises a susceptor core84and a susceptor shell86as illustrated inFIG. 7. The susceptor shell86is provided over this susceptor core84. In addition, the coaxial susceptor wire88may be positioned an equal distance from both a first, lower conductor portion80A and a second, upper conductor portion80B. The coaxial susceptor wires are preferably electrically insulated from these conductor portions80A,B. Initially, the application of a first alternating current Ii150by way of a power source (FIG. 5) to the first conductor portion80A produces an alternating magnetic field lines96A that comprise concentric circles around the cylindrically current carrying conductor80A. InFIG. 6, these concentric circles96A may be illustrated as comprising a first magnetic field96which is illustrated as directed perpendicularly out of the paper. Similarly, the application of a second alternating current Ii160(flowing in an opposite direction as the first alternation current Ii150) through the second conductor portion80B produces an alternating magnetic field lines96B that comprise concentric circles around the cylindrically current carrying conductor80B.

Because of the orientation of the first and second magnetic fields96A,B, these fields96A,B will essentially cancel each another out on the outside of the blanket54, below the first conductor80A as they reside in opposite directions. Similarly, above the second or upper conductor80B on the outside of the blanket54, the first and second magnetic fields96A,B will also essentially cancel one another out. In contrast, within the heating blanket matrix78and hence within the coaxial susceptors82, the first and second magnetic fields96A,B will be additive to one another since both fields are oriented substantially parallel to the axis of the susceptor wires82. This substantially parallel combined oscillating magnetic field96A,B will therefore generate eddy currents that travel circumferentially within the coaxial susceptors82.

Initially, the concentration of the magnetic fields96A and96B results in relatively large eddy currents that are generated in the coaxial susceptor outer shell86of the coaxial conductors82. The induced eddy currents result in resistive heating of the coaxial susceptor shell86. The susceptor shell86conductively heats the matrix78and the structure10in thermal contact with the heating blanket54. (FIGS. 5-8) The heating of the susceptor shell86continues during application of the alternating current until the magnetic material of the susceptor shell86approaches its Curie temperature, which again in this illustrated arrangement is lower than the Curie temperature of the susceptor core84. Importantly, during this initial heating process, the susceptor shell86acts to shield the higher magnetic Curie point material of the susceptor core84. Such shielding by the susceptor shell86acts to prevent the higher Curie point material of the susceptor core84from dominating heating at the lower temperatures.

Upon approaching the temperature where the magnetic properties of the susceptor shell86become negligible (i.e., when the thickness of the susceptor shell is on the order or less than the electrical skin depth), the coaxial susceptor shell86becomes non-magnetic. At this non-magnetic point, the magnetic fields96A,B generated by the first conductor portion and the second conductor portion80A,B are no longer effective on the susceptor shell86(which now has a mu of approximately 1). The induced eddy currents and associated resistive heating of the susceptor shell86therefore diminishes to a level sufficient to maintain the temperature of the susceptor shell82at the lower Curie temperature. Once the lower Curie temperature of the susceptor shell82is achieved, temperature regulation by way of the susceptor core84with higher Curie temperature commences.

As the susceptor shell86no longer generates heat and as a result of the close proximity of the susceptor core84of the coaxial susceptor wire88to the conductor80, the concentration of the magnetic field96B results in relatively large eddy currents in the coaxial susceptor core84. The induced eddy currents within the susceptor core84result in resistive heating of the coaxial susceptor core84. The susceptor core84therefore conductively heats the matrix78and the structure10in thermal contact with the heating blanket54(FIG. 3). The heating of the susceptor core84continues during application of the alternating current Ii150and Iii160until the magnetic material of the susceptor core84approaches its Curie temperature, which again in this illustrated arrangement comprises a higher Curie temperature than the Curie temperature of the susceptor shell86. Upon reaching the higher Curie temperature of the susceptor core84, the coaxial susceptor core84becomes non-magnetic. At this non-magnetic point, the magnetic fields96A,B are no longer concentrated in the susceptor core84. The induced eddy currents and associated resistive heating of the susceptor core84therefore diminishes to a level sufficient to maintain the temperature of the susceptor core84at the higher Curie temperature.

