Attachment of Ignition Suppression or Quenching Device to a Fastener Stack

A device which is attachable to a fastener stack, the device comprising: an attachment element made of shape memory material; and a cap comprising a base having an aperture and a shell having an interior space in fluid communication with the opening, wherein the base supports the shell and is coupled to the attachment element.

BACKGROUND

The present disclosure relates generally to devices and methods for suppressing or quenching ignition and, more specifically, to a cover and methods of attaching the cover over a fastener stack to suppress or quench ignition.

As used herein, the term “fastener stack” means a fastener assembly comprising one fastener (e.g., bolt, stud, or pin), one or more mating parts (e.g., nuts or swaged collars), and optionally one or more washers (e.g., the fastener stack may have zero washers). The fastener may include a mating portion having external projections, such as helical threads or annular rings, while the mating part has internal projections which engage the external projections after the mating part has been tightened or swaged. Optionally, the fastener may further include a transition portion disposed between the shank and the mating portion. Fasteners are typically made from metal (e.g., stainless steel, titanium). The fastener stack typically joins two or more plates having respective holes which are aligned. The fastener from the fastener stack is positioned through the aligned holes. The washers (if present) are adjacent to and in contact with respective plates. The plate material may be metal, polymer, ceramic, or composite (e.g., fiber-reinforced plastic).

Separation of the fastener stack is often undesirable and various strategies are employed (e.g., a lock washer, adhesive, etc.) As one example, during a lightning strike on an aircraft, a high electrical current propagates through conductive paths on the aircraft. Due to the non-isotropic electrical conduction which occurs in composite materials used in modern aircraft designs and potentially poor electrical connection at panel interfaces, in order for the current to travel from one composite panel to another, lightning-induced current may pass through a fastener stack. While passing through a fastener stack, the current may damage the fastener stack and surrounding structure.

Various technologies have been developed that impart lightning protection to a fastener stack, including quenching caps, caps with internal sealed volumes, seal nuts, and seal washers. For example, hot particles may form in the space under the cap as a result of the lightning strike. However, typical solutions require a change to the fastener stack, an adhesive bond, or both.

SUMMARY

The subject matter disclosed in detail below is directed to ignition suppression devices and ignition quenching devices (collectively referred to hereinafter as “devices”).

Although various embodiments of devices attached to fastener stacks using shape memory material are described in some detail below, one or more of those embodiments may be characterized by one or more of the following aspects.

One aspect of the subject matter disclosed in detail below is a device which is attachable to a fastener stack, the device comprising: an attachment element made of shape memory material; and a cap comprising a base having an aperture and a shell having an interior space in fluid communication with the opening, wherein the base supports the shell and is coupled to the attachment element.

Other aspects of ignition suppression/quenching devices attached to fastener stacks using shape memory material are disclosed below.

DETAILED DESCRIPTION

For the purpose of illustration, ignition suppression devices and ignition quenching devices attached to fastener stacks using shape memory material will now be described in detail. However, not all features of an actual implementation are described in this specification. A person skilled in the art will appreciate that in the development of any such embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

The illustrative embodiments presented recognize and take into account that fastener systems may be exposed to voltages and currents induced by an electromagnetic event such as lightning. It is desirable to control an electrical current or discharge that may be caused by an electromagnetic event. As an example, it is desirable to inhibit an electrical current or discharge caused by an electromagnetic event from igniting fuel or other flammable material in a storage tank.

The devices described below include an element for attaching the ignition suppression or ignition quenching device to a fastener stack. The attachment element is made of shape memory material. As used herein, the term “shape memory material” includes shape memory alloy (SMA) and shape memory polymer (SMP). A shape memory alloy is an alloy that can be deformed when cold but returns to its pre-deformed (“remembered”) shape when heated. Some shape memory alloys also a two-way shape-memory effect. A shape memory polymer is a polymeric material that has the ability to return from a deformed state (temporary shape) to their original state (permanent shape) induced by an external stimulus (trigger), such as temperature change.

