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HISTORY

The first thing this book will do is explain exactly what honeycomb is and review some of its history. Honeycomb consists of an array of open cells, formed from very thin sheets of material attached to each other. Usually the cells form hexagons (Figure l.l), but there are other cell configurations that will be described and discussed in Chapter 2.

Honeycomb closely resembles the bee's honeycomb found in nature, from which it gets its name. It can be made from any thin flat material, and in the past over 500 different kinds of honeycomb have been manufactured. Paper honeycomb was first made about 2000 years ago by the Chinese, who used it for ornaments and not structurally as it is today. Even now, however, we still see ornament tissue paper honeycomb turkeys being used as decorations for Thanksgiving, and tissue paper honeycomb bells and stars during the Christmas season.

The first honeycomb core patent, covering a manufacturing method for the production of Kraft paper honeycomb, is probably the Budwig Patent, issued in 1905 in Germany. One of the earliest man-made sandwich structures of which we have a record was a tubular railroad bridge in Wales, built in 1845. It consisted of a large rectangular tube, the floor of which supported railroad tracks, and through which trains ran. The tube's

Figure 1.1 Hexagonal honeycomb cells.

T. Bitzer, Honeycomb Technology© Chapman & Hall 1997

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top compressive panel had two flat plates connected to a square cell egg­crate type wood core.

In 1919 the first aircraft sandwich panel was fabricated using thin mahogany facings bonded to an end-grain balsa wood core. It was used as the primary structure of the pontoons of a seaplane. Later, between World War I and World War II, plywood skins glued to a balsa wood core were used as the primary structure in Italian seaplanes. An entire squadron of these aircraft was flown to Brazil in the 1920s and another squadron was flown to the Chicago World's Fair in the 1930s - truly a remarkable demonstration of flight time for that period.

The manufacture of modern structural honeycombs probably began in the late 1930s when J. D. Lincoln manufactured Kraft paper honeycomb for use in the furniture built by Lincoln Industries in Marion, Virginia, USA. The material was used in sandwich panels which consisted of thin hardwood facings bonded to a relatively thick slice of paper honeycomb.

At the outbreak of World War II paper honeycomb was used by the Glen L. Martin Company in radomes - structural enclosures for radar antennas, which were then in their infancy. It was quite successful; however, the paper core did pick up moisture. Martin later developed a honeycomb made of cotton duck fabric and by the end of World War II they had produced honeycomb cores made of cotton fabric, glass fabric and aluminum foil.

Also at this time the de Havilland Airplane Company designed and built the Mosquito bomber, which used sandwich panels in parts of the airframe. The excellent performance displayed by this airplane led to the acceptance of many aircraft designers, particularly in England, of the basic superiority of the sandwich structure as a means of making a more efficient and higher performing airplane. As a result many aircraft design groups began to examine better ways to make sandwich structures and better materials from which to make the cores and facings.

It was not until 1945 that the first all-aluminum sandwich panel was produced. The real breakthrough came just before with the development of better adhesives for the attachment of facings to the cores. Adhesives were developed that had the right rheology (flow during curing) for use with honeycomb. The adhesives stayed on the honeycomb cell edges when the facings were being bonded. Earlier adhesives would not stay on the top honeycomb surface but instead ran down the cell walls; consequently, it was not possible to achieve a good bond to the top skin. It is also interesting to note that in this period of time most adhesives gave off volatiles when curing so the aluminum honeycomb cores had to be perforated (small pin holes put in the foil before being made into core) to allow the gases to escape during cure. If the core was not perforated the buildup of pressure within the cells could prevent a good core-to-facing bond and even blowout the core. Now most modern adhesives are 100%

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Figure 1.2 Honeycomb sandwich panel.

solids and do not give off volatiles; thus, the honeycomb need not be perforated for this reason. Currently core is perforated for some space applications where air is not wanted in the cells.

A summary of the important developments in the history of honeycomb technology is given below .

• 1845 First known man-made sandwich structure - wood egg-crate core used for top compression panel.

.1919 First aircraft sandwich panel - thin mahogany faced balsa wood core used on seaplane pontoons .

• World War II Military aircraft used plywood facings on balsa wood core .