As an example of the heating of the magnetic material to the Curie temperature,FIG. 8illustrates a plot of heat output130measured over temperature132for an exemplary heating blanket comprising an array of coaxial smart susceptors as disclosed herein. Specifically, the heating blanket may comprise an array of coaxial susceptors mounted within a conductor80wherein the conductor80comprises a Litz wire formed as a flattened helix as illustrated inFIG. 5. Specifically, to generate the data presented in this graph, the array of coaxial susceptors comprise a quantity of forty (40) 20 mil diameter/inch and were inductively heated by way of a100e,300 KHz magnetic field. The coaxial susceptors comprised a susceptor core comprising a 13 mil diameter alloy34(34% Ni and 66% Fe) core and a 3.5 mil thick alloy32(32% Ni and 68% Fe) shell. As those of ordinary skill in the art will recognize, alternative core and shell configurations may also be utilized. As can be seen inFIG. 8, this coaxial susceptor arrangement provided an extended useful temperature range for such a coaxial smart susceptor including a controlled temperature range from about 60° F. to about 380° F. It should be noted that typically, in certain applications, more heat is needed to compensate for higher heat losses at higher temperatures as those temperatures illustrated inFIG. 8. In order to provide the required increase in heat, the current and therefore the magnetic fields are increased as necessary by increasing the power supply current. This increase in current will effectively shift the curve inFIG. 8upward so as to provide a desired amount of heat while still maintaining the same negative slope curve shape while providing a greater amount of heat to cooler areas, such as those located near heat sinks (see e.g., heat sink28andFIG. 1).

FIG. 9is an illustration of an alternative coaxial susceptor and conductor arrangement200that may be used in a heating blanket, such as the heating blanket54illustrated inFIGS. 1-3. In this illustrated alternative arrangement200, the coaxial susceptor210comprises a spring shaped coaxial susceptor and is wound around a conductor220. In one preferred arrangement, the coaxial susceptor210comprises a core and shell arrangement as describe and illustrated inFIG. 7, however alternative coaxial susceptor arrangements may also be utilized.

For example,FIG. 10is an illustration of an alternative layout of the alternative coaxial susceptor and conductor arrangement illustrated inFIG. 9. AndFIG. 11illustrates a top view of an alternative heating blanket arrangement254showing the meandering pattern of the conductor220and the array of coaxial susceptor wires210within the housing258. In one preferred arrangement, the array of coaxial susceptor wires210comprise spring formed coaxial wires as illustrated inFIG. 9. Such susceptor wires210may be wound around the conductor220such that a longitudinal axis of the array of coaxial susceptor wires210is substantially perpendicular to an electrical current flowing through the conductor220and generating a magnetic field parallel to the longitudinal axis of the susceptor wires210. In this manner, a varying magnetic field generated by the conductor220induces eddy currents in the array of coaxial susceptor wires210as will be discussed in greater detail herein.

A power supply290providing alternating current electric power may be connected to the heating blanket254by means of the heating blanket wiring256. The power supply290may be configured as a portable or fixed power supply290which may be connected to a conventional 60 Hz, 110 volt or 220 volt outlet. Although the power supply290may be connected to a conventional 60 Hz outlet, the frequency of the alternating current that is provided to the conductor220may preferably range from approximately 1000 Hz to approximately 400,000 Hz. The voltage provided to the conductor220may range from approximately 10 volts to approximately 300 volts but is preferably less than approximately 60 volts. Likewise, the frequency of the alternating current provided to the conductor220by the power supply is preferably between approximately 10 amps and approximately 1000 amps. In this regard, the power supply290may be provided in a constant-current configuration wherein the voltage across the conductor220may decrease as the magnetic materials within the heating blanket254approach the Curie temperature at which the voltage may cease to increase when the Curie temperature is reached as described in greater detail below.

Referring toFIGS. 12 and 13, shown is an embodiment of the magnetic blanket254having a spring coaxial susceptor210formed of magnetic material having a Curie temperature and provided around a conductor220. The coaxial susceptor210may be formed as a solid or unitary component in a cylindrical arrangement in a spiral or spring configuration around the conductor220in order to enhance the flexibility of the heating blanket254. As can be seen inFIG. 13, the coaxial susceptor210may extend along a length of the conductor220within the housing258. The coaxial susceptor210may be coaxially mounted relative to the conductor220. The application of alternating current to the conductor220produces an alternating magnetic field296. The magnetic field296is absorbed by the magnetic material from which the coaxial susceptor210is formed causing the coaxial susceptor210to be inductively heated.

More particularly and referring toFIG. 13, the flow of alternating current through the conductor220results in the generation of the magnetic field296surrounding the coaxial susceptor210. Eddy currents298generated within the coaxial susceptor210as a result of exposure thereof to the magnetic field296causes inductive heating of the coaxial susceptor210. The housing258may include a thermally conductive matrix278material such as silicone to facilitate thermal conduction of the heat generated by the coaxial susceptor210to the surface of the heating blanket254. The magnetic material from which the coaxial susceptor210is formed preferably has a high magnetic permeability and a Curie temperature that corresponds to the desired temperature to which a structure is to be heated by the heating blanket254. The coaxial susceptor210and conductor220are preferably sized and configured such that at temperatures below the Curie temperature of the magnetic material, the magnetic field296is concentrated in the coaxial susceptor210due to the magnetic permeability of the material.

