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Keywords: Nomex paper; Nomex honeycomb

while the core material may either be of metallic/aramidhoneycomb or metallic/polymeric foam.

properties have to be determined for each core type viamechanical testing or analytical approximation. The accu-

are analyzed in terms of their eective properties ratherthan by consideration of their real cellular structure. Con-

models were investigated by Triplett and Schonberg [7],who conducted a numerical analysis for circular honey-comb sandwich plates subjected to low-velocity impact.They found that numerical results were inaccurate whenhoneycomb crushing was ignored for the nite elementmodel. Meraghni et al. [5] modied the classical laminate

* Corresponding author.E-mail address: keithfoo@pmail.ntu.edu.sg (C.C. Foo).

Composite Structures 80 (2For numerical impact analyses of honeycomb sandwichstructures, several modelling approaches have been identi-ed. One approach utilises standard shell nite elements,and is mainly used for approximation of the global behav-iour in thin sandwich panels [2]. Another approach usesstandard two-dimensional shell nite elements for the face-sheets and three-dimensional solid nite elements for thecore [3]. Such models are used to predict both local andgeneral responses in the sandwich panel. However, material

sequently, the determination of eective elastic propertiesfor this continuum core becomes important.

Various analytical techniques have been proposed topredict the eective continuum properties of the core interms of its geometric and material characteristics [46].Gibson and Ashby [1] published analytical formulationsfor the in-plane and out-of-plane stinesses, as well as theupper and lower limits of the transverse shear moduli, fora regular hexagonal honeycomb. Their material properties1. Introduction

Structural sandwich panels are widely used in light-weight construction especially in aerospace industriesbecause of their high specic strengths and stinesses.The typical sandwich panel consists of a lightweight corecovered by two thin facesheets. Each facesheet may be anisotropic material or a bre-reinforced composite laminate

racy of the numerical solution depends on a variety of geo-metric and material characteristics of the constitutivematerials in the core and facesheets.

Computational expenses for nite element honeycombsandwich models increase rapidly as the number of cellsin the core increase. Therefore, to attain eciency innumerical analysis, the honeycomb core is usually replacedwith an equivalent continuum model. The sandwich panelsMechanical propertiesNomex honey

Choon Chiang Foo *, Gin B

Nanyang Technological University, School of Mechanical and Aero

Available online

Abstract

This paper presents extensive test results of linear elastic mechaThe fundamental mechanical properties of the Nomex paper are teycomb structures. The nite element results are then compared witheory from Gibson and Ashby [Gibson LJ, Ashby MF. Cellular sobserved for the moduli of Nomex honeycombs. 2006 Elsevier Ltd. All rights reserved.0263-8223/$ - see front matter 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.compstruct.2006.07.010f Nomex material andmb structure

ay Chai, Leong Keey Seah

ce Engineering, 50 Nanyang Avenue, Singapore 639798, Singapore

September 2006

al properties of Nomex paper and Nomex honeycomb structures.used in the nite element modeling and analysis of Nomex hon-the experimental results and with the results using the well knowns: structures & properties. Pergamon Press; 1988]. Size eects are

www.elsevier.com/locate/compstruct

007) 588594

direction, Et. The specimens were manufactured and sup-plied by DuPont, USA.

Each test specimen was of 0.125 mm thickness, 180 mmlength and 29.5 mm width according to the standard set byTAPPI [8]. Allowances were made at both ends of eachspecimen to allow a pair of pneumatic grips to hold ontothe ends of the paper during the tensile test, as shown inFig. 1. The Instron 5564 tensile testing machine, with a500 N load cell, was used for this test. The pneumatic gripswere attached to the machine such that they were paralleland aligned in a straight line. This was to ensure that theload would act along the length of the specimen. The pres-sure of the grips was set to 6.5 bar to ensure a tight gripthat would give an accurate indication of the elongationof the test specimen. The load and displacement data were

Structures 80 (2007) 588594 589theory and applied it on a unit cell to derive the equivalentelastic rigidities for the honeycomb core. Hohe and Becker[6] also proposed a strain energy-based homogenisationtechnique to derive the eective elastic properties of anygeneral cellular structure by considering a representativevolume element. However, theoretical formulation of theeective elastic constants for the core could be tedious oralmost impossible if the sandwich construction is too com-plicated. Even if it is possible, the mathematical derivationsfor one type of sandwich core might not be applicable toother types.

Extensive literature searches and reviews gave no relatedsignicant publications on mechanical and material proper-ties of Nomex paper used in the manufacturing of commer-cial honeycomb structure. With the escalating use ofhoneycomb in a wide variety of industries especially in theaerospace sector, it is important to determine the mechani-cal behaviour and strength of the hexagonal-celled honey-comb core to within engineering accuracy. The advantageof todays computer technology enables researchers tomodel the geometrically correct honeycomb structure andthereby creating a need for the fundamental mechanicalproperties of the base material of the honeycomb structure.

