Method of making a device for controlling stress in joints at cryogenic temperatures

A composite resin tank includes a wall joined to a dome along a joint. A softening strip is used to control stress in the joint.

BACKGROUND INFORMATION

The present disclosure generally relates to structural joints, especially bonded joints in composite structures, and deals more particularly with a device for controlling stress in the joint at low temperatures, and with a method of making the device.

Certain structures may be specifically designed for use in cryogenic environments. For example, spacecraft and launch vehicles may employ tanks to store solid propellants or liquid fuels such as liquid hydrogen and/or liquid oxygen at cryogenic temperatures, typically below −238 degrees Fahrenheit. In order to reduce weight of the spacecraft or launch vehicle, the tank may be formed of several composite parts that are attached to each other by one or more joints. In one such application, the composite tank includes a cylindrical outer wall having a skirt joined to dome-shaped ends by bonded lap joints that are Y-shaped in cross section.

In composite cryogenic tanks of larger diameters, e.g. diameters of greater than approximately than 14 to 16 feet, the line loads in the bonded joints may result in unacceptably high levels of peak stresses in the joint, presenting the possibility of a joint leak. In order to increase the strength of the Y-joint, it may be possible to place a softening strip in a notch area of the joint. The softening strip may reduce shear stress peaking that occurs in the bond line by enabling a smoother load transmission between structures. This could be accomplished at ambient temperatures using a softening strip formed of a material such as rubber, however, at cryogenic temperatures, rubber and many other conventional materials may not remain soft, but instead harden. Upon hardening at cryogenic temperatures, the softening strip may no longer be capable of controlling shear stress in the joint.

Known materials that may be used as softening strips present several other problems in cryogenic environments. For example, such materials typically have a coefficient of thermal expansion (CTE) that may be incompatible with other composite components of the joint which have a relatively low CTE. This mis-match of CTE's may create undesirable thermally-induced stresses in the joint at cryogenic temperatures. Adding to the challenge of designing an effective softening strip is the fact that the softening strip must remain stable at the temperatures at which the other composite resin components of the tank are thermally cured. Also, it is desirable that that softening strip be able to be machined into a shape that is specific to the particular joint application.

Accordingly, there is a need for a softening strip device that is effective in controlling stresses in joints at cryogenic temperatures. There is also a need for a method of making a softening strip of the type mentioned above that is relatively simple, effective and well-suited for use in production environments.

SUMMARY

The disclosed embodiments provide a device for reducing peak stresses in joints at cryogenic temperatures. The device comprises a softening strip placed in a notch in the joint that remains soft, flexible and compliant at cryogenic temperatures, and which is effective in linearizing the stress in the joint. The softening strip may be used to transfer line loads in relatively large composite structures, and remains stable at the temperatures at which thermosetting resins used in the structures are typically cured.

According to one disclosed embodiment, a device is provided for controlling stress in a joint between two surfaces. The device comprises a three dimensional fabric adapted to be placed in the joint. The three dimensional fabric is porous and includes fibers coated with a plastic binder. The three dimensional fabric includes one of woven fibers, braided fibers, stacked layers of fibers, stitched fibers, and pinned layers of fibers. The fibers are selected from the group consisting of graphite fibers, glass fibers, aramid fibers and metal fibers. The fibers include fibers extending along three substantially orthogonal axes, and may include fibers having a diameter within the range of approximately 2 and 15 microns. The plastic binder is a polymer that is compliant at both room temperature and at cryogenic temperatures. The plastic binder may be a fluoropolymer exhibiting elastic compliance at temperatures below approximately −150° F. and stability at temperatures up to approximately 400° F. or more.

According to another embodiment, a structural assembly comprises a first composite resin structure having a first surface and a second composite resin structure having a second surface. The assembly further comprises a joint between the first and second surfaces, and a softening strip for controlling stress in the joint. The joint may be Y-shaped, and the softening strip may be wedge shaped in cross section. The softening strip may be configured to linearize the stress in the joint at cryogenic temperatures. The softening strip is a three dimensional fabric including fibers having a plastic coating that is elastically compliant at cryogenic temperatures. The plastic coating is a polymer having a melting point higher than a temperature required to cure the first and second composite resin structures. The fabric is porous, and the fibers are flexible and movable relative to each other in response to loads transmitted through the joint while at cryogenic temperatures.

