Patent Publication Number: US-2022234309-A1

Title: Expandable tooling systems and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 16/459,492 filed Jul. 1, 2019. The complete disclosure of the above-identified patent application is hereby incorporated by reference for all purposes. Also incorporated by reference, for all purposes, is U.S. patent application Ser. No. 16/053,733, filed Aug. 2, 2018 by The Boeing Company, and issued as U.S. Pat. No. 11,046,027 on Jun. 29, 2021. 
    
    
     FIELD 
     This disclosure relates to systems and methods for manufacturing composite parts. More specifically, the disclosed examples relate to composite part manufacture using expandable tooling. 
     INTRODUCTION 
     Engineered composite materials are used in many applications, typically where the composite material can be made stronger, lighter, and/or less expensively than a traditional material. A variety of modern composite materials exist, but the most common are varieties of fiber-reinforced polymer composites, such as fiberglass or carbon fiber composites. 
     For many composite materials, the manufacturing process includes curing the fiber-reinforced matrix material, typically under elevated temperatures and pressures. An industrial autoclave is often used for curing composite materials, as autoclaves permit the application of both temperature and pressure under controlled conditions. 
     Unfortunately, for processes requiring an autoclave, a bottleneck may be created in the manufacturing process, with throughput dependent upon the capacity of the autoclaves available, and requiring transport of either raw materials or preassembled but uncured components to the autoclave, and subsequent transport of the cured components from the autoclave to where they will be utilized. 
     So-called “Out of Autoclave” composite manufacturing (or OOA) provides an alternative to traditional industrial curing processes typically used for composite manufacture. An ideal OOA curing process would achieve the same quality of composite component as an industrial autoclave, without requiring treatment within a traditional autoclave. 
     SUMMARY 
     The present disclosure provides systems, apparatus, and methods relating to expandable tooling for curing composite structures. 
     In some examples, the present disclosure relates to methods of manufacturing a composite workpiece that include adding an expandable element to an internal volume of the constraining container proximate to an uncured composite workpiece supported on a rigid form, where the unexpanded element is configured to expand when a predetermined change is produced in an attribute of the unexpanded element; expanding the expandable element by producing the predetermined change in the attribute of the unexpanded element so that an expansion of the expandable element applies a resulting pressure to the workpiece supported on the rigid form within the internal volume; and curing the composite workpiece while the resulting pressure is applied to the workpiece supported on the rigid form. 
     In some examples, the present disclosure relates to methods of manufacturing a composite component that include adding a thermally-activated expandable element to an internal volume of a constraining container housing an uncured composite component supported on a rigid form; expanding the thermally-activated expandable element by heating the thermally-activated expandable element to at least a predetermined temperature; and curing the composite component within the internal volume of the constraining container while the expanded element applies pressure to the uncured component. 
     Features, functions, and advantages can be achieved independently in various examples of the present disclosure, or can be combined in yet other examples, further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart depicting steps of an illustrative aircraft manufacturing and service method. 
         FIG. 2  is a schematic diagram of an illustrative aircraft. 
         FIG. 3  is an isometric view of an illustrative aircraft. 
         FIG. 4  is a back perspective view of a portion of an illustrative aircraft bulkhead including illustrative stiffeners. 
         FIG. 5  is a perspective view of a portion of an illustrative aircraft wing including stiffeners. 
         FIG. 6  is a perspective view of an illustrative composite aircraft hat stiffener attached to a skin. 
         FIG. 7  is an exploded isometric view of the stiffener and skin of  FIG. 6 . 
         FIG. 8  depicts an illustrative workpiece assembly including an uncured composite workpiece disposed upon a rigid form. 
         FIG. 9  depicts an illustrative workpiece assembly including an uncured composite workpiece disposed upon a rigid form. 
         FIG. 10  is an illustrative functional block diagram depicting a constraining container according to the present disclosure, enclosing an uncured composite workpiece disposed upon a rigid form, in the presence of expandable pellets. 
         FIG. 11  is a cross-sectional view of the illustrative workpiece assembly of  FIG. 9  disposed within an illustrative constraining container. 
         FIG. 12  is a cross-sectional view of the workpiece assembly and constraining container of  FIG. 11  with the addition of an expandable element in the form of multiple pellets. 
         FIG. 13  is a cross-sectional view of the workpiece assembly and constraining container of  FIG. 12  after expansion of the expandable pellets. 
         FIG. 14  depicts the addition of multiple volumetrically invariant adjuncts to the workpiece assembly and constraining container of  FIG. 12 . 
         FIG. 15  is a cross-sectional view of the workpiece assembly and constraining container of  FIG. 14  after expansion of the expandable pellets. 
         FIG. 16  is a cross-sectional view of the workpiece assembly and constraining container of  FIG. 12  with the addition of an illustrative contractible element. 
         FIG. 17  is a cross-sectional view of the workpiece assembly and constraining container of  FIG. 16  after expansion of the expandable pellets 
         FIG. 18  is a cross-sectional view of the workpiece assembly and constraining container of  FIG. 17  after the contractible element has been decreased in volume. 
         FIG. 19  is a cross-sectional view of the workpiece assembly and constraining container of  FIG. 12  with the addition of an alternative contractible element that includes a bladder. 
         FIG. 20  is a cross-sectional view of the workpiece assembly and constraining container of  FIG. 11  with the addition of an expandable element in the form of expandable pellets retained within multiple bags. 
         FIG. 21  is a cross-sectional view of the workpiece assembly and constraining container of  FIG. 20  after expansion of the expandable pellets within the multiple bags. 
         FIG. 22  is a cross-sectional view of the workpiece assembly and constraining container of  FIG. 12  with the addition of a heat-generating substance in the form of multiple packets. 
         FIG. 23  is a cross-sectional view of the workpiece assembly and constraining container of  FIG. 12  with the addition of a heat-generating substance in the form of multiple pellets. 
         FIG. 24  is a flowchart depicting steps of an illustrative method for manufacturing a composite workpiece. 
         FIG. 25  is a flowchart depicting steps of an alternative illustrative method for manufacturing a composite workpiece. 
     
    
    
     DETAILED DESCRIPTION 
     Various aspects and examples of an expandable tooling system, as well as related methods, are described below and illustrated in the associated drawings. Unless otherwise specified, an expandable tooling system, and/or its various components may, but are not required to, contain at least one of the structures, components, functionalities, and/or variations described, illustrated, and/or incorporated herein. Furthermore, unless specifically excluded, the process steps, structures, components, functionalities, and/or variations described, illustrated, and/or incorporated herein may be included in other similar devices and methods, including being interchangeable between disclosed examples. The following description of various examples is merely illustrative in nature and is in no way intended to limit the examples, their applications, or their uses. Additionally, the advantages provided by the examples and embodiments described below are illustrative in nature and not all examples and embodiments provide the same advantages or the same degree of advantages. 
     This Detailed Description includes the following sections, which follow immediately below: (1) Definitions; (2) Overview; (3) Examples, Components, and Alternatives; (4) Illustrative Combinations and Additional Examples; (5) Advantages, Features, and Benefits; and (6) Conclusion. 
     Definitions 
     The following definitions apply herein, unless otherwise indicated. 
     “Substantially” means to be predominantly conforming to the particular dimension, range, shape, concept, or other aspect modified by the term, such that a feature or component need not conform exactly, so long as it is suitable for its intended purpose or function. For example, a “substantially cylindrical” object means that the object resembles a cylinder, but may have one or more deviations from a true cylinder. 
     “Comprising,” “including,” and “having” (and conjugations thereof) are used interchangeably to mean including but not necessarily limited to, and are open-ended terms not intended to exclude additional, unrecited elements or method steps. 
     Terms such as “first”, “second”, and “third” are used to distinguish or identify various members of a group, or the like, in the order they are introduced in a particular context and are not intended to show serial or numerical limitation, or be fixed identifiers for the group members. 
     “Coupled” means to be in such relation that the performance of one influences the performance of the other, may include being connected, either permanently or releasably, whether directly or indirectly through intervening components, and is not necessarily limited to physical connection(s). 
     “Expandable” means able to be expanded, or having the potential or capability of increasing in size and/or volume. A substance or discrete element that is expandable may be capable of increasing in size or volume symmetrically, or asymmetrically. Where the expandable substance is capable of symmetric expansion, the substance undergoes an a substantially equivalent degree of expansion along each axis. Where the expandable substance exhibits asymmetric expansion, the substance can undergo a greater relative expansion along a first axis, or first and second axes, than along a different axis. 
     Overview 
     In general, an expandable tooling system includes an expandable element configured to apply positive pressure to a composite workpiece during the process of curing the workpiece. Typically, the uncured composite workpiece is disposed upon, or supported by, a rigid form which, in turn, is placed within an internal volume of a constraining container. The expandable element is then added to the internal volume of the constraining container so that it is at least proximate to the uncured composite workpiece, and on the opposing side from the rigid form. 
     Prior to and/or during the curing process, the unexpanded element is caused to expand such that it applies pressure to the interior surfaces of the constraining container, as well as the surface of the uncured composite workpiece. The expanded element typically applies the pressure resulting from the expansion of the expandable element to the composite workpiece during some or all of the curing process to facilitate consolidation. After the part has been cured, the expanded element can be removed from the constraining container prior to, simultaneously with, or after the cured composite workpiece is removed from the constraining container. 
     By employing constraining containers that are minimally larger than the uncured composite workpiece, the amount of expandable element can be minimized. Simultaneously, the use of such constraining containers for applying pressure to the uncured composite workpiece permits composite manufacturing to occur without the necessity of employing an industrial autoclave. 
