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i

NASA/TM—2009–215900

Hot/Wet Open Hole Compression Strength

of Carbon/Epoxy Laminates for Launch 

Vehicle Applications

A.T. Nettles

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

June 2009

National Aeronautics and

Space Administration

Marshall Space Flight Center • MSFC, Alabama 35812
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076 –1320

443–757–5802

This report is also available in electronic form at

<https://www2.sti.nasa.gov>

TRADEMARKS

Trade names and trademarks are used in this report for identification only. This usage does not constitute an official 

endorsement, either expressed or implied, by the National Aeronautics and Space Administration. 
iii

TABLE OF CONTENTS

1. INTRODUCTION ............................................................................................................ 1

2. ENVIRONMENTAL EFFECTS ON OPEN HOLE COMPRESSION STRENGTH ..... 2

3. MATERIALS AND TESTING ........................................................................................ 5

4. RESULTS .......................................................................................................................... 10

4.1 Unconditioned Specimens ........................................................................................... 10

4.2 Conditioning of Specimens ......................................................................................... 11

4.3 Open Hole Compression Strength of Conditioned Specimens 

Varying Test Temperature ............................................................................................ 14

4.4 Conditioned Specimens Varying Humidity .................................................................. 16

5. CONCLUSIONS ............................................................................................................... 20

REFERENCES ...................................................................................................................... 23
iv
v

LIST OF FIGURES

 1. OHC strength versus temperature for ‘wet’ and ambient 

specimens of AS4/PR500 ......................................................................................... 3

 2. Results of beach exposure specimens of 18-ply IM7/8552 laminates ....................... 4

 3. Schematic of specimens used in this study ............................................................... 5

 4. Test specimen and fixture in environmental chamber 

prior to compression testing .................................................................................... 6

 5. Front (a) and side (b) view of failed OHC specimen ................................................ 6

 6. Environmental humidity test chamber used to wet condition specimens .................. 7

 7. OHC test specimens in the humidity chamber ......................................................... 8

 8. Specimen being weighed .......................................................................................... 9

 9. OHC strength versus temperature for unconditioned specimens .............................. 11

10. OHC strength versus dwell time at 220 °F for unconditioned specimens ................. 13

11. Percent moisture weight gain (loss) versus square root of days ................................ 14

12. OHC strength versus temperature for 0.7% weight gain conditioned specimens ...... 15

13. OHC strength versus percent moisture weight gain for specimens at 220 °F ............ 17

14. Plot of all OHC data tested at 220 °F ...................................................................... 19
vi

LIST OF TABLES

 1. Open hole compression strengths of quasi-isotropic carbon/epoxy laminates .......... 2

 2. Reduction in OHC strength of AS4/PR500 due to temperature and moisture ......... 3

 3. Test temperatures and results for OHC strength versus temperature 

of unconditioned specimens .................................................................................... 10

 4. Data from all unconditioned OHC specimens tested at various temperatures .......... 12

 5. Results for OHC strength versus dwell time at 220 °F of unconditioned 

specimens ................................................................................................................ 12

 6. Data from all unconditioned OHC specimens tested at 220 °F 

for various dwell times ............................................................................................. 13

 7. Results for OHC strength versus temperature for 0.7% weight gain 

conditioned specimens ............................................................................................. 15

 8. Data from all 0.7% weight gain OHC specimens tested at various temperatures ...... 16

 9. Results for OHC strength versus moisture weight gain at 220 °F ............................. 17

10. Data from OHC specimens tested at 220 °F and varying moisture content ............. 18
vii

LIST OF ACRONYMS

ASTM American Society for Testing and Materials

CMH Composite Material Handbook

CSZ Cincinnati sub-zero

ETW elevated temperature wet

KS Kennedy Space Center

OHC open hole compression

RH relative humidity

RT room temperature

TM Technical Memorandum

Tg glass transition temperature
viii
1

TECHNICAL MEMORANDUM

HOT/WET OPEN HOLE COMPRESSION STRENGTH OF CARBON/EPOXY LAMINATES 

FOR LAUNCH VEHICLE APPLICATIONS

1. INTRODUCTION

Measuring the open hole compression (OHC) strength of polymer matrix composites is a 

useful way to test the effects of discontinuities that cause a stress concentration.1 The ‘environmental knockdown’ or ‘hot/wet’ factor for laminates is usually found by testing conditioned specimens 

to determine OHC strength.2 This approach is easier, less costly, and can be better controlled than 

testing impact-damaged laminates at environmental extremes.

The design of structural composite laminates typically takes into account the hot/wet performance of the material since matrix dominated properties tend to be reduced with increasing 

temperature and humidity. Aircraft must consider these extreme environmental factors since they 

can experience high temperatures for hours and the humidity levels the airplane may experience 

are not known. However, since single-use launch vehicles have a lifetime of minutes, the composite 

materials used on these vehicles may not be subjected to the extreme and lengthy environmental 

conditions imposed on aircraft composite structures. 

