AlllDE ISMSflfl

^^[.L INST OF STANDARDS i TECH R.I.C.

TA435 .US8 \743;1978 C.I NBS-PUB-C 1974 COMMERCE

PUBLICATION

BUILDING SCIENCE SERIES

Jm 2 7 1972

U.S.

DEPARTMENT

OF

COMMERCE

National

Bureau

of

lards

.U58

NO. 43

19;^

Paper

Honeycomb Sandwich

Panels

as Lightweight

Structural Components
The Building Science Series

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UNITED STATES DEPARTMENT OF COMMERCE • Peter G. Peterson, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Direcior

Paper Honeycomb Sandwich Panels

as Lightweight Structural Components

T. W. Reichard

Building Research Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Building Science Series 43

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 43, 7 pages (Apr. 1972) CODEN: BSSNBV

Issued April 1972

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 (Order by SD Catalog No. C 13.29/2:43). Price 25 cents Stock Number 0303-0959
Contents

Page

1. Introduction 1

2. Honeycomb Cores 3

3. Facings 5

4. Adhesive Bonding 5

5. Summary 5

Library of Congress Catalog Card Number: 71—190625

II
Paper Honeycomb Sandwich Panels as Lightweight Structural Components

T. W. Reichard

This paper presents a resume of current practice in the U.S. with regard to the use of sandwich panels in single-story buildings. Descriptions and properties of typical

paper honeycomb cores are given. Some of the factors which are considered in choosing

sandwich facings and adhesives are given.

Key words: Adhesive; paper honeycomb; sandwich facings; sandwich panels.

1. Introduction

Light weight in buildings is not necessarily a panacea to the building industry. It can be advantageous when the saving in weight results

in some functional or economic gain. When a

reduction in weight is required it can be achieved

by either one or both of two methods : first, by

the use of material in a more efficient manner

;

and second, by the use of materials with high

strength-to-density ratios. Structural sandwiches for buildings can be an example of both

of these methods, but usually only the first is used because of the higher cost of high strength

materials.

Structural sandwiches are a logical choice

when designing lightweight buildings and they

Figure 1. Typical paper honeycomb sandwich panel.

are being use quite extensively in the U.S. at the present time. Several of the sandwiches

being used were developed by the aerospace

industry. This industry requires very high

strength-to-weight ratios in their structural

components. Structural sandwiches with honeycomb cores and high strength facings (see fig.

1) provide the highest flexural rigidities with

very low weights.

The honeycomb cores for aircraft and space

vehicle components are being made from a

variety of materials, such as paper, plastic, or even titanium. The facings used on these sandwiches vary from materials such as aluminum

to graphite or boron fiber reinforced plastics.

Table 1 is a listing of some of the sandwiches,

which are being used or are being proposed for use in buildings at the present time. It should

be noted that most of these include paper honeycomb as a core material.

The plastic foam listed here is not used widely

in structural sandwiches because of the low

shear strength of most of these foams. When

used as a core, the plastic foam is relied on only

to provide lateral stiff"ening for the facing and

auxiliary structural members are attached to the facings.

Figures 2 and 3 illustrate two structures

which are being produced in the U.S. and which

utilize sandwich panel material. The building

shown in figure 2 was constructed in our laboratory using aluminum-faced, paper honeycomb

panels 76mm (3 inches) thick. These panels,

which were used in walls, roof, and floors, were

connected together by means of sheet metal screws and aluminum extrusions.

The building shown in figure 3 is constructed

of sandwich panels using paper honeycomb

cores and steel facings. This building is a

single-story, single-family house prefabricated

by one of the large corporations in the U.S. The

complete building is designed to be set up in a matter of hours on concrete piers.

1
2

Figure 3. Single family house constructed with steel-faced sandwich panels.
Table 1. Typical structural sandwich panels for buildings

Type

Core Structural facing Additional

Thickness Material Thickness Material Facing Weight'

mm (in) mm (in) kg/m^ (Ib/ft'^)

1 76. (3) Paper honeycomb l/.O Aluminum none

2 76. (3) Paper honeycomb

and plastic foam

0.5 (.020) Steel none 10.7 (2.2)

3 51. (2) Aluminum honeycomb

1.6 ( 064) High pressure laminate none

4 76. (3) Paper honeycomb 3.6 (.140) FRP 2 Gypsum

board

32.2 (6.6)

5 102. (4) Paper honeycomb 3.2 (.125) Cement asbestos board none 18.6 (3.8)

6 56. (2.25) Plastic foam'^ 3.2 (.125) Cement asbestos board

Gypsum

board

7 89. (3.5) Corrugated FRP 2.5 (.100) FRP none 14.6 (3.0)

1 Weight per unit area of panel.

2 FRP indicates fiberglass reinforced plastic.

3 This panel has a structural aluminum frame.

2. Honeycomb Cores

Honeycombs suitable for use as cores in sandwich panels are being produced from many different materials but essentially from those materials which can be formed into thin sheets.

