Sandwich

SANDWICH CONSTRUCTION 12 Andrew C. Marshall

12.1 INTRODUCTION

This chapter covers a unique form of composites known as ’structural sandwich construction’.

A structural sandwich consists of three elements, as shown in Fig. 12.1:

I

Fig. 12.1 The elements of a sandwich structure are as follows: (a) two rigid, thin, high strength facings; (b) one thick, low density core; and (c) an adhesive attachment which forces the core and facings to act as a continuous structure. The facings of a sandwich panel act similarly to the flanges of an I-beam, resisting the bending loads and increasing the bending stiffness of the structure by spreading the facings apart. However, unlike the I-beam’s web, the core gives continuous support to the flanges or facings.

Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7

1. a pair of thin, strong facings; 2. a thick, lightweight core to separate the fac-

ings and carry loads from one facing to the other; and

3. an attachment which is capable of transmit- ting shear and axial loads to and from the core.

This chapter provides a general background and a brief summary of the various materials in common use; the design steps used to cal- culate loads; some design details for solving load point, edging and attachment problems; and a few tables, charts and graphs containing useful information for the designer. An attempt is also made throughout the chapter to provide suggestions and perspectives to help a new user of sandwich structures technology to avoid some of the errors of his predecessors.

Structural sandwich construction is one of the first forms of composite structures to have attained broad acceptance and usage. Virtually all commercial airliners and helicopters and nearly all military air and space vehicles make extensive usage of sandwich construction. In recent years, most commercial space vehicles have also adopted this technology for many components. The effectiveness of sandwich construction is shown in Fig. 12.2.

In addition to air and space vehicles, the system is commonly used in the manufacture of cargo containers, relocatable shelters and air- field surfacing, navy ship interiors, small boats and yachts, duplicate die models and production parts in the automobile and recreational

Fncing material 255

Fig. 12.2 A striking example of how conversion to sandwich stiffens a structure without materially increasing its weight. This example uses 1.6 mm (0.063 in) thick aluminum facings and 1/4-5052 37 kg/m7 (2.3 lb/fPj aluminum core.

vehicle industry, snow skis, display cases, resi- dential construction materials, interior partitions, doors, cabinets and a great many other everyday items.

Although the employment of sandwich design to produce lightweight or special purpose load-carrying members is thought to have originated as early as 1820, routine commercial use of the idea did not occur until about 110 years later. What started this sudden acceptance was the successful commercial production of structural adhesives, starting in both UK and USA in the 1920s and 1930s.

This early production began with the use of casein glue and later urea-formaldehyde and phenolics, with wood facings and cores. The search for better adhesives subsequently resulted in the development of the rubber- phenolics and the vinyl-phenolics, which were suitable for use with metals. Commercial adhesives such as ’Cycleweld,’ (from Chrysler Motors), ’Plycosite,’ (from US Plywood) and ’Redux’ (from Bonded Structures, in Duxford, UK) adhered well to both wood and metals and possessed rather high and predictable strength.

The result was the beginning of a revolution in bonding technology. Many further developments followed in only a few years. They included improved cleaning methods for metal skins; low weight, high strength/stiffness honeycomb core materials; ‘B’ staged tape adhesives which could be stored for long times; glass fabrics and collimated tapes preimpregnated with accurately measured

amounts of ’B’ staged resins; high strength resins; tough, high peel adhesives requiring lower cure temperatures and pressures; as well as the discovery of the resistance of sandwich to sonic fatigue.

12.2 FACING MATERIAL

The primary function of the face sheets in a sandwich structure is to provide the required bending and in-plane shear stiffness and to carry the edgewise and bending loads, as well as the in-plane shear loading. In the aerospace field, facings most commonly chosen are resin impregnated fiberglass cloth or a laminate of unidirectional fibers (commonly called ’prepreg’), graphite prepreg, 2024 or 7075 aluminum alloy, titanium alloy, or any of several stainless steel or refractory metal alloys. Even the most economical of these products represents a substantial cost and customary practice is to choose among them very carefully on a value engineering, or lowest lifetime cost, basis.

12.2.1 SUITABILITY OF MATERIALS

When choosing facing materials (as well as the core, adhesive, or other materials) for an application, it is wise to examine the less obvious properties of the material, such as toughness or brittleness, mode of fracture, durability and weatherability, compatibility with rivets and bolts and other such attributes which may directly affect the usability or success of the

256 Sandwich construction

end product, even though not directly involved in stress analysis or weight savings. An understanding of these requirements has resulted in a switch from aluminum to fiberglass skins and from fiberglass to aramid (Nomex, from DuPont) cores for most aircraft cabin interior panels.

12.3 CORE MATERIALS

The primary function of a core in sandwich structures is that of stabilizing the facings and carrying most of the shear loads through the thickness. In order to perform this job effi- ciently, the core must be as rigid and as light as possible and must deliver uniformly predictable properties in the environment (such as high humidity) in which the finished part is to perform.

12.3.1 TYPES OF CORE MATERIALS

Wood

Several different materials are used exten- sively as sandwich cores. The oldest of these is wood, which continues to be used in many applications as a core for such common applications as doors, partitions and many other ’builder’s supply’ items. It is also used in the majority of snow skis, either flat-grain or end- grain, although a few of the higher performance skis employ honeycomb, foam, or reinforced plastic cores. End-grain balsa has broad acceptance in boat hulls up to lengths of 15.2m (50 ft) or more and is still used for replacement flooring for many older and a few new aircraft.

The traditional advantage of the low cost of wood has been progressively eroded with the passage of time and many users report difficulty in supply, even at prices higher than foam and sometimes approaching that of honeycomb. Even so, the ease of use and excellent durability of the end product has led to substantially increased usage, particularly of the carefully selected grades of end-grain balsa, in

applications such as boat hulls, large tanks and airborne pallets and containers. This broaden- ing usage is also prompted by its excellent compressive strength and modulus properties when compared to all but the aramid paper honeycombs, which are much more expensive. Complete information can be obtained from the leading producer of these materials, BaltekI3, or Balsa Ecuador Lumber Company.

