Patent Publication Number: US-2022234315-A1

Title: Lightweight sandwich structures and methods of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional application of U.S. patent application Ser. No. 15/967,037, filed Apr. 30, 2018, which claims priority to and the benefit of U.S. Provisional Application No. 62/527,773, filed Jun. 30, 2017, the entire content of which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Contract No. NNC15CA16C awarded by NASA. The Government has certain rights in the invention. 
    
    
     FIELD 
     The present disclosure relates generally to sandwich structures and methods of manufacturing the same. 
     BACKGROUND 
     Sandwich structures including a pair of facesheets connected by a low density core are commonly employed in aircraft and spacecraft due to their increased flexural stiffness and buckling resistance compared to stiffened plates having an equivalent mass. In sandwich structures, the facesheets are configured to carry all in-plane loads and the core transmits shear loads and increases the effective moment of inertia of the sandwich structure. 
     In applications such as space launch vehicles it is desirable to reduce core mass and increase facesheet stiffness and strength. Commonly, these properties are achieved by utilizing an ultralight core material (e.g., having a density less than 0.15 grams per cubic centimeter) and fiber reinforced composite facesheets (e.g., carbon fiber epoxy). Ideally, these sandwich structures would be formed by co-curing the facesheets to the core by laying up polymer impregnated composite plies onto exposed surfaces of the core and consolidating the plies with the application of heat and pressure because co-curing can increase the specific strength and stiffness of the facesheets, eliminate parasitic adhesive mass in the facesheets, and reduce tolerance errors for complex assemblies. 
     However, related art sandwich structures with lightweight cores are not formed by co-curing because the pressure utilized to consolidate the facesheets during co-curing exceeds the relatively low compressive strength of the lightweight core. Accordingly, co-curing cannot be utilized with related art methodologies for forming sandwich structures without damaging the lightweight core. Accordingly, some related art sandwich structures with lightweight cores are formed by separately forming and consolidating the facesheets and then attaching the consolidated facesheets to the core, which increases the mass and cost of manufacturing the sandwich structure. Alternatively, related art sandwich structures may be formed by co-curing the facesheets by consolidating the facesheets under a reduced compaction pressure (e.g., a sub-optimal compaction pressure), which limits the performance of the facesheets to carry in-plane loads and increases the parasitic adhesive mass of the sandwich structure. 
     SUMMARY 
     The present disclosure is directed to various methods of manufacturing a sandwich structure. In one embodiment, the method includes at least partially filling an open volume of an open cellular core with a sacrificial mold material, consolidating the sacrificial mold material to form a sacrificial mold, laying up a composite facesheet on each of at least two surfaces of the open cellular core, co-curing the composite facesheets by applying a consolidation temperature and a compaction pressure to the composite facesheets to form the sandwich structure, and removing the sacrificial mold. The compaction pressure is greater than a compressive strength of the open cellular core and less than a combined compressive strength of the open cellular core and the sacrificial mold. 
     The method may also include placing the open cellular core in a chamber of a mold before at least partially filling the open volume with the sacrificial mold material. 
     The at least two surfaces of the open cellular core may be in direct contact with inner surfaces of the chamber. 
     The method may also include pressing the at least two surfaces of the open cellular core into at least one spacer positioned between the open cellular core and inner surfaces of the chamber. The at least one spacer masks the at least two surfaces of the open cellular core from contact with the sacrificial mold material. The material of the at least one spacer may be silicone, rubber, closed cell foam, a polymer film, or a combination thereof. 
     The consolidation temperature may be from about 23° C. to about 180° C. 
     The compaction pressure may be from about 0.1 MPa to about 12 MPa. 
     The method may also include applying a release agent to the open cellular core before the at least partially filling of the open volume with the sacrificial mold material, and masking the at least two surfaces of the open cellular core against exposure to the release agent. 
     The at least partially filling of the opening volume with the sacrificial mold material may be performed by pouring under gravity, filling under vacuum, filling under positive pressure, sifting powder, compaction of powder, or a combination thereof. 
     The sacrificial mold material may be of eutectic salt, plaster, polyethylene glycol (PEG), polyethylene oxide (PEO), ceramic spheres, plaster, wax, or a combination thereof. 
     Each of the at least two composite facesheets may include pre-impregnated fiber reinforced polymers. 
     Each of the at least two composite facesheets may include a dry fabric reinforcement layer and a liquid resin on the dry fabric reinforcement layer. 
     The removing of the sacrificial mold may be performed by burning the sacrificial mold, dissolving the sacrificial mold, etching the sacrificial mold, fracturing the sacrificial mold, evaporating the sacrificial mold, melting the sacrificial mold, or a combination thereof. 