According to still another embodiment, a method is provided of making a softening strip for controlling stress in a joint. The method comprises producing a fabric perform having fibers extending in three dimensions, and coating surfaces of the fibers with a thermoplastic polymer binder. Coating the surface of the fibers includes preparing an aqueous solution containing particles of the thermoplastic polymer binder, immersing the fabric in the aqueous solution, and adhering the binder particles to the surface of the fibers by drying the fabric. Coating the surfaces of the fibers may also includes placing the infused fabric in a vacuum, and using the vacuum to draw air bubbles from the aqueous solution that are trapped within the fabric. Coating the surface of the fibers further includes heating the binder particles adhered to the fibers until the binder particles melt and flow together around the fibers. The method further comprises loosening any bonds formed between the coated fibers by cold working the fabric. Cold working the fabric is performed by passing the fabric through and compressing the fabric between a pair of rollers, or pressing the fabric between two dies. The method may also comprise shaping the fabric as the bonds between the coated fibers are being loosened, and machining the fabric with the coated fibers to a desired shape suited to the joint.

DETAILED DESCRIPTION

Referring more particularly to the drawings, embodiments of the disclosure may be described in the context of the spacecraft manufacturing and service method100as shown inFIG. 1and spacecraft200as shown inFIG. 2. Turning first toFIG. 1, a diagram illustrating a spacecraft manufacturing and service method is depicted in accordance with one disclosed embodiment.

During pre-production, exemplary spacecraft manufacturing and service method100may include specification and design102of spacecraft200inFIG. 2and material procurement104. During production, component and subassembly manufacturing106and system integration108of spacecraft200inFIG. 2takes place. Thereafter, spacecraft200inFIG. 2may go through certification and delivery110in order to be placed in service112. While in service by a customer, spacecraft200inFIG. 2is scheduled for routine maintenance and service114, which may include modification, reconfiguration, refurbishment, and other maintenance or service.

Each of the processes of spacecraft manufacturing and service method100may be performed or carried out by a system integrator, a third party, and/or an operator. In these examples, the operator may be a customer. For the purposes of this description, a system integrator may include, without limitation, any number of spacecraft manufacturers and major-system subcontractors; a third party may include, for example, without limitation, any number of venders, subcontractors, and suppliers; and an operator may be a country, leasing company, military entity, service organization, and so on.

With reference now toFIG. 2, a diagram of a spacecraft is depicted in which embodiments may be implemented. In this example, spacecraft200may be produced by spacecraft manufacturing and service method100inFIG. 1. Spacecraft200may include structure202with a plurality of systems204and interior206. Examples of systems204include, for example, without limitation, one or more of propulsion system208, electrical system210, hydraulic system212, and environmental system214. Any number of systems may be included. Further, in some implementations some of the systems may not be needed. For example, when spacecraft200takes the form of a launch vehicle, environmental system214may be unnecessary.

Apparatus and methods embodied herein may be employed during any one of stages of spacecraft manufacturing and service method100inFIG. 1. For example, components or subassemblies produced in component and subassembly manufacturing106inFIG. 1may be fabricated or manufactured in a manner similar to components or subassemblies produced while spacecraft200is in service112inFIG. 1.

Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during production stages, such as component and subassembly manufacturing106and system integration108inFIG. 1as an example, without limitation. These embodiments may substantially expedite the assembly of or reduce the cost of spacecraft200.

In these examples, embodiments may be implemented to attach components in propulsion system208to structure202in spacecraft200. The different embodiments may be applied to attach any structure within spacecraft200to any other structure in other implementations.

The different embodiments take into account that it may be desirable to increase the strength of currently available joints between the structure of a spacecraft and a tank containing a propellant above current levels with respect to various forces. Forces on the joint may include, for example, without limitation, shear forces at the joint between the structure of a spacecraft and a tank. Current tanks may be metal tanks that typically use welded joints. In these examples, the joints may take the form of Y-joints. Composite tanks may be employed to provide weight savings over metal tanks. With these types of composite tanks, however, Y-joints with increased strength may not be possible with currently available Y-joint designs and techniques.

The different embodiments may use a softening strip to extend the strength of a bonded Y-joint between two structures. For example, without limitation, currently available large solid rocket motors may use a rubber softening strip in the Y-joints. The different embodiments take into account and recognize that a difficulty may be present in finding an appropriate material for a softening strip that may be usable for the different operating temperatures. With cryogenic tanks, these temperatures may be very low temperatures, such as, for example, without limitation, below −238 degrees Fahrenheit. These temperatures may also be referred to as cryogenic temperatures.

The different embodiments recognize that an appropriate material may remain soft at the different operating temperatures. Rubber materials, as currently used in solid rocket motors, may not be usable at cryogenic temperatures because these materials may become too stiff. The material needed may remain soft relative to the adherends while at cryogenic temperatures. An adherend may be a body attached to another body by the means of adhesive substance.

The different embodiments also take into account and recognize that a softening strip may have a coefficient of thermal expansion that may be matched or within some threshold limits of the adherends to prevent joint failure from thermally induced stresses.