Foam

The use of foam as a structural core has been and is now, extensive. Recent developments in the technology of foam injection have sharply increased the use of these materials. The most novel of these is use of a cold-cavity die, in which the foam is injection molded in a single production step. A careful adjustment of the mixing and curing reaction of the foam, together with the heat-sink effect of the mold results in a part with facings which are simply an un-foamed, higher density form of the same polymer which constitutes the foamed core. The high productivity and modest cost of this scheme have resulted in many applications in the automotive and industrial fields. Another fast-growing form of the material is in cores for fiberglass snow skis and tennis rackets, in which an assembly of facings and close-out details is placed in a closed cavity mold and foam injected to form both the core and the adhesive attachment to the pre-cured glass fiber skins and various edge details. The saving in labor over conventional assembly methods has resulted in rapid acceptance of the process and the construction of many new factories.

Foams can also provide special properties such as insulation or radar transparency, when used with appropriate facing materials.

The very low cost polystyrene foams are used primarily in non-sandwich applications, their role in structural parts for refrigerated vehicles and buildings having been largely taken over by the urethanes. The single major

Cove materials 257

exception to this statement lies in the extensive use of polystyrene foams as cores in several thousand amateur-built composite aircraft. This application was pioneered by Burt Rutan, in his ’moldless construction’, used in his series of high performance small aircraft and the many similar designs offered by others in subsequent years.

The polyvinyl chloride (PVC) foams, which made an impact on the transport aircraft industry as flooring cores, have been largely replaced by the more efficient high density aramid honeycombs.

The foam-in-place system of producing sandwich structures has been used for more than 35 years, because of its simple concept. However, users of this system have always had difficulty with the continuing problem of producing uniform properties from one mix to the next and in achieving uniformly high core and bond strengths to the metal or pre-cured glass fiber skins. The use of systematic incom- ing inspection, automatic mixing and dispensing equipment and, in the case of critical airframe parts, test coupons, produced integrally with the basic part, have all helped to keep the problems under control.

It will be noted that Table 12.1 does not list the shear strength of many of the various

Roll c T r

A

HOBE Block HOBE Slice + Expanded Panel

Expansion Process of Honeycomb Manufacture I

foams, even though this value is needed for sandwich panel design. This property, even where listed, cannot be considered to be a reliable value. The actual value for an application at hand must be determined for the actual materials and conditions of use in order to be considered reliable. When a value for shear strength is not available, it may be roughly estimated to be about 0.7 times the compressive strength shown. Even the compressive strength cannot be considered to be reliable, however, as many differing methods of mea- suring this value are commonly used and each results in a substantially different value reported.

12.3.2 HONEYCOMB

Honeycomb types in common usage include products made from uncoated and resin- impregnated kraft paper, various aluminum alloys, aramid paper and glass or carbon fiber reinforced plastic in a number of cloth weaves and resin systems. Honeycombs based on titanium, stainless steel and many others are used in lesser quantities. Most honeycomb cores are constructed by adhesively bonding strips of thin material together, as shown in Fig. 12.3.

In the case of aramid paper honeycomb, the

Roll Corrugating Rolls

Corrugation Process of Honeycomb Manufacture

Corrugated Sheet Corrugated Block

Fig. 12.3 Most honeycomb is produced by the expansion process. Actual cell shape produced by either method may vary greatly.


Table 12.1 Properties of several foam materials used as cores*

TYP Compressive Tensile strength Maximum

strength at 10% deflection service Density (ASTM 01623) (ASTM 01621) temperature

lb/ft3 kg/m3 psi MPa psi MPa "F "C

ABS (acrylonitrile bu tadiene-styrene)

Injection molding type pellets 40-56

Cellulois acetate Boards and rods (rigid, closed cell foam) 6.0-8.0

Epoxies Rigid closed cell, 5.0 precast blocks, 10.0 slabs, sheet 20.0

Phenolics Foam-in-phase 'X-1% liquid resin 2-5

7-10

Polypropylene

Polypropylene"

Polyurethaneb

Pellets

Skinned molded (rigid) Skin Core

Polyvinyl chloride Rigid closed cell

641-897

96-128

80 160 320

5-24 32-80

112-160

50 801

35.0 561

1.3-3.0 2148 4-8 64-128 9-12 144192 13-18 208-288 19-25 30p400

25-65 400-1041 3-30 48481

3 48

boards and billets 6 96

20004000 13.8-27.6 2300-3700 15.8-25.5 176-180 80-82

170 1.2 125 0.86 350 177

51 0.35 90 0.62 350 177 180 1.2 260 1.8 350 177 650 4.5 1080 7.4 350 177

3-17 0.021-0.12 2-15 0.014-0.10 20-54 0.1384.372 22-85 0.15-0.58 Continuous 80-130 0.552-0.896 158-300 1.09-2.07 service at 145

300

5500 37.9 7500 51.7 270 132

1600 11.03 2100 14.4

15-96 0.104.65 15-60 0.10-0.41 180-250 82-121 90-290 0.62-1.99 70-275 0.48-1.90 200-250 93-131 230450 1.58-3.10 290-550 1.99-3.79 250-275 121-135 475-700 3.284.83 650-1100 4.48-7.58 250-300 121-149 775-1300 5.34-8.96 1200-2000 8.27-13.8 250-300 121-149

100-2700 0.68-18.6 40-3000 0.28-20.7 150-250 66-121 15-1500 15-1500 150-250 66-121

1000 6.90 95 0.65

200 1.38 andup andup

* Where shear strength and modulus properties are not shown, use a figure of 0.7 times the compressive strength shown as a first approximation for design feasibility consideration. Always test actual material used for true value of shear strength and modulus. a High density, foam, molded, parts and shapes, with solid, integral skin.

foam-in-place; for spray, pour, or froth-pour techniques. Rigid (closed cell) molded parts; boards, blocks, slabs; pipe covering; one-shot, two- and three-package systems for

Core materials 259

Table 12.1 Continued

Type Thermal conductivity Shear Shear

strength modulus BTU in

Wm-' K-I psi MPa psi MPa ~ _ _ h-'Pf2

ABS (acrylonitrile butadiene-styrene)

Injection molding type pellets 0.58-2.1 0.08-0.30

Cellulose acetate Boards and rods (rigid, closed cell foam) 0.31 0.04

Epoxies Rigid closed cell, 0.26 0.04 precast blocks, 0.28 0.04 slabs, sheet 0.32 0.05

Phenolics Foam-in-p hase 0.2 1-0.28 0.03-0.04 liquid resin 0.20-0.22 0.03-0.04

0.24-0.28 0.03-0.04

Polypropylene Pellets 1.05 0.15

Polypropylene" 4.2 0.61

Polyurethaneb 0.11-0.21 0.2-0.4 20 0.14 226 1.56 0.15-0.29 0.02-0.04 90 0.62 1500 10.3 0.19-0.35 0.03-0.05 180 1.24 4500 31.0