This paper hopes to address the current deciency andalso provides new ndings on the analysis of honeycombstructures. The rst and foremost step in this paper is todetermine the material properties of the Nomex paper usedin themanufacture of theNomex honeycomb. Standard ten-sile tests according to TAPPI standard [8] are conducted ontheNomexpaper in the bre and transverse directions. Theseresults are then used in a numerical simulation to determineYoungs moduli of the honeycomb structure in three direc-tions. Experimental in-plane tensile and out-of-plane com-pressive tests are also carried out on bare honeycombcores, and results are compared with the numerical values.

2. Experimental investigation

Tensile tests were performed on the Nomex paper of thehoneycomb to determine the mechanical properties in thebre (or machine) direction and transverse (or cross-machine) direction. After which, in-plane tensile tests andout-of-plane compressive tests are conducted to determinethe three fundamental Youngs moduli for the honeycombstructures. The core was a 15 mm thick HexWeb A1 Nomexhoneycomb core coated with phenolic resin. Each honey-combhada cell size of 13 mmandawall thickness of 0.3 mm.

2.1. Testing of Nomex paper

Nomex honeycomb is made from Nomex paper, whichis a form of paper made of aromatic polyamide (aramid)bers. The initial paper honeycomb is usually dipped in aphenolic resin to produce a honeycomb core with highstrength and very good re resistance. Tensile tests were

C.C. Foo et al. / Compositeperformed on Nomex paper to determine Youngs modulialong the machine direction, Ef and the cross machineautomatically acquired by a computer via the load cell andan extensometer. All test specimens were pulled at a con-stant rate of 25 mm/min.

2.2. Testing of Nomex honeycomb core

To determine the two in-plane moduli for the bare hon-eycomb cores, E11 and E22, tensile tests were carried out inaccordance to the ASTM standard for delamination test[9]. The test specimens measured 130 mm wide by260 mm long with a test section of 220 mm between thelocating pins, as shown in Fig. 2. The specimen width isparallel to the node bonded areas. In a honeycomb cell,the node refers the bonded portion of adjacent ribbonsheets of paper, while the free wall is the cell wall sectionof single unbonded sheet.

Eight aluminium end plates were fabricated for the ten-sile tests and holes were drilled in them. Nine locating pinswere inserted in each pair of end plates for the tests in theX2-direction, and six locating pins for the tests in theX1-direction. The honeycomb specimen was then mountedonto the pins, as shown in Fig. 2. The subsequent assem-bled rig for a typical test in X1- and X2-direction is shownrespectively in Figs. 3 and 4. For these tests, the laboratoryInstron 5564 machine, together with a 5 kN load cell andFig. 1. Experiment set-up of tensile test on Nomex paper using thepneumatic grips.

an accompanying computer with data logging software,was used. To eliminate the slack in the honeycomb speci-men, a preload was also applied prior to the test. The testspecimens were then pulled at a displacement rate of 5 mm/min. The test was considered void whenever failureoccurred at the ends, and a new test was performed.

The compressive tests as shown in Fig. 5 were carried out

glued sheets, each cell has four free walls of thickness tand two nodes of double thickness 2t. Fig. 6 depicts thedimensions and symbols pertaining to the honeycomb core.The unit cell was replicated to produce many identical cells,and these cells were then merged to assemble the 200 130 15 mm3 honeycomb core. Cores with a single cell and multi-ple cells are herein termed as unit-cell and multi cells honey-combs, respectively. The adhesive bonding between eachcell was assumed to be perfect. Youngs moduli of theNomex paper in the machine and cross machine directionswere taken to be 3.13 GPa and 0.955 GPa respectively.

To simulate the in-plane tensile tests, the nodes at oneedge of the honeycomb core were pinned, while at theopposite edge, the nodes were constrained to move onlyin the pulling direction as shown in Fig. 7. For the out-of-plane compressive test, a downward displacement loadwas prescribed on all the nodes located at the top of thecore. The nodes at the bottom were clamped. A maximumapplied strain of 0.002 mm/mm is used in all tests. A meshconvergence study was then carried out to determine the

Fig. 2. Tensile test specimen with pinned ends (X2-direction test).

590 C.C. Foo et al. / Composite Structures 80 (2007) 588594to determine the elastic modulus of the bare honeycombcore in the out-of-plane direction, E33, for specimens with9, 30, 60 and 196 cells. Flat metal plates were used to crushentire specimens at a slow displacement rate of 0.5 mm/min,so as to ensure an even distribution of load throughout thecore. It was assumed that during crushing, the change incross-sectional area of the cell walls was negligible, and itwould not aect the elastic modulus signicantly.

