Characterization of Nomex honeycomb core

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Characterization of Nomex honeycomb core
constituent material mechanical properties
Article in Composite Structures · November 2014
DOI: 10.1016/j.compstruct.2014.06.033

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Composite Structures 117 (2014) 255–266

Contents lists available at ScienceDirect

Composite Structures
journal homepage: www.elsevier.com/locate/compstruct

Characterization of Nomex honeycomb core constituent material
mechanical properties
Rene Roy a, Sung-Jun Park a, Jin-Hwe Kweon a,⇑, Jin-Ho Choi b
a
Department of Aerospace and System Engineering, Research Center for Aircraft Parts Technology, Gyeongsang National University, 900 Gajwa-dong, Jinju, Gyeongnam
660-701, Republic of Korea
b
School of Mechanical Engineering, Research Center for Aircraft Parts Technology, Gyeongsang National University, 900 Gajwa-dong, Jinju, Gyeongnam 660-701, Republic of Korea

a r t i c l e

i n f o


Article history:
Available online 1 July 2014
Keywords:
Nomex honeycomb
Nomex paper
Phenolic resin
Mechanical properties
Finite element analysis (FEA)

a b s t r a c t
Nomex honeycomb cores have been widely used in composite sandwich panels. To accomplish mesoscale finite element modeling of these cores, cell wall mechanical properties are required, for which limited data are available. In this work, tensile testing was performed on Nomex paper, phenolic resin, and
Nomex paper coated with phenolic resin. Flatwise tension and compression tests were also performed on
two types of Nomex honeycomb cores. Test results were calibrated in finite element modeling to account
for strain gage local stiffening effects and thickness normalization. Identified cell wall material properties
were implemented in a honeycomb core finite element model and further calibrated by matching simulation results to manufacturer test data. The cells’ double-wall thickness was also adjusted. These calibrations and adjustments led to an exact simulation match with test data. Numerically matching cell wall
material properties depends on modeling sophistication and is subject to core test result variability
and core construction differences among manufacturers.
Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Polymer composite sandwich construction is an attractive
design option, as it typically offers an excellent stiffness-to-weight
ratio. Honeycomb type cores used in sandwich construction are
among the best performers in this regard and have been used
extensively. Nomex (E.I. du Pont de Nemours Corp., Wilmington,
DE, USA) honeycomb cores can be a valuable choice given their
flammability properties, dielectric properties, environmental resistance and galvanic compatibility with face materials. Nomex honeycomb cores are composed of adhered strips of Nomex paper
dipped in phenolic resin. Nomex paper is made from Nomex fibers,
a meta-aramid chemical component. Nomex paper strips are typically adhered by bands of thermoset epoxy adhesive. This assembly, once expanded to form honeycomb cells, is dipped into
phenolic resin and cured to form the final core. These cores are sold
in different cell sizes (1.6–19 mm) and core densities (29–144 kg/m3)
and can be ordered with a specific core height. Core manufacturers provide mechanical properties for the core block, namely
⇑ Corresponding author. Tel.: +82 10 9535 6104.
E-mail addresses: rroy@gnu.ac.kr (R. Roy), isungjun@nate.com (S.-J. Park),
jhkweon@gnu.ac.kr (J.-H. Kweon), choi@gnu.ac.kr (J.-H. Choi).
http://dx.doi.org/10.1016/j.compstruct.2014.06.033
0263-8223/Ó 2014 Elsevier Ltd. All rights reserved.

compression strength and modulus, and main direction shear
strength and modulus. Core tensile strength is often evaluated

from a fabricated sandwich with a flatwise tensile test [1]. This test
can also serve to evaluate face-to-core bond quality [2]. A designer
using a Nomex honeycomb core can therefore consider these
mechanical properties in his calculations. Sandwich in-plane compression instabilities, namely panel buckling, intra-cell buckling
and skin wrinkling, must also be verified and analytical formulas
are available to do so [3,4]. For honeycomb core applications
involving phenomena such as impact loading [5–7], concentrated
loading [8], inserts in the sandwich [9–11], or where the vibration
properties of the sandwich are of interest, experimental testing or
finite element modeling (FEM) analysis may be desirable. FEM
models of honeycomb cores can rely on equivalent 3D solid elements, where a block of solid elements is made to have the same
properties as the core (compression, tension, and shear). Honeycomb core failure may happen through cell material fracture or cell
wall buckling, depending on the loading regime and the core configuration [12,13]. Generally speaking, if the cell wall is thin and
long, it may be prone to buckling, whereas if it is thick and short,
it may reach its critical failure stress before buckling. In the case
of cell wall buckling, this phenomenon cannot be represented
directly with an equivalent solid element FEM model. Recently,

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R. Roy et al. / Composite Structures 117 (2014) 255–266

several researchers proceeded to model the core with its detailed
geometry using shell elements (meso-scale modeling) [6,8,13–
17]. In this case, it is also desirable that the material properties
used for shell elements are such that the core’s global properties
are reproduced (namely the core’s compression, tension and shear
moduli). A wide range of material property values have been used
in the literature for meso-scale FEM modeling of Nomex honeycomb cores [7,8,13,20]. In the references listed, the Young’s modulus (E) used for honeycomb shell elements in the core thickness
direction ranged from 1.9 to 3.5 GPa for different core size models
(cell size, density). Apart from these material properties, different
modeling choices are possible, including basic cell geometry
[4,21], geometrical and property variability within the cell (imperfections) [13,15], or sandwich core/face boundary conditions
[14,17]. It can be argued that modeling parameters such as accurate cell wall thickness, cell geometry, and incorporation of imperfections have a non-negligible influence and will therefore dictate
the cell wall material property required to match a given core’s
global properties. Material test data of the cell wall material, which
is essentially Nomex paper coated with phenolic resin, is somewhat rare and is not provided by core manufacturers. Tsujii et al.
[22] conducted tensile tests on Nomex paper coated with phenolic
resin, although the study is published in Japanese. Foo et al. [23]
tested the tensile properties of plain Nomex paper and measured