0.34-0.52 0.05-0.07 450 3.1 15000 103.5 0.26-0.40 0.04-0.06

Skinned molded (rigid) Skin 0.12-0.80 0.02-0.12 Core 0.21-0.55 20-500 225-15 000

Polyvinyl chloride Rigid closed cell 2.0 at 70 65 0.45 1200 8.3 boards and billets 120 0.83 2200 15.2

* Where shear strength and modulus properties are not shown, use a figure of 0.7 times the compressive strength shown as a first approximation for design feasibility consideration. Always test actual material used for true value of shear strength and modulus. a High density, foam, molded, parts and shapes, with solid, integral skin.

foam-in-place; for spray, pour, or froth-pour techniques. Rigid (closed cell) molded parts; boards, blocks, slabs; pipe covering; one-shot, two- and three-package systems for


inherent toughness and abuse resistance of the material makes cores of 1648kg/m3 (1-3 lb/ft3) an excellent choice for aircraft cabin interior walls and ceilings, even with glass fabric-reinforced skins as low as 0.254 mm (0.010 in) in thickness.

Physical and mechanical properties of the honeycomb core materials are strongly influ-

Fig. 12.4 Thermal conductivity through sandwich panels can be isolated into the contribution of each component: facings, core and adhesive. The resistances (or reciprocal of conductivity) can simply be added - including the effect of boundary layer conditions. The thermal properties of typical facing materials may be found in many handbooks. Thermal resistance values for typical core to facing adhesives are typically 0.03 for film adhesives with a scrim cloth support and 0.01 for unsupported adhesives. These graphs give the resistance for aluminum and non-metallic honeycomb at a mean temperature of 23.9"C (75°F). Note that for non-metallic honeycomb, it has been found that the cell size is more critical than core density. The reverse is true with aluminum honeycomb. To correct for mean temperature, divide the resistance at 23.9"C (75°F) by coefficient Q.

.028

N

E13021 W

-=x &j .014

0) w U

P

007

(4 0

enced by the properties of the materials from which they are manufactured. Some of these differences are obvious in the thermal conductivity information shown in Fig. 12.4 and Fig. 12.5. However, several significant properties of honeycomb cores are peculiar to the materials and should be separately noted.

Thermal Resistance - Aluminum Honeycomb

25(1 0) 5 0 (2 0) 76(30 lO(40) Core Thickness- cm (in )

Thermal Resistance-Non Metallic Honevcomb 70 4

cu $13 53 3

W

9 2 35 2

PI e U

18 1

1 3 (0 5) 25(10) 3 8(1 5) 5 0 (2 0) Core Thickness- cm (in )

Effect of Mean TemDerature

-1 29 -17.8 93 204

LIVE GRAPHClick here to view



/knovel2/view_hotlink.jsp?hotlink_id=432288904



Core materials 261

1.2 , I I I I I I I I 1.1

1 .o

a .9 Y

8 .e 4: Y

.7

F .6

.5 0 0 .4

5 8

I 1 I I I I I

I

Fig. 12.5 Measured core shear strength will vary depending upon the test method, core thickness, skin thickness and many other factors. The above curves may be used only for preliminary correction factors. Physical tests of the final design must be used to confirm actual values obtained, as the curves shown above are only approximate.

1 125 L E KRAFT PAPER, 1 &"INCH XEXAGON CELLS 3003 - H I 9 ALUMINUM

PHENOLIC RESIN,

00024 -INCH FOIL ;':INCH HEXAGON' CELLS I

I I I I (bl 0 ' I 2 3 4

CORE THICKNESS (INCHES J

Density

All mechanical properties increase with higher density, as shown in Fig. 12.6.

Cell shape

All honeycombs are anisotropic and the resulting directional properties should be adapted to

the loads anticipated. Figure 12.7 shows typical differences in shear strength for the L and W directions. In addition, some cell shapes allow easy forming or curving at a small loss in strength/weight ratio. This attribute can be of great importance in manufacturing curved parts of appreciable thickness.

Fig. 12.6(a) Typical stabilized compressive strengths.


1 PCF I L Fig. 12.6(b) Typical 'L' shear

0 2 0 40 60 8 0 100 120 140 160kglm' strengths. Density

Fig. 12.7 Plate shear test values may be signifi- cantly different from test results obtained from testing beams. Values shown above are typical for 5052 aluminum honeycomb.

Cell shape variations

Cell shape variations may be either furnished to specification by the core manufacturer, or, in certain materials such as aluminum, shapes may be intentionally or inadvertently altered by the core user. It should be noted that the under- or over-expansion of the core changes its cell shape and density. The over-expanded version of Fig. 12.8(c) changes directional properties such that the L direction becomes slightly the weaker of the two major axes.

8

C D

E F

G H

Fig. 12.8 A few of the many cell configurations in common usage. 8, G and H are only produced by the corrugating method. F is a cell configuration nearly always used in the manufacture of welded metal honeycomb. C is flexible in one axis, while G and H are flexible in both axes. A, C and D are expanded from identical unexpanded slices, A being normal expansion, C fully over-expanded and D 50% expanded. B is a reinforced corrugated core, with an extra layer of uncorrugated web material placed between each layer of corrugated web material. Reinforcing layers may be added in dis- crete locations or patterns and may be of the same or different web material or thickness.

Core materials 263

Since the drop in the L direction strength can amount to as much as 30%, such changes in cell shape must not be allowed to occur by error.

Cell size

Although cell size tends to be a secondary variable for most mechanical properties of core materials, it is primary in fixing the strength level of the core-to-face attachment (or, more accurately, in fixing the required lower limit on core-to-panel adhesive weight) and in determining stress levels at which intracell buckling or face dimpling of facings occurs.

Thickness v :

The shear and compressive properties noted for a 'pecific 'Ore type can Only be when test methods are controlled and the correct thickness of core is tested. Failure to allow for the effect of thickness can affect observed values by a factor of 4 or more, as noted in Fig. 12.5. It should be emphasized that the correction factor shown may be considerably different, depending on skin material and thickness, as well as the exact test method used.

Fig. 12.9 Plate shear test for honeycomb shear strength and modulus 1.27 cm (0.50 in) thick steel plates are oven-cleaned and may be reused many times.