     The open cellular core may include a series of struts arranged in a lattice structure. Each strut of the series of struts may have a solid cross-section or a hollow cross-section. Each strut of the series of struts may be a photopolymer waveguide. 
     The open cellular core may include foam. 
     The open cellular core may include a partially connected honeycomb structure or a grid architecture. 
     A method of forming a sandwich structure according to another embodiment of the present disclosure includes at least partially filling an open volume of an open cellular core with a sacrificial mold material, consolidating the sacrificial mold material to form a sacrificial mold, laying up a composite facesheet on each of at least two common surfaces of the open cellular core and the sacrificial mold, co-curing the composite facesheets by applying a consolidation temperature and a compaction pressure to the composite facesheets to form the composite sandwich structure, and removing the sacrificial mold. The open volume of the open cellular core extends along three orthogonal axes. The compaction pressure is greater than a compressive strength of the open cellular core and less than a combined compressive strength of the open cellular core and the sacrificial mold. 
     The present disclosure is also directed to various embodiments of a sandwich structure. In one embodiment, the sandwich structure includes an open cellular core defining an open volume, a sacrificial mold at least partially filling the open volume of the open cellular core, and at least two composite facesheets bonded to at least two surfaces of the open cellular core. 
     This summary is provided to introduce a selection of features and concepts of embodiments of the present disclosure that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used in limiting the scope of the claimed subject matter. One or more of the described features may be combined with one or more other described features to provide a workable device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of embodiments of the present disclosure will become more apparent by reference to the following detailed description when considered in conjunction with the following drawings. In the drawings, like reference numerals are used throughout the figures to reference like features and components. The figures are not necessarily drawn to scale. 
         FIGS. 1A-1F  illustrate steps of forming a sandwich structure without utilizing a spacer layer according to one embodiment of the present disclosure; 
         FIGS. 1G-1H  are detail views illustrating steps of forming the sandwich structure without utilizing the spacer layer according to the embodiment illustrated in  FIGS. 1A-1F ; 
         FIGS. 2A-2D  illustrate steps of forming a sandwich structure utilizing a spacer layer according to one embodiment of the present disclosure; and 
         FIG. 3  is a flowchart illustrating steps of forming a sandwich structure according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A-1H  depict steps of a method of manufacturing a sandwich structure  100  including an open cellular core  101  and first and second facesheets  102 ,  103  coupled to the open cellular core  101  according to one embodiment of the present disclosure. 
     As illustrated in  FIG. 1A , the method includes a step of obtaining or manufacturing the open cellular core  101 . The open cellular core  101  defines an open volume (e.g., a porosity)  104 . In one or more embodiments, the open volume  104  of the open cellular core  101  may extend laterally, longitudinally, and transversely through the open cellular core  101  (e.g., the open volume  104  may extend in a thickness direction, a length direction, and a width direction of the open cellular core  101 ). That is, in one or more embodiments, the open volume  104  of the open cellular core  101  is open along three orthogonal axes. In the illustrated embodiment, the open cellular core  101  includes a series of interconnected struts  105  arranged in a lattice structure  106  (e.g., a series of repeating unit cells or half unit cells). In one or more embodiments, the open cellular core  101  may include a foam, a grid, or a partially-connected honeycomb structure. In one or more embodiments in which the open cellular core  101  includes a series of interconnected struts  105  arranged in a lattice structure  106 , the struts  105  may be solid or hollow. The open cellular core  101  may include any suitable material depending on the desired properties of the sandwich structure  100 . For instance, in one or more embodiments, the open cellular core  101  may be made out of metal (e.g., aluminum, nickel, copper), silicon carbide, silicon oxycarbide, alumina, silicon carbonitrile, polymer (e.g., acrylate, methacrylate, thiol, epoxy, urethane, polyimide), ceramic, or any combination thereof. In one or more embodiments, the open cellular core  101  may have a thickness T from approximately (about) 0.5 mm to approximately (about) 50 mm. In one or more embodiments, the open cellular core  101  may have a density from approximately (about) 0.02 grams per cubic centimeter (g/cc) to approximately (about) 1 g/cc. In one or more embodiments, the open cellular core  101  may include hollow nickel struts  105  arranged in a lattice  106  structure having a density of approximately (about) 0.4 g/cc and a thickness T of approximately (about) 13 mm. 