tensile moduli of 3.40 GPa (machine direction) and 2.46 GPa (cross
direction). Staal [4] chose to perform miniature three point flexural
tests on rectangular cell wall sections cut out of a core. He obtained
an average flexural modulus of 3.81 GPa (in the core’s thickness
direction) but found this approach sensitive to error in the thickness measurement of the specimens. In this paper, experimental
mechanical tests on Nomex paper, phenolic resin and phenolic
resin-coated Nomex paper are reported. Characterizations of
Nomex honeycomb core density and geometry are also presented.
Flatwise tension and compression tests are also performed on
Nomex honeycomb cores installed with strain gages. Where applicable, the mechanical property results obtained are calibrated
through FEM to account for strain gage local stiffening effects
and extra phenolic coating on the specimens tested compared to
a typical honeycomb material. These calibrated mechanical properties are then used in a meso-scale shell element FEM model of
a core, and the resulting simulated core properties are compared
to manufacturer test data and discussed.

2. Test procedure
2.1. Nomex paper tension test
Tensile tests were performed on Nomex type 410 paper with a
nominal thickness of 0.05 mm [24]. A universal testing press with a

10kN load cell was used (model LR 10K from Lloyd Instruments
Ltd.), as was the case for all load tests in this paper Fig. 1a. A
constant head displacement rate of 25 mm/min was applied. The
overall dimensions of the specimen were 50 mm (width) 
350 mm (length). Tabs consisting of epoxy-adhered 25  50 mm
#100 sandpaper sheets were used, giving an effective gage length
of 300 mm. Tests were performed in the paper roll direction (0°),
transverse direction (90°), and 45° for five specimens each
Fig. 1b. Strain data were derived by dividing the machine head displacement by the initial gage length. This approach may be vulnerable to grip slippage and machine compliance affecting the
displacement data. However, the force level in the tests remained
relatively low (maximum force range 92–187 N), so we assumed
that machine compliance is small. Additionally, no grip slippage
was apparent during the tests and in the load/displacement result
plots.
2.2. Phenolic resin tension test
Tensile tests were performed on resol phenolic resin plates
(HirenolÃ’ KRD-HM2, Kolon Industries Inc., South Korea). These
tests were performed to characterize a constituent material present in the composition of Nomex honeycomb cores. Phenolic resin
was mixed with 25 wt% ethanol, and 250  185  2 mm dimensions plates were fabricated. To prevent excessive foaming, a gradual cure cycle was used as follows: 4 h each at 50, 60, 65, 70, 75
and 80 °C, 2 h each at 90 and 100 °C, 1 h at 120 °C, and 1.5 h at

150 °C [25]. Shrinkage of approximately 2.5% was observed upon
curing. Five tensile specimens were cut according to the ASTM
D638-03 standard with an overall length of 200 mm and a narrow
section width of 13 mm [26]. Both specimen main surfaces were
polished up to 3 lm grit; the average final specimen thickness
was 1.76 mm. Glass/epoxy laminates were used as tabs, measuring
42  19  1.6 mm. Specimens were installed with a 2-element 90°
cross rosette strain gage (model UFCA-3-17-3L, Tokyo Sokki
Kenkyujo Co. Ltd., Tokyo, Japan) at mid-length after sanding the

Fig. 1. (a) Nomex paper tension test set-up, (b) Nomex paper roll.

R. Roy et al. / Composite Structures 117 (2014) 255–266

257

was as follows: 2 h at room temperature, 1 h each at 70, 100 and
120 °C, and finally 1.5 h at 150 °C [25]. Initially, seven specimens
were installed at once in the holder, but later only three specimens
were installed to prevent accidental contact between the dipped

paper sections. We therefore advise in this case to use a minimum
gap of approximately 8 mm between paper sections. After removal
from the holder, the lower 10 mm and longitudinal extremities of
the specimens were cut-out, giving a final specimen area of
40 mm (width)  290 mm (length). Tabs consisting of 25  40
mm #100 sandpaper sections adhered with epoxy were used, giving an effective gage length of 240 mm. From random measurements, we observed that the specimen thickness varied by
approximately 0.02 mm from top to bottom widthwise. This may
be due to a gravity effect during the dipping process. Specimens
were installed with a 2-element 90° cross rosette strain gage at
mid-length (model FCA-3-11-1L, Tokyo Sokki Kenkyujo Co. Ltd.,
Tokyo, Japan). Three specimens for each paper direction were
tested with a constant head displacement rate of 1 mm/min (see
Fig. 4).
Fig. 2. Phenolic resin tension test specimens and tension test set-up.

