40207_12

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 transmitting 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 calculate 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

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

264 Sandw

ich constructiotz

Core materials 265

266 Sandw

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m

n

Core materials

267

xx

xx

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22

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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 bottom skin attachment.

272 Sandw

ich construction

Lolo

o

mm

m

OO

Lo

m

am

+

em

Adhesive matevials 273

s; 6

N

... ..-2 u m

u

moo

dN

N

om

0

dm

N

ma

-


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 ribbon 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.

Designing a sandwich 277

POOR

(1

GOOD ~ . . .

Fig. 12.12 The square edge close-out shown here using a channel may result in a neat, clean edge, but requires machining both the top and bottom of the core and squeezing adhesive and core into the channel during assembly. The alternative shown on the left would be much better.

/

Densified Core 2

Fig. 12.13 Densified core edge treatment.

HIGH -S TR€NG TH INSERT'

ME TAL CHA NNEL ' HIGH-STRENGTH INSERT,

ME TAL CHANNEL ' HIGH - S TRENG TH INSER T\

METAL 2''

DENSIFIED CORE '

EDGE CELLS FILLED'

FACINGS FORMED '

CRUSHED AND BONDED

r RE/NFORCEMENT

FO RMEO RING ---. Fig. 12.14 Several common edge treatments.


Strong. Uses standard angle. Pop rivets to locate and apply pressure during bonding.

Very strong. Special extrusion. Difficult to apply adhesive uniformly and assemble

Low strength. Very low cost. Inside facing and core scarfed then bent. Fill corner with epoxy or foam to stiffen.

Strong. Special extrusion. Seals can be incorporated.

Very strong with inside tie-bar. Can include external seal or gasket.

Fig. 12.1 5 Several suggestions for corner designs, edge close-outs and splices.

Ex trurion, Weldd f

b \

Locking Bar

Fig. 12.16 Additional joints and corner treatments.

Fig. 12.17 Typical core splice using a foam-tape adhesive. Foaming of the tape adhesive permits a less-than-perfect fit of core details, but requires that the core be precisely fixed in position during the cure to avoid a step in the surface at the splice line.

ACROSS RIBBON 1Wl DIRECTION

C

Designing a sandwich 279

IN RIBBON /LI DIMCTION

Fig. 12.18 Joint A may be formed by simply crushing one piece of glass fabric honeycomb into the adjoining section. This method will work to some extent with some aluminum honeycombs, but not with most other core materials. Joints B and C require a perfect match of cell shape and cell pitch and are very difficult to produce on any realistic and cost-effective basis.

but become prohibitively expensive to produce for splices more than a few inches long. Do not hesitate to use several joining methods in the same part. Fittings to be included in a bonded sandwich may be produced from weldments, forgings or riveted assemblies, or may themselves be bonded assemblies. Available adhesives permit secondary bonding to be performed at temperatures from 16°C (60°F) up to 177°C (350°F) without degrading the integrity of the previously bonded sub-assemblies. Use bolts and rivets for carrying loads (not for soothing fears). Where space is not available for progressive doublers or wide-area bonded overlaps to carry high loads, the addition of rivets or bolts is sometimes the only solution. Their use, however, often results in lower (sometimes dramatically lower) fatigue life of the structure, in addition to increased weight. The use of 'chicken rivets', added for the sole purpose of appearance, is to be particularly avoided, since they often defeat much of the advantage which would otherwise result from use of the bonded structure.

5. Use doublers where needed, instead of a heavier facing over the entire part. The use of doublers, although adding labor cost in assembly, often improves the part quality. Where skins are formed of glass or graphite prepreg, the problem is even simpler, since extra plies can be added to carry extra loads exactly where and as needed.

