FORMED CORE SANDWICH STRUCTURE AND METHOD AND SYSTEM FOR

MAKING SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. application serial no. 11/702,884, filed on February 05, 2007, entitled "Formed Core Sandwich Structure and Method and System for Making Same" which is a continuation-in-part of U.S. application serial no. 11/285,096, filed on November 22, 2005, entitled "Formed Metal Core Sandwich Structure and Method and System for Making Same". The disclosures of each of which are herein incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

Technical Field

[0002] The present invention relates to metal sandwich sheet structures and, more particularly, to formable metal core sandwich sheet structures and methods and systems for manufacturing same.

Description of Related Art

[0003] Metal core sandwich structures are known in the art. For example, U.S. Patent No. 3,525,663 to Hale and U.S. Patent No. 4,035,536 to Morrison both disclose metal sandwich structures having a formed metal core brazed to two outer metal sheets. However, it is believed that the structures disclosed by Hale and Morrison, and other similar conventional formed metal core sandwich structures, cannot be easily formed into compound curves and other complex shapes without delaminating, wrinkling and/or otherwise losing its structural integrity. Therefore, prior metal core sandwich structures are not well-suited for applications that require formation of compound curves, for example, without delaminating or wrinkling during the forming process.

[0004] Other types of metal core sandwich structures, such as honeycomb sandwich structures, are described in U.S. patent no. 6,544,623 to Straza, for example. Although this

honeycomb core sandwich structure provides improved formability when compared to other metal core sandwich structures, it is relatively expensive to manufacture and, therefore, not well- suited for many commercial, cost-sensitive applications (e.g., automobile bodies).

[0005] Therefore, there is a need for an improved metal core sandwich structure that is more economical to manufacture and which exhibits improved formability characteristics without sacrificing structural integrity and strength.

BRIEF SUMMARY OF THE INVENTION

[0006] The invention addresses the above and other needs by providing an improved metal sandwich sheet structure that includes a formed metal core that is brazed or bonded to two outer flat metal sheets. This improved metal sandwich structure is capable of being formed into complex shapes while retaining its structural integrity. This improved metal sandwich structure is also easier and more economical to manufacture than prior honeycomb core sandwich structures, while providing similar strengths and characteristics as the honeycomb core structures.

[0007] In one embodiment of the invention, a method of producing a metal core sandwich structure includes providing a formed metal core that is lightweight and many times stiffer than regular sheet metal, and which is easily formable into curved structures as well as structures having compound curves that curve in two or more directions simultaneously, for example. In one embodiment, a metal core sheet is rolled, stamped or pressed into a corrugated shape having a plurality of cells comprising alternating front and rear projections extending outwardly in front of and behind a median plane, with each projection having a bonding surface area or land configured to be brazed or bonded with corresponding external metal sheets on both sides of the formed metal core sheet. A plurality of micro- abrasions or indentations are formed on the bonding lands, allowing stronger brazing joints to be formed between the metal core and the external metal sheets by facilitating improved capillary action by the metal core during the brazing process. The improved brazing joints provide better bonds so the resulting multi-layer sheet structure will not fracture or delaminate during forming. The resulting sandwich structure is easier to form into complex curves and shapes while maintaining its structural integrity. Thus, the improved metal sandwich structure is well suited for various applications where formability and structural integrity are important (e.g., automotive body applications).

[0008] In another embodiment of the invention, an apparatus for forming an improved core metal sheet is provided. The apparatus includes a press or mold that presses (via stretching and/or gathering) the core metal sheet to form a desired corrugated core structure. At desired locations on the surface of the press, micro-protrusions and/or micro-ridges and/or micro-lines (collectively referred to herein as "micro-protrusions") extend outwardly from the press surfaces so that during forming of the core metal sheet, micro-abrasions and/or micro-grooves and/or micro-channels (collectively referred to herein as "micro-abrasions") are formed to extend inwardly on desired surface areas of the formed metal core. These micro-abrasions allow for improved brazing between the formed metal core and the two outer metal sheets through improved capillary action by the metal core during brazing.