2.4. Nomex honeycomb core test
surface with #1000 sandpaper. A constant head displacement rate
of 5 mm/min was applied in the tests (Fig. 2). Strain gage signals
were recorded at 10 Hz through a NEC AS1203 strain amplifier
(NEC Corporation, Tokyo, Japan) and a National Instrument USB6251 data acquisition module (National Instruments Corporation,

Austin, USA), as was the case with all strain gages used in this
paper. During the tensile tests, the specimens failed at the grips;
therefore, the measured strength was not considered. The cured
resin was brittle and it could crack while cutting specimens too
quickly. Nonetheless, a sufficiently high load was reached in the
tests to measure significant modulus and Poisson’s ratio values
(maximum strains higher than 0.4%).
2.3. Nomex/phenolic tension test
Specimens made of Nomex paper dipped into phenolic resin
were fabricated. Specimens were fabricated with the paper in the
roll direction (0°), transverse direction (90°), and 45°. Nomex paper
specimen sections initially measured 50 mm (width)  350 mm
(length)  0.05 mm (thickness). Paper sections were secured
straight in a metal holder (Fig. 3a). This holder was then dipped
in a bath of phenolic resin (same resin as in Section 2.2) mixed with
25 wt% ethanol for 5 min (Fig. 3b). After removal from the bath, the
holder was left to drip above the bath for 5 min. The specimens
were then cured in an oven while still in the holder. The cure cycle

Flat sandwich panel specimens with Nomex honeycomb cores
were prepared to perform flatwise tension and compression tests.
In the tests, strain gages were installed on the honeycomb walls.
These tests served to provide additional core material stress/strain
behavior data and also allowed for the investigation of possible differences in the material’s tensile and compressive moduli. With
aramid fibers for example, there is evidence that contrary to strain
stiffening in tension, strain softening can occur in compression
[27]. The sandwich panels tested had faces consisting of glass/
epoxy 3/1 weave prepreg fabric laminates (GEP 224, SK Chemicals,
South Korea). These laminates were first vacuum bag-cured alone
in an autoclave at 3.0 atm with a stacking sequence of [0/90]2S
and total thickness of 1.0 mm. Two Nomex honeycomb cores were
evaluated: both with a 4.8 mm cell size and 25.4 mm height and
with densities of 32 and 64 kg/m3 (models HD322 and HD342,
M.C. Gill Corporation, El Monte, USA). These cores were provided
by Korea Aerospace Industries Ltd. The faces were lightly sanded
(#1000 grit sandpaper) and adhered to the cores with an epoxy
adhesive film (Hysol EA9696.030, Henkel Corporation, Düsseldorf,
Germany) in vacuum bag autoclave molding with a maximum
pressure of 2.0 atm. The bag vacuum was released when the autoclave pressure climbed past 1.0 atm. The faces were adhered to the
core one at a time so that the effect of gravity on the adhesive line
would be the same on both sides. The specimen’s nominal area

Fig. 3. (a) Nomex paper sections in holder; (b) Nomex paper sections in phenolic resin bath.

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R. Roy et al. / Composite Structures 117 (2014) 255–266

stopped before failure when the recorded gage strain reached
approximately 0.3%. Specimens were then removed from the flatwise tensile test jigs, keeping the adhered steel block in place. Flatwise compression tests were then performed on these specimens
until failure with the same machine head displacement rate of
0.2 mm/min [28]. Three specimens of each core type were tested.
2.5. Density measurement
The density of the specimens tested was measured by taking
their weight in air and in water with an electronic density meter
(VIBRA DME, Shinko Denshi Co., Jakarta, Indonesia) (see Fig. 6).
This density meter model automatically calculates the density
from these two weight measurements. The density of the
Nomex/phenolic specimen was measured after the tensile test
from an un-cracked area section of 40  50 mm cut from each
specimen. The average thickness (t) of this section was then calculated with the following formula: t = m/(qA), where m is the specimen’s dry mass, q its measured density, and A its surface area. The
average constituent material density (qmat) of five different Nomex
honeycomb cores was also measured. This measurement was done
by submerging a honeycomb core section in the density meter.
Core specimens with an area measuring approximately 6  6 cm
were used. When the core specimen was submerged in the density
meter, it was slightly shaken to dislodge any air bubbles from it.
3. Test results and discussion
Fig. 4. Nomex/phenolic tensile test set-up.

dimensions were 50  50 mm with a total height of 27.4 mm. To
first perform a flatwise tensile test, specimens were adhered to
steel blocks with a liquid epoxy adhesive cured at room temperature. Two small strain gages were then installed on the specimens
(model UFLK-1-17-3L, Tokyo Sokki Kenkyujo Co. Ltd., Tokyo,
Japan), one on each side parallel to the L-direction, on a honeycomb double-wall section (Fig. 5). Flatwise tensile tests were performed at a machine head displacement rate of 0.2 mm/min and

3.1. Nomex paper
Fig. 7 presents representative stress/strain curves from Nomex
paper tensile tests. To calculate the stress, the recorded load was
divided by the specimen’s section, considering an actual thickness
of 0.056 mm, based on measurement data from the manufacturer
[24]. Strain was calculated by taking the machine displacement
divided by the specimen’s initial gage length of 300 mm. The plastic-like behavior observed in the curves is similar to what Foo et al.
previously reported [23]. Likewise, wrinkling of the paper began at

Fig. 5. (a) Flatwise tension test; (b) flatwise compression test.

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R. Roy et al. / Composite Structures 117 (2014) 255–266
Table 1
Nomex T410 paper tension test average results.
Paper direction

E [GPa]

ru [MPa]

eu [%]
7.27
0.75
10.3%

0°

Mean
Sn1
C.V.

3.18
0.088
2.85%

64.2
2.6
4.04%

45°

Mean
Sn1
C.V.

2.36
0.10
4.24%

42.6
0.53
1.23%

90°

Mean
Sn1
C.V.