'Pecified and

Specimen geometry and test method

Like thickness, these must be specified and carefully controlled in order to obtain compa- rability -with test values obtained by others. Shear strength values obtained using plate shear test methods of Fig. 12.9 are quite normally up to 25% below those obtained when using the flexure method shown in Fig. 12.10. Both methods are accepted and used and any

Fig. l2*l0 Short beam shear test for core. Note the ample bearing area provided at each load and support point to preclude core crushing prior to ,-hear failure.

lack of understanding of the differences can lead to monumental, if nonsensical, problems. Paper honeycomb

It will be noted that the tables of mechanical Paper honeycomb is the first predecessor of all properties for various honeycombs, Tables the types of honeycomb, having been pro- 12.4-12.12, specify the shear test method used duced for some 2000 years. Early forms were in producing the data shown. not used as structural cores, but were

Table 12.2(a) Properties of 5052 alloy hexagonal aluminum honeycomb" m N

Horltyomb dtwgt~ation, cell - material - g ~ u g e .- -

--

Bn re,

- ---

typical

6.3 870p 9.0 1480x 3.1 270 1.5 520 8.1 1400 12.0 2200p 2.6 200 8.4 1530 2.0 130 3.1 270 5.7 770 8.1 1400 1.6 85 4.3 480 6.0 850 7.9 1360 1.0 30 1.6 85 3.0 260 6.5 970

min

200 375 1000

150 1 070 90 200 560 1000 60 350 630 970 20 60 190 700

typical min

91op 1500x 290 215 545 405 1470 1100

2325p 215 160 1600 1180 135 100 290 215 810 600 1470 1100 95 70 505 370 880 660 1420 1050 45 20 95 70 270 200 1020 750

typical

275p 420x

75 150 350 90op

55 370 34 75

220 350 20 140 235 340 10 20 70 265

typical typical

5101.7 775x

130 210 260 340 750 725

ll0Op YO 165 800 760 60 120 130 210 390 460 750 725 40 85 230 320 430 495 725 700 25 15 40 85 120 200 505 545

min

155 285 670

120 690 80 155 410 670 60 265 435 650 32 60 145 500

ksi - --

typical

90P 105x 45.0 70.0 135

37.11 140 27.0 45.0 90.0 135 21.0 66.0 96.0 130 12.0 21.0 43.0 105

psi

typical

320p 520x 130 220 455

6251) 100 475 70 130 300 455 50 210 315 440 30 50 125 350

" Corrugated 5052 and 5056 aluminum honeycomb is ~vailable in higher densities with crush strenghts up to 6000 psi. Test data obtained at 0.626 in thickness. *" Crush strength values shown are avrragc or typical; actual values may vary because of density tolerances, etc.

p = preliminary properties; x = predicted values Note: contact core producer for complete information

Core materials 265

Table 12.3a Properties of several commonly used glass-reinforced plastic honeycombs* N 5% '3

Honcycom b designat ion, material - cell - density -

Hexagonal HRP - 3/16 - 4.0 HRP - 3/16 - 5.5 HRP - 3/16 - 8.0 HRP - 3/16 - 12.0 HRP - 1/4 - 3.5 H W - 1/4-6.5 HRP - 3/8 - 2.2 HRP - 3/8 - 4.5 HRP - 3/8 - 6.0 HRP - 3/8 - 8.0

-- -

Bare - ----

Strength, flsr

-

typical min 500 350 800 600 1400 1100 2280 1600 350 260 1025 850 150 105 610 450 900 750 1060 920

Corizpresszve - -

Stahdrzed -

Streizgih, Mo~iulus, ps i ks r

- - - -- --

typical min typical 600 480 57 940 750 95 1600 1280 164 2300 1800 260p 500 400 46 1180 900 120 200 145 13 690 550 65 1000 750 100 1200 150p

Plate d i ~ a r ~ -- - - 2

'W' Direction 3 'L' Divection

~ -~ F Strength, Modulus, Strength, Modulus,

psi !is i psi ksi "1 ~

-- ~ - - - - 0 5

typical min typical typical min typical 2 260 210 11.5 140 110 5.0

1 E

425 370 19.5 220 190 8.5 2 5' 660 600 34.0 400 370 15.0

940p 815 55p 570 500 25p 230 170 9.0 120 100 3.5 450 25.0 260 11.0 105 75 5.0 60 45 2.0 300 260 14.0 170 150 6.0 400 340 22.5 260 210 10.0 520 31p 320 13p

' Test data obtained at 0.500 in thickness. p = preliminary properties.

Table 12.3b Properties of several commonly used glass-reinforced plastic honeycombs* (metric)

Honeycomb designation material - cell - density

Hexagonal HRP - 3/16 - 4.0 HRP - 3/16 - 5.5 HRP - 3/16 - 8.0 HRP - 3/16 - 12.0 HRP-1/4-3.5 HRP - 1/4 - 6.5 HRP - 3/8 - 2.2 HRP - 3/8 - 4.5 HRP - 3/8 - 6.0 HRP - 3/8 - 8.0

- -

Bare

Strength, kPa

-

typical min 3447 2417 5516 4137 9653 7584

15 720 11 032 2413 1793 7067 5861 1034 724 4206 3103 6205 5171 7308 6343

Compressive -- - -- -- -- -- Plate shear

Stabilized --

'L' Direction - -- - -

Strength, Modulus, Strength, Modulus, kPa MPa kPa MPa

-- -

typical 4137 6481

11 032 15 858 3447 8136 1379 4757 6895 8274

4309p 8481p 2930p 5654p

1655 4137 2930

6067p

min typical 3309 393 5171 655 8825 1131

12 411 1793p 2758 317 6205 827 1000 90 3792 448 5171 689

1034p

-- -

typical 1793 2930 4551

6481p 1586 3103 724 2068 2758 3585

1448 2723p

965 1655

862p 1931 1344

2689p

- - -. -- -

min typical 1448 79 2551 134 4137 234 5619p 379p 1172 62

172 517 34 1793 97 2344 155

214p

- -

'W' Direction -

Strength, Modulus, kPa MPa --- -

typical min typical 965 758 34 1517 1310 59 2758 2551 103

3930p 3447p 172p 827 689 24 1793 76 414 310 14 1172 1034 41 1793 1448 69

2206p 9 0 ~

* Test data obtained at 12.70 mm thickness. p = preliminary properties.


employed as decoration - and are still fre- quently seen today as seasonal decorations in department stores in the form of expanded bells, spheres and so forth.