3. Numerical investigation

A linear elastic numerical analysis was carried out usingthe commercial nite element software ANSYS v6.0. Shellelements were used to model the honeycomb cells. As manycommercial honeycombs are made by expanding strip-Fig. 3. Tensile test of a Nomex hooptimum mesh density for the analysis.Determination of the E-values for the simulated honey-

comb requires stress and strain values. The reaction forcesin each node on the constrained bottom edge are added upto give the total reaction force, F. The stress is calculatedfrom

r FA

1

The theoretical area for a single cell is A11 = (h + l sinh)bin x11 direction and A22 = (l cosh)b in x22 direction wherethe variable b is the core thickness (or height) and the othervariables are dened in Fig. 6(a). For a multi cell honey-comb in x11 direction, the width is p cells (p = 12 for13 mm cell size) and therefore the area isneycomb in the X1-direction.

StrC.C. Foo et al. / CompositeA11 ph l sin hb 2In x22 direction, the width is q cells (q = 10 for 13 mm cellsize)

A22 ql cos hb 3A prescribed displacement is applied on the upper line ofthe model. The strain is calculated from

e L L0L0

dL0

4

Fig. 4. Tensile test of a Nomex ho

Fig. 5. Compressive tests on

2t

t

l

h

X1

X2L

W

Fig. 6. Co-ordinates and geometrical parameters of unit honeycomb celland honeycomb structure.uctures 80 (2007) 588594 591where d is the displacement and L0 is the initial length ofthe multi cell honeycomb. The height for the multi cell hon-eycomb in x11 direction is r cells (r = 15 for 13 mm cellsize). The initial length is then dened by

L0 2r 1 c2

5

In x22 direction, the height is s cells (s = 18 for 13 mm cellsize), and then the initial length is

L0 sh s 1l sin h 6Youngs modulus is then calculated from Hookes law

E re

7

4. Results and discussion

Tensile tests were carried out on the base paper materialto determine its mechanical properties. In total, eight testswere performed for the bre and transverse directions, andtypical loadstrain curves are shown in Fig. 8. The testresults are consistent and thus reliable. Wrinkling of the

neycomb in the X2-direction.

bare honeycomb cores.

eyc

592 C.C. Foo et al. / Composite Structures 80 (2007) 588594Fig. 7. Finite element model of the honspecimens was observed when the load reached about300 N and 170 N in the bre and transverse directions,respectively. Hence to get a good prediction of the linearelastic material properties, the stressstrain data were ana-lyzed up to the load before wrinkling. The average resultsof Youngs moduli obtained from the eight tests in bothdirections are 3.40 GPa and 2.46 GPa along the bre andtransverse directions, respectively.

Experimental in-plane pinned tensile tests were con-ducted to examine the in-plane behaviour of the bare hon-eycomb cores. The load response of the honeycomb

Fig. 8. Tensile curves of Nomex paper. (a) F

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60Extension, mm

Load

, N

Fig. 9. Load versus displacement curves for honeycomb test specimens. (a) Specomb structure in x11 and x22 direction.specimens with incremental end elongation for theX1- and X2-directions are plotted in Fig. 9. As can be seenin the gure, the results are consistent and thus reliable.Elastic region was observed over a displacement of38 mm and up to a load of 91.0 N in the X1-direction,and 50 mm at a load of 111.5 N in the X2 direction (seeFig. 9). Plasticity then occurred due to debonding betweenthe bonded papers, resulting in permanent failure to thespecimens. Fig. 10 further illustrates the deformation ofthe honeycomb core under tension for both FEM andexperimental cases. The results agree well, and the

iber direction; (b) Transverse direction.

020406080

100120140

160180

0 20 40 60 80 100Extension, mm

Load

, N

imens loaded in the X1-direction; (b) Specimens loaded in the X2-direction.

Str(a) experiment 37mm

(b) FEM 25mm

(c) Experiment 31mm

(d) FEM 20mm

At an applied strain of 0.125 At an applied strain of 0.100

Fig. 10. Comparison between the deformation of experiment and niteelement method.

C.C. Foo et al. / Compositedeformed shape of the honeycomb is represented well usingthe numerical simulation.

