February 2013 

NASA/TM–2013-217802 

Evaluation of Ceramic Honeycomb Core 

Compression Behavior at Room Temperature 

R. Keith Bird 

Langley Research Center, Hampton, Virginia 

Thomas S. LaPointe 

Oklahoma State University, Stillwater, Oklahoma 
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February 2013 

NASA/TM–2013-217802 

Evaluation of Ceramic Honeycomb Core 

Compression Behavior at Room Temperature 

R. Keith Bird 

Langley Research Center, Hampton, Virginia 

Thomas S. LaPointe 

Oklahoma State University, Stillwater, Oklahoma 
Available from: 

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7115 Standard Drive 

Hanover, MD 21076-1320

443-757-5802 

Acknowledgments 

The authors would like to thank Fred Dynys at NASA Glenn Research Center for 

providing the honeycomb core compression specimens machined from the ZAL-45AA 

rigid insulation tile material. 

Thomas LaPointe contributed to this work as a research intern student under the Langley 

Aerospace Research Summer Scholars (LARSS) program during the Summer 2012 

session. 

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not 

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National Aeronautics and Space Administration 
Abstract 

Room temperature flatwise compression tests were conducted on two 

varieties of ceramic honeycomb core specimens that have potential for 

high-temperature structural applications. One set of specimens was 

fabricated using strips of a commercially-available thin-gage "ceramic 

paper" sheet molded into a hexagonal core configuration. The other set 

was fabricated by machining honeycomb core directly from a 

commercially-available rigid insulation tile material. This paper 

summarizes the results from these tests. 
2

Introduction 

 Honeycomb core sandwich structures based on ceramic materials can offer the high-temperature 

strength and stiffness needed for hot structure applications on aerospace vehicles. Filling the core with 

fibrous insulation gives the sandwich structure the potential to provide thermal insulation from the 

extreme operating environment while carrying the required structural loads. A key challenge is 

development of thin-gage ceramic material that has a high load capacity and is a strong thermal insulator. 

One possible material system is a lightweight "ceramic paper" honeycomb core sandwich structure. 

Another material system with potential is a honeycomb core machined directly from rigid insulation tile 

material. These honeycomb core systems are of interest due to their low density and rigid insulating walls 

as well as the commercial availability of the starting materials. 

 The potential of these honeycomb systems for hot structures is being investigated under research 

and development activities at the National Aeronautics and Space Administration (NASA). As part of 

these activities, a study was conducted to generate room temperature compression properties of these 

honeycomb core systems to provide an initial evaluation of their potential for structural applications. This 

report summarizes the results from these honeycomb core compression tests. 

Materials 

 Conventional off-the-shelf ceramic materials were used for this study. The ceramic paper used for 

honeycomb core fabrication was Refractory Sheet 99 (RS99). This material is a commercially-available 

alumina-based, fibrous ceramic material with a density of 130 lb/ft3 and rated for temperatures up to 

3000°F (ref. 1). The material is manufactured in various sheet thicknesses and can be conformed to any 

shape once moistened with water. The material can be rigidized by curing at 570°F. 

 The rigid insulation tile material used for directly machining honeycomb core was commercially 

available ZAL-45AA. This material is composed of alumina fibers and inorganic binders and is rated for 

use at temperatures up to 2800°F. It has a density of 45 lb/ft3 and is a porous material with good thermal 

insulation properties (ref. 2). 

Honeycomb Core Specimen Fabrication 

 Individual flatwise compression (FWC) specimens were fabricated for this study. Ceramic paper 

honeycomb core specimens were manufactured by Steve Miller and Associates Research Foundation 

(SMARF) under NASA Cooperative Agreement NNX08AD14A. The core was fabricated by cutting the 

RS99 sheet into strips. The sheet thickness was nominally 0.025 inch. The strips were moistened with 

water and molded into the core ribbons with a 7/8-inch hexagonal core cell configuration. The ribbons 

were joined together at the nodes during the ceramic paper curing process at 570°F. The test specimens 

were fabricated individually with nominal dimensions of 3 inches by 3 inches by 1.5 inches. After the 

specimens were fabricated, the core was coated with S-Coat protective coating and cured. The final 

honeycomb core density was approximately 10 lb/ft3. Figure 1 shows one of the ceramic paper 

honeycomb core FWC specimens. Five of these specimens were provided for room temperature 

compression testing. 