1.96
0.069
3.52%

33.6
0.61
1.82%

6.50
0.57
8.71%
6.69
0.85
12.8%

Table 2
Nomex paper tensile property comparison.
Paper nominal
thickness

Source

0.05 mm

0.13 mm

Fig. 6. Electronic density meter.

r0

r90

[MPa]

[MPa]

E0
[GPa]

E90
[GPa]

Current
results
Dupont [23]

3.18

1.96

–

–

Foo et al.
[22]
Dupont [23]

3.40

2.46

–

–

103.7

50.0

64.2

33.6

69.8

32.2

108.5

54.2

approximately the mid-level force amplitude. Every specimen we
tested failed in its middle length portion, away from the tabs.
The tensile modulus was calculated from a linear regression in
the 0.2–0.4% strain interval. Table 1 presents the average test
results for the different paper directions. Compared to Foo et al.
[23], the obtained average moduli test results differ by 7–23%
and reveal a higher orthotropic ratio (1.62 vs 1.38 for Foo et al.)
(Table 2). While it is premature to make a conclusion because of
limited data, these differences could possibly be caused by the different paper thickness tested (0.05 mm vs 0.13 mm for Foo et al.).
In terms of strength, both results are comparable to published data
from the manufacturer [24]. The listed strength values of Foo et al.
were determined by estimating the maximum forces on the test
curves in their published work.

C.V. = 4.05%), and the average Poisson’s ratio was 0.389
(Sn1 = 0.005, C.V. = 1.35%), measured from five specimens. The
modulus obtained is higher than a stated guideline value of
3.9 GPa [29] but slightly lower than the measured result of Redjel
[30] (5.16 GPa). Giglio et al. [14] recently used a modulus value of
3 GPa in their numerical model. The Poisson ratio result of
m = 0.389 compares to a measured result of 0.36 from Redjel [30].
By isotropic material theory, a shear modulus of 1.78 GPa was calculated from the current results {G = E/(1 + 2m)}. The average cured
resin density, measured from three specimens of approximately
13  40  1.76 mm dimensions, was qphenol = 1.342 g/cc.

3.2. Phenolic resin

3.3. Nomex/phenolic

The phenolic resin tensile test results displayed a linear elastic
stress/strain behavior, as expected. The tensile modulus and Poisson’s ratio were calculated from a linear regression of the stress/
strain and ey/ex slope, respectively, in the 0.1–0.3% strain range.
The average tensile modulus was 4.94 GPa (Sn1 = 0.20 GPa,

Representative stress/strain curves from the Nomex/phenolic
tension tests are presented in Fig. 8. Linear elastic behavior was
observed until failure. Compared to plain Nomex paper (Fig. 7),
the addition of a phenolic resin coating eliminated plastic deformation and greatly reduced maximum strain. This behavior was also

Fig. 7. Nomex T410 paper representative stress/strain curves.

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R. Roy et al. / Composite Structures 117 (2014) 255–266

Fig. 8. Nomex/phenolic specimen representative stress/strain curves.

observed by Hähnel [31], who reported semi-qualitative stress/
strain slopes of Nomex paper impregnated with phenolic resin.
When Hähnel used only a single resin impregnation, the material
retained some plastic behavior, but with two resin impregnations,
the material exhibited completely linear elastic behavior. Considering this result, we assumed that our single resin impregnation
was relatively thick in comparison. For the 0° Nomex/phenolic
specimens, the average failure strength of these specimens
(42.9 MPa) corresponds to a force of 204.5 N. Let us assume that,
in this test, the phenolic coating is the first to fail. This assumption
is based on the Nomex/phenolic specimen’s relatively low failure
strain compared to plain Nomex paper (0.83% vs 7.27%). Failure
of the coating would then transfer a total stress of 89.6 MPa
{204.5 N/(40.75  0.056) mm2} to the paper alone, higher than its
previously measured average strength of 64.2 MPa. In this scenario, the phenolic coating and the paper would fail virtually
simultaneously. This scenario also assumes a simplified representation where there is no phenolic resin impregnation in the paper.
The tensile modulus and Poisson’s ratio were calculated from a linear regression of the stress/strain and ey/ex slopes, respectively, in
the 0.2–0.4% strain range. From the density measurement, calculated average individual specimen thicknesses were in the range
of 0.109–0.145 mm. These thicknesses were in agreement with
random individual caliper measurements on the specimens. Average test results for the different paper directions are presented in
Table 3. The moduli and Poisson’s ratios here listed are raw test
data. Later in this text, the local effect of the strain gage will be
modeled in FEM analysis. Varying specimen thickness will also
be normalized in the model. The test data will therefore be corrected to obtain typical Nomex honeycomb core constituent material properties.

3.4. Nomex honeycomb core constituent material density
The measured constituent material densities (qmat) of five
Nomex honeycomb cores are listed in Table 4. Despite the different core densities and manufacturers evaluated, constituent
material densities are fairly close, within a range of 1.11–1.16 g/cc.
Using the core and material densities, the cell size (H), and a
theoretical perfect hexagon cell geometry, a theoretical honeycomb constant single-wall thickness (tth) can be calculated (Eq.
(1)). This is an idealized representation because resin accumulation in cell corners is not considered. From these calculations, a
slight trend is apparent: the higher the wall thickness, the higher
the material density. Only the ‘McGill HD322’ specimen departs
from this trend. This trend is consistent with the fact that the
density of Nomex paper increases with thickness: for example,
for t = 0.056 mm, the density is q = 0.72 g/cc, and for
t = 0.079 mm, the density is q = 0.80 g/cc [24]. Furthermore, this
trend is consistent if the increase in wall thickness comes from
extra phenolic resin coating because its density is greater than
Nomex paper (qphenol = 1.342 g/cc).

tth ¼

6 qcore  103
H
16
qmat

ð1Þ

3.5. Nomex honeycomb core flatwise test
Fig. 9 shows the global core stress/strain behavior in the flatwise tension and compression tests. Core stress was taken as the
applied load divided by the core area (A = 50  50 mm), and core
strain was taken as the machine displacement divided by the core

Table 3
Nomex/phenolic tension test raw results.
E [GPa]

m

ru [MPa]

eu [%]

q [g/cc]

t [mm]

0°

Mean
Sn1
C.V.