Thus, the different embodiments provide a method and apparatus for reinforcing or forming joints, such as, for example, without limitation, Y-joints. In the different embodiments, a three-dimensional preform may be present in which a plastic matrix is impregnated into the three-dimensional preform to form a softening strip. This softening strip may be capable of remaining flexible at a temperature at which a material has a liquid form and in which the material has a gas form at an ambient temperature. In other words, the different embodiments may be used at temperatures at which a gas has a liquid state or form. As an example, without limitation, the softening strip may be capable of remaining flexible at cryogenic temperatures. A cryogenic temperature may be a temperature at which constituents of air may liquefy. A cryogenic temperature may be, for example, temperatures below around −150 degrees centigrade.

With reference now toFIG. 3, a diagram illustrating a spacecraft is depicted in accordance with another embodiment. Spacecraft300is an example of spacecraft200inFIG. 2. In this example, spacecraft300may include structure302and structure304. Structure302may be the structure of the spacecraft providing a frame or other support for other components in spacecraft300. Structure304, in this example, may be, for example, without limitation, tank306. Of course, structure304may be any other component that may be located within spacecraft300that is to be attached to structure302.

Surface308of structure302may be attached to surface310of structure304to form joint312. In these examples, joint312may take the form of Y-joint314. The attachment, in these examples, may be performed by bonding surface308to surface310. Bonding may be a process for fastening components or structures to each other. In these examples, bonding may be performed in a number of different ways. Bonding may include, for example, without limitation, using adhesives, welding, fasteners, curing processes, or some other suitable process.

The different embodiments may be applied to any type of structural joint such as, for example, without limitation, a joint in which loads peak near the shortest, stiffest load paths into a joint and then fall off to longer, softer load paths. Any extensive shear carrying lap joint may tend to transfer load more rapidly at the edges of the joint than away from the edges. This situation may be a consequence of load distribution through a structure being proportional to the relative stiffness of the available load paths.

Given the same cross-sectional areas, short load paths may be stiffer than long load paths. This situation may create the familiar phenomenon of shear peaking at joint ends. If an additional soft load path can be created outside the start of such a joint, part of the total load can be transferred through this additional path, lowering the load to be carried by the original joint and reducing shear peaking. This may be a feature of the softening strip.

A refinement in some joints may be to taper the softening strip so that the stiffness of the secondary load paths may be continually increased as the loads approach original joint. This feature may result in a more uniform transfer of shear across the joint at a consistent low level. This transfer may occur instead of the abruptly peaking shear characteristic of the simple lap joint or the two, more moderate, shear peaks that may be created by using a uniform thickness softening strip.

In this depicted example, strip316may be attached to surface308and surface310in and/or near joint312. Strip316may act as a softening strip to increase the strength of joint312with respect to forces that may be applied to joint312. These forces may include, for example, without limitation, shear forces on structures302and304. In these examples, a shear force may be a force applied to structures302and304in the direction of line318.

In the different embodiments, strip316may include three-dimensional preform320and plastic matrix322. Three-dimensional preform320may be a three-dimensional structure and may take the form of three-dimensional fabric324. This fabric may be, for example, without limitation, a fabric comprised of at least one of woven fibers, braided fibers, stacked fabric layers, and/or some other suitable material. When fibers are used, these fibers may include, for example, without limitation, graphite fibers, glass fibers, aramid fibers, metal fibers, or any other suitable structure fiber.

Plastic matrix322may be comprised of any plastic material that may retain or provide flexibility at operating temperatures for tank306. Operating temperatures for tank306may be temperatures at or near cryogenic temperatures. In these examples, a plastic may be any synthetic or semi-synthetic polymerization material or product. Polymerization may be a process of reacting monomer molecules together in a chemical reaction to form linear chains or a three-dimensional network of polymer chains. Examples of types of plastics that may be used include, for example, without limitation, fluorocarbons and urethanes. One example of a fluorocarbon that may be used is a Teflon®. polymer. Teflon® is a registered trademark of DuPont Company.

In this manner, tank306may be attached to structure302with joint312being formed in which strip316may be present. The use of strip316may provide additional reinforcement to add strength to resist various forces, such as, for example, without limitation, shear forces that may be applied to structure304and tank306.

The illustration of spacecraft300inFIG. 3is provided to illustrate one implementation for various embodiments. This illustration is not meant to imply architectural or physical limitations to a manner in which the different embodiments may be implemented. For example, structure302and structure304may be structures within another vehicle other than spacecraft300. For example, the structures may be ones present in an aircraft. In other embodiments, spacecraft300may take the form of a spacecraft such as, for example, without limitation, a shuttle, a launch vehicle, a combination of a space shuttle and launch vehicle, or some other suitable vehicle capable of space travel.