[0009] In a further embodiment, the outer layer sheets are electro-tacked to the formed core prior to brazing or bonding, which allows for improved subsequent brazing and an improved quality sandwich structure. In one embodiment, the invention provides a novel electro-tacking apparatus for tack welding the formed core to the outer metal sheets prior to brazing.

[0010] In various embodiments of the invention, various cell shapes and geometries such as hexagonal, square, octagonal or other shape may be formed in the core sheet as well as different cell sizes in accordance with desired formability, stiffness and crush characteristics. For example, it may be desirable for some zones of the sandwich structure to be more easily crushed (e.g., to provide "crush zones" in an automobile body frame) or to be more rigid or more flexible. For example, if crush zones are desired, the sandwich structure (or at least designated areas of the sandwich structure) can be designed to be more easily crushed which may help avoid automobile impact injuries while other areas of the automobile body (e.g., roofs) will be stiffer, allowing for more protection from metal crushing into passenger compartments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 illustrates a perspective view of a metal core sheet after it has been formed, in accordance with one embodiment of the invention.

[0012] Figure 2A illustrates a top view of the formed metal core sheet of Figure 1.

[0013] Figure 2B illustrates a cross-sectional side view of the formed metal sheet of Figures 1 and 2A, taken along a cross-section indicated by lines 2B-2B of Figure 2A.

[0014] Figure 3A illustrates a top view of a metal core sheet formed in a corrugated checkerboard pattern, in accordance with another embodiment of the invention.

[0015] Figure 3B illustrates a cross-section side view of the formed metal core of Figure 3A taken along lines 3B-3B of Figure 3A.

[0016] Figure 4 illustrates a side view of a metal core sheet being formed between two press plates or molds, in accordance with one embodiment of the invention.

[0017] Figure 5 illustrates an exploded side view of a metal core sandwich structure prior to brazing, in accordance with one embodiment of the invention.

[0018] Figure 6 illustrates the metal core sandwich structure of Figure 4 after brazing.

[0019] Figure 7 illustrates a perspective view of an electro-tack welding apparatus used to tack weld the metal core sandwich structure prior to brazing, in accordance with a further embodiment of the invention.

[0020] Figure 8 illustrates a cross-sectional side view of one embodiment of a double cell sandwich structure.

[0021] Figure 9 illustrates a top view of a press plate used to form a core sheet used in the embodiment of Figure 8.

[0022] Figure 10 is cross-sectional side of the embodiment of Figure 8 showing exemplary dimension.

[0023] Figure 11 illustrates a cross-sectional side view of a further embodiment of a double cell sandwich structure.

[0024] Figure 12a illustrates a top view of an upper press used to form a first core sheet used in the embodiment shown in Figure 11.

[0025] Figure 12b is a cross-sectional side view of the upper press of Figure 12 and a lower press used for forming a first core sheet, in accordance with one embodiment of the present invention.

[0026] Figure 13a illustrates top view a top view of an upper press used to form a second core sheet used in the embodiment shown in Figure 11.

[0027] Figure 13b is a cross-sectional side view of the upper press of Figure 13a, taken along the lines 13b- 13b of that figure, and a lower press for forming a second core sheet, in accordance with one embodiment of the present invention.

[0028] Figure 14 illustrates a cross-sectional side view of an exemplary embodiment of a double cell sandwich structure having a middle sheet, in accordance with one embodiment of the present invention.

[0029] Figure 15 illustrates a cross-sectional side view of another exemplary embodiment of a double cell sandwich structure having a middle sheet, in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE

INVENTION

[0030] Preferred embodiments of the invention are described in detail below with reference to the figures wherein like elements are referenced with like numerals throughout. It is understood that the figures are not necessarily drawn to scale but intended to merely illustrate some of the novel aspects, features and processes of the invention.