 ZAL-45AA honeycomb core FWC specimens were machined using water jet cutting at NASA 

Glenn Research Center. The nominal specimen dimensions were 3 inches by 3 inches by 1 inch. The core 

cells were 1-inch hexagons with nominal wall thickness of 0.14 inch. The specimens were coated with SCoat protective coating and cured. The final specimen core density was approximately 16 lb/ft3. Figure 2 

shows a honeycomb core FWC specimen machined from the ZAL-45AA tile. Three of these specimens 

were provided for room temperature compression testing. 
3

Specimen Preparation 

 Each specimen was measured, weighed, and photographed. The dimensions for each specimen are 

tabulated in table 1. 

 Due to the method of ceramic paper specimen fabrication in which individual RS99 strips were 

joined together, the surfaces were not flat because some of the strips protrude above the adjacent strips. 

In order to produce a specimen with flat and parallel surfaces for compression testing, the two faces of 

each specimen were potted in a polymeric mounting compound to a depth of approximately 0.25 inch. 

The potting material was allowed to cure at room temperature, then the faces were machined flat and 

parallel. The exposed height of honeycomb core between the two potted faces was measured and used as 

the specimen gage length (approximately 1 inch). One of the potted specimens is shown in figure 3. 

 The ZAL-45AA honeycomb core specimens were machined flat and parallel during the specimen 

fabrication process. However, during the coating process the coating was applied to the entire specimen, 

including the compression surfaces. Therefore, the compression surfaces of these specimens were lightly 

ground to remove the coating and provide flat and parallel surfaces. These specimens were not potted. 

Test Procedures 

 Figure 4 shows one of the potted RS99 ceramic paper FWC specimens in the compression test 

system. The tests were conducted in accordance with ASTM Standard C365 (ref. 3). Extensometers 

were mounted to the front and back of the compression platens to measure relative displacement between 

the two platens. Tests were conducted at a platen displacement rate of 0.050 inch/minute. 

 FWC strength was defined as the maximum load divided by the overall cross sectional area (~9 in2

). Strain was 

calculated by dividing the displacement by the specimen gage length. The gage length for the potted 

specimens was defined as the height of the exposed core while the gage length for the unpotted specimens 

was the specimen height. The specimen modulus was calculated by conducting a linear regression over 

the linear portion of the stress-strain curve. 

Results and Discussion 

 A representative flatwise compression stress-strain curve for the RS99 ceramic paper honeycomb 

core specimens is shown in figure 5. The initial "toe" region of the curve is due to seating of the 

specimen and the load train upon loading. ASTM C365 procedures were used to remove this strain from 

the data (ref. 3). The intersection of the strain axis with the dashed line in the figure represents the zero 

strain point. The stress-strain curves had a linear region over which the modulus was calculated. At the 

FWC strength, the specimen fractured and failed catastrophically. 

 Figure 6 shows a typical stress-strain curve for the ZAL-45AA honeycomb core specimens. As was 

the case with the RS99 specimens, the "toe" region was removed from the strain calculations. The curves 

had a linear region over which modulus was calculated. The onset of specimen failure occurred at the 

FWC strength value, but unlike the RS99 specimens, the ZAL-45AA specimens continued to carry load after 

the peak stress. The sustained crushing stress following the onset of failure was about half the FWC 

strength value. 

 The FWC compression results for the RS99 ceramic paper and ZAL-45AA tile honeycomb core specimens are 

tabulated in table 2 and plotted in figures 7-9. The FWC strength for the two honeycomb core systems 

is plotted in figure 7. Each data point represents one specimen. The RS99 honeycomb core had 

substantially greater strength than did the ZAL-45AA core. The ZAL-45AA specimens exhibited 

consistent strength values while the RS99 specimens had a very large range of scatter in the strength data. 
4 

This scatter is related to nonuniformity issues associated with individual specimens and how well the 

ceramic paper ribbons were bonded to each other during the core fabrication process. 