In accordance with some embodiments, the attachment element is made of a shape memory alloy such as nickel-titanium alloy. In accordance with other embodiments, the attachment element is made of a shape memory polymer. By configuring the shape memory attachment element to shrink upon application of a stimulus, a radially compressive load can be placed upon a nut, washer(s), fastener head, and/or fastener external threads in a fastener stack. The compression may create either a cold-weld or a substantial frictional resistance to keep an ignition suppression device or an ignition quenching device in place over a fastener stack. For example, embodiments herein resist separation of the fastener stack to reduce or eliminate dielectric breakdown of the ambient air (e.g., to prevent a spark).

FIG. 1shows a top view of an attachment element made of shape memory material (hereinafter “shape memory attachment element10”) in accordance with one embodiment.FIG. 1Ais a sectional view of shape memory attachment element10, the section being taken along a plane indicated by the dashed line1A-1A inFIG. 1. As best seen inFIG. 1, the shape memory attachment element10is in the form of a circular ring1having an aperture28which is sized to allow a projecting portion of a fastener stack to pass through.

The shape memory attachment element10further includes a plurality (e.g., six) of tabs11projecting radially outward from circular ring1at intervals (e.g., equiangular intervals). The tabs11may be integrally formed with and made of the same shape memory material as the circular ring1. The tabs11are configured to project into respective slots formed in the base of an ignition-suppressing cap (not shown inFIG. 1), eventually reaching a relative position where the cap is twist-locked to the shape memory attachment element10.

FIG. 2Ashows a sectional view of a shape memory attachment element10(of the type depicted inFIGS. 1 and 1A) attached to a fastener stack2. The portions of fastener stack2depicted inFIG. 2Ainclude a fastener4and a mating part6. The fastener4passes through a hole26in a support structure30comprising a pair of plates22and24, which are thus fastened together. The plates22and24have respective holes of equal diameter which are aligned to form hole26in support structure30.

The support structure30with fastener stack2may be installed in a structure such that the volume of space above plate22may include a combustible environment. For example, the combustible environment may include a fuel (e.g., hydrogen, gaseous, liquid, and/or aerosolized hydrocarbon, and/or suspended particulate such as sawdust, etc.), an oxidizer (e.g., oxygen, fluorine, and/or nitrous oxide), and optionally a non-reactive diluent (e.g., nitrogen, argon, and/or helium) with concentrations within the flammability limits of the fuel/oxidizer mixture. As another example, the combustible environment may include a gas that undergoes explosive decomposition (e.g., acetylene, nitrous oxide). Additional specific examples of fuels include motor fuels such as automotive fuel, diesel fuel, aviation fuel, and/or jet fuel. The combustible environment may include gases, vapors, aerosols, and/or particulate.

If the installed fastener stack2were left uncovered, the portion of fastener stack2which projects above the support structure30would be exposed to the combustible environment. The fastener stack2includes metal and/or conductive components that could shunt electrical current and/or be associated with electromagnetic effects that may become ignition sources. For example, the fastener stack2may be subject to electromagnetic effects that may produce arcing at the fastener stack2. Protection against such ignition may be provided by covering the projecting portions of fastener stack2with a cap12shown inFIG. 2B.

Still referring toFIG. 2A, the fastener4comprises a mating portion having external projections (not shown inFIG. 2A). The mating part6has internal projections (not shown inFIG. 2A) that are interengaged with the external projections of the mating portion of the fastener4. The fastener stack2depicted inFIG. 2Afurther includes a washer8that surrounds a portion of the fastener4. The shape memory attachment element10is shown in a compressed state attached to the washer8. In the scenario depicted inFIG. 2A, the unthreaded shank of fastener2passes through hole26in support structure30. The washer8is disposed between the plate22and the mating part6.

The plate material may be metal, polymer, ceramic, or composite (e.g., fiber-reinforced plastic). For example, plate22may be a wall of an aircraft fuel tank, which wall is made of carbon fiber reinforced plastic.FIG. 2Ashows mating part6and washer8(and a portion of fastener4) on the inside of the fuel tank. Other fastener stack elements (not shown inFIG. 2A) involved in the fastening of plates22and24are disposed outside the fuel tank (such as the head of a bolt if fastener4is a bolt). (Nor are such other fastener stack elements shown inFIGS. 2B, 3A, 3B, 4B, and 5B.)