     The expandable element is typically configured to expand when a predetermined change is produced in the expandable element. The predetermined change is typically a change in a physical property or chemical property or any combination thereof, and/or any other suitable property of the expandable element that is associated with expansion of the expandable element. Unless otherwise specified, expansion of the expandable element refers to an increase in the volume of the expandable element, surface area of the expandable element, and/or spatial extent of the expandable element in one or more dimensions. For example, the expandable element can be configured to expand when the temperature of the element is raised from a lower temperature, such as an ambient temperature, to a predetermined higher temperature. Accordingly, in cases in which curing the workpiece includes raising the temperature of the workpiece, the expandable element expands inside the internal volume during the curing process. The expanding or expanded element exerts pressure against the interior of the constraining container as well as the uncured composite workpiece during the curing process. 
     The expandable element can be selected so that upon expansion within the interior of a constraining container, the expanding element exerts sufficient pressure to effectively consolidate a composite material as it cures For some composite materials, an applied pressure of less than 1 atmosphere can be sufficient for consolidation and curing, while other composite materials can be more effectively cured at an applied pressure of 1 atmosphere or greater. The expandable element can be selected to exert sufficient pressure that pressures can be applied that have typically previously required an autoclave (for example, 1-5 atmospheres). 
     In one aspect, the curing process is simplified and facilitated by adding the expandable element as a plurality of expandable pellets, where the expandable pellets are configured to undergo volumetric expansion when heated to at least the predetermined temperature. The expandable element can comprise one or more expandable pellets (also called expandable beads) each configured to expand (e.g., to a predetermined volume) when heated to a predetermined temperature. For example, the composition of the expandable pellets can be designed to achieve a desired relationship between the volume of each expandable pellet and the temperature of the expandable pellet as a function of time. 
     The extent of the expansion of a given expandable pellet composition can be measured and recorded, as can the forces generated by the expansion. The formulation of the pellet composition can therefore be varied in order to obtain a desired degree of expansion and expansion force. In this way, the number and composition of expandable pellets employed can be selected such that the expansion of the plurality of expandable pellets within the known volume will apply a desired pressure upon the uncured composite workpiece at one or more stages of the curing process. After the composite workpiece has been cured, the expanded pellets can be easily removed from the constraining container. 
     The disclosed systems and methods are useful for a variety of composite materials, used in manufacturing desired components for any suitable industrial application. The presently described systems and methods are particularly useful for out-of-autoclave manufacturing of composites, such as may be desirable at a large or a remote worksite. The presently described systems and methods are additionally useful for the manufacture of components having a unique or awkward shape that may not readily be processed in an industrial autoclave. 
     Composite stiffeners or stringers, for example, are typically applied to fuselage sections and wing skins, in order to confer stiffness and strength to the aircraft panels to which they are attached while economizing on weight. For strength and rigidity, a stiffener may exhibit a concave cross-section with projecting extensions. The stiffener may additionally incorporate an overall curvature in order to match the curve of the fuselage to which it will be attached. Yet further, the stiffener may include one or more bends, or joggles, in order to accommodate one or more aircraft systems. Due to these constraints, the resulting stiffener may have a size and shape that makes it difficult to transport to and from an industrial autoclave, or may even prevent the autoclave from accommodating the uncured stiffener. 
     However, such a composite stiffener can be readily accommodated by a constraining container specifically sized and shaped for that stiffener, and the uncured composite stiffener can be disposed upon a rigid form constructed so as to define and incorporate the desired cross-sectional profile, the desired curvature, and the desired joggles in the stiffener. The composite stiffener can then readily be cured while disposed upon the rigid form while the requisite pressure is applied to the composite by a suitable expandable element. 
     Examples, Components, and Alternatives 
     The following sections describe selected aspects of exemplary removable expandable tooling, as well as related systems and/or methods. The examples in these sections are intended for illustration and should not be interpreted as limiting the entire scope of the present disclosure. Each section can include one or more distinct embodiments or examples, and/or contextual or related information, function, and/or structure. 
     A. Illustrative Applications and Associated Methods 
     The presently disclosed systems and methods may be used in any suitable industry, for the manufacture of any desired composite material. Although the examples provided herein are described in the context of aircraft manufacturing and service, these are merely illustrative examples, and should not be considered limiting the applicability of the disclosed systems and methods in any way. 
       FIGS. 1-3  depict an illustrative aircraft manufacturing and service method  100  and an illustrative aircraft  120 . Method  100  includes a plurality of processes, stages, or phases. During pre-production, method  100  can include a specification and design phase  104  of aircraft  120  and a material procurement phase  106 . During production, a component and subassembly manufacturing phase  108  and a system integration phase  110  of aircraft  120  can take place. Thereafter, aircraft  120  can go through a certification and delivery phase  112  to be placed into in-service phase  114 . While in service (e.g., by an operator), aircraft  120  can be scheduled for routine maintenance and service  116  (which can also include modification, reconfiguration, refurbishment, and so on of one or more systems of aircraft  120 ). While the examples described herein relate generally to component and subassembly manufacturing phase  108  of aircraft  120 , they can be practiced at other stages of method  100 . 
     Each of the processes of method  100  can be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator can include, without limitation, any number of aircraft manufacturers and major-system subcontractors; a third party can include, without limitation, any number of vendors, subcontractors, and suppliers; and an operator can be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIGS. 2-3 , aircraft  120  produced by illustrative method  100  can include a frame  122  with a plurality of systems  124  and an interior  126 . Examples of plurality of systems  124  include one or more of a propulsion system  128 , an electrical system  130 , a hydraulic system  132 , an environmental system  134 , a cargo system  136 , and a landing system  138 . Each system can comprise various subsystems, such as controllers, processors, actuators, effectors, motors, generators, etc., depending on the functionality involved. Any number of other systems can be included. Although an aerospace example is shown, the principles disclosed herein can be applied to other industries, such as the automotive industry, rail transport industry, and nautical transport industry. Accordingly, in addition to aircraft  120 , the principles disclosed herein can apply to other structures, such as other vehicles, e.g., land vehicles, marine vehicles, etc. 
     Apparatuses and methods shown or described herein can be employed during any one or more of the stages of the manufacturing and service method  100 . For example, components or subassemblies corresponding to component and subassembly manufacturing phase  108  can be fabricated or manufactured in a manner suitable for components or subassemblies used while aircraft  120  is operating during in-service phase  114 . Also, one or more examples of the apparatuses, methods, or combinations thereof can be utilized during production stages  108  and  110 , for example, by substantially expediting assembly of or reducing the cost to manufacture or use aircraft  120 . Similarly, one or more examples of the apparatus or method realizations, or a combination thereof, can be utilized, for example and without limitation, during maintenance and service phase  116 . 
     Any component or substructure of an aircraft that lends itself to composite manufacture can be compatible with the illustrative methods and processes described herein, including without limitation structural components, fuselage panels, bulkhead sections, and the like. In one aspect, the presently described methods are particularly useful for the manufacture of stiffeners, or stringers, used in aircraft manufacture. 
     As shown in  FIGS. 3-5 , an aircraft  120  can include one or more stiffeners  150  configured to carry loads. In some examples, stiffeners  150  are attached to skins  155  to improve the strength, stiffness, and/or buckling resistance of the skins. Stiffeners  150  can be included in any suitable part of aircraft frame  122  and/or any other suitable part of aircraft  120 .  FIGS. 3-4  depict stiffeners  150  reinforcing skin  155  in an illustrative aircraft bulkhead  160 .  FIG. 5  depicts stiffeners  150  reinforcing skin  155  in an illustrative aircraft wing  165 . 
     B. Illustrative Aircraft Stiffener 
     This section describes illustrative hat stiffener  180 , as shown in  FIGS. 6-7 . Hat stiffener  180  is an example of stiffener  150 , described above. 
     As depicted in  FIG. 6 , hat stiffener  180  includes a cap section  182  and first and second sidewalls  184  and  186  extending from opposing side portions of the cap section. In the example depicted in  FIG. 6 , first and second sidewalls  184  and  186  extend from cap section  182  at obtuse angles; in other examples, first and second sidewalls  184  and  186  can form acute angles or substantially right angles with cap section  182 . The angle between cap section  182  and first sidewall  184  may or may not be equal to the angle between cap section  182  and second sidewall  186 . Cap section  182  can be substantially planar, as depicted in  FIG. 6 , or can include curved and/or angled portions. 
     Hat stiffener  180  further includes first flange  188  extending from first sidewall  184 , and second flange  190  extending from second sidewall  186 . First and second flanges  188  and  190  extend away from each other in opposing directions and can be parallel to cap section  182  (e.g., the first and second flanges can be coplanar and can define a plane that is parallel to a plane generally defined by the cap section). First and second flanges  188  and  190  have respective bottom surfaces that can be attached to skin  155  so that hat stiffener  180  is configured to reinforce, stiffen, and strengthen the skin. A plurality of hat stiffeners  180  can be attached to an expanse of skin  155 . 
     Hat stiffener  180  is a composite part comprising one or more composite layers (also called plies) that are adhered together by curing (e.g., by application of heat and/or pressure). Skin  155  can also be a composite part comprising one or more composite layers.  FIG. 7  is an exploded view depicting illustrative composite stiffener layers  195   a,    195   b,  and  195   c  of hat stiffener  180  and illustrative composite skin layers  197   a,    197   b,  and  197   c  of skin  155 . Alternatively, hat stiffener  180  and/or skin  155  can comprise more composite layers, or fewer composite layers, than are depicted in  FIG. 7 . Hat stiffener  180  can be attached to skin  155  by curing the stiffener and the skin while they are held together, or by curing the stiffener and the skin separately and then fastening the stiffener to the skin. Hat stiffener  180  and skin  155  can each comprise one or more polymer materials, thermoplastic materials, thermosetting materials, and/or any other suitable materials depending on the desired properties for the finished workpiece. 