Current materials used as sandwich cores are different, in that much stronger kraft paper is employed and 11-35% phenolic resin is fre- quently used to improve mechanical properties, as well as moisture and fungus resistance. Many variations are available in cell sizes of 10,13 and 19 mm (%, X and % in) or even larger sizes. The higher strength versions are only produced in the smaller cell size, with the 10mm (% in) cell available as a water- migration resistant grade meeting military specification MIL-H-2104Q.

Most applications are found in non-aircraft uses, where cost saving is the one primary objective. Usage is growing rapidly in recreational vehicles; for doors, walls and partitions; for factory produced kitchen cabinets; in packaged patio room additions for homes; in curtain wall panels; and in bearing walls for commercial building.

Some of the above alloys are also available as corrugated, corrugated and reinforced, over- expanded and flexible cell configurations. Some have also been produced in a specially tailored geometry to make all the cell axes lie on a true radius of a cylinder, a sphere, or other unique configurations. These same alloy foils can also be wound as a corrugated spiral to form a cylinder or tube for very light energy absorption applications.

The aluminum honeycomb cores remain the most used, as well as the most versatile of the various core materials obtainable and are often found to possess the most favorable performance/cost ratio available. Expanded aluminum cores commercially available ranges from a low of about 32 kg/m3 (2 lb/ft3) to a high of 192kg/m3 (12.0Ib/ft"). Corrugated aluminum cores, however, start at under 128kg/m3 (81b/ft3) and can be pur- chased up to 880 kg/m3 (55 lb/ft3). At densities below 128 kg/m3 (8 lb/ft3) corrugated core suffers a serious penalty in shear properties when compared to expanded core.

Aluminum honeycomb Glass fiber-reinforced plastic honeycomb

This family of materials has been in production and growing since about 1947. Aluminum honeycomb now includes four alloys, at least five cell shapes and many foil gauges to provide a range of densities. The alloys generally available are:

0 3003-H19, the lowest strength of the group, usually used for non-aircraft applications;

0 5052-H39, the most often used aircraft grade, available with a corrosion resistant surface treatment. Mechanical properties are listed in Table 12.2;

0 5056-H39, the strongest of the regular aircraft grades, available with a corrosion resistant surface treatment;

0 2024-T3 or T81, the most heat-resistant alloy and slightly stronger in some properties than 5056-H39. Available with a corrosion resistant surface treatment.

This family of materials is most commonly used in electrically sensitive parts, such as radomes and antennae, or where a heat resistant resin and low thermal conductivity make it a natural choice. It has also seen distin- guished service as a matrix for retaining non-structural ablative materials, such as soft silicone rubbers or syntactic rigid epoxy foams, which otherwise could not have been used effectively as ablative heat shields on the Gemini and Apollo re-entry vehicles.

Only high temperature phenolic and poly- imide cores are generally produced. They are commonly available in cell sizes of 5, 6.3 and 10 mm (K, X and X in) with a 3 mm (% in) cell available in a bias weave glass reinforcement. Densities range from 32 to 192 kg/m" (2 to 12 lb/ft3). Mechanical properties of several commercially available glass fiber-reinforced cores are shown in Tables 12.3-12.6.

Core materials 269

Table 12.4(a) Properties of glass-reinforced phenolic honeycomb (bias weave reinforcement)*

Conipressiue Plate shear -. ~ ~~- - .--_____ -~_____.

Honeycomb Bare S tabilized ~ 'L' Direction 'W' Direction

designation Strength, Strength, Modulus, Strength, Modulus, Strength, Modulus, material - cell - density __ psi psi ksi psi ksi psi ksi

__

typical typical typical typical typical typical typical

HFT - 1/8 - 3.0 300p 350p 22p 185p 17P 95P 7P HFT - 1/8 - 4.0 390p 575p 45p 300p 32P HFT - 1/8 - 5.5 52533 960p 67p 425p 42P

HFT - 3/16 - 1.8 75P 120p 14p 105p 13P 5% 4P HFT - 3/16 - 2.0 loop 170p 17p 115p 15P 6OP 5P HFT - 3/16 - 3.0 27513 375p 32p 200p 24P loop 9P

150p 12p 225p 17p

HFT - 1 /8 - 8.0 1450p 1625p 1OOp 575p 48P 340p 25p

HFT - 3/16 - 4.0 435p 550p 45p 275p 3% 140p 12p HFT/OX - 3/16 - 6.0 lOOOp 11OOp 67p 290p 13P 335p 30p * Test data obtained at 0.500 in thickness. Honeycomb is normally not tested for bare compressive strength.

Table 12.4(b) Properties of glass-reinforced phenolic honeycomb (bias weave reinforcement)* (metric)

Compressive ~ _ _ ~-

Plate shear _____

Bare Stabilized 'L' Direction 'W' Direction Honeycomb designation Strength, Strength, Modulus, Strength, Modulus, Strength, Modulus, material - cell - density kPa kPa MPa kPa MPa kPa MPa

HFT - 1/8 - 3.0 HFT-1/8-4.0 HFT - 1/8 - 5.5 HFT - 1/8 - 8.0 HFT - 3/16 - 1.8 HFT - 3/16 - 2.0 HFT - 3/16 - 3.0 HFT - 3/16 - 4.0 HFT/OX - 3/16 -

typical

2068p 2688p 3619p

517p 689p 1896p 2999p

6.0 6894p

9997p

typical

2413p 396413 6618p 11 203p

827p 1172p 2585p 3792p 7584p

typical

151p 310p 461p 689p

117p

310p 461p

97P

220p

typical

1275p 206813 2930p 3964p 724p 792p 1378p 1896p 1999p

typical

117p

289p 331p

103p 165p 206p

220p

89P

89P

typical

655p 1034p 1551p 2344

413p 68913 965p 2309p

344p

typical

48P 82P

27P 34P 6% 82P

117p 172p

206p

* Test data obtained at 12.70 mm thickness. Honeycomb is normally not tested for bare compressive strength.