5.31
0.258
4.86%

0.292
0.060
20.6%

42.9
4.0
9.38%

0.829
0.099
12.0%

1.334
0.038
2.88%

0.117
0.009
7.53%

45°

Mean
Sn1
C.V.

4.40
0.217
4.94%

0.323
0.020
6.09%

31.3
3.5
11.1%

0.722
0.027
3.71%

1.281
0.047
3.64%

0.141
0.005
3.66%

90°

Mean
Sn1
C.V.

4.39
0.178
4.05%

0.322
0.021
6.52%

20.7
4.4
21.2%

0.471
0.069
14.6%

1.318
0.022
1.63%

0.138
0.007
4.80%

Paper

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R. Roy et al. / Composite Structures 117 (2014) 255–266
Table 4
Measured material density of Nomex honeycomb cores.
Manufacturer

Nominal cell size (H) [mm]

qcore [kg/m3]

Core height (T) [mm]

qmat [g/cc]

tth [mm]

McGill
Hexcel
Plascore
McGill HD322
McGill HD342

3.18
3.18
3.18
4.76
4.76

51.7
76.6
51.6
31.4
60.0

8.85
12.70
17.80
25.40
25.40

1.114
1.142
1.115
1.148
1.160

0.0553
0.0800
0.0552
0.0489
0.0924

Fig. 9. Flatwise tension and compression core stress/strain curves: (a) 32 kg/m3 core; (b) 64 kg/m3 core.

height (T = 25 mm). In compression, the 32 kg/m3 core yielded at
approximately 85% (0.6 MPa) of its maximum strength, while
the 64 kg/m3 core had a more brittle behavior, with no significant
yield before failure. Combined stress/strain curves derived from
the strain gage signals are presented in Fig. 10. Honeycomb wall
stress was considered in this case and was calculated by taking
the applied load divided by an idealized honeycomb wall section
area [20]. Conversion from core stress to wall stress was calculated
as in Eq. (2), with l = 2.75 mm the cell wall length, u = 60° the hexagonal cell angle, and wall thicknesses of tð32 kg=m3 Þ = 0.0489 mm
and t ð64 kg=m3 Þ = 0.0924 mm (from Table 4).

rwall lð1 þ cos uÞ sin u
¼
rcore
2t
¼ f36:54ð32 kg=m3 Þ; 19:33ð64 kg=m3 Þg

ð2Þ

On the tensile side of the strain gage signals, linear elastic
behavior was strictly observed for the 32 kg/m3 core, while the
64 kg/m3 core had one or two strain gages showing some nonlinear
drift. We attribute this behavior to experimental imprecision,
namely load misalignment and strain gage location imprecision
on the honeycomb wall. In compression, at low stress, behavior
is generally linear, while some strain gages showed pronounced

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R. Roy et al. / Composite Structures 117 (2014) 255–266

Fig. 10. Flatwise tension and compression strain gage data: (a) 32 kg/m3 core; (b) 64 kg/m3 core.

nonlinear drift with increasing stress. Again, we attribute this drift
to experimental imprecisions. We found that our flatwise compression test was sensitive to specimen alignment (placing the specimen center in-line with the load). Keeping the steel blocks in the
test, and hence adding height, might have amplified this sensitivity. For the 32 kg/m3 core, even the strain gage signals that
remained fairly linear with increasing stress diverged at approximately 85% of the maximum compressive stress (around
0.6 MPa). This behavior is consistent with the yielding observed
in the global core stress/strain curves. The 64 kg/m3 core’s strain
gage signals remained fairly linear until failure; this behavior is
again consistent with the global core brittle behavior observed. A
linear regression of the stress/strain slope in the 0.1–0.2% strain
range (tension) and {0.1%, 0.2%} strain range (compression)
was calculated from the strain gage signals. We define this quantity as the cell wall elastic modulus, and individual results are presented in Table 5. The 32 kg/m3 core had similar tension/
compression average moduli (Etens = 5.29 GPa, Ecomp = 5.30 GPa),
while the 64 kg/m3 core was 7.22% softer in compression
(Etens = 5.26 GPa, Ecomp = 4.88 GPa). In both cases, the variability
on the moduli was relatively high (C.V.  10%), and few specimens
were tested (3 each). We therefore will not make any definite conclusions from the differences in tension and compression moduli.

Fig. 11. Honeycomb core cell geometry description.

As for the Nomex/phenolic test results, the flatwise tests will be
modeled in FEM and the results compensated for the strain gage
local stiffening effect. Because of the Nomex honeycomb core’s
expansion fabrication process [32], it is most likely that the Nomex
paper’s 90° direction will be aligned with the core’s through-thickness direction. This direction was the main direction of the load

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Table 5
Core cell wall strain gage raw elastic modulus results.
Specimen
3

Tension [GPa] {S.G. 1, S.G. 2}

Compression [GPa] {S.G. 1, S.G. 2}

Specimen
3

Tension [GPa] {S.G. 1, S.G. 2}

Compression [GPa] {S.G. 1, S.G. 2}

32 kg/m – 1
32 kg/m3 – 2
32 kg/m3 – 3

{5.17, 4.86}
{5.64, 5.78}
{4.86, 6.01}

{4.75, 4.88}
{5.28, 6.37}
{5.73, 5.35}

64 kg/m – 1
64 kg/m3 – 2
64 kg/m3 – 3

{5.52, 6.56}
{6.56, 4.99}
{5.16, 4.83}

{4.91, 4.57}
{6.11, 5.37}
{5.56, 4.70}

Mean
Sn1
C.V.

