Patent Publication Number: US-2022220995-A1

Title: Attachment of Ignition Suppression or Quenching Device to a Fastener Stack

Description:
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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, functions and advantages discussed in the preceding section may be achieved independently in various embodiments or may be combined in yet other embodiments. Various embodiments will be hereinafter described with reference to drawings for the purpose of illustrating the above-described and other aspects. None of the diagrams are drawn to scale. 
       In the appended assembly drawings, various components which extend 360 degrees around a fastener are shown in section without hatching to avoid clutter. Also, the external projections of the mating portion of the fastener and the internal projections of the mating part (e.g., nut or collar) are not shown. 
         FIG. 1  is a diagram representing a top view of a shape memory attachment element in the form of a tabbed circular ring made of shape memory material in accordance with one embodiment. 
         FIG. 1A  is a diagram representing a sectional view (with hatching) of the shape memory attachment element depicted in  FIG. 1 , the section being taken along a plane indicated by the dashed line  1 A- 1 A in  FIG. 1 . 
         FIG. 2A  is a diagram representing a sectional view (without hatching) of the shape memory attachment element depicted in  FIG. 1  attached to a fastener stack. 
         FIG. 2B  is a diagram representing a sectional view (without hatching) of an ignition suppression cap sealed to a plate depicted in  FIG. 2A  and attached to the shape memory attachment element depicted in  FIG. 2A . 
         FIG. 3A  is a diagram representing a sectional view (without hatching) of a shape memory attachment element that includes a circular ring (attached to a fastener stack) with external threads. 
         FIG. 3B  is a diagram representing a sectional view (without hatching) of an ignition quenching cap sealed to a plate and threadably coupled to the externally threaded shape memory attachment element depicted in  FIG. 3A . 
         FIG. 4A  is a diagram representing a sectional view (with hatching) of a circular ring made of shape memory material and configured to have an offset. 
         FIG. 4B  is a diagram representing a sectional view of an ignition suppression device consisting of a shape memory attachment element of the type depicted in  FIG. 4A  and a gasket made of thermoplastic material in accordance with a further embodiment. 
         FIG. 4C  is a diagram representing a sectional view (without hatching) of the ignition suppression device depicted in  FIG. 4B  attached to a fastener stack. 
         FIG. 5A  is a diagram representing a sectional view (with hatching) of an ignition suppression device comprising a non-porous cap with knife-edge base seal in accordance with another embodiment. 
         FIG. 5B  is a diagram representing a sectional view (without hatching) of the ignition suppression device depicted in  FIG. 5A  attached to a fastener stack using a shape memory attachment element. 
         FIG. 6  is a flowchart identifying steps of a method for covering a portion of a fastener stack in accordance with one embodiment. 
         FIGS. 7A and 7B  are diagrams representing respective top views of C-shaped and U-shaped shape memory attachment elements. 
     
    
    
     Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals. 
     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&#39;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. 1  shows a top view of an attachment element made of shape memory material (hereinafter “shape memory attachment element  10 ”) in accordance with one embodiment.  FIG. 1A  is a sectional view of shape memory attachment element  10 , the section being taken along a plane indicated by the dashed line  1 A- 1 A in  FIG. 1 . As best seen in  FIG. 1 , the shape memory attachment element  10  is in the form of a circular ring  1  having an aperture  28  which is sized to allow a projecting portion of a fastener stack to pass through. 
     The shape memory attachment element  10  further includes a plurality (e.g., six) of tabs  11  projecting radially outward from circular ring  1  at intervals (e.g., equiangular intervals). The tabs  11  may be integrally formed with and made of the same shape memory material as the circular ring  1 . The tabs  11  are configured to project into respective slots formed in the base of an ignition-suppressing cap (not shown in  FIG. 1 ), eventually reaching a relative position where the cap is twist-locked to the shape memory attachment element  10 . 
       FIG. 2A  shows a sectional view of a shape memory attachment element  10  (of the type depicted in  FIGS. 1 and 1A ) attached to a fastener stack  2 . The portions of fastener stack  2  depicted in  FIG. 2A  include a fastener  4  and a mating part  6 . The fastener  4  passes through a hole  26  in a support structure  30  comprising a pair of plates  22  and  24 , which are thus fastened together. The plates  22  and  24  have respective holes of equal diameter which are aligned to form hole  26  in support structure  30 . 
     The support structure  30  with fastener stack  2  may be installed in a structure such that the volume of space above plate  22  may 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 stack  2  were left uncovered, the portion of fastener stack  2  which projects above the support structure  30  would be exposed to the combustible environment. The fastener stack  2  includes 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 stack  2  may be subject to electromagnetic effects that may produce arcing at the fastener stack  2 . Protection against such ignition may be provided by covering the projecting portions of fastener stack  2  with a cap  12  shown in  FIG. 2B . 