The shape memory attachment element10is configured so that in a pre-stimulus state, the shape memory attachment element10is adjacent to the fastener stack2along at least a portion of a periphery of the fastener stack2, whereas in a post-stimulus state (depicted inFIG. 2A), the shape memory attachment element10is compressed onto and in contact with (attached to) the fastener stack2. More specifically, the aperture28of shape memory attachment element10shrinks in response to a stimulus, and thus the shape memory attachment element10clamps onto part of the fastener stack.

In the example depicted inFIG. 2A, the shape memory attachment element10is clamped onto washer8. In alternative implementations, the shape memory attachment element10may clamp onto part or all of a bolt head, exposed threads of a bolt, more than one washer, and/or a nut. The clamping may provide a frictional force to keep the ignition suppression device (shown in its entirety inFIG. 2Bdescribed below) affixed to the fastener stack2. The clamping may cold weld part or all of shape memory attachment element10to a portion of the fastener stack2. In one proposed implementation, the stimulus is a change in temperature, preferably an increase in temperature.

The shape memory attachment element10may also include other materials. For example, a shape memory alloy ring may be mechanically connected to a polymer ring. Additionally or alternatively, a shape memory alloy ring may be coated with another material for one or more of the following purposes: (a) corrosion protection; (b) to assist in cold welding; (c) to increase frictional force between the attachment portion and the fastener stack; and (d) to facilitate attachment to a base seal portion, containment portion, and/or porous material portion of an ignition suppression device or a or ignition quenching device.

Following attachment of shape memory attachment element10to fastener stack2, a cap12is coupled to the shape memory attachment element10by twist-locking, as depicted inFIG. 2B. When assembled in an ignition-suppressing system, the cap12and fastener2are co-located at the fastener site, with the cap12covering and/or enclosing the fastener stack2. The cap12and fastener stack2may be axisymmetric about an axis of the fastener stack2and perpendicular to a local plane of the support structure30. Hence, the schematic view ofFIG. 2Brepresents a cross-sectional view of the ignition suppression system.

As seen inFIG. 2B, the cap12is configured to provide a containment portion that covers the portion of fastener stack2that projects above plate22(hereinafter “projecting portion”). The cap12is configured to define an open volume32between the cap12and the projecting portion of the fastener stack2. In accordance with some embodiments, the cap12, when combined with other elements (e.g., base seal20) of the ignition suppression device, hermetically seals the open volume32from the combustible environment inside the fuel tank. During a lightning strike event, if hot particles or hot gases are ejected from or near the fastener stack2, they will be contained within the open volume32and will be isolated from the combustible environment inside the fuel tank. In accordance with other embodiments, the cap12may have a porous region so that the cap performs a quenching function.

In the example depicted inFIG. 2B, the cap12includes a base18having an opening34and a shell14having an interior space32in fluid communication with the opening34in base18. The shell14and base18may be integrally formed and made of the same material or may be made of different materials and attached to each other by adhesive bonding, mechanical coupling, etc. The base18is in contact with plate22, is coupled to shape memory attachment element10, and supports shell14in a position such that the interior space32of shell14is partially occupied by a portion of the fastener stack2. The base18is configured with retention slots17which receive and retain respective tabs11of shape memory attachment element10.

FIG. 2Billustrates an example of an ignition suppression system in which the cap12is mechanically coupled to shape memory attachment element10by a twist-lock mechanism. The cap12may be coupled to shape memory attachment element10by placing the cap12over the fastener stack2and by twisting cap12relative to fastener stack2a small fraction of a turn (typically about one eighth of a turn).

As previously described, the shape memory attachment element10includes the cap attachment features, specifically, a series of radially projecting tabs11. The attachment structure of cap12includes a plurality of entry slots (not shown inFIG. 2B, but see U.S. Pat. No. 10,501,202) formed in the base18of cap12. The tabs11and the entry slots are configured to fit together such that cap12may be applied over the fastener stack2after the fastener stack2is installed in the support structure30.