6. Use external doublers rather than infernal doublers wherever possible. The use of internal doublers usually means that a relief cut must be made in the thickness dimension of the core to prevent bridging and a conse- quent unbonded area where the doubler ends. Figure 12.19 shows a panel where the loads which can be carried are the same at each end of the panel. The design detail on the left end can cost substantially more to manufacture than that on the right end. Figure 12.20 shows the same panel with both ends produced at low cost, while still achieving an unbroken outer skin line on one side. In the case of some skin materials, such as 0.25mm (0.010in) aluminum, or most weights of pre-impregnated glass or graphite cloth, it is feasible to use thin doublers without a relief cut in the core, since


Fig. 12.19 Internal and external doubler treatment.

I 1 Fig. 12.20 Low cost doubler treatment.

Fig. 12.21 Doublers at a skin splice.

the gap caused by bridging is small enough to be within the capacity of the adhesive to fill. Sometimes an extra layer of adhesive film is added to help. An example of a dou- ble skin splice using this method is shown in Fig. 12.21.

12.6 STRUCTURAL ANALYSIS FOR SPECIFIC CASES

The following notations are used in sandwich design formulas.* This chapter’s formulas are only for honeycomb beams and columns which have the same facings on each side of the core.

Structural analysis for specific cases 281

E t h2 D = flexural stiffness; D = JL-

2% Ec = modulus of elasticity of the honeycomb, Pa (psi) E, = modulus of elasticity of facing material, Pa (psi) Gc = shear modulus of rigidity of the honeycomb, Pa (psi) Kb = bending deflection constant Ks = shear deflection constant L = beam span length or column height, mm (in) b = beam width, mm (in) M = maximum moment, kg/m width (Ib/ in width) P = load, kg (lb) Pcr = column critical load, kg/m (lb/in) P = column facing yield load, kg/m (lb/in) 9 = maximum shear force, kg/m width (Ib / in width) d = sandwich total thickness, mm (in) h = distance between facing centroids, mm (in); h = tc + t , s = core cell size, mm (in) tc = core thickness, mm (in) t , = facing thickness, mm (in) w = uniform beam load, Pa (psi) A = maximum beam deflection, mm (in) ,I6 = 1 - Poisson’s ratio of the facing material squared = 1 -p2 p = facing material’s Poisson’s ratio of = maximum facing stress, Pa (psi) uy = yield stress of facing material, Pa (psi) zc< = maximum core compressive stress, Pa

zcs = maximum core shear stress, Pa (psi) (psi)

The need to be able to accurately calculate the exact performance for many forms of sandwich structures had led to the development of a substantial body of literature on the subject. This chapter will cover in detail only the commonly used analyses and will provide reference sources for a number of others.

12.6.1 DESIGN REQUIREMENTS

Sandwich structures should be designed to meet the basic structural criteria listed below, when these criteria pertain to the type of loading under consideration.

1. The facings should be thick enough to withstand the tensile, compressive and shear stresses induced by the design load.

2. The core should have sufficient strength to withstand the shear stresses induced by the design loads.

3. The core should be thick enough and have sufficient shear modulus to prevent overall buckling of the sandwich under load.

4. Compressive modulus of the core and the compressive strength of the facings should be sufficient to prevent wrinkling of the faces under the design load.

5. The core cells should be small enough to prevent intracell dimpling of the facings under design load.

6. The core should have sufficient compressive strength to resist crushing by design loads acting normal to the panel facings or by compressive stresses induced through flexure.

12.6.2 MODES OF FAILURE

Typical modes of failure are shown in Fig. 12.22.

12.6.3 DESIGN STEPS

1. Define Zoads. For multi-point loadings, use the formulas in Roark’s Formulas for Stress and Strain.16

2. Define beam type. The values of Fig. 12.23 provide the simple starting point for these calculations. Some care in using the fixed end type of support is needed, as in actual practice total fixity is not realized and the resulting deflection is greater than that calculated.

3. Determine deflection limitations. For most applications, the allowable deflection of the


Facing failure

Initial failure may occur in either compression or tension face. caused by insufficient panel thickness, facing thickness, or facing strength.