Because moisture may adversely affect the resin-dominated strength properties of carbon/

epoxy laminates at elevated temperatures, the procedure in American Society for Testing and 

Materials (ASTM) Standard D 52293 was developed to force moisture into the test specimen. This 

standard calls for the saturation of the specimen (i.e., no more water can be forced in) before subsequent mechanical testing to assess the effects of moisture on the laminate. Aircraft use this unrealistically high moisture level as a ‘worst case’ scenario to be conservative because of the extreme 

variation in flight patterns and conditions. Following fabrication of a composite launch vehicle, the 

environments experienced by the hardware are known. Since the actual moisture absorption of the 

composite material can be either predicted, or directly measured, applying standard aircraft testing 

procedures to launch vehicles may result in an over-designed structure with increased weight.

The objective of the study described in this Technical Memorandum (TM) is to determine 

the loss of OHC strength of the material system being used on the Ares I composite interstage as 

separate functions of moisture absorption and elevated temperature. This will assist in determining 

if hot/wet testing should be performed for the realistic conditions experienced by the interstage and 

if so, what magnitude of environmental knockdown can be expected. Since composite materials are 

considered for use on launch vehicles solely to reduce weight, it is prudent to minimize the mass of 

the structure while still maintaining proper safety margins.
2

2. ENVIRONMENTAL EFFECTS ON OPEN HOLE COMPRESSION STRENGTH

Composite Material Handbook 17 (CMH-17), Vol. 24 contains laminate property data 

including the hot/wet OHC strength of a quasi-isotropic laminate for some fiber/resin systems. 

Table 1 is a sampling of some of the OHC strength data (available in the open literature) for quasiisotropic carbon fiber/epoxy material systems. For the data listed in table 1, the specimens were 

saturated prior to testing although the dwell times of the specimens at temperature are not known 

and the test methods may have varied from ASTM D 6484.5 The limited data does provide a qualitative example of hot/wet knockdown effects published for notched quasi-isotropic compression 

specimens.

Table 1. Open hole compression strengths of quasi-isotropic carbon/epoxy laminates.

Carbon Fiber/Epoxy Resin

Room Temperature 

(ksi)

180 W 

(ksi) % Decrease

220 W 

(ksi) % Decrease

AS4 Fabric/PR500(a) 45.3 36.3 19.9 – –

AS4/997(a) 53.0 45.3 14.5 – –

IM7/8552(a) 48.7 43.3 11.1 40.6 16.6

T300 Fabric /977-2(a) 43.3 39.1 9.7 – –

IM7 Fabric /PR500(a) 44.7 – – 35.2 21.3

IM7/977-2(a) 43.0 34.9 18.8 – –

G30-500 Fabric /5276-1(b) 49 35 28.6 – –

T800/T3900(b) 42 33 21.4 – –

IM7/977-2(b) 45 37 17.8 – –

IM7/8551-7(b) 42 37 11.9 36 14.3

AS4/8551-7(b) 44 38 13.6 – –

IM7/977-3(b) 46.7 – – 37.0 20.8

AS4/3501-6(c) 44.0 36.7 16.6 – –

(a) Data from CMH-174

(b) Vendor data

(c) DSTO-TR-2077 Australian DoD Technical Report, Air Vehicles Division6

The OHC strength knockdown factor for various quasi-isotropic carbon/fiber epoxy systems tested at 180 °F in hot/wet conditions is on the order of 10% to 20% with variation between 

the systems being tested. Variation in resulting OHC strengths for the same resin with different 

fibers is illustrated by the resin 977-2. The OHC strength for this resin with T300 fabric shows a 

9.7% decrease at 180 °F wet verses ambient (room temperature (RT)), but with the IM7 unitape, 

the decrease is almost doubled at 17.8%. 

Typically OHC strength data only include ambient dry and elevated wet results so an identification of the contribution of moisture versus heat on resulting OHC strength data is not usually 

possible. In CMH-17,4 OHC strength data for AS4/PR500 carbon/epoxy is given at ambient versus 
3

elevated dry conditions so a comparison can be made for this material. Figure 1 shows the data 

in graphical form. The ‘wet’ specimens were conditioned at 160 °F and 85% RH for 2 weeks. The 

exact amount of moisture uptake was not given.

Ambient

Wet

OHC Strength (ksi)

Temperature (F)

50

45

40

35

30

25

20

50 100 150 200 250 300 350

[0f

/45f/90f/–45f]

S

Figure 1. OHC strength versus temperature for ‘wet’ and ambient specimens of AS4/PR500.

From this figure, it is evident that moisture and temperature interact to cause more OHC 

strength degradation as temperature increases for saturated specimens. In addition, it appears that 

increasing temperature is more detrimental to the OHC strength than the moisture content. Table 2 

summarizes the percent drop in OHC strength from temperature and moisture separately.