     With continued reference to the embodiment illustrated in  FIG. 1A , the method also includes a step of cleaning the open cellular core  101  (e.g., surfaces of the struts  105 ) to remove any contaminants from the surfaces of the open cellular core  101 , such as particulates, dust, and/or oil. In the illustrated embodiment, the method also includes a step of applying a release agent (e.g., silicone, lecithin, wax, or combinations thereof) to at least a portion of the open cellular core  101 . In one or more embodiments, the release agent may be applied to surfaces of the open cellular core  101  (e.g., surfaces of the struts  105 ) defining the open volume  104 . The release agent is configured to promote or aid in removal of a sacrificial mold (described below in a subsequent step of the method) from the open volume  104  of the open cellular core  101 . Additionally, in one or more embodiments, the method may include a step of masking one or more portions (e.g., opposing upper and lower surfaces  107 ,  108 ) of the open cellular core  101  before applying the release agent. Masking one or more portions of the open cellular core  101  is configured to prevent or protect these portions of the open cellular core  101  from being exposed to the release agent during the step of applying the release agent to the open cellular core  101 . In one or more embodiments, the surfaces  107 ,  108  of the open cellular core  101  along which the first and second facesheets  102 ,  103  will be coupled to the open cellular core  101  during a subsequent step of the method described below may be masked against exposure to the release agent. Otherwise, application of release agent to the surfaces  107 ,  108  along which the facesheets  102 ,  103  will be coupled to the open cellular core  101  might weaken the connection between the facesheets  102 ,  103  and the open cellular core  101  (e.g., masking the upper and lower surfaces  107 ,  108  of the open cellular core  101  is configured to promote interfacial adhesion between the facesheets  102 ,  103  and the open cellular core  101 ). 
     With reference now to the embodiment illustrated in  FIG. 1B , the method includes a step of inserting the open cellular core  101  into a chamber or cavity  109  defined by a mold  110 . In the illustrated embodiment, the mold  110  defines an inlet opening  111  through which sacrificial mold material  115  may be introduced into the chamber  109  in a subsequent step. Additionally, in one or more embodiments, the mold  110  may also define an outlet opening. In one or more embodiments, the mold  110  may be thermally insulated. 
     With reference now to the embodiment illustrated in  FIG. 1C , the method includes a step of introducing the sacrificial mold material  115  into the open volume  104  of the open cellular core  101  and at least partially filling the open volume  104  of the open cellular core  101  with the sacrificial mold material  115  (e.g., infiltrating at least a portion of the open volume  104  of the open cellular core  101  with the sacrificial mold material  115 ). In one or more embodiments, the step of at least partially filling the open volume  104  of the open cellular core  101  may include completely or substantially completely filling the open volume  104  of the open cellular core  101  with the sacrificial mold material  115 . In one or more embodiments, step of at least partially filling the open volume  104  of the open cellular core  101  may include inserting the sacrificial mold material  115  into the chamber  109  of the mold  110  through the inlet opening  111  and allowing the sacrificial mold material  115  to flow into the open volume  104  of the open cellular core  101 . In one or more embodiments, the outlet opening of the mold  110  may be utilized to capture excess of the sacrificial mold material  115  introduced into the chamber  109  and the open volume  104  of the open cellular core  101 , and/or the outlet opening may be utilized to capture entrapped air in the open volume  104  of the open cellular core  101 . In one or more embodiments, the sacrificial mold material  115  may include eutectic salt, plaster, polyethylene glycol (PEG), polyethylene oxide (PEO), ceramic spheres, plaster, wax, or combinations thereof. In one or more embodiments, the sacrificial mold material  115  may be a powder mixture of ceramic spheres, plaster, and PEG. In one or more embodiments, the chamber  109  of the mold  110  may be thermally insulated depending on the type of sacrificial mold material  115  utilized (e.g., the chamber  109  of the mold  110  may be thermally insulated when the sacrificial mold material  115  is molten eutectic salt). 
     In one or more embodiments, the method may include a step of completely or substantially completely filling the open volume  104  of the open cellular core  101  with the sacrificial mold material  115 . In one or more embodiments, the method may include a step of completely or substantially completely filling the chamber  109  of the mold  110  with the sacrificial mold material  115 . In this manner, the chamber  109  of the mold  110  enables integration of the sacrificial mold material  115  with the open cellular core  101  and defines the geometry of the combined open cellular core  101  and the sacrificial mold material  115  (e.g., the chamber  109  of the mold  110  defines the geometry of the parallel core-sacrificial mold material combination). In one or more embodiments, the step of introducing the sacrificial mold material  115  into the open volume of the open cellular core  101  may be performed in any suitable manner depending, for instance, on the type of sacrificial mold material  115  utilized and/or the phase of the sacrificial mold material  115  (e.g., liquid or powder). In one or more embodiments, the step of introducing the sacrificial mold material  115  into the open volume  104  of the open cellular core  101  may include pouring under gravity, filling under vacuum, filling under positive pressure, sifting and/or compaction of powder, or one or more combinations thereof. 