[0031] Figure 1 illustrates a perspective view of a formed metal core 10 made in accordance with one embodiment of the invention. In this embodiment, the metal core 10 is formed in a corrugated pattern having a plurality of cells 12 comprising alternating front and rear projections extending outwardly in front of and behind a median plane 14, with each projection having a bonding surface area or land 16 configured to be brazed or bonded with corresponding external metal sheets (not shown) on both sides of the median plane 14. As shown in Figure 1, a plurality of micro-abrasions or indentations 18 are formed on the bonding lands 16. As explained in further detail below, the micro- abrasions 18 provide stronger brazing joints to be formed between the metal core 10 and the external metal sheets by facilitating improved capillary action by the metal core 10 during the brazing process.

[0032] Figure 2 A illustrates a top view of the formed metal core 10 of Figure 1. The metal core 10 includes a plurality of upwardly projecting cells 12A and a plurality of downwardly projecting cells 12B. The bonding lands 16 of the upwardly projecting cells 12 A are also shown while the bonding lands 16 of the downwardly projecting cells 12B are located on the opposite side of the core 10 and, hence, not shown in Figure 2 A. A plurality of micro abrasions or indentions 18 are formed on the bonding lands 16 of each cell 12A and 12B to facilitate improved capillary action of the metal core 10 during brazing.

[0033] Figure 2B illustrates a cross-sectional side view of the metal core 10 taken along perspective lines 2B-2B of Figure 2A. As shown in Figures 1, 2A and 2B, in one embodiment, each cell 12A and 12B is formed in the shape of an octagon. However, it is understood that the invention is not limited to any particular shape, size and/or configuration of the cells 12A, 12B. Any desired shape, size and configuration of cells may be utilized to achieve various desired

physical characteristics of the resulting metal core sandwich structure, in accordance with the present invention.

[0034] Figure 3A illustrates a top view of an alternative, exemplary formed metal core 30 having a plurality of square-shaped cells 32A and 32B in a checkerboard configuration. The cells 32A represent upwardly projecting cells and the cells 32B represent downwardly projecting cells. Each cell 32A and 32B has a corresponding bonding land 34 and a plurality of micro- abrasions 36 on the surface of each bonding land 34. Figure 3B illustrates a cross-sectional side view of the formed metal core 30 of Figure 3A, taken along lines 3B-3B of that figure.

[0035] Figure 4 illustrates a cross-sectional side view of the metal core 30 as it is formed between two presses 42 and 44 having corresponding molded patterns and configurations to form the metal core 30 into a desired shape and configuration. During forming, a flat sheet of metal material 30 is placed between the top and bottom presses 42 and 44, respectively. As the top and bottom presses 42 and 44 are pressed together the metal core 30 is formed via stretching and/or gathering of the metal sheet material 30 to form the cells 32A and 32B in a checkerboard configuration. It is understood that the presses 42 and 44 are exemplary only and that presses having other desired molded patterns and configurations may be utilized to form a metal core having cells of various shapes (e.g., octagons, hexagons, pentagons, squares, triangles, etc.), and sizes, and configurations.

[0036] In one embodiment, the micro-abrasions 36 illustrated in Figure 3 A, for example, are formed during pressing of the metal core 30. A plurality of micro-protrusions, ridges and/or lines (collectively referred to herein as "micro-protrusions") (not shown) are provided on selected surfaces 46 and 48 of top and bottom presses 42 and 44, respectively. During pressing of the metal core 30, the micro-protrusions form the plurality of micro-abrasions 36 on the bonding lands 34 of the cells 32A, 32B of the formed metal core 30. In one embodiment, the micro-protrusions are formed on the surfaces of the presses 42 and 44 corresponding to the bonding lands 34 of the formed metal core 30. In one embodiment, the micro-protrusions are formed on the corresponding press surfaces by fine etching of press surfaces using known means and techniques (e.g., electrical metal etching, chemical etching, sand blasting, grit blasting, etc.). In one embodiment, the micro-protrusions are formed so as to provide micro-abrasions 36 having a depth and/or width of 0.0005 inches to 0.002 inches and cover approximately 50-80% of the surface area of the bonding lands 34. In various embodiments, the abrasions 36 may be substantially parallel channels, criss-cross configurations, or channels traveling in random