Table 2. Reduction in OHC strength of AS4/PR500 due to temperature and moisture.

Temperature

(°F)

Dry Value

(ksi)

% Reduction from

70 °F due to 

Temperature

Elevated Temperature 

Wet (ETW) Value

(ksi)

Additional %

Reduction due

to Moisture

Total % Reduction 

from 70 °F 

Dry to ETW

70 45.3 – – – –

180 38.2 15.7 36.3 4.2 19.9

240 35.6 21.4 32.8 6.2 27.6

300 32.1 29.1 27.1 11.1 40.2
4

It is evident that large knockdowns must be taken if the structure is to experience aircraftlike elevated temperatures, especially if the structure must be designed to airplane standard ‘saturation’ conditions.

If the actual environment that the launch vehicle hardware will experience is used instead 

of the harsh ones used for aircraft, it is possible that an additional weight savings can be realized. 

Figure 2 shows the results of a Kennedy Space Center (KSC) Corrosion Technology Laboratory 

Atmospheric Corrosion Test Site exposure of laminates made of the material system (IM7/8552) 

to be used for the Ares I composite interstage. The program requirement is for a 6-mo ‘pad stay,’ 

which equates to 6 mo of atmospheric exposure. Even at a longer pad stay, the laminates had a 

weight gain of ≈0.5% that can be considered a worst case scenario.

Series 1*

Series 2

Series 3*

Series 4

Percent Weight Gain

*Series 1 and 3 dried per ASTM D 5229

0.6

0.5

0.4

0.3

0.2

0.1

0

April-08

May-08

June-08

July-08

August-08

September-08

October-08

November-08

December-08

January-09

February-09

Figure 2. Results of beach exposure specimens of 18-ply IM7/8552 laminates.

This TM proposes to provide a better understanding of the effects of moisture and heat on 

the resulting OHC strength of laminates of IM7/8552 carbon/epoxy in order to evaluate whether 

testing to more realistic conditions could result in a weight savings due to lower knockdown factors. In order to accomplish this, OHC specimens were tested at a variety of moisture contents and 

temperatures to evaluate the sensitivity of these parameters.
5

3. MATERIALS AND TESTING

The material used in this study consisted of IM7/8552 carbon/epoxy with a per-ply thickness of 0.006 in. This particular resin has a glass transition temperature (Tg) of 392 °F dry and 

309 °F wet. The layup of the laminate was [+45,0,-45,0,90,0,0,90,0]S which makes an 18-ply directional specimen with more zero-degree plies in the loading direction. Panels were fabricated as a 24- 

by 24-in laminate from which the specimens were cut. Cure was in an autoclave at 55 psi pressure 

and 350 °F for 120 min. From the 24- by 24-in panel, 3- by 1-in specimens were machined with a 

0.125-in-diameter hole at its center as shown schematically in figure 3. The 3-in side was parallel to 

the zero-degree fibers. Since the specimens were to be end-loaded, the loaded edges were machined 

to within a 0.001-in tolerance.

1 in

0.125 in

3 in

Figure 3. Schematic of specimens used in this study.

A ‘Northrop’ fixture7 was used to prevent global buckling of the specimen. Figure 4 shows 

a photograph of the fixture in the test chamber prior to testing. While the majority of OHC is 

based on ASTM D 6484,5 the ‘Northrop’ method has been shown to give comparable results with 

a smaller specimen. The smaller specimen makes mechanical testing less costly and easier to perform.8 The recommended hole size is chosen to give a width/hole diameter ratio greater than six,9

and in this study, a ratio of eight was used. Figure 5 shows a front and side view of a failed specimen after testing.
6

Fixture

Platens

Figure 4. Test specimen and fixture in environmental chamber prior to compression testing.

Figure 5. Front (a) and side (b) view of failed OHC specimen.

(a) (b)
7

The specimens were randomly chosen for the various tests to prevent any bias that might 

result from taking groups of specimens from the same area of the large panel. The specimens were 

conditioned in a Cincinnati Sub-Zero (CSZ) MicroClimate® environmental humidity chamber at 

elevated temperature and relative humidity. A picture of the humidity chamber used is shown in 

figure 6 and the specimens in the chamber are shown in figure 7. The specimens were periodically 

removed and weighed on a Mettler AE200® scale to determine the amount of moisture they had 

absorbed as a percentage of total material weight. A picture of a specimen being weighed is shown 

in figure 8. Once a ‘desired amount’ of moisture had been absorbed, specimens were removed from 

the environmental chamber and placed in plastic bags. The ‘desired amount’ of moisture absorption was scheduled to be from 0.1%–1.0% weight increase in increments of 0.1%, however this was 

not achieved due to testing constraints. Subsequent mechanical testing was performed within 48 hr 

after removal from the chamber using an Instron 5582® with a 22,500-lb load cell. 