     The chamber  109  of the mold  110  is at least as large as the bulk volume of the open cellular core  101 . In the embodiment illustrated in  FIGS. 1A-1H , the chamber  109  is sized such that the upper and lower surfaces  107 ,  108  of the open cellular core  101  along which the facesheets  102 ,  103  will be coupled to the open cellular core  101  are in direct contact with inner surfaces (e.g., inwardly facing surfaces)  112 ,  113 , respectively, of the mold  110 . 
     With reference now to the embodiment illustrated in  FIGS. 1D and 1H , after the step of at least partially filling the open volume  104  of the open cellular core  101  with the sacrificial mold material  115 , the method includes a step of consolidating the sacrificial mold material  115  to solidify the sacrificial mold material  115  into a solid sacrificial mold  116 . The solid sacrificial mold  116 , formed by the step of consolidating the sacrificial mold material  115 , is configured to increase the compressive strength of the open cellular core  101  (e.g., the compressive strength of the combined solid sacrificial mold  116  and the open cellular core  101  exceeds the compressive strength of the open cellular core  101  alone). In one or more embodiments, the step of consolidating the sacrificial mold material  115  may be performed in any suitable manner depending, for instance, on the type of sacrificial mold material  115  utilized and/or the phase of the sacrificial mold material  115  (e.g., liquid or powder). In one or more embodiments, the step of consolidating the sacrificial mold material  115  may include curing (e.g., heating), solidification (e.g., cooling), compaction (e.g., sintering), and/or evaporation of a liquid (e.g., a solvent) in the sacrificial mold material  115 . Following the step of consolidating the sacrificial mold material  115 , the combined sacrificial mold  116  and the open cellular core  101  may be removed from the chamber  109  of the mold  110 . 
     In one or more embodiments in which the sacrificial mold  116  is porous, the method may include a step of applying a sealant on surfaces (e.g., upper and lower surfaces  117 ,  118 ) of the sacrificial mold  116  along which the composite facesheets  102 ,  103 , respectively, will be laid up in a subsequent step (e.g., a sealant may be applied to the upper and lower surfaces  117 ,  118  of the sacrificial mold  116  that will interface with (e.g., contact) the composite facesheets  102 ,  103 , respectively). The sealant is configured to prevent or inhibit the infiltration of excess adhesive into the porous sacrificial mold  116  during a subsequent step of co-curing composite facesheets  102 ,  103  to the open cellular core  101 , and the inhibition of adhesive into the porous sacrificial mold  116  is configured to aid in the removal of the sacrificial mold  116  from the open volume  104  of the open cellular core  101  during a subsequent step of the method described below. Additionally, in one or more embodiments, the method may include a step of applying a release agent on the surfaces  117 ,  118  of the sacrificial mold  116  along which the composite facesheets  102 ,  103  will be laid up in a subsequent step (e.g., a release agent may be applied to the surfaces  117 ,  118  of the sacrificial mold  116  that will interface with (e.g., contact) the composite facesheets  102 ,  103 , respectively). The release agent is configured to aid in the removal of the sacrificial mold  116  from the open volume  104  of the open cellular core  101  during a subsequent step of the method described below. In one or more embodiments, the upper and lower surfaces  107 ,  108  of the open cellular core  101 , along which the composite facesheets  102 ,  103  will be attached, may be masked against exposure to the sealant and/or the release agent applied to the sacrificial mold  116 , which is configured to promote a robust bond between the composite facesheets  102 ,  103  and the open cellular core  101 . 
     With reference now to the embodiment illustrated in  FIG. 1E , the method also includes a step of laying up the composite facesheets  102 ,  103  (e.g. composite plies) on at least two surfaces (e.g., the opposing upper and lower surfaces  107 ,  108 ) of the open cellular core  101 . In the illustrated embodiment, the sacrificial mold  116  is in parallel with the open cellular core  101  and is in series with the composite facesheets  102 ,  103  on the surfaces  107 ,  108  of the open cellular core  101 . In one or more embodiments, the composite facesheets  102 ,  103  may be pre-impregnated fiber-reinforced polymer plies. In one or more embodiments, the composite facesheets  102 ,  103  may be dry fabric reinforcement plies onto which a liquid resin is deposited. In one or more embodiments, each of the composite facesheets  102 ,  103  may have a thickness from approximately (about) 0.1 mm to approximately (about) 13 mm. In one or more embodiments, the composite facesheets  102 ,  103  may include any suitable fiber reinforcement material, such as carbon, glass, alumina, silicon carbide, boron, aramid, polyethylene, or any combination or combinations thereof. In one or more embodiments, the composite facesheets  102 ,  103  may include a matrix material, such as epoxy, silicone, urethane, cyanate ester, polyimide, bismaleimide, acrylate, carbosilane, siloxane, and/or sequisiloxane. In one or more embodiments, the composite facesheets  102 ,  103  may include a fiber reinforcement ply having continuous unidirectional fibers, woven fibers, knit fibers, braided fibers, discontinuous chopped fibers, whiskers, platelets, and/or particulates. In one or more embodiments, each of the facesheets  102 ,  103  may be an approximately (about) 1 mm thick unidirectional carbon fiber reinforced epoxy composite facesheet with a quasi-isotropic layup. 