directions and/or curved channels. In alternative embodiments, the micro-abrasions may be formed by electronic discharge machining (EDM) processes, which are well-known in the art. In other embodiments, the micro-abrasions 36 may be formed on the bonding lands 34 of the metal core 30 after the metal core 30 has been pressed and formed. It is appreciated, however, that providing presses 42 and 44 with preformed micro-protrusions that automatically form micro-abrasions 36 on the metal core 30 during pressing can save substantial time and work when compared to forming the micro-abrasions 36 manually or individually for each metal core 30 subsequent to pressing.

[0037] In alternative embodiments, the metal core 10, 30 may be formed after a brazing material has been placed on each side of the core 10, 30. Thus, the metal core sheet, plus brazing material on each side of the metal core sheets, are pressed and formed together by the presses 42 and 44. The brazing material can be placed onto the surface of the metal core 10, 30 prior to forming the core 10, 30 by spraying a brazing powder onto the core sheet to be formed or by placing sheets of brazing material 50 on either side of the core sheet to be formed. With the proper amount of pressure during forming, the brazing material will be embedded into the core sheet. This facilitates the subsequent brazing process by evenly distributing the braze material across the surface of the core sheet to be formed. In these embodiments, the micro- abrasions 18, 36 are formed and pass through the brazing material onto the bonding lands 16, 34 of the formed metal core 10, 30. Thereafter, the formed metal core 10, 30 plus formed brazing material sheets are sandwiched between two external metal sheets and brazed at high temperature (e.g., 670 to 730 degrees Celsius) in a reduced-atmosphere or vacuum furnace to produce a metal core sandwich structure in accordance with the present invention.

[0038] Figure 5 illustrates an exploded cross-sectional side view of an exemplary formed metal core 10 (Fig. 1) sandwiched between two sheets of brazing material 50, which are in turn sandwiched between two external metal sheets 60. In this embodiment, the brazing material sheets 50 have not been pressed and formed with the metal core 10, as described above. Figure 6 illustrates a cross-section side view of the resulting metal core sandwich structure 70 after brazing of the metal core 10 to the external metal sheets 60 has been completed. The micro- abrasions 18 (Fig. 2A) allow for an improved capillary effect so that the brazing material 50, upon melting, flows into the pores of the bonding lands 16 (Figs. 1 and 2A) more easily and thereby provides a stronger braze joint between the closely fitted surfaces of the bonding lands 16 and the external metal sheets 60. The improved capillary effect and, hence, improved brazing

joint provided by the micro-abrasions 18 produce a stronger metal core sandwich structure 70 that can be formed into compound curves, and other shapes and forms, more easily without delaminating, wrinkling or otherwise losing its structural integrity. In further embodiments, micro- abrasions may also be formed on the interior surface of the external metal sheets 60 in order to improve the capillary effect of the external metal sheets 60 during brazing.

[0039] In a further embodiment of the invention, prior to brazing, all of the layers of the metal sandwich structure 70, comprising a formed metal core 10, brazing material sheets 50 and two outer metal sheets 60, are electro-tack- welded together so as to secure the multiple layers to one another and prevent relative movements during the brazing process. This further improves the strength of the braze joints formed between the bonding lands 16 of the core 10 and the external metal sheets 60 to provide a higher quality finished product.