Aramid paper honeycomb

This is an especially tough and damage resistant product, based on a completely synthetic, calendered 'Nomex' paper material produced by DuPont. The core is expanded very much like aluminum or glass fabric honeycomb and then dip-coated with phenolic or other suitable resin system. The mechanical properties of the material as a structural core are some-

what lower than aluminum, especially in modulus, but it possesses a unique ability to survive overloads in local areas without per- manent damage. This translates into abuse resistance when applied to very light interior aircraft panels or flooring and gives the material a competitive edge even at the higher cost it represents. The base material is relatively incombustible and the small amounts present


Table 12.5 HFT glass-reinforced phenolic honeycomb (Fibertruss bias weave)* ~~

__- Compressive Plate shear _ ~ _ _ _ _ _ _ . _ ~ ~ _ _ _ ~ ~~~~ -

'W' Direction Honeycomb drsignation, Strength, Strength, Modulus, Strength, Modulus, Strength, Modulus, materid - cell - densitu m i asi ksi psi ks i psi ksi

-~ Stabilized 'L' Direction ~ ~ _ _ _ _ ~~

Bare

HFT - 1/8 - 3.0 HFT - 1/8 - 4.0 HFT - 1/8 - 5.5 HFT - 1/8 - 8.0 HFT - 3/16 - 2.0 HFT - 3/16 - 3.0 HFT - 3/16 - 4.0 IIFT/OX - 3/16 -

typical

250p 460p 850p 1600p

250p 460p

90P

6.0 l000p

typical

360y 530p 9501) 1750p 140p 320p 530p 1100p

typical typical typical typical

185p 16P 96P 150p

460p 34P 240p 340p

21P 45P 310p 25P 65P 95P 600p 43P 17P 118p 15p 55P

90P 170p 20P 310p 25P

32P 45P 67P 290p 13P 335p

150p

typical

6 . 4 ~ 9.5p 13.5p 20.0p 4.3p

9.5p 6 . 5 ~

3 0 . 0 ~

* Test data obtained at 0.500 in thickness. p = preliminary properties

in typical panels result in low volumes of smoke and gases given off in fire tests. Typical applications make use of these properties very effectively. As a consequence, they have grown to a commercial volume nearly as large as that of aluminum, for use in aircraft structures. Uses outside the aerospace industry are limited due to the high cost of the material, but despite this it has seen some application in boat hulls up to 10.2 m (40 ft) in length, as well as in skis, racing shells and several other products.

Aramid core is normally produced in cell sizes of 3, 5, 6.5 and 10 mm (%, 36, X and % in), in densities of 24-192 kg/m3 (1.5-12 lb/ft"). Densities higher than 64 kg/m3 (4 lb/ft3) are almost entirely used for aircraft flooring. Mechanical properties of some of these core materials are shown in Table 12.6.

Carbon fiber honeycomb

Reinforced plastic honeycomb has for many years employed glass fabric reinforcement, bu t only rarely employed other fibers. In the past few years, however, both Kevlar and carbon fiber have become much more common as reinforcing fibers for honeycomb. Carbon fibers only now are beginning to be used in

space vehicles. In addition to this small usage, however, carbon fiber honeycomb is now used as the structural core for nacelle assemblies in the Boeing Model 777 transport aircraft. The constant pressure for lighter structures in such designs has led to the use of carbon fiber facings, which have a potential corrosion problem when used with aluminum cores. This concern for corrosion problems has subsequently led to the adoption of a new class of carbon fiber honeycomb materials for this aircraft and will possibly lead to further use in other future designs.

Two types of carbon fiber cores are now being produced. One is for purely structural applications, while the other has a require- ment for heat transfer through the thickness of the panel. The former type uses only the usual pan based carbon fibers, while the latter employs pitch based carbon fibers, which duplicate the heat transfer properties of the aluminum core which it replaces. Although neither of these materials is as yet in large volume production, the economic impact is substantial, since these honeycombs are markedly higher in price than the aluminum or Nomex cores they replace.

Little data is yet available on these new cores, but it is likely they will see substantial

Adhesive materials 271

use and public scrutiny in the next several years.

Kevlar honeycomb

This honeycomb has been in use for a number of years as a core for space vehicle antenna reflectors. The purpose of the Kevlar honeycomb is to allow transmission of radio signals through the panel, while at the same time the Kevlar facing acts as a partial reflecting antenna for a different wavelength of a different signal.

Kevlar honeycomb, based on one of several fabrics woven from Kevlar yarn, is usually produced in cell sizes of 6.3-9.5 mm (%-% in) . Usual densities available range from 16 to 64 kg/m3 (14 Ib/ft3).

Kevlar paper honeycomb

In addition to Kevlar honeycomb made from woven fabric, DuPont has recently introduced a new honeycomb, based on a Nomex-like paper, which is entirely composed of fibers derived from Kevlar. This material has rather surprising mechanical and physical properties, with strengths well above both glass and Nomex honeycombs and dielectric properties somewhat superior to Nomex. This material is trade named 'Kortex' and is available in the usual range of cell sizes and densities. Because the material is somewhat more expensive than Nomex, no large scale replacement of Nomex honeycomb appears likely, although many special purpose applications have been developed in both air and space craft.

12.4 ADHESIVE MATERIALS

Adhesives, as they apply to sandwich structures, constitute a somewhat different family of materials than those required to bond an open cellular core to a stiff and continuous facing. Although these differences are less important with some of the newer modified epoxy materials, they remain basic and must

be understood by the designer and fabricator in order for the otherwise inevitable problems to be avoided. Some factors which merit atten- tion are discussed below.

12.4.1 PRODUCTS GIVEN OFF DURING CURE

Some adhesive types, such as phenolic, give off a vapor as a product of the curing reaction and the presence of these secondary materials can lead to several problems:

0 internal pressure, resulting in little or no bond in some areas, or 'blisters';

0 core splitting, as the gas forces its way through the core to a lower pressure area;

0 core movement, sometimes several inches, resulting in an unusable cured part;

0 subsequent corrosion of core or skins by the chemical action of the vapor or its residual condensate.

12.4.2 BONDING PRESSURE

Adhesives such as the phenolics and some others actually require more than atmospheric pressure in order to prevent excessive poros- ity. Certain forms may be suitable for solid cores like balsa, but cannot be used at all in open cores such as honeycomb or large cell foams. Also, most core materials will not alone withstand compressive bonding loads exceed- ing a few atmospheres and consequently cannot be used with any adhesive system requiring higher pressures.

12.4.3 FILLET FORMING

In order to achieve a good attachment to an open cell core, such as honeycomb, the adhesive must have a unique combination of surface tension, surface wetting and controlled flow during early stages of cure. Controlled flow prevents the adhesive from flowing down the cell wall and leaving a low strength top skin attachment and an overweight bot- tom skin attachment.