       FIG. 8  schematically depicts an assembly  200  that includes an uncured composite workpiece  202  corresponding to an aircraft stiffener, where the uncured composite workpiece  202  is disposed upon an appropriately-shaped rigid form  204 . In this example the upper surface  206  of rigid form  204  defines the desired cross-sectional profile of the desired final stiffener, including extensions  208  and  210  to support what will become first flange  188  and second flange  190 , respectively. Although rigid form  204  defines a concave shell, any configuration of rigid form having a suitable upper surface upon which the shape of the desired composite workpiece can be formed is an appropriate configuration. For example as shown for assembly  212  of  FIG. 9 , rigid form  214  can include a solid form, rather than a concave shell. 
     Rigid form  214  can be substantially resistant to compression, at least when pressure is applied on upper surface  206  of the rigid form, which contacts an undersurface  216  of uncured composite workpiece  202 . In this way pressure applied to the outer surface  228  of uncured composite workpiece  202  acts cooperatively with rigid form  204 ,  214  to generate compressive force upon workpiece  202 . 
     Rigid form  214  can perform the function of a conventional caul plate, or can be used in conjunction with a conventional caul plate. 
     C. Expandable Tooling System Overview 
       FIG. 10  depicts a functional block diagram illustrating a constraining container  220  that encloses a workpiece assembly  212  that includes an uncured composite workpiece  205  disposed upon a rigid form  214 , and that is optionally covered by a barrier film  234 . Constraining container  220  also includes expandable pellets  232  that are configured to expand when a predetermined change is produced in an attribute of the expandable pellets and to thereby exert pressure upon workpiece assembly  212 , and so upon uncured composite workpiece  205 . 
     Constraining container  220  can optionally further include one or more additional elements selected to facilitate or modify the curing process of workpiece  215 , such as one or more bags  251  that can enclose expandable pellets  232  to form one or more bag assemblies  252 , thereby facilitate handling of expandable pellets  232 , as well as the removal of expanded pellets after curing is complete. 
     Additional elements that may modify or moderate the pressures applied by expandable pellets  232  can include one or more volumetrically invariant (i.e. substantially noncompressible) adjuncts  240 , and/or one or more contractible elements  242  (optionally include a fluid-filled bladder  244 ), which can be reduced in volume after curing to facilitate access to workpiece assembly  212 . 
     Where expandable pellets  232  are thermally-expandable pellets, constraining container  220  can be heated externally. Alternatively or in addition, constraining container  220  can include one or more heat-generating substances  254  configured to heat expandable pellets  232  to a predetermined temperature at which the pellets will expand. 
     One or more of the optional additional elements shown in  FIG. 10  may be present in constraining container  220 , without limitation, and in any combination, as will be discussed in greater detail below. 
     D. Expandable Element 
       FIGS. 11-13  semi-schematically depict the components shown in the block diagram of  FIG. 10 .  FIG. 11  schematically depicts workpiece assembly  212  of  FIG. 9 , including uncured composite workpiece  202  disposed upon rigid form  214 . Assembly  212  is disposed within an exemplary constraining container  220 , where container  220  is constructed so as to facilitate the application of pressure upon uncured composite workpiece  205  by the expansion of an expandable element. Constraining container  220  is configured to enclose assembly  212 , so that container walls  222  define a volume  224  within the constraining container and intermediate the inner surfaces  226  of container walls  222  and the outer surface  228  of uncured composite workpiece  205 . The addition of an expandable element to container volume  224 , in an amount sufficient to make contact with both uncured composite workpiece  205  and inner surfaces  226  permits the generation and application of pressure upon surface  228  of workpiece  205  when the expandable element is expanded. 
     Constraining container  220  is typically constructed so that the addition of workpiece assembly  212  and subsequent addition of expandable element  218  is facilitated, as well as removal of the expanded element and workpiece after curing is completed. Container  220  can incorporate a removable upper surface, or lid, or feature one or more removable panels to provide access to the interior of the container. Any type of sealable opening is an appropriate opening, provided that when it is sealed, the container can withstand the pressure generated within the container. 
     In one aspect, constraining container  220  can be prepared using a variety of a cement, a plaster, or a concrete. The creation of inexpensive molded containers using cement, plaster, or concrete can help reduce the cost of composite manufacture, as the materials used for such containers can be inexpensive, and would not require sophisticated tooling to prepare. Alternatively, constraining container  220  can include multiple parts, such as a base, walls, and cover, and the multiple parts of container  220  are assembled and/or clamped together to form constraining container  220 . 
     Alternatively, constraining container  220  can be comprised of multiple parts, such that when disassembled, enhanced access is provided to rigid form  214 , for example to assist in the layup of uncured composite workpiece  205 . Upon assembly of workpiece  205 , constraining container  220  can be partially or fully assembled. In one aspect, constraining container  220  is partially assembled, and only fully assembled after the addition of expandable element  218 . Alternatively, or in addition, constraining container  220  can be configured so that it can be conveniently used to effect repair of composite materials in the field. 
     In order for appropriate compressive forces to be applied to uncured composite workpiece  205  while it is within constraining container  220 , rigid form  214  can be well-supported either by a substantially non-compressible surface, or alternatively, by another source of compressive force to be applied against the underside of rigid form  214 . As shown in  FIG. 11 , it can be sufficient for rigid form  214  to be supported by the lower wall of container  220 , or to be supported by a substantially noncompressible floor  230 . Alternatively, the assembly of workpiece and rigid form can be disposed on a layer of an expandable element, as well as proximate to the workpiece, so as to effectively apply pressure from expanding the expandable element on all sides of the workpiece and form. In yet another aspect, rigid form  214  can be incorporated into the structure of container  220  itself. That is, rigid form  214  can be a portion of, or an extension of, a lower surface of container  220 , for example. 
     Expandable element  218  can take any suitable form, without limitation. The expandable element can be added to constraining container  220 , for example, as a powder or a foam. Alternatively, or in addition, the expandable element can be added to container  220  as discrete portions of a solid or semi-solid, such as layers of an expandable element which can be draped across assembly  212 , or as smaller portions such as pellets, or beads. Where expandable element  218  is used in the form of smaller solid or semi-solid portions, expandable element  218  can be added to container  220  by adding individual sacks or bags of pellets, beads, or other smaller portions. Although  FIGS. 12-13  depict the expandable element as a plurality of pellets  232 , this is a representative depiction and should not be considered in any way limiting. 
     In one aspect, the curing process of the composite workpiece further includes applying a removable barrier film  234  to an outer surface of the uncured composite workpiece before adding expandable element  218  to internal volume  224  of constraining container  220 . In this aspect, selected chemical and/or physical interactions between expandable element  218  and workpiece  205  can be minimized and/or eliminated by the presence of barrier film  234 , which is shown in  FIG. 11 . Barrier film  234  can be selected to be resistant to heat, and to be readily removable after workpiece  205  is cured. Appropriate materials for barrier film  234  can include silicon-based films, polymer-based films, and/or fluorinated polymer-based films. 
     In one aspect, barrier film  234  is incorporated into a vacuum bag that contains workpiece  205 . In this aspect barrier film  234  may be used to reduce the porosity of the surface of workpiece  205 , and additionally or alternatively may be used to further consolidate workpiece  205  during curing by evacuating the vacuum bag while expandable element  218  is applying pressure to the workpiece. 
     Typically, expandable element  218  is added to an internal volume  224  of constraining container  220  while the expandable element is in an unexpanded state, as shown in  FIG. 12 . Prior to and/or during the curing process, expandable element  218  is made to expand (e.g., to increase in volume) to at least partially fill volume  224 , such that the expanded expandable element applies positive pressure directly or indirectly to at least some inner surfaces  226  of constraining container  220  as well as the upper and outer surface of uncured workpiece  205 . The pressure exerted by element  218  as it expands thereby helps to compress and consolidate workpiece  205  as it is cured. 
     Expandable element  218  can be configured to expand (e.g., to a predetermined volume and/or pressure) when a predetermined change is produced in an attribute of the unexpanded element. Typically, expandable element  218  is inserted into container volume  224  in an unexpanded state, the predetermined change is produced in the attribute of the unexpanded element while the unexpanded element is within volume  224 , and the unexpanded element expands in response to the produced predetermined change. The attribute of expandable element  218  can be a physical and/or chemical attribute. 
     In one aspect, the expandable element  218  can be configured to expand in volume when it interacts with water. For example, where the expandable element  218  is or includes a desiccant, the desiccant can increase in volume as water is absorbed. For example, anhydrous calcium sulfate (anhydrite) can exhibit an increase in volume of 61% when it absorbs water to form gypsum. 
     Water can be added to expandable element  218  directly, such as by adding liquid water or water vapor to the interior of constraining container  220 . Alternatively, or in addition, water or water vapor can be generated within container  220  itself, for example by an appropriate chemical reaction. 
     In one aspect, the predetermined change in an attribute of the unexpanded element includes a change in the temperature of expandable element  218  and/or the temperature of one or more portions of the expandable element. Accordingly, producing the predetermined change in the attribute of expandable element  218  can include raising the temperature of the unexpanded expandable element from a lower temperature, such as an ambient temperature (e.g., room temperature), to at least a predetermined temperature greater than the initial or ambient temperature (e.g., the predetermined temperature is a number of degrees above the ambient temperature suitable to produce a predetermined expansion of the expandable element). The expandable element then undergoes thermal expansion as a result of the increase in temperature. 
     In this aspect, the curing process can include adding a thermally-activated expandable element to the internal volume  224  of the constraining container  220 , where the thermally-activated expandable element is configured to expand when the temperature of the element is raised to at least a predetermined temperature. 
     Alternatively, or in addition, expanding the thermally-activated expandable element by heating the thermally-activated expandable element to at least a predetermined temperature can include producing a predetermined pressure against the uncured composite workpiece when the thermally-activated expandable element is heated to at least the predetermined temperature. Typically, the predetermined pressure is a pressure sufficient to adequately cure the composite material. 