5.39
0.49
9.14%

5.39
0.60
11.0%

Mean
Sn1
C.V.

5.60
0.78
13.9%

5.20
0.59
11.3%

and strain measurement in these core flatwise tests. It will thus be
possible to compare identified 90° direction properties from the
Nomex/phenolic specimens and the core flatwise tests.
4. Modeling
4.1. Nomex/phenolic tension test model
A finite element model of the Nomex/phenolic tension tests was
built in order to compensate for the local stiffening effect expected
in the strain gage area. To achieve this, the strain gage material was
included in the model. Based on the properties of epoxy films [33],
the following parameters were used to model the strain gage:
E = 2.5 GPa, m = 0.36, and a 0.03 mm thickness. For every tensile
test group (i.e., 0°, 45°, and 90°), a model was built that considers
each group’s particular average thickness (from Table 3) to determine the extra phenolic coating thickness in the model (Fig. 12).
The models therefore also served to normalize the phenolic coating
thickness of the different specimen groups. From microscope
observation of the Hexcel 80 kg/m3 core, we determined the average total wall thicknesses to be approximately 0.08 mm, of which
approximately 0.055 mm is the Nomex paper only. The properties
of the 0.08 mm thick center portion (material to identify) were varied by trial and error in order to match the model’s effective prop-

erties at the strain gage to the test results. We note that this
0.08 mm thick material includes any resin impregnation in the
Nomex paper. The test moduli could be matched by adjusting
almost independently one material property in the model: E1 for
E0°, E2 for E90°, and G12 for E45°. The simulated effective gage Poisson’s ratios could not be matched exactly to all experimental
results simultaneously. The model’s Poisson’s ratio was therefore
chosen based on a least mean square calculation of the model’s
deviation from the Poisson’s ratio of the three test results (0°,
45°, and 90°). Identified Nomex core constituent material properties are listed in Table 6. The E2 modulus that was identified
(3.22 GPa) is lower than the one obtained by Staal [4] (3.81 GPa)
and the 3.5 GPa value used by Asprone et al. [13] and Roy et al.
[19] in their models. The shear modulus that was obtained
(1.26 GPa) is also lower than what was previously identified in
Roy et al. (1.68 GPa) [19]. These moduli differences will be discussed further in Section 4.3. The in-plane Poisson’s ratio that
was identified (0.24) differs from the value of 0.4 frequently used
in Nomex core FEM model simulations [6–8,18]. However, the inplane Poisson’s ratio is closer to the value of 0.2 previously identified by Roy et al. [19]. In the absence of any stated rationale, the
Poisson‘s ratio value of 0.4 previously cited appears taken from
phenolic resin property, or from earlier work (e.g., [12]). Given
the Poisson’s ratio of the phenolic resin we used (0.389), the current Nomex/phenolic results (m = 0.24) would imply that the
Nomex paper itself has a relatively lower Poisson’s ratio value.
For verification, let’s assume the previously described 0.08 mm
thick Nomex/phenolic material as layered. This material would
have a 0.055 mm thick Nomex paper core with 0.0125 mm thick
phenolic resin coatings and no resin impregnation in the Nomex
paper. From classical laminate theory calculations [34], we found
that a Nomex paper Poisson’s ratio value of 0.193 is required to
obtain a Nomex/phenolic Poisson’s ratio of 0.24. Under these
assumptions, this 0.193 value is comparable or at the low range
of reported Poisson’s ratio values for wood-based paper materials
[35,36].
4.2. Core flatwise test model

Fig. 12. Representation of the Nomex/phenolic tension test FEA model.

The core flatwise tests were modeled in FEM to calibrate the
strain gage measurement results. A meso-scale model was built
with shell elements representing the honeycomb cell geometry
(see Fig. 13). This type of model can consider several aspects of
an actual core’s variable nature, such as cell wall thickness variability [13,37,38], cell wall curvature (in-plane, out-of-plane)
[4,20,21,39], material property variability within a cell wall
[13,40], resin accumulation at the cell corners [14], and other ran-

Table 6
Nomex/phenolic calibrated elastic properties and test-simulation comparison.
Identified material properties

E1 [GPa]
5.20

E2 [GPa]
3.22

G12 [GPa]
1.26

m12

Gage results
Test
Model

E0° [GPa]
5.31
5.31

E45° [GPa]
4.40
4.40

E90° [GPa]
4.39
4.39

m0°–90°

m45°

m90°–0°

0.292
0.309

0.323
0.361

0.322
0.259

0.24

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R. Roy et al. / Composite Structures 117 (2014) 255–266

Fig. 13. Nomex honeycomb core FEA model (compression model dimensions pictured).