Transverse shear failure Caused by insufficient core strength or panel thickness.

Local crushing of core

Caused by low core compression strength

General buckling

Caused by insufficient panel thickness or insufficient core rigidity.

Shear crimping Sometimes occurs following, and as a consequence of, general buckling. Caused by low core shear modulus, or low adhesive shear strength.

Face wrinkling Facing buckles as a ’plate on an elastic foundation‘. It may buckle inward or outward, depending on relative strengths of core in compression and adhesive in flatwise tension.

Intracell buckling (dimpling) Applicable to cellular cores only. Occurs with every thin facings and large core cells. This effect may cause failure by propergating across adjacent cells thus inducing face wrinkling.

Tensile failure in facing

I

Adhesive bond

Core comDression

Faces buckle into core cell

M Of = ~ t , hb

V hb

= -

P ‘ A

u = -

PCr = tcGc

EftC

1/2

Fig. 12.22 Modes of failure in sandwich structures. Sandwich structures must be designed to resist these modes of failure. Failures may occur which combine more than one of the modes shown.


1 192 -

Beam type

1 4 -

Maximum shear force

V

I Simple s tmor t Uniform load 1 . .

1 - I f 1 Both ends fixed Uniform load I

E 8

Simple support Center load

I P A

P 2 -

I I I

1 Both ends fixed Center load 1 1

5 384 __

E 4

1 Cantilever Triangular load 1 I

Shear deflection constant

1 8 -

I

I I I Cantilever Uniform load

l r r r r r r r P = ql

I I I

1 One end simply supported . _ . . one end fixed Uniform load

I

PL 8

“ I + 192

Fig. 12.23 Loaded beam chart, where P = total load (per unit width), L = span, 0, = facing stress, t, = skin thickness, h = centroid distance, zcs = core stress, shear, tcc = core stress, compressive, A = (1 -/,L)~ facing property, E , = modulus of elasticity of facings, Gc = modulus of elasticity of core in shear, Ism, = moment of inertia, sandwich, s = cell size, Ec = modulus of elasticity of core in compressions, F S = factor of safety, T = total sandwich thickness (note that P must be determined for a beam unit width). If deflections are critical, actual deflections should be verified by tests.


4.

5.

6.

7

8.

structure is usually limited to L/360. In some cases, greater deflections may be used, or, as in the case of snow skis, very much greater deflections may be a normal part of the function of the structure. Select skin material . Skin considerations include the weight target, possible abuse and local (denting) loads, corrosion or dec- orative constraints and costs. Select standard thicknesses and make the initial calculation as outlined below. The facing thickness directly affects both the skin stress and the deflection. Calculate f irs t approximation. After the first sandwich thickness, h, is determined, another selection of t, or E , may be made to arrive at more desirable or practical values of h. Most sandwich structures in ordinary usage are in the thickness range of 1.5-150 mm (0.06-6.00 in). Select skin thickness. Keep in mind that materials such as fiberglass cloth and aluminum are available in specific, standard thicknesses. After the skin thickness for deflection is selected, it should be checked for stress. The formula for 6, is used and a factor of safety determined. Select core. Calculate the core shear stress, sc*. Note that the core strength is not the same in the L and W directions. Refine the selection, including considerations of material compatibility, cell sizes and types. Determine the corrections needed to account for the effects of thickness on strength, as shown in Fig. 12.5. Check the factor of safety using the calculated stress and the corrected allowable stress. Other considerations include crushing and compression strengths, modulus in shear, weight and costs. For rolling wheel loadings, the crushing strength and the skin thickness are often the most important considerations. Check def7ection. For many applications, the calculation of the expected deflection may omit the shear deflection portion. With a very small deflection limitation, with a very thick sandwich, or with a very short span,

the shear component should be calculated and the core selection may be influenced by the shear modulus needed.

9. Face wrinkling and intracell dimpling. With thin skins, a local failure of the skin in buckling may be encountered. A check on the afcrit will determine whether this may be a design consideration.