With reference now toFIG. 4, a diagram of a spacecraft is depicted in accordance with an embodiment. In this example, launch vehicle400is an example of one implementation of spacecraft300inFIG. 3. Launch vehicle400has forward section402and aft section404. Launch vehicle400may have longitudinal axis406. Launch vehicle400is an example of a spacecraft in which Y-joint314inFIG. 3may be implemented. Section408may be an example of a portion of launch vehicle400in which Y-joint314may be found and/or implemented.

With reference now toFIG. 5, a diagram of a cross-sectional portion of a launch vehicle is depicted in accordance with an embodiment. In this diagram, a cross-section of section408of launch vehicle400along longitudinal axis406is depicted.

In this example, arrow501points towards the forward part of launch vehicle400, while arrow503points to the aft portion of launch vehicle400.

In this illustrative example, tank504in launch vehicle400may include dome506, dome508, and wall510. Wall510may be a cylindrical wall in these examples. Wall510may be attached to forward skirt512and aft skirt514. Wall510may be located between forward skirt512and aft skirt514. In this example, wall510may be a pressurized portion of tank504, while forward skirt512and aft skirt514may be located at unpressurized portions of tank504.

Forward skirt512and aft skirt514are examples of structures to which tank504may be attached. In these examples, these different structures may be cylindrical in form of which only a cross-section may be seen inFIG. 5. In this depicted example, Y-joint520and Y-joint522may be present. Y-joint520may be formed from the intersection of dome506, wall510and forward skirt512. Y-joint522may be formed at the intersection of dome508, wall510, and aft skirt514. In these examples, Y-joint520and Y-joint522may be continuous in circumferential direction around axis524.

In these examples, tank504may take the form of a composite cryogenic tank. Tank504may hold a propellant, such as, for example, without limitation, liquid hydrogen and/or liquid oxygen. In this example, tank504may hold liquid hydrogen. In this example, tank504may be greater than around 16 feet in diameter. A more detailed illustration of Y-joint520in section530is shown inFIG. 6below.

Turning now toFIG. 6, a diagram of a joint is depicted in accordance with a further embodiment. As illustrated in section530, forward skirt512may include outboard skin600, core602, and inboard skin604. In this example, outboard skin600and inboard skin604may be face sheets with core602being located between those face sheets. Core602may be a low density structure element. Core602may be used, amongst other uses, to transmit loads between outboard skin600and inboard skin604. Core602may take various forms. For example, core602may have flutes, honeycombs, or other suitable forms. In these examples, a fluted structure may be a desired structure for core602.

In this illustrative example, Y-joint520may be formed at the intersection of dome506, wall510, and forward skirt512. Y-joint520may have softening strip606located in or near Y-joint520. Softening strip606may be bonded to inboard skin604and tank504.

In the different embodiments, softening strip606may be co-bonded to inboard skin604and tank504along with the bonding of inboard skin604to tank504. In other words, these different components may be co-bonded to each other at the same time. The co-bonding, in these examples, may take the form of one or more curing processes in which the composite components in softening strip606, inboard skin604, and tank504may be cured to bond these components to each other.

The illustration of tank504is presented for purposes of depicting one manner in which different embodiments may be implemented. The softening strip606, in the different embodiments, may be applied to other tank and/or skirt geometries. For example, although tank504is shown with a cylindrical wall, other tank configurations may be employed. As an example, a conical wall for wall510may be used. Also, other tanks may not be readily symmetric. Regardless of the structure or shape of tank504, a Y-joint may be used.

With reference now toFIG. 7, a diagram illustrating a softening strip is depicted in accordance with an embodiment. In the illustrated example, softening strip606is shown in a perspective view. The illustration of softening strip606in this example is only a portion of softening strip606. Softening strip606may be circular in shape to follow the perimeter of wall510. The dimensions of softening strip606may vary depending on the particular implementation. In this example, end700of softening strip606may have a thickness of around 0.2 inches in section708. In these examples, end702may have a thickness that is as thin as practicable to prevent introduction of a stress riser in Y-joint520. In these illustrative examples, a thickness of end702may be as thin as practical, based on the ease of manufacturing and/or handling.

With respect to the use of softening strip606in Y-joint520, softening strip606may have a length of around 4 inches from end700to end702as shown in section710. Further, softening strip606may extend around the full circumference of tank504in these examples. Other dimensions of softening strip606may vary depending on the particular implementation. Of course, softening strip606may have any dimensions needed to be placed within a Y-section or other space in a joint between structures. Although a constant and/or uniform cross-section is shown for softening strip606, the cross-section may vary depending on the configuration of the joint.

Softening strip606may be shaped in a manner to fit within Y-joint520as shown inFIG. 6. In this example, softening strip606may have a wedge shape in which softening strip606may be thicker at end700than at end702.