The entry slots are connected to locking ramps and then to retention slots17. The locking ramps are configured to guide the tabs11from the entry slots to retention slots17. Hence, the cap12may be installed with tabs11in the entry slots162. A twist of the ignition-quenching cap12causes the tabs11to be driven up respective locking ramps (not shown inFIG. 2B, but see U.S. Pat. No. 10,501,202 or U.S. Patent Application Publ. No. 2020/0080584) until tabs11clear the locking ramps. Once tabs11have cleared the locking ramps, tabs11may snap into the retention slots17. A height differential between the locking ramps and the retention slots17traps the tabs11in the retention slots17(or at least more force may be required to remove the ignition-quenching cap12than to install it).

The locking features help to keep the cap12in place at the fastener site and to resist dislodgement due to vibration and/or environmental perturbations. The cap12may have external features to facilitate twisting the cap12(applying torque) to lock the tabs11in the retention slots17. The external features may be configured to accept a socket wrench.

The example shape memory attachment element10depicted inFIG. 1has six tabs11, and the cap12has six retention slots17. However, the number of tabs may be different than six in alternative implementations. Furthermore, tabs11may be circumferentially distributed substantially uniformly (as shown inFIG. 1) or asymmetrically.

In an alternative embodiment, the cap12is coupled to the shape memory attachment element10by threadably coupling internal threads of the cap12with external threads formed on shape memory attachment element10. A device comprising a cap12threadably coupled to a shape memory attachment element10will be described in some detail below in the context of an ignition quenching device having a porous cap, but threadably coupling is equally applicable to a non-porous cap.

The ignition suppression device depicted inFIG. 2Bfurther includes a base seal20seated in an annular groove5formed in a bottom surface of the base18of cap12. The base seal20is configured to compress when the bottom surface of base18is in contact with plate22, thereby sealing the open volume32under cap12. The base seal20may be an0-ring or a gasket. The gasket may be reversibly compressible (e.g., a foam gasket or a rubber gasket). In alternative implementations, the gasket may be irreversibly compressible. For example, the gasket may have a sharp edge that irreversibly deforms when pressed against the surface of plate22. The sharp edge may be a knife edge. The gasket may be made of a polymer, preferably a thermoplastic. The polymer may be, e.g., polyamide, polyimide, polyamide-imide, polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE), perfluoroalkoxy alkane (PFA), polyaryl-amide, or acetal.

The walls of groove5in base18may be made of a metal, polymer, ceramic, composite, etc. The groove5may have complete walls on both sides or may have a periodic wall on one or both sides. The groove5may have a dovetail to retain the base seal20.

In accordance with an alternative embodiment, the bottom surface of base18may be sealed to plate22by means of an uncured or a cured adhesive or sealant (e.g., polysulfide sealant). In the case where cap12is non-porous, the base seal20preferably makes an airtight seal to the plate22. As explained below, if the cap12includes a portion made of porous material, the base seal20is optional.

FIG. 3Ashows a sectional view (without hatching) of a shape memory attachment element10in the form of a circular ring with external threads13in accordance with an alternative embodiment. The shape memory attachment element10, in a post-stimulus state, is attached to a fastener stack2. The only difference between the scenarios respectively depicted inFIGS. 2A and 3Ais that inFIG. 2A, the shape memory attachment element10has tabs11, whereas inFIG. 3A, shape memory attachment element10has external threads13.

Following attachment of shape memory attachment element10to fastener stack2, a cap12is threadably coupled to the shape memory attachment element10, as depicted inFIG. 3B. Cap12and fastener2are co-located at the fastener site, with the cap12covering the fastener stack2. The cap12and fastener stack2may be axisymmetric about an axis of the fastener stack2and perpendicular to a local plane of the support structure30. Hence, the schematic view ofFIG. 3Brepresents a cross-sectional view of the ignition suppression system.

As seen inFIG. 3B, the cap12is configured to provide a containment portion that covers the portion of fastener stack2that projects above plate22(hereinafter “projecting portion”). The cap12is configured to define an open volume between the cap12and the projecting portion of the fastener stack2.

In the example depicted inFIG. 3B, the cap12includes a base18having an opening34and a shell14having an interior space32in fluid communication with the opening34in base18. The shell14and base18may be integrally formed and made of the same material or may be made of different materials and attached to each other by adhesive bonding, mechanical coupling, etc. The base18is in contact with plate22, is coupled to shape memory attachment element10, and supports shell14in a position such that the interior space of shell14is partially occupied by a portion of the fastener stack2.