Out-of-plane compressive tests on bare honeycombswere also carried out. Fig. 11 shows a typical stressstraincurve obtained from the compressive tests. The compres-sive stress increases almost linearly with the strain due tothe elastic bending of the thin cell walls. This linear elasticregime terminates with the onset of fracture: the stress rstreaches a maximum, before it drops to a lower level. Fol-lowing which, the compressive stress is maintained at anearly constant level. Audible cracking was observed dur-ing this process in the tests. Similar behaviour has beenreported by other researchers [10]. Fracture appears to bethe dominant failure mode for these Nomex honeycombsunder compression.

The experimental, numerical and theoretical Youngsmoduli for a Nomex honeycomb core of nine cells are pre-sented in Table 1. The numerical and experimental resultscompare well, with a maximum error of 12% for E33. How-ever, the experimental E33 is 27% greater than the theoret-ical value. One reason for this large discrepancy could bethat the theoretical formulations were derived for an iso-tropic material, but Nomex paper is anisotropic. Anotherreason could be due to the size eect of the whole honey-

Fig. 11. Compressive tests on bare honeycomb cores.comb core. For the theoretical formulations in [6], a unithoneycomb cell was considered. However both numericaland experimental results indicate that Youngs moduli inX3 exhibit a dependency on the size of the honeycomb core.Interestingly, the theory states that the in-plane moduli

Table 1Comparison of Youngs moduli (MPa) for Nomex honeycombs with ninecells

Nomexhoneycomb

Experimentalresults

Theoreticalresults

Numericalresults

E11 0:480:030:05 0.457 0.482

E22 0:4430:0070:003 0.457 0.441

E33 120:683:953:95 88.15 105.64

40

60

80

100

120

140

160

180

0 50 100 150 200 250 300No. of cells

E 33,

MPa

FEMExp

theory

Fig. 12. Variation of E33 with number of cells.

uctures 80 (2007) 588594 593(E11 and E22) are independent of the core size, and E11 isequivalent to E22. However, the numerical and experimen-tal values for E11 and E22 show otherwise.

Fig. 12 illustrates the variation of E33 with the numberof cells in the core. The theoretical value is independentof the number of cells. On the other hand, both numericaland experimental values decrease as the number of cellsincrease, and the former converges to a value of100.7 MPa. Since the modulus is inversely proportionalto the area of the specimen in Eq. 1, this result could beexpected. This also highlights that Youngs moduli for aparticular conguration of honeycomb core are not solelydependent on the cell geometry, but also on the numberof cells. Tests on a larger sample of specimen sizes maybe required to form a clearer pattern of the inuence ofthe number of cells on E33. One constraint of the Instrontesting machine was that it was not suitable for carryingout compression on large honeycomb cores whose contactareas were larger than that of the load cell.

5. Conclusion

Experimental tests were performed on the base materialof Nomex honeycomb to ascertain its properties. Thesendings were then used in numerical analyses for static ten-sion and compression on bare honeycombs. Youngs mod-uli of the bare honeycomb obtained numerically showed

good comparison to the experimental values, with dier-ences up to a maximum of 12%. It was also found thatE33 decreases with increasing number of cells. Youngsmoduli of the bare honeycomb are dependent on the sizeof the specimen.

References

[1] Gibson LJ, Ashby MF. Cellular solids: Structures & properties. Perg-amon Press; 1988.

[2] Meo M, Morris AJ, Vignjevic R, Marengo G. Numerical simulationof low-velocity impact on an aircraft sandwich panel. Compos Struct2003;62:35360.

[3] Aktay Levent, Johnson Alastair F, Holzapfel Martin. Prediction ofimpact damage on sandwich composite panels. Comput Mater Sci2005;32:25260.

[4] Burton WS, Noor AK. Assessment of continuum models forsandwich panel honeycomb cores. Comput Meth Appl Mech Eng1997;145:34160.

[5] Meraghni F, Desrumaux F, Benzeggagh ML. Mechanical behavior ofcellular core for structural sandwich panels. Composites: Part A1999;30:76779.

[6] Hohe Jorg, Becker Wilfried. A mechanical model for two-dimen-sional cellular sandwich cores with general geometry. Comput MaterSci 2000;19:10815.

[7] Triplett Matt H, Schonberg William P. Static and dynamic niteelement analysis of honeycomb sandwich structures. Struct Eng Mech1998;6:95113.

[8] TAPPI. Tensile properties of paper and paperboard (using constantrate of elongation apparatus), 1996.

[9] ASTM. General products, chemical specialities and end use products,1998.

[10] Zhang J, Ashby MF. The out-of-plane properties of honeycombs. IntJ Mech Sci 1992;34:47589.

594 C.C. Foo et al. / Composite Structures 80 (2007) 588594

Mechanical properties of Nomex material and Nomex honeycomb structureIntroductionExperimental investigationTesting of Nomex paperTesting of Nomex honeycomb core

Numerical investigationResults and discussionConclusionReferences