10. Other considerations. Often, honeycomb panels are supported on more than two sides. If the ratio of length to width is greater than 3: 1, the calculations using the shorter span and designing as a unit beam are quite adequate. The formulas in Roark16 are useful where the shear deflection may be ignored, using the following formulas.

bt3 . I , =--

t,h2b - -~ 'sandw,ich 2 ' solid 12

So, for plate calculations:

Isolid = 6t,h2

Use of these formulas for deflections may give lower values than actually experienced, since the shear deflection may be important. Table 88 of RoarkI6 gives some approximate multi- pliers to use for plates when supported as noted.

12.6.4 SIMPLE FORMULAS

Bending stress in facings:

M a, = ~

t , h where M is determined by Fig. 12.23.

Core shear stress:

V cs h

= -

where V is from Fig. 12.23.

Deflection: ~ ~ 1 x 3 2 K ~ P L

A = - +- E, t , h2 hGc


( K , and K, from Fig. 12.23).

For most beams, the second term is relatively

material is to be woven roving, polyester and core to be KI'-3/8-60(25).

small, but should be checked if deflection is critical or span is short.

Moment of inertia, sandwich: t,h2b

2 I,, = ~

Face dimpling: 2E, t,

u fcnt . = -[-I 1

Load, P': P' = 120/144 = 0.833 psi

Span, L: L = 8 x 12 = 96 in

Kb, Ks, M, V from Fig. 12.23:

Kb = 0.013, K, = 0.125, M = 8, V= 2.

Skin, t,: Try t , = 0.090 in

Face wrinkling: A, E , for fiberglass use:

A = 0.98, E , = 1.85 x lo6. E,tc Calculate h: Factor of safety:

FS = K,P 'L~~P

tPE, Allowable or typical stress A =

calculated stress

- 0.013 x 0.833 x 964 x 2 x O.98ll2

h = 5.518 (round out to 5.5 = panel total thick-

- 0.090 x 96/270 x 1.85 x lo6 12.6.5 SAMPLE PROBLEM: ANALYSIS OF FLAT

RECTANGULAR SANDWICH BEAMS ness, h = 5.41) Try thicker skin t, = 0.150 Design a flat roof panel for a bus stop. Use a

snow load of 120 lb/ft2. Use a simple panel with a simply supported span of 0.203 m (8 ft). Deflection is to be limited to L/270 and the factor of safety is to be greater than 2.0. Skin

0.013 x 0.833 x 96* x 2 x O.98ll2 0.150 x 96/270 x 1.85 x lo6

h =

= 4.274

Use 4.00 overall thickness, h = 3.850. Since either construction is practical check out the skin and core stresses:

- - 0.833 ~ 9 6 ~ = 1971 psi P'L2 Uf = ~

t$Mc 0.090 x 5.41 x 8

Uf = 962 = 1662 psi 0.833 0.150 x 3.85 x 8

te that the skin stresses are quite close; therefore, the factors of safety would be similar.

38000 = 23 = 19;FS = ~

38 000 FS = ~

Fig. 12.24 Schematic diagram of a flat sandwich panel. 1971 1662

286 Sandwich construcfion

For core: 12.6.6 ANALYSIS OF FLAT RECTANGULAR SANDWICH COLUMNS: COLUMN DESIGN EXAMPLE - P'L - - 0.833 x 96 = 739 psi

' r s - 5.41 x 2

or, L = B f i / (20.32 cm) t

0'833 96 = 10.38 psi + P 3.85 x 2

Note that the core stresses are quite low and there is not much difference in the stresses for the two thicknesses chosen. For KP-3/8-60(25), W shear strength = 60 psi. From Fig.12.5: thickness factor = 0.42, W shear modulus = 5800.

W, shear, corrected = 60 x 0.42 = 25 psi

FS = 25/ 10.4 = 2.4.