     With continued reference to the embodiment illustrated in  FIG. 1E , after the step of laying up the composite facesheets  102 ,  103  on the surfaces  107 ,  108  of the open cellular core  101 , the method includes a step of co-curing the composite facesheets  102 ,  103  onto the surfaces  107 ,  108  of the open cellular core  101 . The step of co-curing the composite facesheets  102 ,  103  onto the surfaces  107 ,  108  of the open cellular core  101  includes consolidating the composite facesheets  102 ,  103 . In one or more embodiments, the step of consolidating the composite facesheets  102 ,  103  includes applying a consolidation temperature and a compaction pressure to the composite facesheets  102 ,  103 . In one or more embodiments, the consolidation temperate applied to the composite facesheets  102 ,  103  is from approximately (about) 23° C. to approximately (about) 180° C. In one or more embodiments, the consolidation temperature may be greater than approximately (about) 180° C. In one or more embodiments, the consolidation temperature may be selected depending on the type of matrix material in the composite facesheets  102 ,  103  (e.g., a consolidation temperature greater than 180° C. may be utilized in the step of consolidating the composite facesheets  102 ,  103  when the matrix material is a high temperature polymer, such as bismaleimides and polyimides). In one or more embodiments, the compaction pressure applied during the step of consolidating the composite facesheets  102 ,  103  may be applied by differential atmospheric pressure (e.g., a vacuum bag), hydrostatic pressure (e.g., a pressurized bladder), a platen press, and/or an autoclave. In one or more embodiments, the step of consolidating the composite facesheets  102 ,  103  may include applying a consolidation temperature of approximately (about) 177° C. under vacuum, and applying a compaction pressure of approximately (about) 1.4 MPa with a heated platen press to the composite facesheets  102 ,  103 . In the illustrated embodiment, the step of consolidating the composite facesheets  102 ,  103  includes placing the open cellular core  101 , the sacrificial mold  116 , and the composite facesheets  102 ,  103  onto a caul plate  119 , and covering the open cellular core, the sacrificial mold  116 , and the composite facesheets  102 ,  103  with a vacuum bag  120  that is sealed to the caul plate  119  with vacuum sealant  121 . Additionally, in the illustrated embodiment, the system for consolidating the composite facesheets  102 ,  103  on the surfaces  107 ,  108  of the open cellular core  101  includes a pair of rigid hard stops  122  on opposite sides of the open cellular core  101  configured to control the compaction pressure applied to the composite facesheets  102 ,  103 . In the illustrated embodiment, the system for consolidating the composite facesheets  102 ,  103  on the surfaces  107 ,  108  of the open cellular core  101  also includes a breather layer  123  and a peel ply  124  on the upper composite facesheet  102  configured to facilitate removal of the vacuum bag  120  after the step of consolidating the composite facesheets  102 ,  103 . 
     In one or more embodiments, the compaction pressure applied during the step of consolidating the composite facesheets  102 ,  103  may be from approximately (about) 0.1 MPa to approximately (about) 12 MPa. In one or more embodiments, the compaction pressure exceeds the compressive strength of the open cellular core  101 , but the compressive strength of the combined sacrificial mold  116  and the open cellular core  101  exceeds the compaction pressure. In this manner, the sacrificial mold  116  is configured to increase the compaction pressure that may be applied to consolidate the composite facesheets  102 ,  103  compared to a related art process in which the open cellular core  101  is not reinforced by a sacrificial mold. 