• 1945 First all-aluminum sandwich panel made - aluminum facings bonded to aluminum honeycomb.

Figure 1.2 shows a typical honeycomb sandwich panel which consists of facings, adhesive and honeycomb core. Chapter 5 discusses sandwich panel constructions and bonding procedures.

Honeycomb is not just used in sandwich panels but has many other applications such as energy absorption, air directionalization, thermal panels, acoustic panels, light diffusion and radio frequency shielding. These other applications will be discussed in Chapter 7.

WHY HONEYCOMB?

Why and when should a honeycomb sandwich structure be used? The basic reason is to save weight; however, smooth skins and excellent fatigue resistance are also attributes of a honeycomb panel. Figure 1.3(a) shows a sheet and stringer air foil with thin skins under load. If the skins are thin

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(1;1)

Figure 1.3 Aerodynamic smoothness. (a) Sheet and stringer air foil with thin skins under load. (b) A DC-IO honeycomb sandwich vane with a mirror-like surface.

and the stringer spacing large, the skins will deform and cause additional unwanted drag on the air foil, while the honeycomb air foil retains a smooth surface even under load. Figure 1.3(b) shows a DC-I0 honeycomb sandwich vane with a man's reflection on the mirror-like surface. Honeycomb panels are used in classified space mirror projects and even over some beds in Las Vegas hotel suites!

Another real plus for sandwich construction is its fatigue resistance. Figure 1.4 shows the results of sonic fatigue tests comparing a honeycomb panel with a skin-stiffened structure. Notice that the honeycomb panel lasted 460 h at 167 dB while the conventional structure only lasted 3 min. The honeycomb panel therefore lasted 9200 times longer. The reason for the greater fatigue resistance of the honeycomb panel is that the sheet and stringer construction uses rivets which are stress risers and cause premature failure. The honeycomb panel facings are continuously bonded to the core and therefore no stress concentrations are present.

But the main reason for using honeycomb is to save weight. Table 1.1 compares the strength and stiffness values of different honeycomb structures made using a 0.064 in. (1.6mm) thick piece of aluminum split in

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200

ill 180 ~

Qi >

.J!! "0 c: 160 :::I 0 en

140

120 0

/ 3 min at 200 dB

/ Sandwich structure

\::: 3 min at 167 dB

\,20 h at 162 dB 460 h at 167 dB

" " / Skin-stiffened '- ...... structure

...... -100 200 300

Time (h) 400 500

Figure 1.4 Comparative sonic fatigue resistance of conventional and sandwich structures. Reproduced from WADC T.R. 58-655.

half as the top and bottom facings of the sandwich. The sandwich on the far right is 37 times stiffer than the flat aluminum sheet and 7 times stronger in bending strength, yet it only weighs 9% more than the solid plate. However, it does cost more.

When light weight is a design criterion, honeycomb should be used if the skins have a buckling problem. If the loads are very high and thick skins are required (no buckling problems) a sheet and stringer or extruded shape may be the most economical solution. Another situation where honeycomb may not be the best alternative is when the loads

Table 1.1 Honeycomb sandwich efficiency

c=JIT QillI2T ITmlT Relative stiffness 7 37

Deflection (in.) 1.000 0.140 0.027

Relative bending strength 3 7

Weight (pst) 0.910 0.978 0.994

Assumes 0.064 in. (1.626 mm) aluminum, 3.0 pcf(48 kg/m3) core, 0.03 psf(I.4 N 1m2) adhesive.

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-L t

T

(a) s

(b)

(c) Fer = 3.6E [fr W=1.7t[~J~

Figure 1.5 Sheet and stringer design.

are very light and some minimum gauge skin must be used for damage considerations. For example if the minimum skin to be used is 0.040 in. (1.0mm), a one-faced sheet and stringer structure would be lighter than a 0.040 in. (1.0mm) two-faced honeycomb panel. However, these two situations are not the norm, and in most cases a honeycomb panel can give a large weight saving.