In many instances, specimens were not conditioned before mechanical testing. These specimens could contain some moisture with the amount depending on many factors such as type of 

epoxy, cure time, and laboratory storage conditions before mechanical testing. In this study, some 

specimens were not conditioned to see how much the unconditioned OHC strength deviated from 

specimens whose moisture content was strictly controlled. This was done in an attempt to determine the criticality of the moisture conditioning of specimens before performing OHC strength 

tests on laminates rather than simply machining the specimens, storing them in laboratory conditions, and then performing the mechanical testing. 

Figure 6. Environmental humidity test chamber used to wet condition specimens.
8

Figure 7. OHC test specimens in the humidity chamber.
9

Figure 8. Specimen being weighed.
10

4. RESULTS

4.1 Unconditioned Specimens

The first set of OHC testing was performed on unconditioned specimens to obtain a 

strength versus temperature profile for IM7/8552 with a layup sequence of [+45,0,–45,0,90,0,0,

90,0]S . A total of five specimens were tested at each of five elevated temperatures between 150 °F 

and 300 °F. Ten specimens were tested at room temperature and 15 were tested at 120 °F. A thermocouple was placed in the hole of the specimen being tested and once the thermocouple indicated 

the specimen had reached the desired temperature, the specimen was held at that temperature for 

10 additional minutes before beginning compression testing at a constant crosshead rate of 0.05 in/

min. Table 3 lists the test temperatures and resulting OHC strengths with a plot of the data given in 

figure 9. The raw data are presented in table 4.

Table 3. Test temperatures and results for OHC strength versus temperature 

of unconditioned specimens.

Temperature

(°F)

Strength

(ksi)

% Decrease From

Room Temperature

Standard Deviation 

(ksi)

RT 72.6 – 1.7

120 70.4 3.0 4.2

150 70.4 3.0 4.1

180 67.3 7.3 3.6

220 65.8 9.4 4.9

260 64.6 11.0 2.1

300 60.6 16.5 3.0

The data show that on unconditioned specimens, the strength drops about 7% from RT 

to 180 °F and about 9% from RT to 220 °F. At temperatures greater than ≈150 °F, the strength 

drop is basically linear with increasing temperature through the range of temperatures tested. 

A second set of OHC testing was performed on unconditioned specimens to obtain strength 

versus dwell time data at 220 °F for this material. This set of experiments was to determine the 

importance of the laminate being subjected to a given temperature for short versus long periods 

of time since all composite components on a launch vehicle will experience only brief excursions 

to the upper use temperature. A thermocouple was not used in these tests since the material was 

not intended to have a uniform temperature throughout its volume to simulate actual flight conditions for a launch vehicle.

A total of five specimens were tested at each of five dwell times at 220 °F. The test fixture 

was heated to 220 °F before each specimen was loaded. After loading the specimen, the fixture 
11

OHC Strength (ksi)

Temperature (F)

80

75

70

65

60

55

50

50 100 150 200 250 300 350

Figure 9. OHC strength versus temperature for unconditioned specimens.

was placed in the oven for the prescribed time period and then compression testing commenced. 

Table 5 shows the test dwell times and results for this set of data and figure 10 shows this data plotted as OHC strength versus dwell time. The raw data are presented in table 6.

The data show that on unconditioned specimens, the strength is independent of dwell time 

at 220 °F for up to 30 min. Much longer dwell times may have a more pronounced effect, but time 

at temperature for such a length of time is not applicable to launch vehicle hardware. The specimens that ‘dwelled’ for 10 and 30 seconds actually experienced a hot environment for a longer 

period due to the time needed to secure the specimen in the hot fixture plus the time needed to 

apply load until the specimen failed. Regardless, the data demonstrate that the ‘dwell time’ is of 

no consequence to the notched compression strength of composite laminates used on expendable 

launch vehicles.

4.2 Conditioning of Specimens

The specimens were conditioned in three batches. The first batch was conditioned at 180 °F 

and 83% relative humidity (RH) to ≈0.7% moisture gain by weight. Some specimens were tested for 

OHC strength and others were left in laboratory conditions (70 °F, 50% RH) to be monitored for 

weight loss. The second batch was conditioned at 150 °F and 100% RH to saturation (≈1.2% weight 

gain). The third batch was first dried and then conditioned at 150 °F and 100 % RH and periodically measured for moisture content. At each measurement for moisture content, some of the 
12

Table 4. Data from all unconditioned OHC specimens tested at various temperatures.

Specimen I.D.