In accordance with the embodiment depicted inFIG. 3B, the cap12is coupled to shape memory attachment element10by placing the cap12over the fastener stack2and screwing the base18of cap12onto the shape memory attachment element10. The base18of cap12comprises internal threads9. The internal threads9of base18are interengaged with the external threads13shape memory attachment element10.

The cap12depicted inFIG. 3Bis configured to perform an ignition quenching function, such as by having a part or all of the shell14made of a porous material. Unlike conventional cap seals, the ignition-quenching covers disclosed herein do not attempt to seal in all of the kinetic and thermal energy of electromagnetic effects at fastener stacks (as might be generated by a lightning strike). Instead, the ignition-quenching covers proposed herein permit gas, liquid, and/or some (non-ballistic) particles to flow through the cap12while removing the thermal energy that may ignite a combustible environment such as a fuel mixture in a fuel tank. Hence, ignition sources, ignition events, and/or combustion within the ignition-quenching covers do not propagate outside of the ignition-quenching covers. Additionally, because there is no need for an airtight seal, installation of ignition-quenching covers may be simplified relative to conventional cap seals. Further, the porous structures on the disclosed ignition-quenching covers may save weight and add useable fuel volume relative to conventional cap seals.

The porous elements incorporated in shell14permit the combustible environment to permeate into and through the cap12and to contact the fastener stack2. e. The porosity is “open porosity”, i.e., the majority of pores are interconnected and not isolated. The porous portions of shell14may be configured to prevent, mitigate, and/or suppress one or more aspects of an ignition event triggered (ignited) by an ignition source associated with the fastener stack2.

More specifically, a porous cap may be configured to prevent formation, propagation, and/or maturation of an ignition kernel therein by dissipating heat energy associated with the ignition source and/or the ignition kernel. An ignition kernel may mature into a self-propagating combustion reaction (e.g., a deflagration wave) when heat energy from the reaction sufficiently heats neighboring combustion reactants (e.g., when energy released is greater than energy losses). The ignition-quenching cover may be configured to dissipate heat energy that may otherwise serve to sustain a combustion reaction. For example, the porous material may have a surface area to pore volume ratio that is high enough to prevent combustion from propagating through the shell14.

The porous elements of shell14may be layered and/or arranged to create pores. Two or more (optionally all) porous elements may have the same characteristics. In some embodiments, at least one of the porous elements has characteristics (e.g., pore size, pore shape, pore orientation, material, etc.) that differ from the characteristics of other porous elements. For example, and as discussed further herein, the shell14may be a porous body constructed of sintered polymeric particles (e.g., sintered nylon spheres), forming a network of varied pores.

The cap12with porous material forms an ignition-quenching cover which may be configured to prevent the ignition of the combustible environment by preventing a hot particle that is emitted from fastener stack2from travelling through the porous body. As used herein, the term “hot particle” refers to a particle that is emitted from the fastener stack2and/or due to an ignition source at the fastener stack2that has a size and/or a thermal energy sufficient to cause ignition of the combustible environment.

The porous body of cap12may be configured such that there are no straight-line trajectories through a pore in the porous body from the interior surface to the exterior surface. If such a straight-line trajectory exists, the size of the pores may be small enough to prevent the traversal of particles having an effective diameter larger than a predetermined size. Particles traversing the ignition-quenching cover along a convoluted (or at least a non-straight) path generally will collide with the porous body in the pores and thereby lose at least a portion of their thermal and/or kinetic energy.

Any porous element may be a mass of bonded particles, a mass of sintered particles, a salt-templated polymer, an inverse-cast polymer, a polymeric mesh, a woven or non-woven polymeric fabric, a polymeric lattice or scrim, or a stochastic open-cell polymeric foam. Pores may also be paths formed in one or two injection-molded parts as disclosed in U.S. Patent Application Publ. Nos. 2020/0080584 and 2020/0080585. If different materials are used, they could be combined in a single layer or stacked in multiple layers.