The use of KP-3/8-60(25) with a factor of safety of 2.4 could be marginal, which may vary from lot to lot of material. The other properties, compression strength and density are acceptable. Note that if the core is oriented to utilize the L shear properties, KP-1/2-80(11), with rCs = 70 x 0.42 = 29.4 might be satisfactory.

Facings: Tempered hardboard, = 3600 psi

p = 0.99

E , = 0.65 x lo6 psi

Core:

Urethane foam, 6 lb/ft3, tc = 3 in

From Table 12.1:

Lcs = 90 psi

Calculate deflection: KbP'L42 KSP'L2

For 5.50 T, A = ~ + ~

t, h2E, G'h

L r - c = 170 psi

Gr = 1500 psi

Check facing yielding: - 0.013 x 0.833 x 964 x 2

0.090 x 5.412 x 1.85 x lo6 -~

Pp = 2t,oY = 2(0.25)(3600) = 1800 lb/in

0.125 x 0.833 x 962 5800 x 5.41

Check general buckling: + - E , t, h2

Pcr = X'D (where D = = 0.377 + 0.032 = 0.0409 in r l n2D

L'+ ~

Note that the added shear deformation is only t,G' 9% of the total deflection.

0.013 x 0.833 x 96j x 2 0.150 x 3.852 x 1.85 x lo6

For 4.00 T, A =

0.125 x 0.833 x 96: 5800 x 3.85

+

0.65 x lo6 x (0.25)(3.25)' 2(0.99)

D =

= 866 872 lb-in/in of width

n2(866 872) PPV =

= 0.447 + 0.042 = 0.489 in ~ ~ ( 8 6 6 872) (96)2 + 3.0(1500)

Manufacturing sandwich stsuctuses 287

= 112 lb/in or 1352 lb/ft

Check shear crimping:

PCr = tcGc = (3.00)(1500)

= 3214 lb in/in of width

Check dimpling and wrinkling: Since facings are relatively thick and continuously supported by foam core, dimpling or wrinkling will probably not occur.

12.6.7 DESIGN CONDITIONS

In-depth treatments for the design conditions listed in Table 12.7 can be found in MIL- HDBK-23I, available from the US Government Printing Office.

12.7 MANUFACTURING SANDWICH STRUCTURES

temperature (both pressure and temperature in the precise amounts, at the precise time required for cure of the adhesive being used); and the provision for tooling and fixtures to hold the assembly in the desired shape and keep all the details in their proper positions during cure. Many different ways of providing these conditions are currently used, from vacuum bags or simple presses to autoclaves and unit tools, where volume and complexity can justify them. Most of the equipment is similar to equipment used in producing bonded structures or reinforced plastic parts where no sandwich structure is involved. However, bonding of sandwich structures is nearly always performed at lower pressures than is the bonding of structures which do not have a low density core and tooling is sometimes lower in cost as a result. Aside from the need for lower maximum pressure, there is little noticeable difference between a sandwich bonding facility and one which only handles

The manufacture of sandwich structures requires three conditions to be met: the appli- A few suggestions can be offered to aid in cation of pressure; the application of living with the problems of sandwich bonding.

non-sandwich bonding.

Table 12.7

Subject MIL-HDB K-23 * CHAPTER

_ _ ~ ~~~ ~~~ ~~

Wrinkling of sandwich facings under edgewise load Dimpling of sandwich facings under edgewise load Design of flat, rectangular sandwich panels under edgewise compression load Design of flat, rectangular sandwich panels under edgewise shear load Design of flat, rectangular sandwich panels under edgewise bending moment Design of flat, rectangular sandwich panels under combined loads Design of flat sandwich panels under uniformly distributed normal load Design of sandwich cylinders under external radial pressure Design of sandwich cylinders under torsion Design of sandwich cylinders under axial compression or bending

Design of flat circular sandwich panels loaded at an insert

7 8 9 10 11 12 13 19 20

Design of sandwich cylinders under combined loads Design of sandwich strips under torsion load

* M1L-HDBK-23 is revised from time to time, with new chapters sometimes added and older material updated. A check with the Plastics Technical Evaluation Center, US Army Armament Research and Development Command, Dover, New Jersey, USA, can verify that you are in possession of the most recent revision.