     Applying the compaction pressure during the step of consolidating the composite facesheets  102 ,  103  is configured to press excess resin out of the composite facesheets  102 ,  103  and thereby increase the fiber volume fraction of the composite facesheets  102 ,  103 . In one or more embodiments, the fiber volume fraction of the composite facesheets  102 ,  103  may be increased to at least approximately (about) 65% following the step of consolidating the composite facesheets  102 ,  103 . Additionally, in one or more embodiments, the excess resin that is pressed from the composite facesheets  102 ,  103  by applying the compaction pressure may flow to the interfaces between the open cellular core  101  and the composite facesheets  102 ,  103  and thereby bond the composite facesheets  102 ,  103  to the surfaces  107 ,  108  of the open cellular core  101 . Accordingly, the excess resin that is pressed from the composite facesheets  102 ,  103  and bonds the composite facesheets  102 ,  103  to the surfaces  107 ,  108  of the open cellular core  101  saves mass that would otherwise have to be applied to the interfaces between composite facesheets  102 ,  103  and the open cellular core  101  if the composite facesheets  102 ,  103  and the open cellular core  101  were separately formed and subsequently adhered together. In this manner, the step of co-curing the composite facesheets  102 ,  103  to the surfaces  107 ,  108  of the open cellular core  101  by applying a compaction pressure to the composite facesheets  102 ,  103  reduces the parasitic adhesive mass of the sandwich structure  100  compared to related art sandwich structures that are not formed by co-curing. Additionally, co-curing the composite facesheets  102 ,  103  to the open cellular core  101  by applying a compaction pressure to the composite facesheets  102 ,  103  is configured to reduce tolerance errors for sandwich structures  100  having complex geometries. For instance, during the step of co-curing the composite facesheets  102 ,  103  to the open cellular core  101  by applying the compaction pressure, the composite facesheets  102 ,  103  conform to the surfaces  107 ,  108  of the open cellular core  101  because the composite facesheets  102 ,  103  are still in a pliable (e.g., pre-cured) state, which enables complex geometries (e.g., curved facesheets) to be formed in a single step. In contrast, related art methods of forming a sandwich structure with complex geometry requires forming the composite facesheets and the core separately with the desired geometry (e.g., curvature). Forming the composite facesheets and the core separately requires additional tooling and increases the chance of assembly misalignment because the composite facesheets are fully cured before being attached to the core and therefore cannot conform to the core during processing. 
     With reference now to the embodiment illustrated in  FIG. 1F , the method also includes a step of removing the sacrificial mold  116  from the open volume  104  of the open cellular core  101 . In one or more embodiments, the process utilized during the step of removing the sacrificial mold  116  may depend, for instance, on the type of sacrificial mold material  115  and/or the process utilized during the step of consolidating the sacrificial mold material  115 . For instance, in one or more embodiments, the step of removing the sacrificial mold  116  may be performed by dissolution of the sacrificial mold  116  in water or a solvent, etching the sacrificial mold  116  in an acidic or basic bath, melting the sacrificial mold  116 , and/or vaporization or combustion of the sacrificial mold  116  at a temperature greater than the consolidation temperature. In one or more embodiments, the sacrificial mold  116  may be removed utilizing heated (e.g., 60° C.) pressurized water. Following the step of removing the sacrificial mold  116 , the open volume  104  defined by the open cellular core  101  of the sandwich structure  100  is free or substantially free of the sacrificial mold  116 . 
     In an alternate embodiment illustrated in  FIGS. 2A-2D , one or more spacer layers  114  may be introduced in the chamber  109  between the upper and lower surfaces  107 ,  108  of the open cellular core  101  and the inner surfaces  112 ,  113  of the chamber  109  (e.g., one or more spacer layers  114  may be inserted into the chamber  109  before inserting the open cellular core  101  into the chamber  109 , or the one or more spacer layers  114  may be applied to the upper surface  107  and/or the lower surface  108  of the open cellular core  101  before inserting the open cellular core  101  into the chamber  109  of the mold  110 ). In one or more embodiments, the surfaces  107 ,  108  of the open cellular core  101  along which the facesheets  102 ,  103  will be attached are pressed into the one or more spacer layers  114 , thereby deforming or penetrating the one or more spacer layers  114  (e.g., the surfaces  107 ,  108  of the open cellular core  101  may be pressed into the one or more spacer layers  114  during the step of inserting the open cellular core  101  into the chamber  109  of the mold  110 ). In one or more embodiments, the material of the one or more spacer layers  114  may include silicone, rubber (e.g., buna rubber), closed cell foam, and/or a polymer film (e.g., polyethylene terephthalate (PET) film). In one or more embodiments, each of the one or more spacer layers  114  may have a thickness t from approximately (about) 0.05 mm to approximately (about) 3.5 mm. In one or more embodiments, the one or more spacer layers  114  may have a thickness t of approximately (about) 1.6 mm. In one or more embodiments, the method may include a step of pressing each of the surfaces  107 ,  108  of the open cellular core  101  along which the facesheets  102 ,  103  will be attached into the one or more spacer layers  114 . In one or more embodiments, fewer than all of the surfaces  107 ,  108  of the open cellular core  101  along which the facesheets  102 ,  103  will be attached may be pressed into the one or more spacer layers  114 . 