Test Temperature 

(°F)

OHC Strength 

(ksi)

OHC-RT-1 70 73.8

OHC-RT-2 70 71.6

OHC-RT-3 70 72.6

OHC-RT-4 70 75.2

OHC-RT-5 70 72.2

OHC-RT-6 70 71.9

OHC-RT-7 70 71.1

OHC-RT-8 70 71.2

OHC-RT-9 70 75.2

OHC-RT-10 70 70.7

OHC-120-1 120 64.9

OHC-120-2 120 74.3

OHC-120-3 120 67.7

OHC-120-4 120 75.0

OHC-120-5 120 64.8

OHC-120-6 120 76.1

OHC-120-7 120 75.0

OHC-120-8 120 80.3

OHC-120-9 120 65.9

OHC-120-10 120 75.8

OHC-120-11 120 65.8

OHC-120-12 120 72.9

OHC-120-13 120 69.2

OHC-120-14 120 68.1

OHC-120-15 120 70.4

Specimen I.D.

Test Temperature 

(°F)

OHC Strength 

(ksi)

OHC-150-1 150 69.3

OHC-150-2 150 66.3

OHC-150-3 150 70.9

OHC-150-4 150 68.4

OHC-150-5 150 77.0

OHC-180-1 180 65.5

OHC-180-2 180 66.4

OHC-180-3 180 64.1

OHC-180-4 180 67.0

OHC-180-5 180 73.4

OHC-220-1 220 67.1

OHC-220-2 220 62.3

OHC-220-3 220 62.4

OHC-220-4 220 63.5

OHC-220-5 220 73.9

OHC-260-1 260 65.1

OHC-260-2 260 61.2

OHC-260-3 260 64.1

OHC-260-4 260 66.5

OHC-260-5 260 66.2

OHC-300-1 300 57.7

OHC-300-2 300 63.0

OHC-300-3 300 61.2

OHC-300-4 300 57.2

OHC-300-5 300 63.8

Table 5. Results for OHC strength versus dwell time at 220 °F of unconditioned specimens.

Dwell Time 

(min)

Strength 

(ksi)

Standard Deviation 

(ksi)

0.17 66.5 2.7

0.5 66.5 2.0

3 66.4 3.3

10 65.8 4.9

30 67.5 1.8

specimens were removed and tested for OHC strength. Moisture uptake (and loss) versus square 

root of time for the three specimen batches is shown in figure 11.
13

Table 6. Data from all unconditioned OHC specimens tested at 220 °F for various dwell times.

Specimen I.D.

Dwell Time 

(min)

OHC Strength 

(ksi)

OHC-220-017-1 0.17 63.5

OHC-220-017-2 0.17 63.9

OHC-220-017-3 0.17 70.5

OHC-220-017-4 0.17 67.0

OHC-220-017-5 0.17 67.5

OHC-220-05-1 0.5 69.5

OHC-220-05-2 0.5 65.2

OHC-220-05-3 0.5 66.0

OHC-220-05-4 0.5 64.5

OHC-220-05-5 0.5 67.2

OHC-220-3-1 3 67.3

OHC-220-3-2 3 67.5

OHC-220-3-3 3 70.1

OHC-220-3-4 3 61.2

OHC-220-3-5 3 65.9

Specimen I.D.

Dwell Time 

(min)

OHC Strength 

(ksi)

OHC-220-10-1 10 67.1

OHC-220-10-2 10 62.3

OHC-220-10-3 10 62.4

OHC-220-10-4 10 63.5

OHC-220-10-5 10 73.9

OHC-220-30-1 30 66.1

OHC-220-30-2 30 70.1

OHC-220-30-3 30 67.4

OHC-220-30-4 30 65.6

OHC-220-30-5 30 68.4

Stress (ksi)

Dwell Time (min)

Ambient Strength = 72.6±1.7

80

75

70

65

60

55

50

0 5 10 15 20 25 30 35

Figure 10. OHC strength versus dwell time at 220 °F for unconditioned specimens.
14

Percent Moisture Weight Gain

Exposure Time (Days)

1.4

1.2

1

0.8

0.6

0.4

0.2

0

0 5 10 15

Ambient then180 °F, 83% RH

Dry then 150 °F, 100% RH

Ambient then 150 °F, 100% RH

Specimens Removed From

Environmental Chamber

0.003% Loss per Day

Figure 11. Percent moisture weight gain (loss) versus square root of days.

Note that the desorption of the moisture in the specimens after removal from the environmental chamber is about 0.003% weight per day. This indicates that if the percent moisture gain 

is to be known to the nearest 0.01% then the specimens should be isolated from the environment 

or tested within 3.3 days after removal from the humidity chamber.

As expected, the specimens that were dried prior to placement in the humidity chamber 

gained moisture slightly faster than those specimens that were not dried, when exposed to 150 °F 

and 100% RH conditioning. The 180 °F at 83% RH conditioning caused the specimens to gain 

weight more slowly than conditioning at 150 °F and 100% RH. The data show that for this particular laminate, one week of conditioning is all that is needed to obtain the ‘worst case’ value of 0.5% 

as determined in figure 2.