[0040] Figure 7 illustrates a perspective view of a novel tack welding apparatus 100 that may be used in conjunction with the present invention. The apparatus 100 includes a computer and/or electronically controlled actuator box 102 that contains standard electronics and mechanical components (e.g., gears, switches, power supply, etc.) to control movement of an actuator arm 104 that controls movement of a top plate 106. The metal core sandwich structure 70 (Figs. 5 and 6) is positioned between the top plate 106 and a bottom ground plate 108. As shown in Figure 7, the top tack- welding plate 106 comprises a plurality of subsections 110 that are electrically insulated from each other. In one embodiment, the subsections 110 can be made from copper or copper-alloy and are insulated from one another by an insulating material (e.g., ceramic). Each of the subsections 110 are electrically coupled to a corresponding conductive wire 112 via corresponding contacts 114. The conductive wires 112 are coupled to a power switch box 116 that controls when electrical current is supplied to each individual wire 112. The power switch box 116 can be controlled by a computer or processor (not shown).

[0041] In one embodiment, a high energy, low voltage current is sequentially applied to each subsection 110 to tack- weld a corresponding portion of the metal core sandwich structure 70 located between the corresponding subsection 110 and the bottom ground plate 108. In one embodiment, the top and bottom plates 106 and 108, respectively, apply an even pressure throughout the sandwich structure 70. A sequence of tack welds are rapidly performed in different areas of the sandwich structure 70 by sequentially applying an electrical current to each subsection 110 of the top plate 106. By sequentially tack-welding only portions of the sandwich structure 70 at any one time, this tack- welding process allows for controlled heating and cooling

throughout the sandwich structure 70, which in turn allows for improved tack welds. Thus, portions of the sandwich structure 70 following tack- welding are allowed to cool while other portions are being tack-welded. This cooling process provides better tack-welds and the resulting sandwich structure 70 has a lower tendency to delaminate at individual tack welds. In other words, letting the structure 70 heat up and cool in different areas as it is being tack welded helps prevent individual tack welds from separating. After the metal core sandwich structure 70 has been tack welded together, as described above, the structure 70 is then placed in a furnace for brazing.

[0042] The metal core sandwich structure 70 may incorporate any type of metal and/or metal alloys as the formed core 10, 30 and the external metal sheets 60, such as Al or D2 steel, for example. It is understood that any metals, metal alloys, or combinations thereof, that are suitable for brazing are contemplated to be within the scope of the present invention (e.g., copper, aluminum, titanium, etc.). Additionally, any type of known brazing material in the form of sheets, foils, sprays , powders, paste or slurries, for example, may be utilized in accordance with the present invention. Furthermore, it is contemplated that some embodiments of the invention may utilize non-metal core materials and non-metal external sheets. For example, known synthetic and/or polymer materials (e.g., Kevlar) may be used to form an injection- molded core and thereafter bonded (e.g., sonically welded and/or vibration welded) to synthetic and/or polymer outer sheets. Micro-abrasions are formed on the bonding lands of the non-metal core which facilitate gripping between the core and the external sheets during and after welding.

[0043] Further embodiments of metal core sandwich structures may include using a double cell sandwich core. These embodiments also have two external sheets defining a central core; but, instead of using a single core sheet (such as sheet 10 of Figure 6), exemplary embodiments of double cell sandwich structures may use two core sheets.

[0044] One embodiment of a double cell sandwich structure 800 is shown in Figure 8. Double cell sandwich structure 800 can comprise external sheets 802 and 804 sandwiching a first core sheet 806a and a second core sheet 806b. External sheets 802 and 804 may be the same as sheets 60 shown in Figure 6. First core sheet 806a and second core sheet 806b may be made of the same materials as detailed above with reference to core sheet 10. In the embodiment 800, the first core sheet 806a and the second core sheet 806b comprise a plurality of pyramid-shaped cells 832a, 832b.