     The predetermined change produced in the attribute of the unexpanded element can be a combination of two or more properties of expandable element  218 , such as a ratio or a product of quantitative values associated with properties of the expandable element, such as two materials that have different coefficients of thermal expansion. 
     The process of curing workpiece  205  can include producing the predetermined change in the attribute of expandable element  218 . Therefore, the expansion of expandable element  218  can occur automatically during the curing process. For example, the attribute can be a temperature of expandable element  218 , and heat applied to assembly  212  during the curing process can produce the predetermined change in the temperature of the expandable element. That is, heat applied to assembly  212  during the curing process can raise the temperature of expandable element  218  to at least a predetermined temperature associated with a desired volume and/or desired increase in volume. One or more properties of expandable element  218  can be designed such that the temperature change induced in the expandable element during the curing of workpiece  205  causes the expandable element to expand a desired predetermined amount as a result of thermal expansion. Alternatively, or additionally, causing expandable element  218  to expand can require additional steps beyond those required to cure workpiece  205 . For example, causing expandable element  218  to expand can include applying an electric field, injecting a liquid, gas, and/or another suitable material, and/or inducing any other suitable change in the expandable element. 
     As mentioned, expandable element  218  can have the form of a plurality of pellets  232 . In one aspect, pellets  232  can include foamable pellets configured to foam when heated to at least a predetermined foaming temperature. Pellets  232  can include a foamable material, e.g., a thermoplastic material treated with a blowing agent; a gas-filled balloon; hollow microspheres, a metal; any other suitable component configured to expand when heated, or any combination thereof. 
     Thermally-expandable pellets  232  can comprise any material capable of undergoing expansion when the predetermined foaming temperature is reached. In particular, a family of plastic polymers capable of softening when heated are called thermoplastic materials. When heated above its glass transition temperature and below its melting point, a solid thermoplastic material softens, becoming a viscous liquid. In this state, thermoplastics can be reshaped, and more specifically, can be expanded. 
     A variety of classes of thermoplastic materials are known, including acrylic polymers, acrylonitrile butadiene styrene (ABS) polymers, nylon polymers, polylactic acid (PLA) polymers, polybenzimidazole polymers, polycarbonate polymers, polyether sulfone (PES) polymers, polyetherimide (PEI) polymers, polyethylene (PE) polymers, polyphenylene oxide (PPO) polymers, polyphenylene sulfide (PPS) polymers, polyvinyl chloride (PVC) polymers, polyvinylidene fluoride (PVDF) polymers, and polytetrafluoroethylene (PTFE) polymers, among others. In particular, expandable pellets  232  that include acrylonitrile butadiene styrene (ABS) polymers can exhibit favorable physical properties when used in conjunction with the systems described herein. 
     Expandable pellets  232  can additionally include a blowing agent. Typically, a blowing agent is selected so that, when heated to at least a predetermined temperature, it forms a plurality of holes, pockets, or voids within the material of the expandable element, such that the volume of the pellet increases. For example, an appropriate blowing agent can be an inert gas that is permeated into the expandable element under pressure. Such a blowing agent can be configured to expand in a plurality of locations within pellets  232  when the temperature of the pellet is increased from an ambient or initial temperature to a predetermined higher temperature, and the expanded gas forms holes, pockets, or voids within the pellet. A blowing agent, if present, can be applied to the expandable element prior to heating. 
     the blowing agent can be, for example, a gas or liquid, such as carbon dioxide, nitrogen, one or more hydrocarbons, water, and/or any other suitable physical and/or chemical blowing agent. The blowing agent can be introduced to pellets  232  under pressure when the blowing agent is a gas, so that the gas diffuses into the pellet to render it foamable. Alternatively, or additionally, the blowing agent can comprise one or more expandable gas-filled microspheres that are embedded in the pellet when it is initially formed. Suitable microspheres can include expandable thermoplastic microspheres sold by AkzoNobel, Inc. of Chicago, Illinois under the proprietary name EXPANCEL. 
     Where expandable pellets  232  include a blowing agent, the blowing agent can be any appropriate substance capable of producing the desired degree of expansion of the resulting pellets. The blowing agent may include a physical blowing agent such as a chlorofluorocarbon, a hydrochlorofluorocarbon, a hydrocarbon, or liquid CO 2 , among others. Alternatively or in addition, the blowing agent may include a chemical blowing agent selected to react with one or more components of the expandable pellets, such as isocyanate and water for polyurethane, azodicarbonamide for vinyl, hydrazine and other nitrogen-based materials for thermoplastic and elastomeric foams, and sodium bicarbonate for thermoplastic foams, among others. 
     Where the expandable pellets include a blowing agent, the blowing agent can include a foaming agent. Whereas the blowing agent can be selected to form a gas, the foaming agent can be a material that facilitates formation of a foam, such as for example, a surfactant. Suitable foaming agents can include sodium laureth sulfate, sodium lauryl ether sulfate (SLES), sodium lauryl sulfate (also known as sodium dodecyl sulfate or SDS), and ammonium lauryl sulfate (ALS), among others. 
     A suitable number of pellets  232  of expandable element  218  to be placed within volume  224  of constraining container  220  can include any number of pellets, provided that when expanded they are able to apply a positive pressure to the surface of uncured workpiece  205  sufficient to consolidate and shape the desired workpiece during curing, and that number is dependent upon the size of volume  224 . That is, where the constraining container  220  fits more closely around the contours of assembly  212 , fewer pellets  232  may be needed. 
     The number of expandable pellets  232  needed within volume  224  can, for example, be between 10 and 100 pellets, or between 100 and 500 pellets, or between 500 and 1000 pellets, or greater than 1000 pellets, depending on the application and workpiece assembly. Typically, a length of each expandable pellet  232  is less than one centimeter. Expandable pellets  232  can be substantially uniform in size, or can include pellets of different sizes. 
     During the process of curing uncured workpiece  205 , expandable pellets  232  are made to expand from an unexpanded state to an expanded state. As described above, expandable pellets  232  can be configured to expand in response to heat applied to assembly  212  during curing. Expandable pellets  232  expand to at least partially fill volume  224 , such that the expanded expandable pellets apply positive pressure to uncured workpiece  205  as it is disposed upon rigid form  214  and the workpiece is cured.  FIG. 12  depicts expandable pellets  232  within volume  224  prior to expansion, while  FIG. 13  depicts expanded pellets  236  within volume  224  after expansion. 
     Expandable pellets  232  can be formulated so that they are at least partially deformable after, during, and/or before expansion. A degree of deformability allows expandable pellets  232  to squeeze into small gaps that might otherwise exist between pellets, between pellets and inner surfaces  226 , and/or between pellets and upper surface  228  of uncured workpiece  205 . Filling these gaps allows the ensemble of expandable pellets  232  to present a substantially smooth surface to workpiece  205 . 
     After workpiece  205  has been cured, constraining container  220  can be unsealed and/or opened as needed, and partially or fully expanded pellets  236  can be removed from volume  224 . Although expanded pellets  236  are typically readily removed from constraining container  220  after workpiece  205  has been cured, in some examples expanded pellets  236  can remain expanded and tightly packed together after workpiece  205  has been cured and cooled, which may tend to impede their removal from container  220 . Expandable pellets  232  can therefore be additionally configured in one or more ways to be more easily separated from workpiece  205 , rigid form  214 , and/or inner surfaces  226  of container  220 . 
     For example, expandable pellets  232  can be configured so that the shape and/or size of the corresponding expanded pellets  236  can be changed when desired, so that they can be more readily extracted. For example, expandable pellets  232  can be configured to shrink when cooled, so that after workpiece  205  is cured and cooled, the pellets shrink in volume, thereby facilitating their removal from the container. 
     In another aspect, the expandable pellets  232  can be modified so as to minimize sintering (self-adhesion) upon heating and expansion. Alternatively, or in addition, expandable element  218  can be configured to minimize potential adhesion with the surfaces of the container and workpiece assembly, such as by coating pellets  232  with a suitable agent configured prevent adhesion of pellets to one another, and/or to facilitate separation of expanded pellets  236  from each other and/or from container  220  after heating. 
     In one aspect, a suitable agent for adding to expandable pellets  232  can include a lubricating agent, such that adding a lubricating agent to the expandable pellets decreases adhesion between pellets before and/or after volumetric expansion of the expandable pellets. A suitable lubricating agent is one that does not interfere with curing of workpiece  205 , and prevents expanded pellets  236  from substantially adhering to one another, to the container, or to the components of the workpiece assembly. Suitable lubricating agents can include liquids, powders, or combinations thereof. When added as a powder, a suitable lubricating agent can comprise a nano-powder. Alternatively, or in addition, suitable lubricating agents can include silicon-based materials, fluorinated polymers, or other substantially inert substances. For example, a suitable lubricating agent can include polytetrafluoroethylene (PTFE) powder, PTFE nano-powder, silicone, perfluoropolyether (PFPE), perfluoroalkylether (PFAE), perfluoropolyalkylether (PFPAE), and/or the like. Such a lubricant can be applied to expandable pellets  232  before the expandable pellets are inserted into constraining container  220 . Alternatively, or additionally, a suitable lubricant can be applied to expandable pellets  232  while they are disposed inside container  220 . Coating at least some of expandable pellets  232  with a suitable lubricant can include mixing the lubricant with the plurality of pellets and/or pouring the lubricant over the plurality of pellets. Additionally, or alternatively, at least a subset of the plurality of expandable pellets  232  can be coated with a desired lubricant and then mixed in with a plurality of uncoated pellets. 
     Crystallinity and/or semi-crystallinity along the outer surfaces of expandable pellets  232  can help to prevent the pellets from sintering to each other. In one aspect, therefore, at least some of expandable pellets  232  can be configured, such as by preprocessing, to have regions of crystallinity along outer surfaces of the pellets, such that adding the thermally-activated expandable element includes adding a plurality of expandable pellets having surface regions of increased crystallinity in order to decrease adhesion between pellets before and/or after volumetric expansion of the expandable pellets. 