Some of the materials now being used for honeycomb are: kraft paper, nylon paper, polyethylene, fiberglass (and other fibers) reinforced

plastics, aluminum, stainless steel, and titanium.

With each material, the most frequently used

honeycomb has a hexagonal configuration (see

fig. 4), but other configurations are being made.

Some of these other configurations are designed

for applications where special qualities such as double curvature are required.

The structural sandwich panels for buildings are almost always made with kraft paper honeycombs. Kraft paper is available in many diiferent weights and thicknesses and is low in cost

in comparison with other materials used for

honeycomb.

The kraft paper is usually modified by impregnation with a phenolic resin. The phenolic

resin improves both the dry and wet strength,

and imparts fungus resistance to the paper.

Other treatments and resins are sometimes used

to impart special properties to the honeycomb.

The mechanical properties and cost of kraft

paper honeycomb vary with the weight of the

paper, amount and type of resin impregnation,

and cell size. The relative costs of some typical

honeycombs are given in table 2. In general, the

mechanical properties can be estimated from

the unit weight of the expanded honeycomb.

The curves of figures 5 and 6 indicate the

eff'ect of the unit weight of some typical honeyAXES

Figure 4. Standard tei~rns used with hexagonal

honeycombs.

combs on their shear and flatwise compressive

strength. It should be noted that the strength

values indicated in figures 5 and 6 are for "dry"

specimens 25.4mm (1 inch) thick and that the

strengths will be different for "wet" paper
UNIT WEIGHT, lb/ ft'

I 2 3

Hexagonal Honeycomb

25.4 mm ( I in) thick

"TLl' Plane

Structural Grade

Paper 11-18% Resin

32 48

UNIT WEIGHT, kg/m3

Figure 5. Typical shear strength values for hexagonal

honeycomb in the "TL" plane.

UNIT WEIGHT, lb/ft'

I

E .352

\ \ f Hexagonol Honeycomb.

25.4 mm {1 in) thick

Commercial Grade Aluminum

17"

Figure 6. Typical compressive strength values for hexagonal honeycomb.

honeycomb specimens and for other thicknesses.

The term "dry" denotes paper honeycomb at equilibrium with room temperature air at 50

percent relative humidity. The moisture content of the paper at these conditions is about

5 percent by weight. All the mechanical properties of paper are affected by moisture content. The amount of moisture in a paper at a

particular humidity condition is affected significantly by the amount and type of resin impregnation. Thus, it is necessary to determine

the exact effect of various humidity conditions on the mechanical properties of a given paper.

It is reasonable to expect that a paper will have

only 50 to 60 percent of its "dry" strength when

at equilibrium with 100 percent relative humidity, and even less after immersion in water.

Table 2. Comparison of honeycomb costs*

Honeycomb type Relative cost

Nonstructural grade, paper 1.0 (0% resin impregnation)

Structural grade, paper 1.2 (11-18% resin impregnation)

Fire-resistant, structural, paper 1.4 High strength, water resistant, paper 2.6 (25-35% resin impregnation)

Commercial grade, aluminum 3.1

* Based on 25.4mm (1 inch) thick honeycomb weighing approximately 32. kg/m^ (2 Ib/ft^).

V\\

1 \

1 \

\

1 \

1 1

•

1 0 1. 2. 3. - in

CORE THICKNESS

Figure 7. Effect of thickness on shear strength

of honeycomb.

Figure 7 indicates clearly the effect that the

thickness of the honeycomb core has on the

shear strength. The effect of the thickness on

the compressive strength is not very significant

for thicknesses greater than 25.4mm (1 inch).

The method of manufacturing honeycombs

results in shear strengths that vary with direction. Figure 4 illustrates the commonly ac- cepted method of indicating the orientation of

honeycombs. The "L" direction is parallel to the paper ribbon and the "W" direction is perpendicular. For a typical hexagonal honeycomb

the shear strength and modulus of rigidity in

the "TW" plane is about 40 percent less than

that in the "TL" plane. This directional variation in properties is unfortunate because the

strong "L" direction is with the width of the

honeycomb as produced (not over 1.22m at

4
present). If the sandwich designer wants to take advantage of the strong direction in a flex- ural member he must splice a number of pieces

of core together in such a manner that the

shear stresses can be transferred from piece

to piece.

The thermal insulation provided by paper

honeycomb as the core in a sandwich panel is insufficient for many applications. When added

insulation is required, some form of insulating

material is placed in the honeycomb cells. Generally some type of plastic foam is used, but

other materials such as vermiculite, perlite,

fiberglass, or mineral wool are also used in the

cells.