[0045] Further to Figure 8, inwardly projecting lands 834a of the first sheet 806a contact corresponding inwardly projecting lands 834b of the second sheet 806b. In addition, outwardly extending lands 836a of the first sheet 806a contact an interior side of the external sheet 802 and outwardly extending lands 836b of the second sheet 806b contact an interior side the external sheet 804. The inwardly projecting landings 834a of the first sheet can be configured to be brazed or bonded with corresponding inwardly facing landings 834b of the second sheet. Moreover, outwardly facing landings 836a and 836b of the first sheet and the second sheet, respectively, can be configured to be brazed or bonded with the interior surface of corresponding external metal sheets 802 and 804. It is understood that any brazing or bonding technique described above may be similarly used with the lands 834 and 836 and external sheets 802 and 804 to facilitate gripping between the core sheets and the core sheets and external sheets. In one embodiment, micro-abrasions 18 are formed on a plurality of lands 834 and /or 836 to improve capillary action during the brazing process.

[0046] Figure 9 illustrates a top view of a press 900 configured to press or mold core sheets 806a and 806b, in accordance with one embodiment of the present invention. Press 900 has corresponding molded patterns, such as punch 902, and configurations to form the cores sheets 806a and 806b. During forming, a flat sheet of metal material can be placed between the top press 900 and a corresponding bottom press (not shown). As the top press 900 and the bottom press are pressed together, a core sheet is formed via stretching and/or gathering of the metal sheet. It is understood that press 900 is exemplary only and that presses having other desired molded patterns and configurations may be used to form metal core sheets having cells of various shapes, sizes and configurations.

[0047] Figure 10 is a cross-sectional view of structure 800 showing exemplary dimensions of the structure 800. The exemplary dimensions are as follows: an inwardly projecting landing thickness A = .0040"; an inside radii B = .0006"; an outside radii C = .010"; a cell diameter D = .050"; a cell incline E = 70°; an outwardly projecting landing thickness F = .0040"; an external sheet thickness G = .0060"; a core sheets width H = .0300"; and a sandwich cell structure width I = .0720". It is understood that the above dimensions are exemplary only and that a sandwich core structure having other sizes and configurations may be used as desired to fit the needs of a particular application.

[0048] Figure 11 is a cross-sectional side view of a further embodiment of a double cell sandwich structure 1100. Structure 1100 has external sheets 1102 and 1104 sandwiching first

core sheet 1106 and second core sheet 1108. The first core sheet 1106 has a generally sinusoidal cross-sectional shape with a bulge 1110 located at a tip of each inwardly projecting peak 1112. The second core sheet 1108 also has a generally sinusoidal cross-sectional shape, but has a dimple 1114 located at a tip of each inwardly projecting peak 1116. As seen in Figure 11, the bulges 1110 sit in the corresponding dimples 1114.

[0049] The bulges 1110 and dimples 1114, as well as the area near the bulges and dimples, may be brazed or bonded to facilitate gripping with a corresponding surface. Moreover, outwardly facing landings 1118a and 1118b of the first sheet 1106 and the second sheet 1108, respectively, can be configured to be brazed or bonded with the interior surface of corresponding external metal sheets 1104 and 1102. As is appreciated, any suitable brazing or bonding technique described in this disclosure may be used. In one embodiment, micro- abrasions 18 are formed on respective surfaces to improve capillary action during the brazing process.

[0050] Similar to embodiments described above, the first core sheet 1106 and the second core sheet 1108 may be formed between two presses having corresponding molded patterns and configurations to form a flat metal sheet into the desired shape and configuration.

[0051] Figure 12a is a top view of an upper press 1202 having a plurality of pins 1206 for forming the first core sheet 1106. In one embodiment, pins 1206 are spaced an equal distance away from each adjacent pin 1206. In a further embodiment, partial pins that would otherwise be located at edges of the press 1202 are eliminated. This is believed to help avoid sharp edges which could cut the material being formed. Only five pins 1206 are shown in Figure 12a for ease of understanding for the reader. As is appreciated, there is no limit as to the number of pins 1206 that can be used.