     Still referring to  FIG. 2A , the fastener  4  comprises a mating portion having external projections (not shown in  FIG. 2A ). The mating part  6  has internal projections (not shown in  FIG. 2A ) that are interengaged with the external projections of the mating portion of the fastener  4 . The fastener stack  2  depicted in  FIG. 2A  further includes a washer  8  that surrounds a portion of the fastener  4 . The shape memory attachment element  10  is shown in a compressed state attached to the washer  8 . In the scenario depicted in  FIG. 2A , the unthreaded shank of fastener  2  passes through hole  26  in support structure  30 . The washer  8  is disposed between the plate  22  and the mating part  6 . 
     The plate material may be metal, polymer, ceramic, or composite (e.g., fiber-reinforced plastic). For example, plate  22  may be a wall of an aircraft fuel tank, which wall is made of carbon fiber reinforced plastic.  FIG. 2A  shows mating part  6  and washer  8  (and a portion of fastener  4 ) on the inside of the fuel tank. Other fastener stack elements (not shown in  FIG. 2A ) involved in the fastening of plates  22  and  24  are disposed outside the fuel tank (such as the head of a bolt if fastener  4  is a bolt). (Nor are such other fastener stack elements shown in  FIGS. 2B, 3A, 3B, 4B, and 5B .) 
     The shape memory attachment element  10  is configured so that in a pre-stimulus state, the shape memory attachment element  10  is adjacent to the fastener stack  2  along at least a portion of a periphery of the fastener stack  2 , whereas in a post-stimulus state (depicted in  FIG. 2A ), the shape memory attachment element  10  is compressed onto and in contact with (attached to) the fastener stack  2 . More specifically, the aperture  28  of shape memory attachment element  10  shrinks in response to a stimulus, and thus the shape memory attachment element  10  clamps onto part of the fastener stack. 
     In the example depicted in  FIG. 2A , the shape memory attachment element  10  is clamped onto washer  8 . In alternative implementations, the shape memory attachment element  10  may 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 in  FIG. 2B  described below) affixed to the fastener stack  2 . The clamping may cold weld part or all of shape memory attachment element  10  to a portion of the fastener stack  2 . In one proposed implementation, the stimulus is a change in temperature, preferably an increase in temperature. 
     The shape memory attachment element  10  may 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 element  10  to fastener stack  2 , a cap  12  is coupled to the shape memory attachment element  10  by twist-locking, as depicted in  FIG. 2B . When assembled in an ignition-suppressing system, the cap  12  and fastener  2  are co-located at the fastener site, with the cap  12  covering and/or enclosing the fastener stack  2 . The cap  12  and fastener stack  2  may be axisymmetric about an axis of the fastener stack  2  and perpendicular to a local plane of the support structure  30 . Hence, the schematic view of  FIG. 2B  represents a cross-sectional view of the ignition suppression system. 
     As seen in  FIG. 2B , the cap  12  is configured to provide a containment portion that covers the portion of fastener stack  2  that projects above plate  22  (hereinafter “projecting portion”). The cap  12  is configured to define an open volume  32  between the cap  12  and the projecting portion of the fastener stack  2 . In accordance with some embodiments, the cap  12 , when combined with other elements (e.g., base seal  20 ) of the ignition suppression device, hermetically seals the open volume  32  from 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 stack  2 , they will be contained within the open volume  32  and will be isolated from the combustible environment inside the fuel tank. In accordance with other embodiments, the cap  12  may have a porous region so that the cap performs a quenching function. 
     In the example depicted in  FIG. 2B , the cap  12  includes a base  18  having an opening  34  and a shell  14  having an interior space  32  in fluid communication with the opening  34  in base  18 . The shell  14  and base  18  may 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 base  18  is in contact with plate  22 , is coupled to shape memory attachment element  10 , and supports shell  14  in a position such that the interior space  32  of shell  14  is partially occupied by a portion of the fastener stack  2 . The base  18  is configured with retention slots  17  which receive and retain respective tabs  11  of shape memory attachment element  10 . 
       FIG. 2B  illustrates an example of an ignition suppression system in which the cap  12  is mechanically coupled to shape memory attachment element  10  by a twist-lock mechanism. The cap  12  may be coupled to shape memory attachment element  10  by placing the cap  12  over the fastener stack  2  and by twisting cap  12  relative to fastener stack  2  a small fraction of a turn (typically about one eighth of a turn). 
     As previously described, the shape memory attachment element  10  includes the cap attachment features, specifically, a series of radially projecting tabs  11 . The attachment structure of cap  12  includes a plurality of entry slots (not shown in  FIG. 2B , but see U.S. Pat. No. 10,501,202) formed in the base  18  of cap  12 . The tabs  11  and the entry slots are configured to fit together such that cap  12  may be applied over the fastener stack  2  after the fastener stack  2  is installed in the support structure  30 . 