Table 12.6(a) Properties of Nomex paper honeycomb*

Honeycomb designation, material - cell - density (gauge) - --

Hexagonal HRH 10 - 1/8 - 1.8 (1.5) HRH 10 - 1/8 - 3.0 (2) HRH 10 - 1 /8 - 4.0 (2) HRH 10 - 1/8 - 5.0 (3) HRH 10 - 1/8 - 6.0 (3) HRH 10 - 1/8 - 9.0 (3) HRH 10 - 5/32 - 5.0 (4) HRH 10 - 3/16 - 2.0 (2) HRH 10 - 3/16 - 4.0 (3) HRH 10 - 3/16 - 6.0 (5) HRH 10 - 1/4 - 1.5 (2) HRH 10 - 1/4 - 3.1 (5) HRH 10 - 1/4 - 4.0 (5) HRH 10 - 3/8 - 1.5 (2) HRH 10 - 3/8 - 3.0 (5)

Bare

Strength,

-

psi

typical 110 300 500 900 1075 1700 800p 150 500 650 90 275 370 90

285p

HRH 10/OX - 3/16 - 1.8 (2) 110 HRH 10/OX - 3/16 - 3.0 (2) 365 HRH10/OX-1/4-3.0(2) 350

Flex-core HRH 10/F35 - 2.5 (3) 150 HRH 10/F35 - 4.5 (5) 450p HRH 10/F50 - 3.5 (3) 300 HRH 10/F50 - 5.0 (5) 550

-

min 70 180 330 600 800 1400

90 320 580 45 180 310 45

70 250 210

105

189

Stabilized

Strength.

-

psi

typical 130 330 560 925 1125 1800 900p 170 560 700 95 285 400 95

300p

130 400 385

3 70 490p 350 625

min 85 270 470 660 825 1600

105 470 650 55 240 360 55

270 250

119

217 525

- -

Modulus, ks i

typical

20 28

60 90

11 28

6

* Test data obtained at 0.500 in thickness. Nomex is a registered trademark of DuPont.

--

'L' Direction

typical 90 180 245 325 370 520

360p 110 245 390 75 170 240 75 170

60 115 110

Strength, psi -- -

min 65 162 225 235 260 370

72 215 330 45 135 200 45

45 95 90

49

105 300

Moduli~s, ksi

Plate shear

typical 3.7 7.0 9.2

13.0 17.0

1 1 . 5 ~ 4.2 7.8 14.5 3.0 7 . 0 ~ 7.5 3.0

5 . 6 ~

2.0 3.0 3.0

4 . 0 ~ 7.3p 5 . 7 ~ 8.0

Strength, psi

-

Modulus, ksi

typical min 50

typical 2.0 3.5 4.7

6.0 9.0

5 . 0 ~ 2.2 4.7 6.0 1.5 3.0 3.5 1.5

3 . 0 ~

3.0 6.0 6.0

1 . 9 ~ 3.7p 2 . 8 ~ 4.1

'W' Direction -

-

-

- p = preliminary properties

Table 12.6(b) Properties of Nomex paper honeycomb* (metric)

Bare

designation, Strength, material - cell - density (gauge) kPa

- --

Hexagonal typical

HRH 10 - 1/8 - 1.8 (1.5) 758 HRH 10 - 1/8 - 3.0 (2) 2068 HRH 10 - 1/8 - 4.0 (2) 34 473 HRH 10 - 1/8 - 5.0 (3) 6205 HRH 10 - 1/8 - 6.0 (3) 7411 HRH 10 - 1/8 - 9.0 (3) 11 721 HRH 10 - 5/32 - 5.0 (4) 5 5 1 5 ~ HRH 10 - 3/16 - 2.0 (2) 1034 HRH 10 - 3/16 - 4.0 (3) 3447 HRH 10 - 3/16 - 6.0 (5) 4481 HRH 10 - 1/4 - 1.5 (2) 620 HRH 10 - 1/4 - 3.1 (5) 1896 HRH 10 - 1/4 - 4.0 (5) 2551 HRH 10 - 3/8 - 1.5 (2) 620 HRH 10 - 3/8 - 3.0 (5) 1965p

OX-core HRH 10/OX - 3/16 - 1.8 (2) 758 HRH 10/OX - 3/16 - 3.0 (2) 2516 HRH 10/OX - 1/4 - 3.0 (2) 2413

Flex-core HRH 10/F35 - 2.5 (3) 1034 HRH 10/F35 - 4.5 (5) 3102p HRH 10/F50 - 3.5 (3) 2068 HRH 10/F50 - 5.0 (5) 3792

-

min

482 1241 2275 4136 5515 9652

620 2206 3998 310 1241 2137 310

482 1723 1447

723

1303

--

Stabilized

kPa

typical min

Strength, Modulus.

-

MPa

typical

137 193

413 620

75 193

41

41 117p

11 117

8 2 ~ 227p

16 255

* Test data obtained at 0.500 in thickness. Nomex is a registered trademark of DuPont p = preliminary properties

Plate shear --

'L' Dzrectmn --

'W' Direction - -

Strength, Modulus, Strength, Modulus, kPa MPa kPa MPa

--

typical min typical typical min typical


12.4.4 ADAPTABILITY

The requirements noted above must all be met while also meeting all the requirements of a skin-to-skin to skin-to-doubler attachment. In the case of contoured parts, the adhesive must also be a good 'gap-filler ' without appreciable strength penalty, since tolerance control of details is much more difficult to achieve on contoured than on flat panels and a greater degree of latitude for misfit must usually be allowed.

12.4.5 BOND LINE CONTROL

This is a need which exists because of misfitting details and is approximately the opposite of adaptability. It is the capability of the adhesive to resist being squeezed out from between faying surfaces when excessive pressure is applied to a local area of the part during cure. Many adhesives are formulated to achieve good core filleting and are subsequently given controlled flow by adding an open weave cloth or fibrous web, cast within a thicker film of adhesive. This 'scrim cloth' then prevents the faying surfaces from squeezing out all the adhesive, which would result in an area of low bond strength.

12.4.6 TOUGHNESS

The word 'toughness' has many meanings in the world of adhesives. Usually, it refers to the resistance shown by the adhesive to permit- ting bond line cracks to grow under impact loading. In the area of sandwich core-to-facing bonds, it refers to the resistance shown by the adhesive toward loads which act to separate the facings from the core under either static or dynamic conditions. It has been found from experience that greater toughness in the bond

Fig. 12.11 Climbing drum peel test for adequacy of skin adhesion. The difference in diameter of the cylinders to which the straps are attached and the cylinder to which the skin is attached causes the drum to rotate clockwise when tension is applied by the universal testing machine. This arrangement allows duplication of test results from one shop to another.

virtue of being easily duplicated, as well as possessing an obvious relationship to the toughness whose value is sought. Values of peel strength will vary considerably, depend- inn upon:

0

0

toughness of the adhesive; amount of adhesive used; density of the core; cell size of the core; direction of the peel (with or across the rib- bon direction); adequacy of the surface preparation; degradation of the adherend surface subsequent to bonding.

line usually equates to greater durability and thus to longer service life.