The porous body may include an exterior coating configured to decrease reactivity of the underlying materials, decrease susceptibility of the underlying materials to the combustible environment, and/or decrease electrical conductivity of the underlying materials. An example of an exterior coating is a parylene conformal coating.

As used herein, the term “salt templated polymer” means an open-porous polymer made by: (1) partially fusing salt granules into an open porous network, (2) infiltrating a polymer precursor solution into some or all of the open spaces in the salt network, (3) curing the polymer, and (4) removing the salt network (e.g. by dissolving in water). As used herein, the term “woven polymer fabric” incudes: (1) a flat woven sheet or a braided tubular material; (2) gauze; and (3) fabric. As used herein, the term “non-woven polymer” includes, e.g., a polymer felt, which may be sandwiched between two layers of polymer meshes or woven polymer fabrics. As used herein, the term “polymer lattice” includes: (1) a lattice or truss structure made via stereolithography (SLA), self-propagating photopolymer waveguides, or other additive manufacturing technique; (2) a polymer lattice with a fuel-compatible coating; (e.g., parylene); (3) a lattice structure with ballistic ignition-resistant architectures, such as integrated features for non-line of sight and complex graded lattice architectures with basal or angled planes. As used herein, the term “inverse cast structure” means an open-porous polymer template made by: (1) SLA, self-propagating photopolymer waveguides, or other additive manufacturing technique, (2) infiltrating a second polymer precursor solution into some or all of the open spaces in the network to cast the structure, (3) curing or drying the polymer, and (4) removing first polymer template through dissolution, etching, or oxidation.

In accordance with an alternative embodiment, an ignition suppression device includes a shape memory attachment element and a base seal having a knife-edge lip and does not include a cap.FIG. 4Ais a diagram representing a sectional view of a shape memory attachment element10ain the form of a circular ring1configured with an offset that forms an inner circumferential seat7.FIG. 4Bis a diagram representing a sectional view of an ignition suppression device19consisting of a shape memory attachment element10aof the type depicted inFIG. 4Aand a knife-edge base seal16made of thermoplastic material. The knife-edge base seal16has a knife-edge lip15. The knife-edge base seal16is seated on the inner circumferential seat7with the knife-edge lip15projecting (pointing) away from inner circumferential seat7. (In alternative embodiments, the seat could be in a groove, such as the groove5shown inFIG. 2B) instead of seated on an offset against the inner diameter of the SMA ring). The knife-edge base seal16has a circular aperture which is coaxial with the circular aperture of circular ring1. The diameters of the respective circular apertures may be the same.

FIG. 4Cis a diagram representing a sectional view of the ignition suppression device19depicted inFIG. 4Battached to a fastener stack2by means of the shape memory attachment element10a.As the shape memory attachment element10acompresses during the application of a stimulus, knife-edge base seal16is deformed to form an airtight seal with the surface of plate22while circular ring1of shape memory attachment element10aseals to the fastener stack2. (The installer may apply an axial force to compress the knife edge axially into the top plate simultaneous with the stimulus compressing the SMA ring radially against the fastener stack.) During service, these seal prevent any EME event under the fastener from emitting energy from between the fastener stack2and the plate22.

FIG. 5Ais a diagram representing a sectional view of an ignition suppression device19acomprising a non-porous (impermeable to fluids) cap12integrally formed with a knife-edge base seal16in accordance with another embodiment. As previously described, the knife-edge base seal16is seated on the inner circumferential seat7of the shape memory attachment element10a

FIG. 5Bis a diagram representing a sectional view of the ignition suppression device19adepicted inFIG. 5Aattached to a fastener stack2using the shape memory attachment element10a.The shape memory attachment element10a,cap12, and knife-edge base seal16(which no longer has a knife edge due to deformation) combine to seal around the entire fastener stack2and to the plate22, thereby containing hot gases and energy (ignition suppression).