     Expandable pellets  232  can therefore be employed where outer surfaces of the pellets exhibit a high degree of crystallinity (e.g., a high percentage of the volume of regions of each pellet near the outer surface is crystalline). The crystallinity can be induced in expandable pellets  232  by controlling one or more factors including the material composition of the pellets, the production temperatures to which the pellets are heated during production, the times for which the pellet temperatures are maintained at the production temperatures during production, electric and/or magnetic fields applied during production, distribution of a blowing agent in the pellets, composition and/or concentration of blowing agent, and so on. The outer surfaces of pellets  232  can be crystalline before foaming, during foaming, and/or after foaming. 
     E. Illustrative Additional Elements and Associated Systems 
     1. Volumetrically Invariant Adjuncts 
     In some aspects, it may be advantageous to add one or more additional elements to the constraining container  220  with the workpiece assembly  212  and expandable pellets  232 . For example, both the application of appropriate pressure to the workpiece as well as extraction of used and expanded pellets can be improved by the addition of one or more volumetrically invariant adjuncts  240 . In this aspect, the curing process can include inserting a plurality of volumetrically invariant adjuncts into the internal volume  224  of the constraining container  220  with the expandable element  218 . 
     As used herein, a volumetrically invariant adjunct is one that does not expand, or expands only minimally, when heated to the predetermined temperature at which expandable pellets undergo expansion. The volumetric invariance of a given substance can be quantified with reference to the coefficient of thermal expansion (CTE) of the substance. A substance with a higher CTE can be expected to expand to a greater degree than a substance having a lower CTE. The volumetric invariance of two substances can therefore be directly compared by comparing their respective CTE values. 
     For example, an adjunct composed of a steel alloys can be expected to undergo only minimal expansion during heating, as steel alloys have CTE values of 6.3-7.3×10 −6  inch/inch·F. The use of borosilicate glass con offer an adjunct that undergoes even less expansion, as borosilicate glass has a CTE of 2.2×10 −6  inch/inch·F. Aluminum metal, on the other hand, can undergo relatively greater expansion, having a CTE of 1.2-1.3×10 −5  inch/inch·F. 
     A volumetrically-invariant adjunct  240  is an adjunct that is selected to maintain substantially the same volume throughout the range of pressures expected to be generated within constraining container  220  during the curing process. In addition to being selected to exhibit no or only minimal expansion during heating, an appropriate volumetrically-invariant adjunct can be selected to be substantially noncompressible under the applied pressures expected to be generated within constraining container  220 . 
     The addition of a one or more volumetrically invariant adjuncts  240  may permit the application of a desired pressure for curing workpiece  205  while using fewer expandable pellets  232 , because as pellets  232  expand, the volumetrically invariant adjuncts will transfer any unbalanced pressures from one side of the adjunct to the other, with virtually no loss of pressure. In this way, fewer expandable pellets  232  can be sufficient to cure a desired workpiece. Additionally, volumetrically invariant adjunct  240  can be reusable, representing a further increased saving in materials used during the curing process. 
     Typically, volumetrically invariant adjunct can have a volume that is approximately the same size as a single expandable pellet  232 . Alternatively the volumetrically invariant adjunct  240  can have a volume larger than a single expandable pellet  232 . Volumetrically invariant adjunct  240  can have a volume between five times and ten times larger than a volume of one of expandable pellets  232 , or a volume between ten times and twenty times larger than a volume of one of the pellets, or a volume more than twenty times larger than a volume of one of the pellets. 
     Volumetrically invariant adjuncts  240  can include any material that is insensitive to the conductions likely to occur with constraining container  220 . For example, volumetrically invariant adjuncts  240  can include a glass, a ceramic, or a metal, or a combination thereof. Volumetrically invariant adjunct(s)  240  can be spherical, cylindrical, or any other shape suitable for addition to and removal from constraining container  220 . In one aspect volumetrically invariant adjuncts  240  can include solid beads, spheres, or rods. In another aspect, volumetrically invariant adjuncts  240  can include hollow beads, spheres, or rods. 
     In one aspect, the curing process includes inserting a plurality of volumetrically invariant adjuncts into internal volume  224  of the constraining container  220  with expandable element  218 , where the plurality of volumetrically invariant adjuncts include beads or rods. 
     As shown schematically in  FIG. 14 , prior to curing workpiece assembly  200 , volumetrically invariant adjuncts  240  and unexpanded expandable pellets  232  are inserted into volume  224  of constraining container  220 . The position of volumetrically invariant adjuncts  240  within volume  224  can be selected so that some expandable pellets  232  are disposed between any volumetrically invariant adjunct and upper surface  228  of workpiece  205  during curing, as if a substantially noncompressible volumetrically invariant adjunct were instead pushed against portions of workpiece  205  during curing, the volumetrically invariant adjunct can undesirably deform workpiece  205 . Volumetrically invariant adjuncts  240  can provide additional surfaces for expandable pellets  232  to push against as they expand, which can benefit the distribution of pressure throughout volume  224  (e.g., by making the pressure distribution more uniform throughout the volume, and/or within selected portions of the volume). 
       FIG. 15  schematically depicts workpiece assembly  212  after curing and prior to removal of expanded pellets  236 , with pellets  236  having expanded to push against the volumetrically invariant adjuncts  240 , inner surfaces  226  of container  220 , and the outer/upper surfaces of workpiece  205 . During and/or prior to curing of workpiece  205 , the presence of volumetrically invariant adjuncts  240  can help push expandable pellets  232  into edge portions, corners, crevices, pockets, and/or narrow portions of volume  224 . 
     2. Contractible Elements 
     Alternatively, or in addition, the addition of a contractible element  242  to the interior of constraining container  220  along with expandable pellets  232  can provide additional and advantageous results. In this aspect, the curing process can include inserting a contractible element into the internal volume  224  of constraining container  220  with the expandable element, where the contractible element can be configured to volumetrically contract when a predetermined change is produced in an attribute of the contractible element. Although depicted schematically as spheres or cylinders in  FIGS. 16 and 17 , a suitable contractible element may have any shape, size or geometry that facilitates the manipulation of the contractible element (i.e., handling, shipping, and adding the contractible element to constraining container  220 ). 
     Typically, contractible element  242  can be configured to shrink when cooled from a heated curing temperature (e.g., a temperature achieved during curing of workpiece  205 ) to an ambient temperature, or a temperature otherwise lower compared to curing temperatures. Contractible element  242  can be larger in volume than a single one of expandable pellets  232 . For example, contractible element  242  can have a volume between five times and ten times larger than a volume of one of expandable pellets  232 , or a volume between ten times and twenty times larger than a volume of one of the pellets, or a volume more than twenty times larger than a volume of one of the pellets. 
     As shown schematically in  FIG. 16 , prior to curing workpiece  205 , contractible element  242  and unexpanded expandable pellets  232  are inserted into volume  224 . The position of contractible element  242  within volume  224  can be selected so that some expandable pellets  232  are disposed between each contractible element and the upper/outer surface of workpiece  205  during curing of the workpiece. If contractible element  242  were instead pushed against portions of workpiece  205  during curing, the contractible element can undesirably deform the workpiece unless it is configured to conform to the walls of container  220  when expanded. Contractible element  242  can provide a surface for expandable pellets  232  to push against as they expand, which can benefit the distribution of pressure throughout volume  224  (e.g., by making the pressure distribution more uniform throughout the volume, and/or within selected portions of the volume). 
       FIG. 17  schematically depicts workpiece assembly  212  after curing and prior to cooling contractible element  242 , with pellets  236  having expanded to push against the contractible elements and surfaces of assembly  212 . During and/or prior to curing of workpiece  205 , contractible elements  242  can help push expandable pellets  232  into edge portions, corners, crevices, pockets, and/or narrow portions of volume  224 . Contractible elements  242  can be configured to expand during the curing process, or to begin the curing process at their maximal volume, and then contract after the curing process is complete. 
       FIG. 18  schematically depicts assembly  212  after curing and after contractible element  242  has been reduced in volume (e.g., by cooling and/or deflation). Where contractible element  242  is configured to shrink upon cooling, reducing contractible element  242  in volume can include cooling the contractible element with a cooling mechanism (e.g., one or more fans, water chillers, thermoelectric coolers, etc.). Additionally, or alternatively, contractible element  242  can be allowed to cool naturally toward an ambient temperature. As shown in  FIG. 18  when shrunken, contractible element  242  can fit loosely within volume  224  and/or within the plurality of expanded pellets  236  within volume  224 , and therefore can be extracted from the constraining container  220  relatively easily. Typically, contractible element  242  can be removed from container  220 &lt; and then the tightly packed expanded pellets  236  are removed. Removing contractible element  242  leaves yet additional space within volume  224 , allowing expanded pellets  236  to move more easily and therefore to be extracted more easily. Alternatively, contractible element  242  and expanded pellets  236  can be removed substantially simultaneously, or at least some of the pellets can be removed prior to removal of the contractible element. 
     In one aspect, external surfaces of contractible element  242  can be configured to stick to expanded pellets  236 , such that at least some of the expanded pellets  236  are removed from volume  224  along with the contractible element  242  when the contractible element is removed from the constraining container  220 . For example, surfaces of contractible element  242  can include one or more adhesives, high-friction materials, and/or shapes (e.g., ribbing, indentations, and/or relief patterns) configured to capture one or more expanded pellets  236  such that the captured pellets can be more readily removed along with the contractible element. 