dom geometrical imperfections [15]. These characteristics can vary
depending on the core manufacturer. To begin with a relatively
simplified representation, we considered a constant cell wall thickness and some in-plane wall curvature at the cell corners (fillets),
similar to Staal [4]. Staal showed that using rounded cell corners
can have a significant effect on the model’s in-plane face wrinkling
buckling load. From microscope observations of the cores, a cell fillet ratio (w) of 0.2 was implemented in the model geometry,
defined as w = (l  L)/2l, with l the idealized hexagonal cell wall
length and L the actual straight portion of the cell wall (Fig. 11).
Linear 4-node shell elements were used, with a mesh division of
9 elements (in-plane wise) by 106 elements (height wise) for each
cell wall segment. This mesh resulted in an element size of approximately 0.2  0.24 mm, which was considered appropriate based
on published mesh convergence analysis of similar models [15].
The core’s bottom node displacements (ui) were locked in the X,
Y, and Z directions. The core’s top node displacements were coupled in the X, Y, and Z directions. With this we considered that
the in-plane stiffness of the core was negligible compared to the
faces. This coupling permitted a single vertical force to be applied
on a node at the geometrical center at the top of the core. The particular strain gage used was also modeled assuming polyimide film
properties (E = 5.32 GPa, m = 0.35, from [41]) and a thickness of
0.05 mm. For both cores tested (32 and 64 kg/m3), the thicknesses
of single-wall elements closest to cell corners (t3) and second closest to cell corners (t4) were adjusted to obtain the exact core density desired in the model. This adjustment is also designed to
represent resin accumulation present in that area. The cell wall
thicknesses
were
t1 = 0.0489 mm,
t2 = 2t1,
t3 = t2,
and
t4 = 0.05632 mm for the 32 kg/m3 core, and t1 = 0.0924 mm,
t2 = 2t1, t3 = t2, and t4 = 0.12395 mm for the 64 kg/m3 core. The
Nomex/phenolic properties obtained in Section 4.1 were used as
a starting point for the core cell wall material properties. The strain
result of the strain gage elements was then averaged and compared to experimental results. To obtain an exact match to the
strain gage experimental results, moduli were all scaled by a constant. For the 32 kg/m3 core, a proportion of 1.293 applied to the
moduli (E1 = 6.73 GPa, E2 = 4.16 GPa, G12 = 1.63 GPa) replicated
the exact modulus measured at the gage; for the 64 kg/m3 core,
this proportion was 1.482 (E1 = 7.71 GPa, E2 = 4.77 GPa,
G12 = 1.87 GPa). Compared to the 90° direction modulus identified
in Section 4.1 (E2 = 3.22 GPa), the moduli identified with this
approach are 29.2% (32 kg/m3 core) and 48.1% (64 kg/m3 core)
higher. Our best hypothesis for this difference is that the cell wall
material of the cores tested simply has greater stiffness than the
specimens we fabricated. Nomex paper can be tailored and optimized for a given application. This tailoring can involve adjusting
the proportion of fibers, pulp, and binder in the paper, as well as
the percentage of voids. Such tailoring has been shown to have a

significant effect for honeycomb cores [42]. Moreover, Nomex
paper type 412 is typically used in honeycomb cores [13,15,25].
We used general usage Nomex paper type 410 to prepare our specimens, like other authors [43,44], because this type was more easily available to us. This different paper grade and the fact that we
applied one thick phenolic coating at once in our specimens (e.g.,
[45]) may be the reason that our specimens are softer than the core
cell wall material. It is also worth noting that the McGill cores
tested are among the stiffest comparable Nomex honeycomb cores
commercially available [32,46].
4.3. Representative Nomex core model
The material properties identified in Section 4.1 were also evaluated for a core with a cell wall thickness comparable to the one
used in the identification process (0.08 mm). The Hexcel 80 kg/
m3 density, 3.18 mm cell size core was chosen as a basis because
we previously determined that its cell has the same nominal cell
wall thickness of 0.08 mm. A meso-scale shell FEM model of the
core was built in the same fashion as described in the previous section. A cell filet ratio (w) of 0.17 was implemented based on microscope observations. The cell hexagon angle was also measured and
found to be u = 47.3° with an actual cell size (H) of 3.38 mm. The
thicknesses of the single-wall elements closest to cell corners were
again adjusted in order to match the measured core density
(76.6 kg/m3 from Table 4). This model served to compare simulated core moduli with test data published by the manufacturer.
The model’s simulated core moduli were determined by applying
a single force (Fi) on a top node at the geometric center and measuring the corresponding core deformation. For compression, the
model had a square area of 50  50 mm, typical of a flatwise test.
In the case of shear, it is recommended that the specimen length
be 10–12 times that of its thickness [47,48]. Therefore in shear
loading, the model had a rectangular area of 50  138 mm. The
simulated core moduli are presented in Table 7, case 2, and they
are all lower than the manufacturer’s test data. As stated before,
our best hypothesis for this difference is that the specimens we
prepared may have a lower stiffness than commercial honeycomb
materials. The material moduli in the model were subsequently
scaled to get closer to the manufacturer’s core moduli data. The
E2 modulus was scaled to exactly match the core’s compression
modulus. Scaling the E1 modulus had little influence on the different simulated core moduli, so it was left unchanged. The G12 modulus was scaled to approach the core’s shear moduli, as it was not
possible to simultaneously obtain a perfect match to both core
main direction shear modulus. An optimal G12 value was therefore
determined from a least mean square calculation of the percentage
deviation from the manufacturer’s data. A much higher shear modulus was required (G12 = 1.85 GPa), and we observed that, com-

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R. Roy et al. / Composite Structures 117 (2014) 255–266
Table 7
Hexcel 3.18 mm – 80 kg/m3 honeycomb core test and simulation properties.
Case

Compression modulus
[MPa]

L-direction shear modulus
[MPa]

W-direction shear
modulus [MPa]

1- Manufacturer test data
2- FEM with identified properties, u = 47.3° (Table 6)
3- FEM with calibrated properties, u = 47.3° (E1 = 5.20 GPa, E2 = 3.73 GPa,
G12 = 1.85 GPa, m12 = 0.24)
4- FEM with u = 60°, q = 80 kg/m3, t2 = 2  t1 (E1 = 5.20 GPa, E2 = 3.61 GPa,
G12 = 1.79 GPa, m12 = 0.24)
5- FEM with u = 60°, q = 80 kg/m3, t2 = 1.77  t1 (E1 = 5.20 GPa, E2 = 3.60 GPa,
G12 = 1.76 GPa, m12 = 0.24)
6- FEM with u = 60°, q = 80 kg/m3, t2 = 1.74  t1 (E1 = 5.20 GPa, E2 = 3.60 GPa,
G12 = 1.76 GPa, m12 = 0.24)