     The entry slots are connected to locking ramps and then to retention slots  17 . The locking ramps are configured to guide the tabs  11  from the entry slots to retention slots  17 . Hence, the cap  12  may be installed with tabs  11  in the entry slots  162 . A twist of the ignition-quenching cap  12  causes the tabs  11  to be driven up respective locking ramps (not shown in  FIG. 2B , but see U.S. Pat. No. 10,501,202 or U.S. Patent Application Publ. No. 2020/0080584) until tabs  11  clear the locking ramps. Once tabs  11  have cleared the locking ramps, tabs  11  may snap into the retention slots  17 . A height differential between the locking ramps and the retention slots  17  traps the tabs  11  in the retention slots  17  (or at least more force may be required to remove the ignition-quenching cap  12  than to install it). 
     The locking features help to keep the cap  12  in place at the fastener site and to resist dislodgement due to vibration and/or environmental perturbations. The cap  12  may have external features to facilitate twisting the cap  12  (applying torque) to lock the tabs  11  in the retention slots  17 . The external features may be configured to accept a socket wrench. 
     The example shape memory attachment element  10  depicted in  FIG. 1  has six tabs  11 , and the cap  12  has six retention slots  17 . However, the number of tabs may be different than six in alternative implementations. Furthermore, tabs  11  may be circumferentially distributed substantially uniformly (as shown in  FIG. 1 ) or asymmetrically. 
     In an alternative embodiment, the cap  12  is coupled to the shape memory attachment element  10  by threadably coupling internal threads of the cap  12  with external threads formed on shape memory attachment element  10 . A device comprising a cap  12  threadably coupled to a shape memory attachment element  10  will 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 in  FIG. 2B  further includes a base seal  20  seated in an annular groove  5  formed in a bottom surface of the base  18  of cap  12 . The base seal  20  is configured to compress when the bottom surface of base  18  is in contact with plate  22 , thereby sealing the open volume  32  under cap  12 . The base seal  20  may be an  0 -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 plate  22 . 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 groove  5  in base  18  may be made of a metal, polymer, ceramic, composite, etc. The groove  5  may have complete walls on both sides or may have a periodic wall on one or both sides. The groove  5  may have a dovetail to retain the base seal  20 . 
     In accordance with an alternative embodiment, the bottom surface of base  18  may be sealed to plate  22  by means of an uncured or a cured adhesive or sealant (e.g., polysulfide sealant). In the case where cap  12  is non-porous, the base seal  20  preferably makes an airtight seal to the plate  22 . As explained below, if the cap  12  includes a portion made of porous material, the base seal  20  is optional. 
       FIG. 3A  shows a sectional view (without hatching) of a shape memory attachment element  10  in the form of a circular ring with external threads  13  in accordance with an alternative embodiment. The shape memory attachment element  10 , in a post-stimulus state, is attached to a fastener stack  2 . The only difference between the scenarios respectively depicted in  FIGS. 2A and 3A  is that in  FIG. 2A , the shape memory attachment element  10  has tabs  11 , whereas in  FIG. 3A , shape memory attachment element  10  has external threads  13 . 
     Following attachment of shape memory attachment element  10  to fastener stack  2 , a cap  12  is threadably coupled to the shape memory attachment element  10 , as depicted in  FIG. 3B . Cap  12  and fastener  2  are co-located at the fastener site, with the cap  12  covering the fastener stack  2 . The cap  12  and fastener stack  2  may be axisymmetric about an axis of the fastener stack  2  and perpendicular to a local plane of the support structure  30 . Hence, the schematic view of  FIG. 3B  represents a cross-sectional view of the ignition suppression system. 
     As seen in  FIG. 3B , the cap  12  is configured to provide a containment portion that covers the portion of fastener stack  2  that projects above plate  22  (hereinafter “projecting portion”). The cap  12  is configured to define an open volume between the cap  12  and the projecting portion of the fastener stack  2 . 
     In the example depicted in  FIG. 3B , the cap  12  includes a base  18  having an opening  34  and a shell  14  having an interior space  32  in fluid communication with the opening  34  in base  18 . The shell  14  and base  18  may 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 base  18  is in contact with plate  22 , is coupled to shape memory attachment element  10 , and supports shell  14  in a position such that the interior space of shell  14  is partially occupied by a portion of the fastener stack  2 . 
     In accordance with the embodiment depicted in  FIG. 3B , the cap  12  is coupled to shape memory attachment element  10  by placing the cap  12  over the fastener stack  2  and screwing the base  18  of cap  12  onto the shape memory attachment element  10 . The base  18  of cap  12  comprises internal threads  9 . The internal threads  9  of base  18  are interengaged with the external threads  13  shape memory attachment element  10 . 