     Contractible element  242  can include a solid material configured to contract when cooled from a heated curing temperature of the curing process to an ambient or other temperature lower than the curing temperature. For example, contractible element  242  can include a metal material. In examples in which at least a portion of constraining container  220  is elongate, for example in order to accommodate an elongate workpiece, such as an uncured aircraft stiffener workpiece, contractible element  242  can comprise a metal rod positioned longitudinally within volume  224 . Where contractible element  242  is substantially elongate, the cross-sectional shape of the element can be circular. Additionally, or alternatively, contractible element  242  can have a different cross-sectional shape, such as oblong, square, triangular, hexagonal, polygonal, and/or irregular. Contractible element  242  can be a solid and/or hollow object having a spherical, planar, rectangular, conical, and/or irregular shape. 
     In an alternative aspect, contractible element  242  can comprise a bladder  244  (see  FIG. 19 ). Bladder  244  (also referred to as a balloon) can contain a fluid  246 , and can be configured to be used in conjunction with expandable pellets  232  in the manner described in the associated description above. 
     Fluid  246  can be confined within one or more bladder walls  248  comprising an elastic material (e.g., silicone). The constituents of fluid  246  and/or bladder walls  248  can be selected to achieve a desired volume and/or pressure of bladder  244  at the temperatures associated with curing workpiece  205 . Fluid  246  can include, without limitation, a liquid, a gas, a super-critical fluid, or a combination thereof. Where contractible element  242  includes a bladder, the bladder can be a sealed bladder and fluid  246  contained therein can be configured to expand and contract as the fluid within expands and contracts, and in particular bladder  244  and fluid  246  can be configured as a contractible element such that the volume of bladder  244  can be reduced when cooled from the curing temperature to a lower or ambient temperature. 
     Alternatively, or in addition, the volume of bladder  244  can be reduced by deflation, such as by puncturing one or more walls  248  of bladder  244 . In examples in which bladder  244  is deflated by puncturing, the bladder can be disposable, and/or can be repairable for reuse. 
     Alternatively, or in addition, bladder  244  can be configured to be opened to allow an input or egress of fluid  246 , to control the volume of the bladder. For example, bladder  244  can be coupled to a valve  249  that can be configured to be closed to retain fluid  246  within the bladder, opened to allow fluid  246  to exit the bladder, or alternatively valve  249  can be configured to be in fluid communication with a source  250  of fluid  246 , such that additional fluid  246  can be added to bladder  244  to achieve a desired bladder volume. 
     Fluid source  250  can include a reservoir for holding fluid  246  and/or a pump for pumping fluid  246  into or out of bladder  244 . Adjusting the volume and/or pressure of fluid  246  within bladder  244  allows the pressure exerted by the bladder on adjacent portions of expandable element  218 , expandable pellets  232 , and/or assembly  212  to be selectively adjusted without directly adjusting the temperature of the bladder or the fluid within the bladder. 
     In one aspect, bladder  244  and/or valve  249  can be configured to not be in fluid communication with source  250  during curing of workpiece  205 . For example, bladder  244  can be at least partially filled with fluid  246  and then disconnected from fluid source  250  prior to inserting the bladder into volume  224  of container  220 . Alternatively, or additionally, bladder  244  can be inserted into volume  224  and at least partially filled with fluid  246  while inside the container, and then disconnected from fluid source  250  prior to curing workpiece  205 . 
     Although useful as a contractible element, the combination of fluid source  250 , valve  249 , and bladder  244  can alternatively or additionally be used to supplement the pressure applied by expandable element  218 . That is, fluid  246  can be added to bladder  244  to enlarge bladder  244  and apply additional pressure to workpiece  205 . Alternatively, or in addition, bladder  244  can be disposed between expandable element  218  and workpiece  205 , so that the pressure applied by expandable element  218  is applied more uniformly to workpiece  205 . 
     Bladder  244  can alternatively or additionally be at least partially filled with a foaming agent configured to expand when heated or release a quantity of gas having sufficient pressure and/or volume to apply a predetermined pressure to inner surface  226  of constraining container  220 . Accordingly, bladder  244  can be an alternative example of an expandable element  218 . 
     2. Additional Pellets 
     Expandable pellets  232  can be combined with a plurality of additional pellets that, while also expandable, are configured to expand to a lesser degree than expandable pellets  232  when heated to a predetermined temperature. Expanding less than expandable pellets  232  when heated to the predetermined temperature can include substantially not expanding when heated to the predetermined temperature (e.g., having substantially the same volume at the predetermined temperature as at an ambient temperature lower than the predetermined temperature.) For example, the additional pellets can have a coefficient of thermal expansion that is less than ten percent of a coefficient of thermal expansion of the expandable pellets. A mixture of expandable pellets  232  and such additional pellets can facilitate the extraction of expanded pellets  236  from constraining container  220 . 
     Alternatively, or in addition, expandable pellets  232  can be combined with a plurality of additional pellets configured to shrink when cooled from the heated curing temperature toward an ambient temperature. Such contractible additional pellets are an example of contractible element  242 , described above. 
     3. Pellet Extraction Systems 
     In those cases where the geometry of constraining container  220  can hinder the removal of expanded pellets  236 , a variety of approaches can be employed to facilitate the extraction of expanded pellets  236  from volume  224  of container  220 , such as using magnetically attractable beads in cooperation with a magnetically attractable bead extraction system. For example, magnetically attractable beads can include steel beads, and a complementary magnetically attractable element to aid in removing the magnetically attractable beads can be a permanent magnet. 
     Alternatively or in addition, a pressurized fluid extraction system can be employed to extract expanded pellets  236  from container  220 . A pressurized fluid extraction system can include a pressurized fluid source configured to force a pressurized fluid into container  220  to flush expanded pellets  236  out of the interior of constraining container  220 . The pressurized fluid can comprise any suitable fluid, such as an inert gas, air, and/or any other suitable gas or liquid. 
     Alternatively or in addition, a vacuum extraction system can be configured to extract expanded pellets  236  from volume  224  of constraining container  220 . A vacuum extraction system can include a vacuum source configured to create a region of low gas pressure and/or partial vacuum adjacent to pull expanded pellets  236  out of constraining container  220 , and further include a receptacle configured to collect the expanded pellets. 
     4. Bagged Expandable Pellets 
     Adding the expandable element  218  to the internal volume  224  of the constraining container  220  can include adding a plurality of thermally-expandable pellets  232  to the internal volume of the constraining container while the pellets are retained within a flexible bag. 
     In this aspect, expandable pellets  232  can be prepackaged into portions by placing a predetermined amount of expandable pellets  232  within a flexible sack or bag  250 , where the bags are configured to be added directly to internal volume  224  of constraining container  220 . Typically, the composition of bag  250  is selected so that the bag can withstand the conditions under which workpiece  205  is cured, as well as withstanding the internal pressures created upon expansion of pellets  232 . 
     The principle advantages offered by bagged expandable pellets include the substantially greater simplicity of handling the pellets, both before and after expansion. Bagged expandable pellets  232  can be more readily portioned out at a job site, and can be more easily transported to where at the job site they are needed. Additionally, the amount of foaming agent included in the expandable pellets or the pre-impregnation of expandable pellets can be metered to a certain dosage, which when combined with metering of bag contents with a known amount of expandable pellets can facilitate calculations for an appropriate amount of expandable pellets  232 , and thereby enhance the production rate of workpiece assembly curing. 
     Addition of expandable pellets to a constraining container can include adding one or more bags of expandable pellets to the constraining container before container  220  is sealed and the workpiece cured. Then, after curing is complete, the resulting expanded pellets  236  can be readily removed from container  220  by removing the now expanded bags and their contents from the container. 
       FIG. 20  schematically depicts a plurality of bag assemblies  252 , comprising bags  251  that are at least partially filled with expandable pellets  232 . Bag assemblies  252  are shown disposed within constraining container  220  with assembly  212 . A variety of bags  251  can be used to construct bag assemblies  252 . Such bags  251  can be configured to contain expandable pellets  232  and/or another type of expandable element  218 , and are additionally configured to permit the expandable pellets to expand (e.g., to apply a predetermined pressure to interior surfaces of an internal volume containing the bag, as described above). Bag  251  can be selected so that it simply provides sufficient internal volume that the full expansion of expandable pellets  232  within bag  251  is accommodated. Alternatively, or in addition, the material of bag  251  can be selected to be partly or wholly expandable (stretchable) itself, so that the expansion of expandable pellets  232  can be accommodated by bag  251 . 
       FIG. 21  depicts the constraining container  220  of  FIG. 20  after workpiece  205  has been cured. Volume  224  of constraining container  220  is substantially filled with bag assemblies  253 , which comprise bags  251  that now enclose expanded pellets  236 . The expanded pellets  236  can be extracted from volume  224  of constraining container  220  by opening the container and removing expanded bag assemblies  253  from the container while some or all of the expanded pellets  236  remain contained in bag  251 . Where expanded bag assemblies  253  are removed from container  220  intact, with expanded pellets  236  remaining confined within bag  251 , no clean-up of spilled or lost pellets is required. However, expanded pellets  236  can alternatively be removed from constraining container  220  by opening one or more bag assemblies  253  and extracting the expanded pellets  236  from bag  251 . In order to facilitate the removal of expanded pellets  236  from bag  251 , the walls of bag  251  can include a hatch, door, zipper, and/or any other closure assembly configured to be opened and closed again without damaging bag  250 . 
     F. Heating Procedures and Materials 
     The various manufacturing processes for manufacturing a composite workpiece as described herein include heating, both to effectively cure the engineered composite workpiece, and, where the expandable element is configured to undergo expansion upon a predetermined increase in temperature, to expand the expandable element so as to create adequate pressure to cure the engineered composite workpiece. 
     Heating the thermally-activated expandable element to at least the predetermined temperature within the internal volume  224  of the constraining container  220  can include externally heating the constraining container. The uncured composite workpiece can be satisfactorily cured without the requirement of an autoclave, simplifying the curing process, as any method of heating conventionally used in manufacturing can be used to heat container  220  and its contents. 