Many types of tests have been devised to measure toughness, but the most common one used for sandwich structures is the climbing drum peel test (Fig. 12.11). This test has the

Because these variables can lead to widely differing peel strengths for the very same adhesive, all of them must be properly understood and controlled if the peel test is to be used and its value compared to other test results.

Adhesive materials 275

The peel test is used to control quality 12.4.9 NITRILE RUBBER MODIFIED EPOXIES throughout the sandwich industry. Values obtained, provided the adhesive weight and core material are in balance, will give indica- tions of tooling or cure problems and of adherend surface preparation problems. It is particularly useful for this when an environ- mental exposure involving both elevated temperature and high humidity is interposed between manufacture and test. It is also adapt- able to use with nearly any skin material, except that it becomes impractical with very thick or very stiff skins.

It can be readily seen that a number of points

These make up a broad group of more recent materials which provide much of the flow and toughness shown by the nylon-epoxies, along with the durability and weather resistance of the vinyl-phenolics. They are the most common of the 'toughened' thermosetting adhesives and are usually limited to about 149°C (300°F) service temperature. Some of these materials routinely achieve shear strengths of 34500 kPa (5000psi) and most can be cured over a wide range of temperatures and pressures.

of difference separate the sandwich adhesives from other structural adhesives. Fortunately for the sandwich user, many adhesives are available which satisfactorily meet both sets of requirements. me types available, along with some salient features, are as follows.

12.4.10 URETHANES

Urethane based adhesives are used in commercial structures. Both moisture-cured and two-part systems are available.

12.4.7 PHENOLICS BLENDED WITH VINYLS, RUBBERS OR EPOXY

All of these families of adhesives give off at least some water during cure and are therefore used only where their high strength, durability or high temperature mechanical properties are essential. Since the out-gassing cure products usually require venting or perforating the core material and a number of non-out- gassing, high temperature adhesives have become available, their use as sandwich adhesives has sharply declined in recent years.

12.4.8 EPOXIES MODIFIED WITH NYLON OR OTHER POLYAMIDE POLYMERS

These adhesives were the first to have excellent filleting and controlled flow along with both high strength and high toughness, although they are somewhat moisture sensitive. Some versions are provided as one side of a two-sided tape adhesive, in which the other side is a rubber or vinyl-phenolic, to provide both excellent peel and durability at the skin side with excellent peel at the core side.

12.4.11 OTHER POLYIMIDES, THERMOPLASTICS AND HIGHLY SPECIALIZED ADHESIVES

These are used in a number of applications ranging up to about 371°C (700°F) service temperature, but do not represent either a very large group of materials or a large volume of usage. In addition to categorizing the available adhesives by chemical type, they can be grouped by the form in which they are available. Generally these are as follows.

Light liquids, heavy liquids, thixotropic liquids, pastes, putties, or syntactic foams

Only a few are used as a core-to-face bond, but many such materials are used in sandwich construction to splice pieces of core to each other in order to provide high strength edges, areas, or surfaces, or to carry shear loads from fittings, inserts, or end ribs. Most of the materials so used are epoxies, modified epoxies, epoxy polyamides or epoxy polyimides. Curing temperatures vary from as low as 4.4"C (40°F) for some two-part systems up to


216°C (420°F) for some of the materials intended for service at elevated temperatures.

Supported films

Films or tapes having a carrier of light glass fiber, cotton, nylon, or polyester fabric, or spunbonded synthetic fiber are provided either dry or with slight to moderate ’tack’ or stickiness, so that the parts of the assembly stay in place as they are being assembled.

Unsupported films, containing only the adhesive

The very low weight films are nearly always furnished without a carrier, as the weight of the carrier itself becomes quite appreciable in very light sandwich structures. They are often hard to handle and sometimes have bond line control problems.

Reticulating films

These are intended for use at very low weights, with the adhesive being melted by hot air after placing on the core, so that it draws back to the cell edge and provides material to form the largest possible fillet without wasting any on the inside facing surface in the middle of the cell.

Cell-edge adhesive

This is a material pre-placed on the cell edge by the honeycomb manufacturer to provide the same results as those produced with reticulating films.

Self-adhesive skins

These skins are usually structural fabrics of glass, graphite, quartz, or aluminum coated glass fibers, pre-impregnated with a resin, which is then cured so that the fiber-filled resin becomes both the face structure and the attaching material.

All the above forms of adhesive are in current use at substantial volume and most are available from many sources.

12.5 DESIGNING A SANDWICH

The usual objective of a sandwich design is to save weight or to increase stiffness or to use less of an expensive skin material, or perhaps all three. Sometimes other objectives, such as reducing tooling or manufacturing costs, achieving aerodynamic smoothness, reducing reflected noise, or increasing durability under exposure to acoustic energy, are also involved. The designer’s problems sift down to relatively few, such as getting the loads in, getting the loads out and attaching small or large load-carrying members, under constraints of deflection, contour, weight and cost.

Understand the fabrication sequence and methods. The cost of a sandwich structure is fundamentally fixed at the design stage and a considerable difference in cost can result from alternate solutions to the design problem. Both of the edge close-out details shown in Fig. 12.12 perform essentially the same job at the same weight. Placing the legs of the channel facing outward instead of inward saves the cost of two relief cuts into the core and the very difficult step of sliding the edge of the core and adhesive into the channel. Another alternative at even lower cost for either fixed or simply supported edges is shown in Figs.

Use the right core. Several densities of core can be used in a single panel, each appropriate to the load carried in the area and adhesively bonded to its neighbor, as shown in Fig. 12.17. In many cases, however, the weight saved in lower density areas of core is added back in the form of core splice adhesive weight. Core splices, such as those shown in Fig. 12.18(b) or (c), have been used to produce ablative matrix structures for large re-entry heat shields,

12.1 3-12.16.

Next Page

Sandwich

Documents

bias weave

design feasibility

reinforced

aramid paper

compressive

reinforced

test data

bare compressive