4.3 Open Hole Compression Strength of Conditioned Specimens Varying Test Temperature

The OHC strength testing of conditioned specimens was performed at various temperatures 

and humidity levels to determine the criticality of each of these variables. This section will examine 

the effects of varying test temperature with a given moisture level in the specimens.

The first set of tests was intended to determine the effect of temperature on OHC strength 

on specimens with 0.7% moisture at a given dwell time of 10 min. A total of three specimens were 

tested at each of five temperatures between 70 °F and 220 °F (five specimens were tested at 300 °F). 

Table 7 shows the results of the tests. The 0.7% moisture weight gain is a value higher than what 
15

can be expected for actual hardware. Also included in the data are RT wet OHC strength values 

for specimens conditioned to 1.0% weight gain. The results are plotted in figure 12 with the data 

from figure 9 superimposed. The raw data are presented in table 8.

Table 7. Results for OHC strength versus temperature for 0.7% weight gain conditioned specimens.

Temperature 

(°F)

Strength 

(ksi)

% Decrease from 

Room Temperature 

(0.7%)

% Decrease 

due to Moisture

Standard Deviation 

(ksi)

RT 73.7 – 0.15 (increase) 1.7

RT (1.0%) 74.1 – 2.1 (increase) 1.7

150 69.2 6.1 1.7 3.2

180 65.8 10.7 2.2 1.6

220 63.7 13.6 3.2 2.1

300 59.9 18.7 0.11 2.9 OHC Strength (ksi)

Temperature (F)

80

75

70

65

60

55

50

50 100 150 200 250 300 350

Unconditioned

Conditioned (0.7%)

Conditioned (1%)

Figure 12. OHC strength versus temperature for 0.7% weight gain conditioned specimens.

The conditioned specimens did exhibit a lower OHC strength at elevated temperatures, but not 

to a large degree since the average value of the conditioned specimen is within the error bars of the 

unconditioned specimens. The specimens tested at RT show that the conditioned specimens actually failed at a higher OHC stress value than the unconditioned specimens, although not by an 
16

Table 8. Data from all 0.7% weight gain OHC specimens tested at various temperatures.

Specimen I.D.

Test Temperature 

(°F)

OHC Strength 

(ksi)

OHC-07-RT-1 70 71.8

OHC-07-RT-2 70 74.4

OHC-07-RT-3 70 75.0

OHC-1.0-RT-1 70 76.0

OHC-1.0-RT-2 70 74.0

OHC-1.0-RT-3 70 74.9

OHC-1.0-RT-3 70 76.9

OHC-1.0-RT-3 70 71.2

OHC-1.0-RT-3 70 73.7

OHC-1.0-RT-3 70 73.9

OHC-1.0-RT-3 70 74.0

OHC-1.0-RT-3 70 74.8

OHC-1.0-RT-3 70 71.7

OHC-07-150-1 150 69.3

OHC-07-150-2 150 72.4

OHC-07-150-3 150 66.0

OHC-07-180-1 180 64.0

OHC-07-180-2 180 66.5

OHC-07-180-3 180 66.8

OHC-07-220-1 220 65.8

OHC-07-220-2 220 61.6

OHC-07-220-3 220 63.6

OHC-07-300-1 300 64.1

OHC-07-300-2 300 61.3

OHC-07-300-3 300 56.6

OHC-07-300-4 300 58.6

OHC-07-300-5 300 59.1

appreciable amount. This highlights the reason that RT wet tests are usually not performed. The 

decrease in OHC strength from ambient to 180 °F wet is 10.7% which is comparable to the value 

(11.1%) given in reference 4 even though the laminate in this study has a larger percentage of zerodegree plies. The decrease in OHC strength from ambient to 220 °F wet is 12.3% which is smaller 

than the value (16.6%) given in reference 4. From the data it is evident that environmental conditioning of this laminate for notched compression strength will have little or no effect on the results.

4.4 Conditioned Specimens Varying Humidity

This section examines the effects on OHC strength of varying moisture levels at a given 

temperature of 220 °F. Table 9 shows the results that are plotted in figure 13. The raw data are 

presented in table 10.
17

Table 9. Results for OHC strength versus moisture weight gain at 220 °F.

% Weight Gain*

Strength 

(ksi)

% Decrease 

From Dry

Standard Deviation 

(ksi)

0.0 66.2 – 3.4

0.189 63.0 4.8 2.1

0.332 62.8 5.1 2.9

0.572 63.2 4.5 3.2

0.754 63.5 4.1 1.8

0.894 63.6 3.9 2.5

1.2 62.2 6.0 2.9

*Specimens originally dried per ASTM D 5229.

OHC Strength (ksi)

Weight Gain (%)

70

65

60

55

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Figure 13. OHC strength versus percent moisture weight gain for specimens at 220 °F.