3. Facings

The structural facings for sandwich panels can be made of almost any material with reliable mechanical properties. The higher

strength materials such as the metals or fiber

reinforced composite are generally used as the

structural facings, but service factors such as

fire, impact, abrasion, and concentrated loads,

greatly influence the selection.

Closely allied with these selection factors is the cost of lamination because the choice of the

adhesive and the subsequent bonding process

are interrelated with the choice of the facing

material. Damage to the facings in handling

and shipping is many times an unexpected cost

item.

Aluminum and steel are widely used as

facings, because they can be purchased with

the surface already prepared for adhesive bonding. This prepared surface greatly simplifies

the sandwich bonding process for the panel producer. In addition, both aluminum and steel are available in coil form with a variety of millapplied, finished surfaces which have proven

to be very durable.

Fiberglass reinforced plastic (FRP) are being used for facing on sandwiches which

are laminated by using the matrix-plastic as

the bonding adhesive.

In some applications additional sheet material is bonded to the exposed surface of the

primary structural facing. This is done when

some characteristics of the panel must be modified. For example, the Type 4 sandwich panel

of table 1 has primary structural facings of

FRP, but gypsum board was bonded to each

face in order to provide better fire resistance

and lower sound transmission.

Sandwich panels will always have a tendency

to bow or bend out of plane because of differ- ences in the temperature and/or in the moisture content of the two facings. This tendency

can be reduced by proper choice of the facing

materials and thickness. The usual practice is to have a symmetrical (balanced) sandwich,

i.e., identical facings on both sides of the sandwich.

Bowing of the sandwich panel is usually partially restrained by other components in the

building. When this restraint and tendency to bow are both significant, unusual stress conditions within the sandwich may induce delamination of the panel.

4. Adhesive Bonding

The subject of the adhesive bonding of sandwich panels is so evolutionary and intricate that

it can not be adequately covered in this paper.

However, a brief mention of some basic precautions is in order. Further information can be

gained by consulting the references cited

below^'^ and reputable adhesive manufacturers'

literature.

Stated simply, a sandwich panel has a relatively thick, low strength core bonded to relatively thin, high strength facings where the

important word in this description is "bonded."

A failure of the bond results in a loss in struc- tural integrity of the sandwich panel.

Under ideal conditions many adhesives will

supply the required bond, but ideal conditions

seldom occur in a building. For example, significant amounts of moisture, usually as vapor,

will penetrate, sandwich panels under service

conditions even when they have metal facings.

High humidity within the panel can degrade

the bond, but when the high humidity is combined with the high temperature induced by

solar heating the degradation can be severe. Moisture combined with elevated temperatures can degrade the bond by several methods

;

one, by degradation of the adhesive itself, two,

by corrosion of the facing material and three,

by chemical reaction with one or more of the

other materials within the panel.

Some adhesives are water soluble while others

allow the moisture to penetrate through and

deposit on the substrate and thus weakening

the bond.

Adhesives can be classified in a number of

ways, but for sandwich panels they should be

classified according to the bond strength and

durability attained when used with the particular facings and core chosen. In general,

wood and wood products (kraft paper for in- stance) are easily bonded, whereas, metals re- quire greater care and cleanliness.

5. Summary

The construction industry in the U.S. is now

using several different sandwich panel systems

and is contemplating the use of others. Many

^ Symposium on Structural Adhesives Bonding, Editor, Michael J. Bodnar ( Interscience Publishers, New York City, New York, 1966).

- U. S. Department of Defense, Military Handbook, Structural Sandwich Composites, MIL-HDBK-23A, 1968.

5
of these systems utilize paper honeycomb as the core material in the sandwiches.

Paper honeycomb is available in a variety of

weights, thicknesses and impregnations which

control the mechanical properties of the core. Insulating material is often placed in the honeycomb cells to improve the thermal characteristics.The facing materials most widely used for

structural sandwiches are prefinished aluminum

or steel and fiberglass reinforced plastic. Additional facings, such as gypsum board, are sometimes added to improve the performance of the

sandwich.

The most important single element in a struc- tural sandwich is the bond between the facings

and core. The choice of the adhesive and the

bonding process should be based on the assumption that the system will be exposed to adverse

conditions in service.

6
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession

BIBLIOGRAPHIC DATA ^BS BSS-43

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Paper Honeycomb Sandwich Panels as Lightweight Structural

Components

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

Thomas W, Re i chard

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Address 13. Type of Report & Period Covered interim, Nov /U—Apr /I

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This paper presents a resume of current practice in the U.S. with regard to the use

of sandwich panels in single-story buildings« A description and the properties

of typical paper honeycomb cores are given. Some of the factors which are considered in choosing sandwich facings and adhesives are given.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Adhesives; paper honeycomb; sandwich facings; sandwich panels.

18. AVAILABILITY STATEMENT

rX| UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22. Price

25^

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