FIG. 6is a flowchart identifying steps of a method100for covering a portion of a fastener stack2in accordance with one embodiment. After a fastener stack has been installed in aligned holes of two or more plates of a support structure, an attachment element made of shape memory material having a first shape is placed so that the attachment element is adjacent to the fastener stack along at least a portion of a periphery of the fastener stack (step102). Optionally, an axial force is directed toward the support structure (step104). Then a stimulus is applied that is sufficient to cause the shape memory material in place to transform to a second shape in which the attachment element is compressively attached to the fastener stack (step106). In accordance with one embodiment, the stimulus is heat. The heat may come from ambient temperature (e.g., if the ignition suppression device is placed over the fastener stack when the ignition suppression device is at a temperature below ambient temperature). The stimulus causes the shape memory attachment element to deform and clamp onto part of the fastener stack. Thereafter, the stimulus is removed along with any axial force being applied (in either order or concurrently) (step108). Optionally, a cap is installed by coupling the cap to the shape memory attachment element (step110). The cap has an interior space that is sized to receive a portion of the fastener stack. In accordance with the various embodiments disclosed above, the cap includes a shell that is either impermeable to fluids or has at least a portion which is porous. In accordance with alternative embodiments, the cap may have a double-shell construction as disclosed in U.S. Patent Application Publ. No. 2020/0080584. The completed installation is then inspected (step112).

The shape memory attachment element need not be circular.FIG. 7Ashows a top view of a pre-stimulus C-shaped shape memory attachment element10b.FIG. 7Bshows a top view of a pre-stimulus U-shaped shape memory attachment element10chaving a pair of parallel legs3aand3b.The post-stimulus shape of U-shaped shape memory attachment element10cmay be a circle (or part of a circle) or a smaller U.

The ignition suppression systems and ignition quenching systems disclosed herein may be part of a fuel tank, such as a wing fuel tank in a composite wing aircraft. The fastener stack2may be a fastener exposed to the fuel volume and/or ullage (ullage is the space within the fuel tank which is not occupied by fuel) and embedded in and/or coupling one or more plates of a support structure30which is exposed to the fuel volume and/or ullage. The support structure30may comprise carbon-fiber composite panels, partitions, stringers, etc. that are in the interior of the fuel tank and/or define at least a portion of the interior of the fuel tank. The cap12covers the fastener stack2and is collocated with the fastener stack2. The cap12may be either porous or non-porous.

The ignition suppression and ignition quenching devices have an attachment element made of shape memory material for attachment to a fastener stack. By configuring a shape memory attachment element to shrink upon application of heat, a radially compressive load can be placed upon a portion of a fastener stack. This can create either a cold-weld or a substantial frictional resistance to keep an ignition suppression or ignition quenching device in place over a fastener stack. Initially, the shape memory attachment element is placed around or partly around a fastener stack. Then a stimulus is applied. In response to the stimulus, the shape memory attachment element attaches to the fastener stack by compression. The remainder of the device (including a cap and a base seal) may be (a) pre-attached to a shape memory attachment element and then the entire device is attached as a unit to the fastener stack; or (b) post-attached to a shape memory attachment element which was previously attached to the fastener stack

Although the aircraft wing fuel tank example is detailed to explain some potential advantages of attaching caps to fastener stacks using shape memory attachment elements, the technology proposed herein may be utilized and/or incorporated within other types of structures. For example, caps attached using shape memory attachment elements may be useful in other applications requiring ignition hazard consideration, including fuel transport, fuel storage, mining operations, chemical processing, metal fabrication, power plant construction and operation, and operations which involve combustible particulate such as suspended dust, sawdust, coal, metal, flour, and/or grain.

Thus, the disclosed technology for attaching ignition suppression/quenching devices to fastener stacks is applicable to aerospace companies for lightning protection in commercial and military aircraft, and in other industries wherein ignition protection of tanks containing a flammable gaseous mixture and/or flammable aerosolized mixture may be exposed to lightning strike. Such industries include, but are not limited to, oil and gas, chemical manufacturing/processing plants, and grain storage.

While devices attached to fastener stacks using shape memory material have been described with reference to various embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the teachings herein. In addition, many modifications may be made to adapt the teachings herein to a particular situation without departing from the scope thereof. Therefore, it is intended that the claims not be limited to the particular embodiments disclosed herein.

In the method claims appended hereto, the alphabetic ordering of steps is for the sole purpose of enabling subsequent short-hand references to antecedent steps and not for the purpose of limiting the scope of the claim to require that the method steps be performed in alphabetic order.