The data suggests that the amount of moisture in the specimen has little effect on the 

OHC strength for a given temperature. Drying the specimen before testing may increase the OHC 

strength slightly, but typically launch vehicle hardware is not ‘dried out’ before flight.

Results from the earlier tests at 220 °F are superimposed on figure 13 and presented 

in figure 14.
18

Specimen I.D. % Weight Gain

OHC Strength 

(ksi)

OHC-DRY-1 0 61.4

OHC-DRY-2 0 65.3

OHC-DRY-3 0 67.5

OHC-DRY-4 0 65.8

OHC-DRY-5 0 73.9

OHC-DRY-6 0 64.6

OHC-DRY-7 0 63.4

OHC-DRY-8 0 65.4

OHC-DRY-9 0 66.8

OHC-DRY-10 0 68.3

OHC-189-1 0.189 64.1

OHC-189-2 0.189 64.9

OHC-189-3 0.189 61.3

OHC-189-4 0.189 64.8

OHC-189-5 0.189 66.6

OHC-189-6 0.189 59.7

OHC-189-7 0.189 61.4

OHC-189-8 0.189 61.5

OHC-189-9 0.189 62.7

OHC-189-10 0.189 62.5

OHC-332-1 0.332 61.4

OHC-332-2 0.332 62.8

OHC-332-3 0.332 63.7

OHC-332-4 0.332 63.7

OHC-332-5 0.332 62.4

OHC-332-6 0.332 68.4

OHC-332-7 0.332 59.5

OHC-332-8 0.332 57.7

OHC-332-9 0.332 64.3

OHC-332-10 0.332 64.5

OHC-572-1 0.572 60.8

OHC-572-2 0.572 62.7

OHC-572-3 0.572 63.0

OHC-572-4 0.572 68.9

OHC-572-5 0.572 65.0

OHC-572-6 0.572 59.9

OHC-572-7 0.572 59.2

OHC-572-8 0.572 60.9

OHC-572-9 0.572 67.1

OHC-572-10 0.572 64.4

Specimen I.D. % Weight Gain

OHC Strength 

(ksi)

OHC-754-1 0.754 63.0

OHC-754-2 0.754 63.5

OHC-754-3 0.754 62.7

OHC-754-4 0.754 60.1

OHC-754-5 0.754 64.7

OHC-754-6 0.754 65.1

OHC-754-7 0.754 63.9

OHC-754-8 0.754 63.7

OHC-754-9 0.754 66.5

OHC-754-10 0.754 50.3

OHC-894-1 0.894 65.1

OHC-894-2 0.894 57.3

OHC-894-3 0.894 64.2

OHC-894-4 0.894 66.1

OHC-894-5 0.894 62.6

OHC-894-6 0.894 64.3

OHC-894-7 0.894 65.2

OHC-894-8 0.894 65.0

OHC-894-9 0.894 63.5

OHC-894-10 0.894 62.5

OHC-SAT-1 1.20 62.0

OHC-SAT-2 1.20 62.9

OHC-SAT-3 1.20 60.3

OHC-SAT-4 1.20 61.9

OHC-SAT-5 1.20 61.6

OHC-SAT-6 1.20 60.7

OHC-SAT-7 1.20 59.3

OHC-SAT-8 1.20 63.1

OHC-SAT-9 1.20 54.7

OHC-SAT-10 1.20 66.4

OHC-SAT-11 1.20 64.5

OHC-SAT-12 1.20 60.5

OHC-SAT-13 1.20 58.2

OHC-SAT-14 1.20 63.3

OHC-SAT-15 1.20 65.4

OHC-SAT-16 1.20 64.9

OHC-SAT-17 1.20 60.3

OHC-SAT-18 1.20 63.2

OHC-SAT-19 1.20 64.8

OHC-SAT-20 1.20 66.0

Table 10. Data from OHC specimens tested at 220 °F and varying moisture content.
19

OHC Strength (ksi)

Weight Gain (%)

Unconditioned

Unconditioned Dwell Averages

0.7% Conditioned

70

65

60

55

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Figure 14. Plot of all OHC data tested at 220 °F.
20

5. CONCLUSIONS

It should be noted that all testing was below this resin’s wet Tg so no dramatic changes in 

strength were to be expected. The conclusions to be drawn from the tests of this study include the 

following:

• The OHC strength knockdown for the material and layup used in this study, which separated 

moisture effects from temperature, was at the lower end of the 10%–20% strength drop seen in 

table 1. This is not unexpected as the laminate in this study was directional, not quasi-isotropic.

• Unconditioned OHC specimens have about 5% more strength than conditioned specimens 

at 220 °F.

• The same trend in OHC strength behavior was seen in the specimens in this study as those presented in figure 1, however the magnitude of strength drop was more severe than in this study. 

An explanation for this could be the high directionality of the laminates in this study.