     Constraining container  220 , or one of its component parts, can be vibrated during the curing process, in order to help settle the contents of the constraining container and apply pressure to workpiece  205  more uniformly. 
     Although the presently described processes do not require an autoclave to apply pressure to the composite workpiece, container  220  can optionally be heated to a desired temperature using an industrial oven, kiln, or furnace that is relies upon combustion, electrical resistance, induction, solar, or geothermal heating, among others. Other heat transfer techniques, such as circulating heated fluid around the container, can also be useful for the present processes. 
     The disclosed processes can be particularly useful when incorporating heating materials within the constraining container  220  itself, as doing so can eliminate the requirement for external heating of constraining container  220 . In this aspect, curing the composite material can include adding a heat-generating substance  254  to the internal volume  224  of the constraining container  220  with the thermally-activated expandable element; and heating the thermally-activated expandable element to at least the predetermined temperature using the heat-generating substance. The heat-generating substance  254  can be added to container  220  as discrete or distinct packages of material, or as pellets or particulates that can be mixed with a thermally-activated expandable element  218 . In particular, where thermally-activated expandable element  218  includes expandable pellets  232 , the heat-generating substance  254  can be added to container  220  in the form of similarly-shaped and sized pellets that can be mixed with pellets  232 . 
     Heat-generating substance  254  can be selected to heat the thermally-activated expandable element to at least the predetermined temperature by undergoing an exothermic (heat-producing) change of state, or undergoing an exothermic (heat-producing) chemical reaction, as will be discussed below. 
     1. Exothermic Chemical Reaction 
     Where a heat-generating substance is selected to generate heat via an exothermic reaction, the substance typically includes the reactants required for the desired chemical reaction. The chemical reaction itself is typically energetically favored, and one or more of the reactants can therefore be isolated from the others, to prevent the reaction from proceeding until heating is desirable. The reaction is typically substantially self-sustaining, so that upon initiation by intermixing the necessary reactants, the reaction will proceed until complete. 
     A variety of appropriate heat-generating chemical reactions can be utilized for the purposes of heating the expandable element described herein, including the following exemplary illustrations. 
     One reaction typically used for flameless heating is the reaction of calcium oxide (or quicklime) with water. Calcium oxide reacts vigorously with water to produce heat, and this reaction is already used for self-heating food containers. Dry calcium oxide and water are combined, typically by piercing a wall or membrane between compartments enclosing the reactants. The exothermic reaction then proceeds, with an enthalpy of reaction of −64.8 kJ/mol: 
       CaO(s)+H 2 O(aq)→Ca(OH) 2 (s)ΔH=−64.8 kJ/mol
 
     The combination of one mole of calcium oxide with one mole of water would therefore yield −64.8 kJ of energy in the form of heat. Using the molecular weights for calcium oxide (56.1 grams/mol) and water (18.0 grams/mol), we can calculate that 74.1 grams of combined reactants would be needed to generate 64.8 kJ of heat energy. If the reaction were carried out by adding excess water to the calcium oxide, the water would act as both reactant and solvent, and in addition serve as a heat transfer medium. That is, the reaction would heat the water, and the heated water could be used to heat expandable element  218 . 
     For the purposes of illustration, a heat-generating packet that employs the calcium oxide-water reaction to generate heat might include 56.1 grams of dry calcium oxide, with 268 grams of water (or 268 mL of water) in a separate chamber. When combined, for example by piercing the membrane between the two chambers, the reaction will consume 18.0 grams of water, and the remaining 250 grams of water will be heated by the 64.8 kJ of energy released by the reaction. 
     The specific heat of liquid water is relatively high, at 4,182 J/K/Kg (Joules/degree Kelvin/Kg), but the addition of 64.8 kJ to 250 grams of water will raise the temperature of the 250 grams of water by 62 degrees Celsius. That is, if the water was initially at 20 degrees Celsius, and assuming no loss of heat to the environment, the water would be heated to approximately 82 degrees Celsius, or 180 degrees Fahrenheit. In addition to its heating capabilities, this reaction has the additional advantage of using inexpensive reactants. Further, although calcium oxide can be an irritant to skin, it is generally safe, and is sometimes even used as a dietary supplement. 
     An alternative heat-generating reaction is the reaction of magnesium metal and water to generate magnesium hydroxide and hydrogen gas. This reaction has been employed by the U.S. military to heat military rations using flameless ration heaters (or FRHs). The reaction is slow, however, and FRHs include metallic iron particles and sodium chloride to accelerate the reaction. An exemplary FRH utilizes 7.5 grams of powdered magnesium-iron alloy and 0.5 grams of salt, with the addition of 30 mL of water, to heat a 230 gram meal packet by 56 degrees Celsius (100 degrees Fahrenheit) in approximately 10 minutes. This corresponds to a release of approximately 50 kJoules of heat energy at about 80 watts. 
     Another alternative heat-generating reaction is the oxidation of iron with oxygen. Although the rusting of iron is typically not associated with heat generation, this reaction is used by some portable hand warmers. The sealed packets include moist, finely-divided iron particles, salt, and optionally appropriate catalysts for the reaction. The packets can additionally include activated charcoal and vermiculite, to help dilute the iron powder to slow the reaction, as well as diffusing the generated heat. When the sealed packet is opened, exposing the contents to oxygen, the packet can generate significant warmth for up to several hours, for some hand warmers up to 57 degrees Celsius (135 degrees Fahrenheit). 
     Yet another alternative heat-generating reaction is the reaction of copper sulfate with powdered zinc, with an enthalpy of reaction of approximately −200 kJ/mol. 
     As discussed above expandable element  218  can have the form of a plurality of foamable pellets configured to foam when heated to at least a predetermined foaming temperature. In one aspect, the foamable pellets are configured so that the foaming process is itself exothermic, such that the foaming process contributes to the heating of the expandable element  218  as well as workpiece  205 . In this aspect, an additional heating heat-generating substance may not be needed. Alternatively, the foamable pellets can be configured so that the foaming process is endothermic (heat-absorbing). A mixture of exothermic and endothermic foamable pellets may be used in order to fine-tune the temperatures reached within container  220 . 
     Where heat-generating substance  254  relies upon an exothermic chemical reaction, the substance can be added to the constraining container  220  as individual packets or packages  256  of the reactants, which can be activated by mixing or combining two or more components of the packet. Packets  256  can be layered with the expandable element  218 , arranged to be adjacent the composite workpiece  205 , or dispersed within volume  224  of container  220 , as shown in  FIG. 22 . 
     Alternatively, or in addition, heat-generating substance  254  can include a reactant that reacts with an additional component, such as oxygen or water, to produce heat. In this aspect the heat-generating substance can be opened or unsealed and then added with the expanding element  218  to container  220 . The heat-generating substance  254  can be added to container  220  in the form of pellets  258  which can be similar to, or distinct from, expandable pellets  232  in size and shape, as depicted in  FIG. 23 . 
     If desired or needed, any required additional component for the heat-generating reaction can also be added to container  220 , for example by the addition of a reagent solution containing the additional component to volume  224  of container  220 , either before or after the container is sealed. In one aspect, the heat-generating substance undergoes an exothermic reaction with water, and water is added to container  220  prior to sealing the container. In another aspect, a solution containing a necessary reactant is pumped into container  220  after it is sealed, to initiate the heat-generating reaction. In yet another aspect, where the heat-generating substance reacts with oxygen, container  220  can incorporate sufficient ventilation that oxygen can reach the heat-generating substance while container  220  is closed, without compromising the integrity of container  220  to contain the pressures created by expandable element  218 . 
     2. Exothermic Change of Phase 
     An alternative class of heat-generating substances suitable for the presently disclosed processes may not require a chemical reaction, but instead generates heat by undergoing a physical change of state. Depending on the thermodynamics of the specific molecular system, a solid undergoing dissolution in a solvent can release a significant amount of heat. Conversely, crystallization of a solid from a saturated solution can release useful heat energy. 
     For example, the dissolution of anhydrous calcium chloride is an exothermic process, and this system has been used in portable heating pads. Typically, an amount of anhydrous calcium chloride and a supply of water are contained in separate compartments, and the heating pad is activated by mixing the contents of the separate compartments, for example by squeezing the heating pad to rupture a membrane disposed between the compartments. 
     Similar to the illustrative calcium oxide-water based heating packet above, an illustrative heat-generating packet utilizing the dissolution of calcium chloride to generate heat might include 55.5 grams of dry calcium chloride (0.5 mol), separated from 200 mL of water in a separate chamber. The enthalpy of dissolution for calcium chloride is −82.8 kJ/mol, and the molecular weight of calcium chloride is 110.98 g/mol. When allowed to combine, for example by piercing a membrane between the two chambers, the calcium chloride will dissolve in the water, and the dissolution will generate 41.4 kJ of heat energy. Assuming ideal conditions where all of that energy is used to heat the 200 mL of water, and that the water is initially at 20 degrees Celsius, and the water will reach a temperature of 70 degrees Celsius (158 degrees Fahrenheit). 
     A common alternative heating system includes a packet containing a supersaturated aqueous solution of sodium acetate. Crystallization of sodium acetate trihydrate can be initiated by flexing a small disc of notched ferrous metal within the solution, which creates a nucleation site for the sodium acetate. Crystallization then occurs very rapidly, and generates significant amounts of heat. 
     The sodium acetate crystallization system offers an additional advantage that it is completely reusable, as placing the packet containing the sodium acetate crystals in boiling water redissolves the sodium acetate in the water contained in the package. Permitting the packet to cool to room temperature recreates a supersaturated solution, and the packet can be used for heating again. 
     Where the heat-generating substance relies upon a change in phase to generate heat, the heat-generating substance can be added as an individual packet or packages of the phase-changing component, which can be activated by mixing or combining two or more components of the packet, or otherwise triggering a change in phase. The packets can be layered with the expandable element  218 , arranged to be adjacent the composite workpiece  205 , or dispersed throughout volume  224  of container  220 , as shown for packets  256  in  FIG. 22 . 