255
219
255

70.0
52.9
77.4

37.0
21.8
31.9

255

72.3

35.9

255

70.1

36.8

255

70.0

37.0

pared to the manufacturer’s test data, the model is relatively stiffer
in the L-direction than in the W-direction (Table 7, case 3). We verified that the rectangular area size chosen showed convergence
within below 1% on the G12 value identified, compared to a slightly
smaller size model. It was also verified that shear loading on the
square model would generate a 6% increase difference in the G12
value identified. Based on the diverging directional core shear stiffness obtained, we reasoned that it is more likely that the manufacturer data comes from a core with nominal dimensions, that is:
u = 60°, H = 3.18 mm, qcore = 80 kg/m3, and w = 0.2 (from [4]).
Therefore, another model was built with these parameters. With
an increased cell hexagon angle (u = 60°), we could expect a correction in the GL/GW ratio according to analytical models [49]. This
was the case as shown in Table 7, case 4, where the shear moduli
are now within 3.3% of the test data. Calibrated E2 and G12 values
are also slightly lower with this model, most likely in great part
due to the increased core density modeled. Another possible consideration in the model is the double-wall (t2) thickness allocation.
In reality, this double-wall may not have exactly double the thickness of a single-wall (t1). The double-wall should have twice the
Nomex paper thickness as a single-wall, but sensibly the same phenolic coating thickness. Microscope observations of core cells confirms this reasoning, and on average we measured a thickness of
t2 = 1.77  t1. This cell wall thickness proportion was incorporated
into the nominal dimensions model (u = 60°), which was also
adjusted to obtain a 80 kg/m3 core density (t1 = 0.08 mm,
t2 = 0.14175 mm, t3 = t2, t4 = 0.11368 mm). The simulated shear
moduli ratio obtained with this configuration were then within
0.6% of the target value (Table 7, case 5). For the sake of obtaining
a perfect match, a thickness proportion of t2 = 1.74  t1 was used
(t1 = 0.08 mm, t2 = 0.0001392 mm, t3 = t2, t4 = 0.0001206 mm). This
configuration gave simulated core moduli that exactly matched the
manufacturer’s test data, along with corresponding calibrated cell
wall material elastic properties (Table 7, case 6). This result demonstrated possible approaches to model calibration and also the
extent of their effect, specifically in terms of shear moduli ratio.
Using slightly different material properties for the double-wall
may also be another way to calibrate the model, ideally with some
analytical rationale or test data to back it. The obtained calibrated
cell wall material elastic properties are comparable to some used
in recent published work on meso-scale FEM modeling of Nomex
honeycomb cores: E2 = 3.60 GPa (3.50 GPa in [13], 3.52 GPa in
[19]), G12 = 1.76 GPa (1.68 GPa in [19]), and m12 = 0.24 (0.2 in [19]).

strength. Measured phenolic resin elastic properties were in the
range of published results. Phenolic resin coated Nomex paper displayed a strictly linear elastic behavior; we attribute this behavior
to the relatively thick resin coating that was used. These Nomex/
phenolic test results were calibrated in finite element modeling
(FEM) to account for strain gage local stiffening effects and for
specimen thickness normalization. The identified Nomex/phenolic
elastic properties were E1 = 5.20 GPa, E2 = 3.22 GPa, G12 = 1.26 GPa,
and m12 = 0.24. Flatwise tension and compression tests were also
performed on Nomex honeycomb cores with strain gages installed.
Finite element models of these tests were also built and E2 material
values of 4.16 or 4.77 GPa were determined from the models. We
attribute these relatively stiffer values to a more optimized material formulation in the commercial Nomex cores compared to our
fabricated Nomex/phenolic test specimens. The identified
Nomex/phenolic material property set was then used in a finite
element model of a core. Simulated core moduli were compared
to manufacturer’s test data. Scaling of the material moduli E2 and
G12 was required to approach the test data. It was also observed
that the honeycomb cell’s double-walls do not exactly have double
the thickness of its single-walls. This observation was also implemented in the model, and it was possible to achieve a perfect
match between core simulations and test results. The calibrated
cell wall material elastic properties in this matching model were
E1 = 5.20 GPa, E2 = 3.60 GPa, G12 = 1.76 GPa, and m12 = 0.24. We consider that some level of model calibration should be expected with
meso-scale Nomex honeycomb core modeling. The need for calibration may come from the level of modeling sophistication used
(imperfections, cell corners), core construction differences among
manufacturers, or core test results variability.

5. Conclusions

References

In this work, tensile tests were first performed on Nomex paper,
phenolic resin, and Nomex paper coated with phenolic resin.
Nomex paper test results were comparable to previously published
test data. These results also confirmed that paper properties are
dependent on the paper’s nominal thickness, at least in terms of

Acknowledgments
This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(2009-0094104). This research was financially supported by the
Ministry Of Trade, Industry & Energy (MOTIE), Korea Institute for
Advancement of Technology (KIAT) and Dong-Nam Institute For
Regional Program Evaluation (IRPE) through the Leading Industry
Development for Economic Region. Part of this work was presented
at the 17th International Conference on Composite Structures
(ICCS-17), 17-21 June 2013, in Porto, Portugal.

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