[0052] Figure 12b illustrates the cross-sectional side view of the upper press 1202, taken along the lines 12b- 12b of Figure 12a, and lower press 1204 forming the first core sheet 1106 in accordance with one embodiment of the present invention. As can be seen, pins 1206a, 1206b, 1206c have a generally conical shape and extend outwardly from a baseline 1208 of the upper press 1202. Located at a tip of each pin 1206a, 1206b, 1206c is a projection 1210. The lower press 1204 has a plurality of conically-shaped cavities 1212a, 1212b, 1212c, each cavity 1212 having an indentation 1214 located at a central bottom area of each cavity 1212. Pins 1206a, 1206b, 1206c extends into respective cavities 1212a, 1212b, 1212c when the upper press 1202 and lower press 1204 are pressed together to form the first core sheet 1106.

[0053] Figure 13a is a top view of an upper press 1302 having a plurality of pins 1306 for forming the second core sheet 1108. As with the upper press 1202, pins 1306 may be spaced an equal distance away from each adjacent pin 1306 and partial pins that would otherwise be located at edges of the press may be eliminated. Eliminating partial pins on the edges of a press is believed to help avoid sharp edges which could cut the material being formed. Also, only five pins 1306 are shown in Figure 13 for ease of understanding for the reader. As is appreciated, however, there is no limit as to the number of pins 1306 that can be used.

[0054] Figure 13b illustrates a cross-sectional side view of upper press 1302, taken along the lines 13b- 13b of Figure 13a, and lower press 1304 forming a second core sheet 1108 in accordance with an embodiment of the present invention. The upper press 1302 and lower press 1304 can be similar to the upper press 1202 and lower press 1204, respectively, with an exception being that that the upper press 1302 has indentations 1306 instead of projections 1210 and the lower press 1304 has projections 1308 instead of indentations 1214. In other words, the placement on respective presses of the projections and indentations for forming the first core sheet 1106 is reversed for forming the second core sheet 1108. Also, the indentations 1306 and projections 1308 are preferably larger than the indentations 1214 and projections 1210 to permit the bulges 1110 formed in the first sheet 1106 to sit in the dimples 1114 formed in the second sheet 1108, as shown in Figure 11.

[0055] Figure 14 illustrates a cross-sectional side view of a double cell sandwich structure 1600 having a middle sheet in accordance with another embodiment of the present invention. Some parts of the structure 1400 are identical to those of the previously described embodiments, and like reference numerals have been used for like parts as appropriate. As shown in Figure 14, a middle sheet 1450 is positioned between the first core sheet 806a and the second core sheet 806b.

[0056] Figure 15 illustrates a cross-sectional side view of a further embodiment of a double cell sandwich structure 1500 having a middle sheet, in accordance with another embodiment of the present invention. Some parts of the structure 1500 are identical to those of the previously described embodiments, and like reference numerals have been used for like parts as appropriate. As shown in Figure 15, a dimpled middle sheet 1550 is positioned between the first core sheet 1106 and the second core sheet 1108. The dimpled middle sheet 1550 has dimples 1552 formed at locations corresponding to the locations of the bulges 1110 of the first core sheet 1106 and the dimples 1114 of the second core sheet 1108.

[0057] It is understood that sheets forming the respective structures 1400 and 1500 may be bonded or brazed together to facilitate gripping using the bonding or brazing techniques described in this disclosure.

[0058] The double cell sandwich structures provide many advantages. For example, it is believed that embodiments of double cell sandwich structures provide many of the advantages of the single cell structures; yet double cell structures can provide even more strength and further reduce dimpling and wrinkling on the surface of the structure during and after forming of the sandwich structure. Embodiments using of a middle sheet between the core sheets can strengthen the structure even further.

[0059] Various preferred embodiments of the invention have been described above. However, it is understood that these various embodiments are exemplary only and should not limit the scope of the invention as recited in the claims below. Various modifications of the preferred embodiments described above can be implemented by those of ordinary skill in the art, without undue experimentation. These various modifications are contemplated to be within the spirit and scope of the invention as set forth in the claims below.