With reference now toFIG. 8, a diagram of a three-dimensional preform is depicted in accordance with an embodiment. Preform800is an example of three-dimensional preform320inFIG. 3.

In the illustrative example, preform800may be a woven three-dimensional graphite fiber preform. Of course, other types of materials and other types of structures may be used for preform800. Other examples of materials that may be used in preform800include, for example, without limitation, fiberglass fibers, boron fibers, aramid fibers, polyethylene fibers, and other suitable materials. Preform800may be formed from braided fibers or stacked or layered fabric. The stack of fabric materials may then be held together using z-pinning in which pins may be inserted and pushed into the stack of fabric in an autoclave with pressure. As another example, fabric materials for preform800may be stitched to each other in the third dimension as indicated by arrow802.

With reference now toFIG. 9, a diagram illustrating a softening strip is depicted in accordance with an embodiment. In this example, preform800may have been infused with plastic matrix900. The plastic material within plastic matrix900may be placed into preform800using any method or process suitable for placing plastic matrix900into preform800.

In these examples, plastic matrix900may be any plastic material capable of maintaining flexibility at temperatures in which a material may be in a liquid form when that material normally may be in a gas form at ambient temperatures. In these examples, ambient temperature may be the temperature of the environment around the spacecraft. In other words, the ambient temperature may be the temperature in the air around the spacecraft when the spacecraft is on the ground.

This combination of preform800and plastic matrix900forms softening strip902. Softening strip902may then be machined, cut, and/or formed in some other suitable fashion in a shape, such as softening strip606inFIG. 6for use in a Y-joint.

Preform800may restrain softening strip902against excessive thermal shrinkage that may be associated with high coefficients of expansion. These high coefficients of expansion may be typical of softening strip matrix materials, such as those for plastic matrix900, which may remain flexible at cryogenic temperatures. Excessive shrinkage in any direction may create thermally induced stresses in the bond lines between softening strip902and any adjacent structure. These types of stresses may weaken the joint. As a result, softening strip902may be constructed with preform800to provide three-dimensional reinforcement, rather than two-dimensional reinforcement.

With reference next toFIG. 10, a flowchart of a process for creating a Y-joint in a structure is depicted in accordance with an embodiment. The process illustrated inFIG. 10may be used to create a joint between a first structure and a second structure.

The process begins by placing a softening strip into a joint region for a first structure (operation1000). The softening strip is capable of remaining flexible at a temperature at which the gas has a liquid form. The process then lays up and/or positions a second structure (operation1002). Thereafter, the softening strip is bonded to the first structure and the second structure to form the joint (operation1004), with the process terminating thereafter. This bonding may be performed using various mechanisms, including co-curing, curing, or other suitable bonding techniques.

With reference toFIG. 11, a flowchart of a process for creating a Y-joint for a tank is depicted in accordance with an embodiment. The process illustrated inFIG. 11may be a detailed example of one implementation of the processes inFIG. 10. The process illustrated in FIG. may be used to create a Y-joint between a composite cryogenic tank and a composite skirt.

The process may begin by laying the dome and wall of the tank (operation1100). Thereafter, a softening strip may be placed into the joint region (operation1102). In operation1002, the softening strip may be placed into the region with an adhesive film.

Next, a skirt structure is laid up (operation1104). The skirt structure may be laid up over the area where the dome and wall may be located along with the softening strip. The components are then cured (operation1106), with the process terminating thereafter. The curing step may be performed using heat and/or pressure. This curing may be performed using an autoclave or some other suitable oven. In other embodiments, these components may be cured using an electron beam. The result of curing operation1106may be a joint similar to joint520inFIG. 5.

With reference now toFIG. 12, another flowchart of a process for creating a Y-joint between a composite cryogenic tank and a composite skirt is depicted in accordance with an embodiment.

The process may begin by laying up the dome and inboard face sheet of the wall of the tank on a tool and curing the lay up (operation1200). The dome may be, for example, without limitation, dome506, and the face sheet of the tank wall may be for a wall such as, for example, without limitation, wall510inFIG. 5. In these examples, the tool may be a mold for the shape of the different composite components.

The process may place a film adhesive on faying surfaces704and706on the softening strip (operation1202). The softening strip in operation1202may be a softening strip such as, for example, without limitation, softening strip606inFIG. 7. In these examples, faying surfaces704and706are the surfaces of a structure that are to be bonded to another structure. In other words, when two structures are bonded to each other, the surfaces that contact each other to form the joint may be referred to as the faying surfaces.