     The cap  12  depicted in  FIG. 3B  is configured to perform an ignition quenching function, such as by having a part or all of the shell  14  made 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 cap  12  while 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 shell  14  permit the combustible environment to permeate into and through the cap  12  and to contact the fastener stack  2 . e. The porosity is “open porosity”, i.e., the majority of pores are interconnected and not isolated. The porous portions of shell  14  may 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 stack  2 . 
     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 shell  14 . 
     The porous elements of shell  14  may 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 shell  14  may be a porous body constructed of sintered polymeric particles (e.g., sintered nylon spheres), forming a network of varied pores. 
     The cap  12  with 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 stack  2  from travelling through the porous body. As used herein, the term “hot particle” refers to a particle that is emitted from the fastener stack  2  and/or due to an ignition source at the fastener stack  2  that has a size and/or a thermal energy sufficient to cause ignition of the combustible environment. 
     The porous body of cap  12  may 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. 
     The porous material of cap may be a polymer, a ceramic, a glass, a metal, a non-metal, a composite material, or combinations thereof. Examples of polymers include polypropylene, polystyrene, polyurethane, PEEK, PTFE, polyimide, acetal, PFA, polyamide (e.g., NYLON-brand polymer), polyamide-imide (e.g., TORLON-brand polymer), epoxy, melamine phenolic, polybutylene terephthalate, EVA, polysulfide, and polysulfone. Examples of metals include aluminum, steel, and titanium. Examples of composite materials include filled polymers and fiber reinforced polymers (e.g., carbon-fiber reinforced polymer, fiberglass, and/or LYTEX-brand chopped fiber filled epoxy composite). 
     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. 4A  is a diagram representing a sectional view of a shape memory attachment element  10   a  in the form of a circular ring  1  configured with an offset that forms an inner circumferential seat  7 .  FIG. 4B  is a diagram representing a sectional view of an ignition suppression device  19  consisting of a shape memory attachment element  10   a  of the type depicted in  FIG. 4A  and a knife-edge base seal  16  made of thermoplastic material. The knife-edge base seal  16  has a knife-edge lip  15 . The knife-edge base seal  16  is seated on the inner circumferential seat  7  with the knife-edge lip  15  projecting (pointing) away from inner circumferential seat  7 . (In alternative embodiments, the seat could be in a groove, such as the groove  5  shown in  FIG. 2B ) instead of seated on an offset against the inner diameter of the SMA ring). The knife-edge base seal  16  has a circular aperture which is coaxial with the circular aperture of circular ring  1 . The diameters of the respective circular apertures may be the same. 
       FIG. 4C  is a diagram representing a sectional view of the ignition suppression device  19  depicted in  FIG. 4B  attached to a fastener stack  2  by means of the shape memory attachment element  10   a.  As the shape memory attachment element  10   a  compresses during the application of a stimulus, knife-edge base seal  16  is deformed to form an airtight seal with the surface of plate  22  while circular ring  1  of shape memory attachment element  10   a  seals to the fastener stack  2 . (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 stack  2  and the plate  22 . 
       FIG. 5A  is a diagram representing a sectional view of an ignition suppression device  19   a  comprising a non-porous (impermeable to fluids) cap  12  integrally formed with a knife-edge base seal  16  in accordance with another embodiment. As previously described, the knife-edge base seal  16  is seated on the inner circumferential seat  7  of the shape memory attachment element  10   a    
       FIG. 5B  is a diagram representing a sectional view of the ignition suppression device  19   a  depicted in  FIG. 5A  attached to a fastener stack  2  using the shape memory attachment element  10   a.  The shape memory attachment element  10   a,  cap  12 , and knife-edge base seal  16  (which no longer has a knife edge due to deformation) combine to seal around the entire fastener stack  2  and to the plate  22 , thereby containing hot gases and energy (ignition suppression). 
       FIG. 6  is a flowchart identifying steps of a method  100  for covering a portion of a fastener stack  2  in 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 (step  102 ). Optionally, an axial force is directed toward the support structure (step  104 ). 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 (step  106 ). 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) (step  108 ). Optionally, a cap is installed by coupling the cap to the shape memory attachment element (step  110 ). 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 (step  112 ). 
     The shape memory attachment element need not be circular.  FIG. 7A  shows a top view of a pre-stimulus C-shaped shape memory attachment element  10   b.    FIG. 7B  shows a top view of a pre-stimulus U-shaped shape memory attachment element  10   c  having a pair of parallel legs  3   a  and  3   b.  The post-stimulus shape of U-shaped shape memory attachment element  10   c  may 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 stack  2  may 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 structure  30  which is exposed to the fuel volume and/or ullage. The support structure  30  may 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 cap  12  covers the fastener stack  2  and is collocated with the fastener stack  2 . The cap  12  may 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.