• OHC strength tends to drop in a linear fashion with increasing temperature. For the fiber/resin 

tested in this study, about a 10% decrease in strength was found in both conditioned and unconditioned specimens from RT to 220 °F.

• Time that the specimen dwells at temperature (up to 30 min at 220 °F in this study) has no effect 

on the OHC strength.

• When conditioned specimens are removed from the environmental chamber and placed in laboratory conditions, moisture can be slowly lost. For the material in this study, testing should be 

completed within 3.3 days to prevent a moisture loss of more than 0.01%.

• Specimens conditioned to 0.7% moisture weight gain showed essentially no difference in OHC 

strength compared to unconditioned specimens when tested at elevated temperatures. Specimens 

tested at RT actually demonstrated a slight strength increase. Actual flight hardware can realistically expect a maximum moisture weight gain of ≈0.5%.

• The OHC strength drop from RT to 180 °F wet was 10.7% which is close to the 11.1% given in 

reference 4. The drop was 12.3% from RT to 220 °F wet which is smaller than that given in reference 4 (16.6%).

• Once the moisture content has reached approximately 0.2% weight gain, additional moisture gain 

has an insignificant effect on the OHC strength for the material and layup sequence examined in 

this study.
21

• For the relatively small drop in ‘hot-wet’ notched strength for the specimens in this study, the cost 

and schedule of testing conditioned specimens should be balanced with the actual fidelity of the 

allowable that is needed. Other knockdown factors such as factor of safety, A-Basis, and damage 

tolerance will all contribute much more to the lower allowable value used to design the structure.

• Weight savings on the interstage are not likely to be realized by assessing realistic conditions as 

opposed to harsh aircraft environment since such little knockdown (≈5%) was caused by the 

environmental conditioning.
22
23

REFERENCES

1. Adams, Don: “Open-Hole Compression Testing,” High Performance Composites, March, 2005. 

<http://www.compositesworld.com/articles/open-hole-compression-testing.aspx>.

2. Strong, A.B.: Fundamentals of Composites Manufacturing: Materials, Methods and Applications, 

2nd ed.,Chapter 20: “Damage Prevention and Repair,” Society of Manufacturing Engineers 

(SME), Dearborn, MI, 2007.

3. ASTM Standard D 5229M-92 (Reapproved 2004), “Standard for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite Materials,” American Society 

for Testing and Materials, West Conshohocken, PA 19428, March 2004.

4. Composite Materials Handbook-17F, Polymer Matrix Composites, Volume 2, “Material Properties,” Materials Sciences Corporation, Horsham, PA 19044, June 2002.

5. ASTM Standard D 6484-99 (Reapproved 2004), “Open Hole Compression Strength of Polymer Matrix Composite Laminates,” American Society for Testing and Materials, West Conshohocken, PA 19044, April 2004.

6. Document NAI-1504 (Revision C), “Preimpregnated Carbon/BMI Material for 375 °F Service,” 

Northrop Corp., Aircraft Division, Hawthorne, CA, May 1988.

7. Callus, P.J.: “The Effects of Hole-Size and Environment on Mechanical Behaviour of a QuasiIsotropic A54/3501-6 Laminate in Tension, Compression, and Bending,” DSTO–TR—2077, 

AR–014–060, Australian Department of Defense, Air Vehicles Division, November, 2007.

8. Coguill, S.L.; and Adams, D.F.: “A Comparison of Open-Hole Compression Fixtures by Experimental Evaluation,” Proceedings of the 45th International SAMPE Symposium, Long Beach, 

CA, May 2000, pp. 1095-1105.

9. Composite Materials Handbook-17F, Polymer Matrix Composites, Volume 1, “Guidelines,” 

Materials Sciences Corporation, Horsham, PA 19044, June 2002.
24

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Hot/Wet Open Hole Compression Strength of Carbon/Epoxy 

Laminates for Launch Vehicle Applications

A.T. Nettles

George C. Marshall Space Flight Center

Marshall Space Flight Center, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 24

Availability: NASA CASI 443–757–5802

Prepared by Materials Processes and Manufacturing Department, Engineering Directorate

M–1259

Technical Memorandum

NASA/TM—2009–215900

Composites, hot/wet, environmental effects, open hole compression

01–06–2009

UU 32

NASA

U U U

This Technical Memorandum examines the effects of heat and absorbed moisture on the open hole 

compression strength of carbon/epoxy laminates with the material and layup intended for the Ares 

I composite interstage. The ‘knockdown’ due to temperature, amount of moisture absorbed, and the 

interaction between these two are examined. Results show that temperature is much more critical than 

the amount of moisture absorbed. The environmental knockdown factor was found to be low for 

this material and layup and thus obtaining a statistically significant number for this value needs to be 

weighed against a program’s cost and schedule since basis values, damage tolerance, and safety factors all 

contribute much more to the overall knockdown factor. 

STI Help Desk at email: help@sti.nasa.gov

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