     G. Illustrative Method of Manufacturing a Composite Workpiece 
     This section describes steps of an illustrative method of manufacturing a composite workpiece, as shown in flowchart  300  of  FIG. 24 . Expandable element  218  and/or associated systems can be utilized in the method steps described below. Where appropriate, reference can be made to components and systems that can be used in carrying out each step. These references are for illustration, and are not intended to limit the possible ways of carrying out any particular step of the method. 
       FIG. 24  is a flowchart illustrating steps performed in an illustrative method. Based on the present disclosure, it should be understood that additional steps can be performed, without departing from the present claims. Although various steps of flowchart  300  are described below and depicted in  FIG. 24 , the steps need not necessarily all be performed, and in some cases can be performed simultaneously or in a different order than the order shown. 
     The present illustrative method can include adding an expandable element to an internal volume of a constraining container proximate to an uncured composite workpiece supported on a rigid form, the unexpanded element being configured to expand when a predetermined change is produced in an attribute of the unexpanded element, as set out at step  302  of flowchart  300 . The method can further include expanding the expandable element by producing the predetermined change in the attribute of the unexpanded element, so that an expansion of the expandable element applies a resulting pressure to the workpiece supported on the rigid form within the internal volume, as set out at step  304  of flowchart  300 . The method can further include curing the composite workpiece while the resulting pressure is applied to the workpiece supported on the rigid form, as set out at step  306  of flowchart  300 . Optionally, the illustrative method can further include removing the expanded element from the internal volume of the constraining container, as set out at step  308  of flowchart  300 . 
     H. Illustrative Method of Manufacturing a Composite Workpiece 
     This section describes steps of an illustrative method of manufacturing a composite workpiece, as shown in flowchart  320  of  FIG. 25 . Expandable element  218  and/or associated systems can be utilized in the method steps described below. Where appropriate, reference can be made to components and systems that can be used in carrying out each step. These references are for illustration, and are not intended to limit the possible ways of carrying out any particular step of the method. 
       FIG. 25  is a flowchart illustrating steps performed in an illustrative method. Based on the present disclosure, it should be understood that additional steps can be performed, without departing from the present claims. Although various steps of flowchart  320  are described below and depicted in  FIG. 25 , the steps need not necessarily all be performed, and in some cases can be performed simultaneously or in a different order than the order shown. 
     The illustrative method of flowchart  320  can include adding a thermally-activated expandable element to an internal volume of a constraining container housing an uncured composite component supported on a rigid form, as set out at step  322 . The method can further include expanding the thermally-activated expandable element by heating the thermally-activated expandable element to at least a predetermined temperature, as set out at step  324  of flowchart  320 . The method can further include curing the uncured composite component within the internal volume of the constraining container while the expanded element applies pressure to the uncured component, as set out at step  326  of flowchart  320 . 
     I. Illustrative Combinations and Additional Examples 
     This section describes additional aspects and features of expandable tooling, presented without limitation as a series of paragraphs, some or all of which can be alphanumerically designated for clarity and efficiency. Each of these paragraphs can be combined with one or more other paragraphs, and/or with disclosure from elsewhere in this application in any suitable manner. Some of the paragraphs below expressly refer to and further limit other paragraphs, providing without limitation examples of some of the suitable combinations.
     A1. A method of manufacturing a composite workpiece, comprising adding an expandable element to an internal volume of a constraining container proximate to an uncured composite workpiece supported on a rigid form, the unexpanded element being configured to expand when a predetermined change is produced in an attribute of the unexpanded element; expanding the expandable element by producing the predetermined change in the attribute of the unexpanded element, so that an expansion of the expandable element applies a resulting pressure to the workpiece supported on the rigid form within the internal volume; and curing the composite workpiece while the resulting pressure is applied to the workpiece supported on the rigid form.   A2. The method of paragraph A1, where adding the expandable element to the internal volume of the constraining container includes adding a thermally-activated expandable element to the internal volume of the constraining container, where the thermally-activated expandable element is configured to expand when the temperature of the element is raised to at least a predetermined temperature.   A3. The method of paragraph A2, where heating the thermally-activated expandable element to at least the predetermined temperature within the internal volume of the constraining container includes externally heating the constraining container.   A4. The method of paragraph A2, further comprising adding a heat-generating substance to the internal volume of the constraining container with the thermally-activated expandable element; and heating the thermally-activated expandable element to at least the predetermined temperature using the heat-generating substance.   A5. The method of paragraph A4, where the heat-generating substance is selected to heat the thermally-activated expandable element to at least the predetermined temperature by undergoing an exothermic change of state, or undergoing an exothermic chemical reaction.   A6. The method of paragraph A2, where adding the thermally-activated expandable element includes adding a plurality of expandable pellets, where the expandable pellets are configured to undergo volumetric expansion when heated to at least the predetermined temperature.   A7. The method of paragraph A6, further comprising adding a lubricating agent to the expandable pellets to decrease adhesion between pellets before and/or after volumetric expansion of the expandable pellets.   A8. The method of paragraph A2, where adding the thermally-activated expandable element includes adding a plurality of expandable pellets having surface regions of increased crystallinity in order to decrease adhesion between pellets before and/or after volumetric expansion of the expandable pellets.   A9. The method of paragraph A1, further comprising inserting a contractible element into the internal volume of the constraining container with the expandable element, the contractible element being configured to volumetrically contract when a predetermined change is produced in an attribute of the contractible element.   A10. The method of paragraph A1, further comprising inserting a plurality of volumetrically invariant adjuncts into the internal volume of the constraining container with the expandable element.   A11. The method of paragraph A10, where inserting the plurality of volumetrically invariant adjuncts into the internal volume of the constraining container includes combining a plurality of volumetrically invariant beads or rods with the expandable element.   A12. The method of paragraph A1, further comprising applying a removable barrier film to an outer surface of the uncured composite workpiece before adding the expandable element to the internal volume of the constraining container.   B1. A method of manufacturing a composite component, comprising adding a thermally-activated expandable element to an internal volume of a constraining container housing an uncured composite component supported on a rigid form; expanding the thermally-activated expandable element by heating the thermally-activated expandable element to at least a predetermined temperature; and curing the composite workpiece within the internal volume of the constraining container while the expanded element applies pressure to the workpiece to form the component.   B2. The method of paragraph B1, further comprising inserting a contractible element into the internal volume of the constraining container with the thermally-activated expandable element, the contractible element being configured to volumetrically contract when a predetermined change is produced in an attribute of the contractible element; effecting the predetermined change in the attribute of the contractible element; and removing the contracted element and the expanded element from the internal volume of the constraining container.   B3. The method of paragraph B2, where expanding the thermally-activated expandable element by heating the thermally-activated expandable element to at least a predetermined temperature includes producing a predetermined pressure against the uncured composite workpiece when the thermally-activated expandable element is heated to at least the predetermined temperature.   B4. The method of paragraph B2, where adding the thermally-activated expandable element to the internal volume of the constraining container includes adding a plurality of thermally-expandable pellets to the internal volume of the constraining container while the pellets are retained within a flexible bag.   B5. The method of paragraph B2, where heating the thermally-activated expandable element includes heating the thermally-activated expandable element with a heat-generating substance added to the internal volume of the constraining container.   B6. The method of paragraph B5, where heating the thermally-activated expandable element includes heating the thermally-activated expandable element with a heat-generating substance while the heat-generating substance is undergoing an exothermic change of state, or undergoing an exothermic chemical reaction.   B7. The method of paragraph B1, where the composite component is a composite aircraft component.   B8. The method of paragraph B1, where the step of curing the composite workpiece within the internal volume of the constraining container is carried out in the absence of an autoclave.   

     Advantages, Features, and Benefits 
     The different examples of the expandable tooling systems and methods described herein provide several advantages over known solutions for applying positive pressure to a composite workpiece assembly while curing the assembly. For example, illustrative examples described herein allow tooling that is adaptive to various shapes, and typically does not need to be tailored to specific dimensions and/or geometry of the composite workpiece assembly. Furthermore, illustrative embodiments and examples described herein allow for manufacturing composite parts having a complicated shape, without manufacturing custom tooling having a corresponding complicated shape. Accordingly, the high cost of manufacturing the complicated tooling is saved. 
     Additionally, and among other benefits, illustrative examples described herein allow tooling that is tailorable to the pressure and temperatures associated with curing a specific composite workpiece assembly. 
     Additionally, and among other benefits, illustrative examples described herein allow tooling that is environmentally preferred. For example, expandable foam pellets can comprise a recyclable material such as polyethylene terephthalate (PET), a material from renewable resources, and/or a biodegradable material such as polylactide (PLA). 
     Additionally, and among other benefits, the expandable tooling systems and methods described herein facilitates the “out-of-autoclave” curing of composite workpieces. Appropriate constraining containers can be fabricated on site for conducting the present methods at a desired location, saving on the cost of providing an industrial autoclave, the cost of transporting workpieces to and from the industrial autoclave, and preventing the types of bottlenecks in production flow that can typically result when production resources are limited in number but widely required. 
     No known system or device can perform these functions. However, not all examples described herein provide the same advantages or the same degree of advantage. 
     CONCLUSION 
     The disclosure set forth above may encompass multiple distinct examples with independent utility. Although each of these has been disclosed in its preferred form(s), the specific examples thereof as disclosed and illustrated herein are not to be considered in a limiting sense, because numerous variations are possible. To the extent that section headings are used within this disclosure, such headings are for organizational purposes only. The subject matter of the disclosure includes all novel and nonobvious combinations and subcombinations of the various elements, features, functions, and/or properties disclosed herein. The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. Other combinations and subcombinations of features, functions, elements, and/or properties may be claimed in applications claiming priority from this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.