 The strain at maximum stress for the two honeycomb core systems is shown in figure 8. The RS99 

honeycomb core had a relatively low strain level. The ZAL-45AA core had about an order of magnitude 

greater strain than did the RS99 core. The porous nature of the ZAL-45AA tile material allowed these 

specimens to deform and accommodate a substantial amount of strain prior to the onset of failure.

Figure 9 shows the modulus for the two honeycomb core systems. The ZAL-45AA core had very 

low modulus values. As was seen with the strain data, the porous nature of this material allowed the 

specimen to absorb a significant level of strain during loading, thus resulting in a low stiffness. The rigid 

RS99 core, however, had much greater modulus values. The modulus data for the RS99 core had a 

bimodal distribution that may be indicative of how well the core ribbons were bonded together at the 

nodes within the individual specimens.

 Representative fractured specimens are shown for RS99 and ZAL-45AA honeycomb core specimens 

in figures 10 and 11, respectively. The RS99 core fractured catastrophically at the FWC strength value. 

The ZAL-45AA core, however, had a controlled crushing behavior after it attained the FWC strength and 

remained intact even after a significant level of strain. The failure modes and the stress-strain behavior 

indicated that the ZAL-45AA honeycomb core demonstrated a greater degree of damage tolerance for 

compression loading than did the RS99 core. 

Concluding Remarks 

 Room temperature compression tests were conducted on two varieties of ceramic honeycomb core 

specimens. One set of specimens was fabricated using strips of thin-gage RS99 sheet molded into a 

hexagonal core configuration with a 7/8-inch cell size and density of 10 lb/ft3

. The other set was 

fabricated by machining honeycomb core directly from ZAL-45AA rigid insulation tile material. This 

core had a 1-inch hexagonal cell size and density of 16 lb/ft3

. 

 The ZAL-45AA core had relatively low FWC strength and modulus, but accommodated higher 

strain levels. Even after fracture, the specimens continued to carry a significant stress level until the test 

was stopped. The sustained crushing stress was about half the FWC strength. 

 The RS99 core had much greater strength and modulus than did the ZAL-45AA core, but 

significantly lower strain. The specimens failed catastrophically at the FWC strength value. The RS99 

core specimens exhibited a high degree of scatter for the strength, modulus, and strain data. 

 The results indicate that the RS99 honeycomb core has potential to produce a ceramic structure with 

relatively high compression strength and stiffness. However, the fabrication techniques must be 

improved to produce a system with much more consistent mechanical properties. In addition, a better 

level of damage tolerance must be developed in this system. The ZAL-45AA core had relatively low 

strength, but demonstrated damage tolerance in which the system could sustain compression load 

following the onset of specimen failure. 

 
5 

References 

1. "High Alumina Matrix Sheets: Types RS-D and RS-99." ZRCI Refractory Composites 

Technical Data Bulletin No. ZRCI-020, July 2005. 

2. "Alumina Insulation Type ZAL-45 & ZAL-45AA." Zircar Ceramics website: 

www.zircarceramics.com/pages/rigidmaterials/specs/zal45.htm . 

3. “Standard Test Method for Flatwise Compressive Properties of Sandwich Cores.” Annual Book 

of ASTM Standards. Designation C365-05 American Society for Testing and Materials, West 

Conshohocken, PA 2005. 

 
6 

Tables 

Table 1. Dimensions and density of honeycomb core flatwise compression specimens. 

 

Spec. No. Core 

Wall 

Thick.

Length Width Height 

(exposed)

Height 

(total) 

Mass Density Density 

(in.) (in.) (in.) (in.) (in.) (g) (g/cm3

) (lb/ft3)

RS99-278 0.028 3.100 2.822 1.223 1.558 39.272 0.176 10.98

RS99-282 0.025 3.019 2.780 1.060 1.558 34.449 0.161 10.03

RS99-284 0.026 3.051 2.775 1.110 1.556 34.006 0.158 9.84

RS99-286 0.026 3.024 2.777 1.120 1.508 32.275 0.155 9.71

RS99-287 0.028 2.960 2.820 1.097 1.518 33.783 0.163 10.16

average 0.027 --- --- --- --- --- 0.162 10.14

ZAL-45AA-C5 0.143 2.987 3.125 0.934 0.934 34.037 0.238 14.87

ZAL-45AA-C6 0.135 2.984 3.139 0.993 0.993 42.946 0.282 17.59

ZAL-45AA-C7 0.147 3.006 3.119 0.975 0.975 37.647 0.251 15.69

average 0.142 --- --- --- --- --- 0.257 16.05
7 

Table 2. Results from room temperature honeycomb core flatwise compression tests. 