288 Sandzuick construction

1. Make sure the core is properly sized to fit the space it is intended to occupy. If it has been stretched a little, to make the distance from one edge member to the opposite one, it will probably shrink back as the cure cycles starts, leaving mysterious voids next to an edge member. If it is undersize in thickness at an edge, the adjoining edge member or fitting will hold the facing away from the core and result in an unbonded area.

2. If a honeycomb core is being used, remember that the adhesive between the core and the faces will end up much thinner than the same adhesive between the edges or solid inserts and the facings. For this reason, it is common to require the core to be as much as 0.25 mm ( 0.010 in) thicker than adjoining solid parts in the same assembly.

3. The elevated temperatures which most core-to-facing adhesives require for curing are often inaccurately measured. A good point to remember is that only the adhesive being cured can give you the cure temperature you are trying to measure. Some shops insert thermocouples directly into the bond line to determine temperature and then leave the thermocouple permanently in the part after cure is completed.

4. Most adhesives flow at an early point in the cure cycle. At this time, the bond lines will change in thickness by substantial amounts. The tooling employed to establish the shape of the part and hold details in place must also allow the details to move into their final cured position. Simple examples are a hot platen press, in which the platens close on the sandwich as the bond lines grow thinner, or an autoclave, in which a flexible bag follows the details as the adhesive flows, continuously transmitting the autoclave pressure to the shrinking assembly. Keep in mind that most adhesives are very weak and crack-prone as they go through the gel point.

5. Inserts or heavy members being cured as a part of a very light assembly will heat up

much more slowly, resulting in warpage problems upon cool-down. Warpage on very light parts can also be caused by one side cooling down too fast as a result of having one side removed from the still-hot tooling, while the other side continues to stay at the temperature of the tool. Also one side, next to the bag may be heating faster or to a higher temperature than the opposite face, which is in contact with a massive and still cold tool. Slower heat-up rates or better heat distribution in the tool design will help prevent these problems.

6. Be sure to provide a route for the escape of trapped air and gases from a totally enclosed part while it is being cured. This is particularly important in parts which are vacuum bagged to a female tool and cured in an autoclave. A coarse cloth 'breather' should be enclosed inside the bag to prevent the bag from sealing off portions of the assembly as pressure is being applied. Critical or expensive assemblies should have several vacuum lines attached at different points of the bag, with each monitored separately by a pressure recorder.

7. Caul plates should be carefully matched to the job they are expected to perform. These tooling aids are often used to cover the top of an assembly containing several different pieces of core, inserts, edges, etc., so that a thin skin will not push each detail to the minimum bond line thickness and result in an uneven outer surface. When the caul plate is moderately stiffer than the top skin, the bonding pressure is transmitted more to the thicker inserts and less to the under- sized inserts, allowing all of the details to 'float' in the adhesive before cure, resulting in optimum relative placement of all the internal details in the sandwich. If the caul plate is extremely stiff or thick, this effect is changed to one of simply bridging over the most oversized details and the danger of producing voids or unbonded areas over the thinner details is substantially increased. Generally, the caul plate should

Manufac t uring sandwich structures 289

8.

not be more than two or three times the thickness of the sandwich facing material. Where thicker caul plates are used, the dimensional control over the size of detail parts in the assembly must be correspond- ingly better. The advantage of using such a thick caul plate derives from the ability to make both sides of a sandwich part have the smooth appearance usually associated only with the 'tool side'. Make sure that core, pre-cured or rigid edges, inserts, skins and other relatively unyielding details assembled in the lay-up have close enough dimensional control to allow adhesives or resins to achieve the target strengths. In simple bonded assemblies, a tolerance of + 0.1 mm (+ 0.005 in) is neces- sary, while assemblies having multiple layers of prepreg or many layers of thin metal doublers can sometimes be success- fully produced with much less demanding dimensional control.