The process may then position the softening strip against the adhesive on the dome and inboard face sheet for the wall of the tank (operation1204). The process may then cure the adhesive joining softening strip to the tank using local heater blankets (operation1206). After operation1206, the softening strip may be bonded to the tank wall. The softening strip may then be trimmed into a final shape in place on the tank wall (operation1208). The process may then position a skirt tool and attach the skirt tool to the tank tool (operation1210).

The process may then place film adhesive on faying surfaces704and706for the inboard wall of skirt512(operation1212). The process may then lay up the inner wall of the skirt and cure the component in place (operation1214).

Next, adhesive may be placed on faying surfaces704and706for skirt512and core602(operation1216). Core602may be the structural elements located between two face sheets for the wall of the structure. The process may then lay up the wall sandwich core and outboard face sheet and cure the components in place (operation1218) with the process terminating thereafter. In operation1218, the outboard face sheet may be, for example, outboard skin600inFIG. 6.

With reference now toFIG. 13, a flowchart of a process for creating a softening strip is depicted in accordance with an embodiment. The process illustrated inFIG. 13may be used to create a softening strip for use in the operations inFIGS. 11 and 12.

The process may begin by forming a three-dimensional preform (operation1300). In these examples, operation1300may be performed using, for example, without limitation, braiding fibers, weaving fibers, stacking materials and performing z-pinning, or some other suitable process.

Next, a plastic matrix may be infused into the three-dimensional preform (operation1302). In these examples, the plastic matrix may be a Teflon®. polymer. With this type of polymer, a dry powder may be placed onto the three-dimensional preform and worked into the preform. For example, the infusion in operation1302may involve vibrating the preform and then applying heat and pressure to perform the infusion into the preform. The heat and pressure in operation1302may be performed using an autoclave.

The preform may be shaped (operation1304). In operation1304, the preform may be machined, cut, or shaped using some other suitable process into a shape for use in a Y-joint. The process may etch the surfaces, faying surface704and faying surface706of the softening strip (operation1306). This etching may be an acid etch. Tertra-Etch®. fluorocarbon etchant is an example of a commercially available etchant used to prepare fluorocarbon surfaces for bonding. Tertra-Etch®. fluorocarbon etchant may be available from W. L. Gore and Associates, Inc., and Tertra-Etch®. is a trademark of W. L. Gore and Associates, Inc. This etching may be used to allow an adhesive film to be attached to the surface of the preform. The process may then attach an adhesive film to each side, side708and side707, of the softening strip (operation1308), with the process terminating thereafter. The adhesive strips may allow the softening strip to be attached and cured to the skirt and the dome of the tank during the curing process.

Attention is now directed toFIG. 14which illustrates a further embodiment of a softening strip1400that may be placed in a joint between the surfaces of two structures, such as the joint520(FIG. 6) between the tank dome506and the tank wall510previously described. The softening strip1400functions to control stress in the joint520and comprises a three-dimensional porous fabric preform1424infused with a plastic binder. Specifically, the softening strip1400linearizes stress in the joint520, thereby reducing peak stress in the joint520, even at cryogenic temperatures.

The three dimensional fabric preform1424comprises fibers1402that are woven, braided, layered or otherwise arranged together such that the fiber preform1424is structurally self-supporting, and has a relatively high strain capability. The constituent fibers1402are arranged to provide the fiber preform1424with resiliency when subjected to stress, even at cryogenic temperatures. The fibers1402may comprise, without limitation, carbon fibers that are arranged along substantially orthogonal axes1408,1410,1412, and are generally proportional and evenly distributed throughout the fabric preform1424, providing the fiber preform1424with quasi-isotropic properties. In other embodiments however, the fibers1402need not be orthogonal, proportional or evenly distributed.

The use of carbon fibers provides the softening strip1400with high-strength and a relatively low CTE, resulting in a fiber preform1424that is cohesive, may be easily machined into the desired final shape and can be handled during tank manufacturing processes. Other fibers may be used, including but not limited to glass fibers, aramid fibers and metal fibers, providing they exhibit the necessary strength, and flexibility at cryogenic temperatures. As used herein, “cryogenic temperatures” includes temperatures that are generally below approximately −150 degrees F. In one embodiment, fibers1402may be used that have a diameter within the range of approximately 2 and 15 microns.

The plastic binder, when sintered as described below, forms a plastic coating1414that encapsulates the fibers1402and provides the fiber preform1424with sufficient rigidity to allow it to be machined to a final desired shape suitable for the application, while contributing to the elastic flexibility and compliance of the softening strip1400at cryogenic temperatures. The plastic binder may comprise any of the thermoplastics previously described, such as, without limitation, FEP (fluorinated ethylene propylene), which has a melting point of approximately 500 degree F. Other fluoropolymers may also be suitable. The plastic binder chosen for a particular application should be elastically compliant and have sufficient softness and flexibility both at room temperatures and at cryogenic temperatures, yet remain stable at temperatures at which the composite thermoset resin components of a structure are thermally cured, which typically may be in the range of approximately 300 and 400 degrees F. Additionally, the plastic binder should have a low CTE, relatively close to that of the fibers1402. Selecting materials for the fibers1402and the plastic binder having CTE's that are close to the CTE of the composite resin used in the tank dome506and the tank wall510will minimize any thermally induced stresses in the joint520.