 

Spec. No. Max 

Load 

FWC 

Strength 

Modulus Strain at 

Max 

Stress

(lbs) (psi) (ksi) (%)

RS99-278 5,160 590 136 0.39 

RS99-282 11,071 1,319 421 0.30 

RS99-284 11,800 1,394 465 0.34 

RS99-286 2,954 352 107 0.30 

RS99-287 8,373 1,003 0.25 411

average 7,872 931 0.32 308

ZAL-45AA-C5 1,289 138 3.02 5.4

ZAL-45AA-C6 1,381 148 2.25 8.9

ZAL-45AA-C7 1,381 147 2.71 8.5

average 1,350 144 7.6 2.66 
8 

Figures 

Figure 1. RS99 ceramic paper honeycomb core flatwise compression specimen. 

Figure 2. ZAL-45AA rigid tile honeycomb core flatwise compression specimen. 

 

0.875 in.

0.03 in.

Specimen Nominal Cell Dimensions

1 in.

0.14 in.

Specimen Nominal Cell Dimensions
9 

Figure 3. RS99 ceramic paper honeycomb core flatwise compression specimen with compression faces 

potted. 

Figure 4. Flatwise compression test system with potted RS99 ceramic paper honeycomb core specimen 

installed. 

 

~1 in.

~0.25 in.

~0.25 in.

Potting Compound

Potting Compound

Specimen

Platen

Platen

Specimen

Extensometer

Extensometer
10 

Figure 5. Representative flatwise compression stress-strain curve for RS99 ceramic paper honeycomb 

core specimen. 

Figure 6. Representative flatwise compression stress-strain curve for ZAL-45AA honeycomb core 

specimen. 

 

0

500

1000

1500

0.0 0.2 0.4 0.6 0.8 1.0

Stress,

psi

Average Strain, %

Specimen

failure

Spec. No. RS99-282

0

50

100

150

0 2 4 6 810 12

Stress,

psi

Average Strain, %

Onset of specimen failure

Spec. No. ZAL-45AA-C6

Sustained crush
11 

Figure 7. Room temperature flatwise compression strength for ceramic honeycomb core specimens. 

Figure 8. Room temperature strain at maximum stress for ceramic honeycomb core specimens. 

 

0

500

1000

1500

Flatwise

Compression

Strength,

psi

ZAL-45AA core;

with S Coat

RS99 core;

with S Coat

0

1

2

3

4

Strain at

Maximum

Stress,

%

ZAL-45AA core;

with S Coat

RS99 core;

with S Coat
12 

Figure 9. Room temperature flatwise compression modulus for ceramic honeycomb core specimens. 

Figure 10. RS99 ceramic paper honeycomb core flatwise compression specimen after test. 

 

0

100

200

300

400

500

Modulus,

ksi

ZAL-45AA core;

with S Coat

RS99 core;

with S Coat

RS99-286

Top Bottom
13 

Figure 11. ZAL-45AA honeycomb core flatwise compression specimen after test. 

ZAL-45AA-C6
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Evaluation of Ceramic Honeycomb Core Compression Behavior at Room 

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14. ABSTRACT

Room temperature flatwise compression tests were conducted on two varieties of ceramic honeycomb core specimens that 

have potential for high-temperature structural applications. One set of specimens was fabricated using strips of a 

commercially-available thin-gage "ceramic paper" sheet molded into a hexagonal core configuration. The other set was 

fabricated by machining honeycomb core directly from a commercially-available rigid insulation tile material. This paper 

summarizes the results from these tests. 

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ceramic honeycomb; compression tests; honeycomb cores; room temperature

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