12.7.1 CORE SHAPING

When core materials must be cut, trimmed, carved, or shaped, many special purpose tools are available. Sawing is the most common machining method, using either conventional blade tooth patterns, or, for some trimming operations, a special 'honeycomb band', in which the blade appears to be running back- ward, with the teeth sharpened on the back side, so that each tooth acts as a slicing knife blade. A different type of saw is also used as a mandrel-mounted router bit. Such tools, shown in Fig. 12.25, are very common where sculpturing of honeycomb or foam is to be accomplished. Router speeds vary from 1000-30 000 rpm for blade diameters of

CA

t

- .-

Fig. 12.25 Honeycomb carving bits employing a slitting saw 0.254 mm (0.010 in) thick x 12.5 teeth per cm (32 teeth per in), 50.8 mm (2 in) in diameter at the cutting edge. Turning at 12 000 to 30 000 rpm, these tools leave a smooth, burr-free surface on nearly any core material. The coarse teeth on the tool in the foreground are for the purpose of break- ing up and removing the excessive amounts of core in cut depths of 5.08-50.8 mm (0.2-2.0 in).

1.8-10cm (0.754 in). Roll forming can be accomplished on metal cores, as shown in Fig.

heat formed' In either forming can be much easier if an inherently formable cell configuration, such as that shown in Fig. 12.8 view H, is used. shops.

Fig- 12-26 Metal honeycomb may be roll-formed

be protected during the operation by inclusion of a loose sheet of thin sheet metal between the core and the outer forming rolls. The tool being used is a 'Farham Roll', co-ody used in sheet metal

12.26, while non-metal cores must usually be using Ordinary The surface usuallY must


REFERENCES

1. MIL-HDBK-23, US Government Printing Office,



4. MIL-A-132, US Government Printing Office,

5. MIL-A-25463, US Government Printing Office,

6. MIL-STD-401, US Government Printing Office,

7. Adhesive Bonded Aerosvace Structures Standard

Washington, DC*.

Washington, DC.

Washington, DC.

Washington, DC.

Washington, DC.

Washington, DC.

Repair Handbook, US’ Government Printing Office, Washington, DC, Fig. 12.27 Nose radome core assembly, assembled

by edge-bonding together several post-formed sec- tions of glass fabric-phenolic honeycomb. Nomex core may also be formed in this manner.

8, Hexcel Corporation, TSB-120. 9. Hexcel Corporation, TSB-123.

10. Hexcel Corporation, TSB-124.

I

Fig. 12.28 Effect of roll-forming on aluminum honeycomb. The core on the left has been roll-formed in sheet metal forming rolls, while the piece on the right has not been pre-formed at all. Note the anti- clastic, or ‘saddle shape’, which the unformed piece assumes when forced to a cylindrical form.

11. 12. 13. 14.

15.

16.

17.

Alcore, TR-il2. American Cyanamid, Handbook of Adhesives. Baltek Corporation, Baltek Catalog. Plantema, Frederic J., Sandwich Construction, John Wiley & Sons, New York. 1966. American Plywood Association, Plywood Design Specifica t ion. Roark, R.J., Formulas fo r Stress and Strain, McGraw-Hill, New York, NY., 5th edn, 1975. Timoshenko, S., Woinowsky-Krieger, S., Theo y ofPlates and Shells, McGraw-Hill, New York, NY. 2nd edn, 1959.

*Publications of the US Government may be updated and revised from time to time. Be sure you have the most recent edition. This can be checked by con- tacting the Plastics Technical Evaluation Center, US Army Armament Research and Development Command, Dover, New Jersey. The publication MIL-HDBK-23 was abandoned some years ago. However, because the information it contained continues to be needed by designers of spacecraft structures, the entire publication will in future be included within MIL-HDBK-17.

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