InFIG. 14, the fabric preform1424is shown as comprising individual, generally orthogonal fibers1402. However, as shown inFIG. 15, each of the fibers1402illustrated inFIG. 14may be a yarn1402comprising a plurality of relatively small diameter, interlocked fibers1402athat may be twisted, bonded or otherwise joined together to form a cohesive thread, sometimes referred to in the art as a “single”. Thus, as used herein, the term “fiber” is intended to include a single fiber, a bundle of fibers or a combination of single fibers and bundles of fibers.

Referring toFIG. 16, when initially fabricated into the three-dimensional preform1424, and prior to being coated with a plastic binder, the orthogonal fibers1402may intersect and contact each other at intersections1416. As shown seen inFIG. 17, after the fiber preform1424is infused with a plastic binder, the resulting plastic coating1414surrounds the fibers1402and may encapsulate multiple fibers1402at the intersections1416. However, as shown inFIG. 14, the spacing between the fibers1402, both before and after being coated with the plastic binder, is such that the fabric preform1424contains porosities1420, rendering the softening strip1400porous. This porosity assists in maintaining the softness and flexibility of the softening strip1400at cryogenic temperatures.

Referring toFIG. 17, when loads are transmitted through the softening strip1400, the strong, but flexible fibers1402may bend, flex or otherwise move relative to each other, thereby absorbing some of the stress in the joint and reducing peak stresses. For example, the fibers1402may individually or together, shift or flex to the positions shown by the broken lines1722as loads are transmitted through the joint520(FIG. 6). As the individual fibers1402of the three-dimensional fabric preform1424are displaced in this manner, the plastic coating1414remains sufficiently soft and flexible such that it flexes and moves along with the fibers1402, even at cryogenic temperatures.

FIG. 18illustrates a graph1800showing the stress τ in the joint520(FIG. 6) along its length L, wherein the length of the softening strip1400is indicated as L1. The broken graph line1810represents the stress in the joint520in the absence of the use of the softening strip1400, and reveals that the peak stress1812occurs at the beginning of the joint520. Graph line1804represents the stress τ transmitted through the joint520along the length L1of the softening strip1400. As is apparent from graph1800, the softening strip1400assists in controlling and distributing the stress τ, causing a reduction in the peak stress τ at the beginning of the joint from the level shown at1812to the level shown at1806.

Attention is now directed toFIG. 19which illustrates the overall steps of a method of fabricating the softening strip1400. Beginning at1902a suitable three-dimensional fabric preform1424is fabricated by weaving, braiding or other suitable processes in which fibers1402are assembled together into a substantially uniform, self-supporting structure. As previously described, the fibers1402may comprise carbon or other types of fiber materials that are suitable for the application and exhibit the desired degree of strength and flexibility at cryogenic temperatures. The fabric preform1424fabricated at step1902is then coated with a suitable plastic binder in a series of steps shown at1904.

At step1906, the three-dimensional fabric preform1424is immersed in an aqueous solution of a plastic binder material such as, without limitation, a thermoplastic such as FEP as previously discussed. The aqueous solution may be made by introducing fine dry particles of the plastic binder into water or other suitable liquid carrier, in a concentration that is dependent on the application. The particles of the plastic binder may remain dispersed throughout the aqueous solution, and may not settle out, thus avoiding the need for periodic agitation or mixing of the solution. At1908, it may be necessary to work the aqueous solution into the fabric preform by massaging the fabric preform1424, due to the fact that the fabric preform1424may be tightly woven, and though porous, may not readily admit and fully saturate with the aqueous solution. This “working” of the fabric preform1424in the aqueous solution may be performed by hand or using a suitable machine that causes movement and agitation between the fabric preform1424and the aqueous solution.

At step1910, the fabric preform1424may be subjected to a vacuum while still immersed in the aqueous solution of plastic binder in order to remove any air bubbles entrapped within the fabric preform1424and assure that the fabric preform1424is completely saturated with the aqueous solution. Air bubbles are undesirable because they may prevent wetting of the fibers1402with the solution. The fiber preform1424remains immersed in the aqueous solution for the length of time necessary for the fiber preform1424to become fully saturated, such that all of the fibers1402become wetted. At step1912, the saturated fabric preform1424is removed from the aqueous solution and then dried using any suitable technique, including but not limited to simple air drying. This drying process results in the plastic binder particles in the aqueous solution adhering to each other and to the fibers1402, thereby covering the fibers1402of the fabric preform1424. At step1914, in order to cause the adhered particles of plastic binder to coalesce into a consolidated coating, the infused fabric preform1424is placed in an oven and baked at a temperature high enough to result in fuse-melting or sintering of the binder particles. Where FEP is used as the binder material, sintering may be achieved at a temperature of approximately 620 degrees F.