To give a better idea of how a sheet and stringer structure is designed, look at Figure 1.5. The usual design procedure is called the post-buckling method. The compressive loading causes the sheet stresses to increase to a critical buckling load, Fer (Figure 1.5(a». Now the sheet will buckle between the stiffeners, but the structure will not fail. Additional loads can

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Compression Shear

Density Strength Modulus Strength Modulus Material (pcJ) (psi) (ksi) (psi) (ksi)

Aluminum honeycomb 3.1 300 75 210 45 Nomex honeycomb 3.0 325 20 175 6 Fiberglass honeycomb 3.0 410 23 195 19 Rohacell foam 3.1 128 10 114 3 Klegecell foam 3.0 69 2.7 51 1.1 Rigicell foam 3.0 80 2.5 70 2.5 Divinycell foam 3.1 100 10.2 73 2.5

1 ksi = 1000 psi.

be applied until the stresses near the stiffeners finally reach their maximum values, Fe (Figure 1.5(b». Instead of using this complicated stress distribution a simpler approach is used which assumes that the maximum stress is the only stress on the sheet and is distributed over an effective width that is determined from the given formula (shown as hatched area in Figure 1.3(c». Notice that the whole skin is not under the maximum stress. This is the normal case. A honeycomb sandwich allows the whole skin to obtain the maximum compressive stress as the core supports the skin and does not allow it to buckle.

Another core material that competes with honeycomb is foam. Table 1.2 compares the properties of these two core materials. The honeycomb strengths are much better, and the shear moduli are considerably higher. Consequently, whenever core material mechanical properties govern the sandwich design, honeycomb is the better way to go. Foam has its place in lightly loaded panels and in insulating panels; however, honeycomb can be used in such situations by filling the cells with foam or another type of insulating material. This provides a good structural panel with fair insulation properties.

In summary, honeycomb should be considered whenever there is a skin buckling problem, as a honeycomb core sandwich is hard to beat on weight criteria. Table 1.3 compares some of the standard panel constructions for a given loading. All these panels are 1 in. (2.5 mm) thick and weigh the same 3.0 psf (14.6 kg/m2), replacing the solid plywood beam. The results would be slightly different depending on what span, loading and panel thickness were chosen, but this table does show the general principles involved. Foam has a lower shear strength and modulus than honeycomb; thus the beam has a lower load capacity and gives more deflection. The sheet and stringer and extrusions have thin skins which buckle; therefore they cannot carry as much load as the honeycomb beam.

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Design Relative strength Relative stiffness

IITlITlITlITlITlITlITlITlITlITlITlCOl 100% 100% Honeycomb sandwich

f#@YMy&'WA'%.~1{itlm&. 26% 68% Foam sandwich

1C=:JC=:Jc:::::::Jc:::::::J1 62% 99% Structural extrusion

lL lL 64% 86% Sheet and stringer

3% 17%

Plywood

Usually the stringers or extrusion webs weigh more than the low density honeycomb core. To keep all the beams the same weight some of the facing material must be reduced; therefore, these beams deflect just a little more or, put another way, their stiffness is less.

Ribbon direction

Figure 1.6 Honeycomb terminology.

Cell size

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Before we get too far into this book, let us discuss the honeycomb termin­ology commonly used. Like most disciplines the honeycomb business has its own jargon, and the following list defines some of the more commonly used honeycomb terms, illustrated in Figure 1.6. A more extensive list is contained in the glossary.

• Honeycomb density - the weight of one cubic foot of core, expressed in pounds per cubic foot (pcf) or the weight of one cubic metre of core, expressed in kilograms per cubic metre (kg/m3).

• Cell - a single honeycomb unit, usually a hexagon. • Ribbon - the fiat sheet material constituting the honeycomb, also

referred to as web. • Node - the bonded portion of adjacent ribbon sheets, double sheets. • Free wall - cell wall sections of single unbonded sheets. • L direction - the core ribbon direction, the direction of continuous

sheets. • R direction - the core ribbon direction, same as the L direction. • W direction - the direction in which the core is expanded or perpen­

dicular to the ribbon direction. • T direction - the core direction parallel with the cell openings. • HOBE - HOneycomb Before Expansion, the solid block of bonded

sheets. • CUE - Core UnExpanded, the solid block of bonded sheets.

SUMMARY

The honeycomb concept goes back a long time, but its first major breakthrough came around 1945. There are many reasons to use honeycomb; however the main reason is its light weight and high strength. Like any discipline, honeycomb technology has its own jargon.