     In one or more embodiments in which the surfaces  107 ,  108  of the open cellular core  101  are in direct contact with the inner surfaces (e.g., the inwardly facing surfaces)  112 ,  113 , respectively, of the mold  110  (embodiment illustrated in FIGS. 1 A- 1 H) when the open cellular core  101  is inserted into the chamber  109  of the mold  110 , the upper and lower surfaces  107 ,  108  of the open cellular core  101  are coextensive or substantially coextensive (e.g., co-planar or substantially co-planar) with the upper and lower surfaces  117 ,  118 , respectively, of the sacrificial mold  116  (see  FIG. 1H ). That is, the open cellular core  101  and the sacrificial mold  116  occupying the open volume  104  of the open cellular core  101  share a continuous, common outer mold line (e.g., continuous, common outer surfaces) following the step of at least partially filling the open volume  104  with the sacrificial mold material  115  and consolidating the sacrificial mold material  115  into the sacrificial mold  116 . Accordingly, in one or more embodiments, the composite facesheets  102 ,  103  may be laid up and co-cured on the common surfaces (e.g., the shared surfaces)  107 ,  108 ,  117 ,  118  of the open cellular core  101  and the sacrificial mold  116 . 
     In one or more embodiments in which the surfaces  107 ,  108  of the open cellular core  101  are pressed into the one or more spacers  114  (embodiment illustrated in  FIGS. 2A-2D ) when the open cellular core  101  is inserted into the chamber  109  of the mold  110 , the one or more spacers  114  mask off the surfaces  107 ,  108  of the open cellular core  101  such that the sacrificial mold material  115  does not contact the surfaces  107 ,  108  of the open cellular core  101  when the sacrificial mold material  115  is inserted into the open volume  104  of the open cellular core  101 . Accordingly, in one or more embodiments, the one or more spacers  114  create a discontinuous, offset interface (e.g., a gap G in  FIG. 2D ) between the surfaces  107 ,  108  of the open cellular core  101  and surfaces  117 ,  118 , respectively, of the sacrificial mold  116 . This discontinuous, offset interface between the surfaces  107 ,  108  of the open cellular core  101  and the surfaces  117 ,  118  of the sacrificial mold  116  allows the excess resin pressed from the composite facesheets  102 ,  103  during the step of consolidating the composite facesheets  102 ,  103  to flow (e.g., wick) into the portions of the open cellular core  101  unoccupied by the sacrificial mold  116  and thereby form a finite thickness adhesive interface between the composite facesheets  102 ,  103  and the surfaces  107 ,  108  of the open cellular core  101 . 
       FIG. 3  is a flowchart illustrating steps of a method  200  of forming a sandwich structure including an open cellular core between two composite facesheets according to one embodiment of the present disclosure. In the embodiment illustrated in  FIG. 3 , the method  200  includes a step  210  of cleaning an open cellular core defining an open volume to remove any contaminants from surfaces of the open cellular core. The open cellular core may have any suitable configuration described above, such as a series of hollow or solid interconnected struts arranged in a lattice structure, a foam, a grid, or a partially-connected honeycomb structure. 
     In the illustrated embodiment, the method  200  includes a step  220  of applying a release agent (e.g., silicone, lecithin, wax, or combinations thereof) to at least a portion of the open cellular core (e.g., surfaces of the open cellular core defining the open volume). The release agent is configured to promote or aid in removal of a sacrificial mold (formed during a subsequent step) from the open volume of the open cellular core. Additionally, in one or more embodiments, the method  200  may also include a step of masking surfaces (e.g., upper and lower surfaces) of the open cellular core against exposure to the release agent, which is configured to promote interfacial adhesion between the open cellular core and facesheets applied to these surfaces of the open cellular core during a subsequent step. 
     With continued reference to  FIG. 3 , the method  200  also includes a step  230  of inserting the open cellular core into a chamber of a mold. In one or more embodiments, the surfaces of the open cellular core along which the facesheets will be coupled to the open cellular core are in direct contact with inner surfaces (e.g., inwardly facing surfaces) of the mold. In one or more embodiments, the method  200  may include a step  240  of inserting one or more spacer layers into the chamber before the step  230  of inserting the open cellular core into the chamber, or the method may include a step of applying the one or more spacer layers to the surfaces of the open cellular core before the step  230  of inserting the open cellular core into the chamber of the mold. 