The sintering in step1914may result in the plastic coating causing the fibers1402of the fabric preform1424to be bound together at the intersections1416, rendering the fabric preform1424less flexible than desired. In order to achieve the desired flexibility and softness at cryogenic temperatures, the infused and sintered fabric preform1424may be mechanically worked through cold working or forming at step1916, as will be discussed later in more detail. This cold working of the infused and sintered fabric preform may fracture any bonds that may be created between the fibers1402of the fiber preform1424, particularly at the intersections1416, and/or may break or crack the plastic coating1414, thereby “loosening” the fabric preform1424so that it is more flexible and better suited to linearize the stresses in the joint520at cryogenic temperatures.

As was the case in the embodiments previously described, at step1718, the coated fabric preform1424may be machined to a shape that matches the geometry of the joint520, using any suitable machining technique. In the case of the domed tank previously discussed, the fabric preform1424may be machined into a tapered or wedge-shaped cross section so that the stiffness of the secondary load paths continually increases as the loads approach the original joint. This feature may result in a more uniform transfer of shear across the joint at a consistently low level.

Attention is now directed toFIG. 20which illustrates one technique for cold working a flat, infused three-dimensional fabric preform1424. In this example, apparatus such as a roll forming machine2002may be used to both cold work the fiber preform1424and compress it in order to “loosen” both the fibers1402and the plastic coating1414, as previously described. The roll forming machine1802may comprise a pair of rollers2004rotating in opposite directions. The fabric preform1424is fed2006into a nip2008between the rollers2004. Depending on the positions and configuration of the rollers2004, the fabric preform1424may be rolled into a desired contour or curvature suitable for the application. Also, depending the application, the fiber preform1424may be machined before or after it is cold worked.

FIG. 21illustrates another technique for simultaneously cold working and shaping a fabric preform1424into a desired shape. In this example, a pair of matched dies2100,2102respectively having matched die surfaces2104,2106are used to cold work the fiber preform1424while forming it into the desired shape. The matched dies2100,2102may be placed in a press (not shown) and the fiber preform is placed between the dies2100,2012. The press displaces2108one of the dies2100, forcing the fiber preform1424into the second die2102, thereby compressing and forming the fabric preform1424to shape.

Attention is now directed toFIG. 22which illustrates the overall steps of a method of fabricating a cryogenic tank in which the softening strip1400is used to reduce peak stresses in one or more joints520. Beginning at step2200, a composite resin tank wall510and a tank dome506are laid up. The tank wall510and the tank dome506may each comprise a multi-ply layup of a fiber reinforced thermoset resin, such as, without limitation, an epoxy or BMI (bismaleimide), reinforced with fibers such as carbon fibers. At step2202, a suitable softening strip1400is fabricated using the process previously described, and shown inFIG. 19. At step2204, the softening strip1400is placed in a joint520between the tank wall510and the tank dome506during layup step2200. The softening strip1400functions to reduce stresses in the joint520, including those occurring at cryogenic temperatures. At step2206, the tank dome506, the tank wall510, and the softening strip1400are co-bonded, as previously described in connection withFIGS. 10-13.

The different operations shown in the flowcharts may not be inclusive of all of the different steps that may be performed for the different embodiments. For example, other operations such as, for example, without limitation, preparation for bonding, shimming, caul plating, and other suitable operations may be performed during the different processes illustrated above. Also, in some embodiments, some of the operations may be performed concurrently or in different orders, depending upon the particular implementation.

Thus, the different embodiments provide a method and apparatus for increasing the strength of a joint between different structures. The different embodiments may include a three-dimensional preform with a plastic matrix impregnated or infused into the three-dimensional preform to form a softening strip. This softening strip is capable of remaining flexible at a temperature in which a material having a gas form at an ambient temperature has a liquid form.

The description of the different embodiments has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art.

Although the different illustrative examples show the use of a softening strip in a Y-joint for a spacecraft, this softening strip may be used in objects other than spacecraft. For example, without limitation, a softening strip may be used in a joint for structures in a submarine, an aircraft, a building, a dam, a manufacturing facility, a power plant, a tank, a car, or some other suitable object.

Further, different embodiments may provide different advantages as compared to other embodiments. The embodiment or embodiments selected are chosen and described in order to best explain the principles of the embodiments, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.