As a result of the close proximity of the coaxial susceptor210to the conductor220, the concentration of the magnetic field296results in relatively large eddy currents298in the coaxial susceptor298. The induced eddy currents298result in resistive heating of the coaxial susceptor210. The coaxial susceptor210conductively heats the matrix278and a structure10(FIGS. 1-3) in thermal contact with the heating blanket254. The heating of the core and shell of coaxial susceptor210occurs as previously described herein with reference toFIG. 6.

The magnetic materials of the coaxial susceptor shell and core may be provided in a variety of compositions including, but not limited to, a metal, an alloy, or any other suitable material having a suitable Curie temperature. For example, the coaxial susceptor may be formed of an alloy having a composition of 32 wt. % Ni—64 wt. % Fe having a Curie temperature of approximately 390° F. The alloy may also be selected as having a composition of 34 wt. % Ni—66 wt. % Fe having a Curie temperature of approximately 450° F. However, the coaxial susceptor may be formed of a variety of other magnetic materials such as alloys which have Curie temperatures in the range of the particular application such as the range of the adhesive curing temperature or the curing temperature of the composite material from which the patch may be formed. Metals comprising the magnetic material nay include iron, cobalt or nickel. Alloys from which the magnetic material may be formed may comprise a combination of the above-described metals including, but not lied to, iron, cobalt and nickel.

Likewise, the presently disclosed conductor (such as the conductor80illustrated inFIGS. 4-6and the conductor220illustrated inFIGS. 9-12) may be formed of any suitable material having an electrical conductivity. Furthermore, the conductor is preferably formed of flexible material to facilitate the application of the heating blanket to curved surfaces. In this regard, the conductor may be formed of Litz wire or other similar wire configurations having a flexible nature and which are configured for carrying high frequency alternating current with minimal weight. The conductor material preferably possesses a relatively low electrical resistance in order to minimize unwanted and/or uncontrollable resistive heating of the conductor. The conductor may be provided as a single strand of wire of unitary construction or the conductor may be formed of braided material such as braided cable. In addition, the conductor may comprise a plurality of conductors which may be electrically connected in parallel in order to minimize the magnitude of the voltage otherwise required for relative long lengths of the conductor such as may be required for large heating blanket configurations.

Referring back toFIGS. 12 and 13, the heat blanket housing258may be formed of a flexible material to provide thermal conduction of heat generated by the susceptor sleeve to the structure to which the heating blanket is applied. In order to minimize environmental heat losses from the heating blanket254, an insulation layer268may be included as illustrated inFIGS. 12 and 13. The insulation layer268may comprise insulation272formed of silicone or other suitable material to minimize heat loss by radiation to the environment. In addition, the insulation layer268may improve the safety and thermal efficiency of the heating blanket254. As was indicated above, the housing258of the heating blanket254may be formed of any suitable high temperature material such as silicone or any other material having a suitable thermal conductivity and low electrical conductivity. Such material may include, but is not limited to, silicone, rubber and polyurethanes or any other thermally conductive material that is preferably flexible.

Referring back toFIGS. 5 and 11, the heating blankets54,254may include thermal sensors such as thermocouples or other suitable temperature sensing devices for monitoring heat at locations along the area of the heating blankets54,254in contact with the structure10(FIG. 3). Alternatively, the heating blankets54,254may include a voltage sensor94,294or other sensing devices connected to the power supply90,290as illustrated inFIGS. 5 and 11.

Referring still toFIGS. 5 and 11, sensors94,294may be configured to indicate the voltage level provided by power supplies90,290, respectively. For a constant current configuration of heating blankets54,254, the voltage may decrease as the magnetic material approaches the Curie temperature. Power supplies90,290may also be configured to facilitate adjustment of the frequency of the alternating current in order to alter the heating rate of the magnetic material. In this regard, power supplies90,290may be coupled to a respective controller92,292in order to facilitate adjustment of the alternating current over a predetermined range in order to facilitate the application of a heating blanket to a wide variety of structures having different heating requirements.

The presently disclosed coaxial susceptor provides a number of advantages. For example, it provides for a heating blanket that provides uniform, controlled heating of large surface areas. In addition, a proper selection of the metal or alloy in the susceptors' shell and the susceptors' core facilitates avoiding excessive heating of the work piece irrespective of the input power. By predetermining the susceptor shell and core metal alloys, improved control and temperature uniformity in the work piece facilitates consistent production of work pieces. The Curie temperature phenomenon of both the core and at least one shell (again, more than one shell may be utilized) is used to control both the temperature ranges as well as the absolute temperature of the work piece. This Curie temperature phenomenon is also utilized to obtain substantial thermal uniformity in the work piece, by matching the Curie temperature of the susceptor to the desired temperature of the induction heating operation being performed.