     The method  200  also includes a step  250  of introducing a sacrificial mold material ( 115  in  FIGS. 1C, 1G, and 2B ) (e.g., eutectic salt, plaster, PEG, PEO, ceramic spheres, plaster, wax, or combinations thereof) into the open volume of the open cellular structure and at least partially filling the open volume of the open cellular core with the sacrificial mold material  115 . The step  250  of introducing the sacrificial mold material into the open volume of the open cellular core may be performed in any suitable manner, such as pouring under gravity, filling under vacuum, filling under positive pressure, sifting and compaction of powder, or combinations thereof. 
     The method  200  also includes a step  260  of consolidating the sacrificial mold material  115  to solidify the sacrificial mold material  115  into a solid sacrificial mold ( 116  in  FIGS. 1D, 1H, 2C, and 2D ). The step  260  of consolidating the sacrificial mold material  115  may be performed in any suitable manner depending, for instance, on the type of sacrificial mold material utilized and/or the phase of the sacrificial mold material  115  (e.g., liquid or powder), such as curing (e.g., heating), solidification (e.g., cooling), compaction (e.g., sintering), and/or evaporation of a liquid (e.g., a solvent) in the sacrificial mold material  115 . 
     In one or more embodiments in which the sacrificial mold  116  is porous, the method  200  may include a step  270  of applying a sealant on surfaces of the sacrificial mold  116  to prevent or inhibit the infiltration of excess adhesive into the porous sacrificial mold  116  during a subsequent step of co-curing composite facesheets to the open cellular core, which is configured to aid in the removal of the sacrificial mold  116  during a subsequent step of the method described below. 
     In the illustrated embodiment, the method  200  also includes a step  280  of laying up composite facesheets on at least two surfaces (e.g., two opposing surfaces) of the open cellular structure. The composite facesheets may have any configuration described above, such as pre-impregnated fiber-reinforced polymer plies or dry fabric reinforcement plies onto which a liquid resin is deposited. 
     In the illustrated embodiment, the method  200  also includes a step  290  of co-curing the composite facesheets onto the surfaces (e.g., the upper and lower surfaces) of the open cellular core. The step  290  of co-curing the composite facesheets onto the surfaces of the open cellular core includes consolidating the composite facesheets by applying a consolidation temperature (e.g., approximately (about) 23° C. to approximately (about) 180° C.) and a compaction pressure (e.g., approximately (about) 0.1 MPa to approximately (about) 12 MPa) to the composite facesheets. The compaction pressure applied may be applied in any suitable manner, such as by differential atmospheric pressure (e.g., a vacuum bag), hydrostatic pressure (e.g., a pressurized bladder), a platen press, and/or an autoclave. In one or more embodiments, the compaction pressure may be greater than the compressive strength of the open cellular core, but less than the compressive strength of the combined sacrificial mold  116  and the open cellular core. 
     With continued reference to the embodiment illustrated in  FIG. 3 , the method  200  also includes a step  300  of removing the sacrificial mold  116  from the open volume of the open cellular structure in any suitable manner, such as by dissolution of the sacrificial mold  116  in water or a solvent (e.g., 60° C. pressurized water), etching the sacrificial mold  116  in an acidic or basic bath, melting the sacrificial mold  116 , and/or vaporization or combustion of the sacrificial mold  116  at a temperature greater than the consolidation temperature. Following the step  300  of removing the sacrificial mold  116 , the sandwich structure includes an open cellular core defining an open (e.g., porous) volume between the two composite facesheets. 
     While this invention has been described in detail with particular references to embodiments thereof, the embodiments described herein are not intended to be exhaustive or to limit the scope of the invention to the exact forms disclosed. Persons skilled in the art and technology to which this invention pertains will appreciate that alterations and changes in the described structures and methods of assembly and operation can be practiced without meaningfully departing from the principles, spirit, and scope of this invention. 
     Although relative terms such as “inner,” “outer,” “upper,” “lower,” and similar terms have been used herein to describe a spatial relationship of one element to another, it is understood that these terms are intended to encompass different orientations of the various elements and components of the invention in addition to the orientation depicted in the figures. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. 
     Additionally, as used herein, the term “about”, “substantially,” and similar terms are used as terms of approximation and not as terms of degree, and are intended to account for the inherent deviations in measured or calculated values that would be recognized by those of ordinary skill in the art. Furthermore, as used herein, when a component is referred to as being “on” or “coupled to” another component, it can be directly on or attached to the other component or intervening components may be present therebetween. Further, the use of “may” when describing embodiments of the inventive concept refers to “one or more embodiments of the inventive concept.” Also, the term “exemplary” is intended to refer to an example or illustration. 
     Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of “1.0 to 10.0” is intended to include all subranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 10.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 10.0, such as, for example, 2.4 to 7.6. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein.