NASA / TM--2001-210799

Permeability Testing of Impacted

Composite Laminates for Use

on Reusable Launch Vehicles

A. T. Nettles

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

National Aeronautics and

Space Administration

Marshall Space Flight Center • MSFC, Alabama 35812

February2001
TRADEMARKS

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Available from:

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TABLE OF CONTENTS

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

2. EXPERIMENTAL PROCEDURES ...............................................................................................

2.1 Specimens ...............................................................................................................................

2.2 Impact Testing .........................................................................................................................

2.3 Leak Check ............................................................................................................................

2.4 Permeability Testing ............................................................................................................

3. RESULTS .......................................................................................................................................

3.1 Impact Testing .........................................................................................................................

3.2 Leak Check Testing .................................................................................................................

3.3 Permeability Testing ................................................................................................................

4. CONCLUSIONS ............................................................................................................................

REFERENCES ...................................................................................................................................

1

2

2

2

3

4

13

13

15

16

19

21

iii
l°

2.

3.

4.

.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

LIST OF FIGURES

Schematic of impact apparatus ............................................................................................

Schematic of leak detection apparatus .................................................................................

Specimen displaying a leak using a bubble-type leak detection fluid .................................

Sketch of permeability apparatus ........................................................................................

Schematic of liquid slug in glass tube .................................................................................

Flow rate results of vertical and slanted glass tube .............................................................

Flow rates for specimen 6B using various liquids for slugs ................................................

Flow rate using different lengths of liquid slugs for specimen 5A ......................................

Flow rate measured on specimen 5A using different size tubes ..........................................

Flow rate measured on specimen 6B using different size tubes ..........................................

Plot of viscous pressure on slug as a function of flow rate .................................................

Increase in pressure as a function of flow rate ....................................................................

Comparison of driving and retarding (viscous) pressures for a 3-ram tube ........................

Comparison of driving and retarding (viscous) pressures for a 1.2-mm tube .....................

Comparison of driving and retarding (viscous) pressures for a 0.4-mm tube .....................

Surface views of impacted specimens .................................................................................

Leak check images ...............................................................................................................

Flow rate (permeability) versus applied pressure for specimen 3A ....................................

Flow rate (permeability) versus applied pressure for specimen 3B ....................................

Flow rate (permeability) versus applied pressure for specimen 4A ....................................

2

3

3

5

6

8

8

9

9

9

10

12

12

12

12

13

15

17

17

17

iv
21.

22.

23.

24.

LIST OF FIGURES (Continued)

Flow rate (permeability) versus applied pressure for specimen 4B ....................................... 17

Flow rate (permeability) versus applied pressure for specimen 5A ....................................... 17

Flow rate (permeability) versus applied pressure for specimen 5B ....................................... 17

Flow rate (permeability) versus applied pressure for specimen 6B ....................................... 18

TECHNICAL MEMORANDUM

PERMEABILITY TESTING OF IMPACTED COMPOSITE LAMINATES

FOR USE ON REUSABLE LAUNCH VEHICLES

1. INTRODUCTION

As composite laminates are being considered for use in liquid propulsion systems, microcracking

due to foreign object impact damage becomes very important, especially if a tank or feedline is to be

unlined. If a component that carries liquid or gaseous hydrogen develops an area of microcracking,

hydrogen can leak out of the component and pose a serious threat to the vehicle. Since it has been shown

that nonvisible impact damage can cause a composite feedline to leak, I a better understanding of the

material's resistance to microcracking is needed. Microcracking can also occur due to thermal and

mechanical stresses and fatigue; however, this study will deal only with foreign object impact damage, a

very real threat to all composite parts. The most quantifiable way of determining how much leakage may

occur after an impact event is to test the material for permeability. Permeability testing has been used in

the past on composites to determine the porosity of rocket nozzle material. 2 Fluid permeability has been

tested on some composite structures to be used as fuel tanks. 3,4 ASTM D 1434, Standard Test Method for

Determining Gas Permeabilin' Characteristics _f Plastic Fihn and Sheeting, exists for gas permeability

testing of plastic film and sheeting, and it is from this test methodology that the one in this study was

adapted.

For this study, prepreg composed of IM7 fiber in a five-harness weave architecture impregnated

with Bryte Technologies EX 1522 epoxy resin was used to construct four-ply laminates with the layup

sequence (0/90,0/90) s. These specimens were representative of one of the carbon/epoxy systems being

evaluated for use in constructing feedlines for future launch vehicles. A drop-weight impact tester with

a 0.25-in.-diameter tup was used to impart varying levels of damage from ah-nost nondetectable to near

penetration. The specimens were then secured in a fixture that could supply a positive pressure of helium

(He) gas on one side and allow a leak detection solution to be applied to the other side. This gave a

qualitative assessment of permeability-after-impact. The specimens were then secured in an apparatus

that could give quantitative results. Since this testing technology is new, a great deal of test verification

was performed and will be presented in this study.
2. EXPERIMENTAL PROCEDURES

This section will explain how the impact and subsequent permeability testing were performed.

Results will be given in section 3.

2.1 Specimens

The specimens were manufactured from carbon/epoxy prepreg, which was in a five-harness

satin weave form. A 36 x 24 in. panel was laid up in a bidirectional configuration on a flat aluminum

tool. This gave the laminate a layup sequence of (0/90, 0/90) s. The panel was then vacuum bagged and

autoclave cured according to the manufacturer's recommended cure cycle. The cured laminate was then

cut into 3 x 3 in. specimens. The nominal thickness of the specimens was 10 mils.

2.2 Impact Testing

The 3-in.-square specimens were impacted at various levels using a drop-weight apparatus. The

impactor consisted of a 0.25 in. semispherically ended tup that was attached to a dynamic load cell to

gather instrumented impact data, should it be needed for future analysis. (None of the instrumented data

were used in the study presented in this paper). The falling mass had a total weight of 2.51 lb and drop

heights of 4, 6, 8, 10, and 12 in. were used. The specimen was simply supported over a 2-in.-square

opening. A schematic of the impact setup is given in figure 1. Two specimens were impacted from each

drop height to give a total of 10 impacted specimens in order to check for repeatability of results. After

each specimen was impacted, images of both the front and back surface damage were recorded with a

digital camera at a magnification of approximately X 5.

_'___,_ VelocityFlag

_Velocity Detector

\

SpecimenSupport

Figure I. Schematic of impact apparatus.
2.3 Leak Check

After all specimens had been impacted and the surface damage recorded, the specimens were

mounted in an apparatus to check for leakage of helium gas when a positive pressure was applied to one

side. A "bubble-type" leak detector solution was used, and a digital image was made of the leaks for

each specimen. A schematic of the apparatus used to check for leaks is shown in figure 2. A sample

image of a leak is given in figure 3.

Specimen

Neoprene Gaskets

Figure 2.

_--_- Pressurized Helium

Gas Inlet

Schematic of leak detection apparatus.

Figure 3. Specimen displaying a leak using

a bubble-type leak detection fluid.
2.4 Permeability Testing

After the specimens had thoroughly dried from the leak detection procedure, they were ready for

permeability testing. This section will be devoted to a discussion of the apparatus used and its limitations

and verification.

2.4.1 Permeability Apparatus

The apparatus used in this study is based loosely on ASTM D 1434, using the volumetric

technique. This apparatus could have been used, but modifications would have to be made for the larger

specimen size needed (to include all the impact damage) and a much more simple apparatus could be

used since relatively high rates of permeance were being measured. The apparatus basically consisted of

a specimen holder that isolated the two surfaces and allowed a positive pressure of gas to be applied to

one side and the gas that leaked through the specimen to escape out the other side. The escaping gas rate

was measured via a volumetric displacement method using a liquid "slug" in glass tubes of various

diameters. The rate-of-volume change was measured with a stopwatch so the amount of gas escaping

through the specimen could be calculated in volume-per-unit time.

It should be noted at this point that, strictly speaking, the term "permeability" can only apply to

homogeneous materials and is defined as the "permeance" times the specimen thickness. Permeance is

defined as the transmission rate per applied pressure of gas, which is what is being measured in this

paper. However, the term "permeability" has been used in the composites industry much more than

"permeance" to indicate a transmission rate, and it will be used in this paper in order to avoid confusion.

A sketch of the apparatus is given in figure 4 to aid in the discussion that follows. A test or "run"

was typically conducted in the following manner.

The 3 x 3 in. square composite specimen was centered and sandwiched between the top and

bottom aluminum plates. A small amount of vacuum grease was placed around the edges of the specimen

to ensure a good seal between the specimen and the neoprene gaskets. The eight bolts and nuts were then

used to secure the specimen between the two plates. Each bolt had 100 in.-lb of torque applied. The

specimen holder was then secured to a cantilever beam above a laboratory table with a C-clamp. An inlet

hose from a helium supply tank was then inserted over the bottom nipple to supply the positive pressure

to the specimen. A precision pressure gauge was included in the supply line so an accurate reading of the

amount of pressure being applied to one side of the specimen could be determined. Typical values of

applied pressure ranged from 3 to 50 psi. A liquid "slug" or "blank" was then introduced into the glass

tube. Alcohol was usually used as the liquid and enough to introduce a slug length of approximately

3-10 in. was inserted into the top of the tube. The alcohol was then allowed to fall into the bottom

U-shape of the Tygon ® tubing that was snugly attached to the glass tube. The other end of the tubing

was then attached to the upper nipple of the specimen holder to channel all leaking gas into the tubing

and force the slug into and up the glass. The slug was allowed to reach steady state (constant velocity)

and then the time needed to travel a given distance was recorded. This procedure was repeated for

various applied pressures of helium so a pressure versus flow rate chart could be created. Section 2.4.2

will present equations and experimental results to show how various parameters could affect the flow

rate data.
_ Ruler

Glass Tube

Liquid Slug

Specimen Holder

_ __ ExplodedView

_ of Specimen

/v _ Holder

Pressure I _,_ I

Gauge __J

Aluminum

Top Plate

RecessedArea

(On Both Plates

NeopreneGaskets

/

1/4 in. Hose Nipple

Aluminum

Bottom Plate

Figure 4. Sketch of permeability apparatus.
2.4.2 Analysis of Apparatus

From past experience and intuitive deductions, it was determined that certain key variables in

performing the test may affect the outcome of the flow rate data. These variables were examined using

equations of laminar flow to see what variables would most likely affect the data. Figure 5 will be useful

in the following discussions.

Tube

Directionof

Movement

He21He2

He2| He2

He_1

He2 He2

LiquidSlug

Figure 5. Schematic of liquid slug in glass tube.

Since the end of the tube is unrestricted, P2 will always equal atmospheric pressure Pa" At the

beginning of the experiment, the volume between the top of the specimen and the slug is also at

atmospheric pressure (i.e., Pl also equals Pa)" Thus the additional force needed to keep the slug in the

tube from falling back down is given by

F = 2rr2Lpg ,

where

r

L=

p=

g =

inner radius of glass tube

length of liquid slug

density of slug liquid

acceleration due to gravity.

Thus the additional pressure needed is F/area = F/n'r 2, or

p_ = Lpg .

(I)

(2)

6
Thisequationindicatesthatthepressureneededto keeptheslugfrom falling backdownthetubeis

proportionalto thesluglengthandtheliquid's density.Thusto minimizethis term,ashortslug length

anda low densityliquid aredesired.It shouldbenotedthatif thetubeis slantedtowardthehorizontal

position,lessof theslugliquid will beactingin adownwardmannerandL will be lessened by the

amount sin0 where 0 is the angle the glass tube makes with the horizontal position.

Assuming the slug to be alcohol, the pressure needed to keep the slug from falling back down

is given by

pg = 0.0289(L),

where L is slug length in inches. Thus assuming a 3-in. slug length, p_ will be on the order of 0.086 psi,

making Pt --14.786 psi.

Once the slug is moving at a constant velocity up the tube (steady state), the viscous forces

of the liquid need to be overcome. This force is given by

F = 87tTlLv ,

where

L--

viscosity of liquid used for slug

length of slug

velocity of slug traveling up the glass tube.

Thus the additional pressure needed to overcome the viscous forces of the slug is F/m "2, or

8rlLv

Pq- 2

I"

This equation indicates that the pressure needed to overcome the viscous forces of the slug are

proportional to the liquid's viscosity, slug length, and velocity, and inversely proportional to the square

of the radius of the tube. Thus to minimize this term, a low viscosity liquid, a short slug length, a low

slug velocity, and especially a large tube radius are all desired.

The helium leaking through the specimen and filling the volume of the permeability apparatus

provides the force (or pressure) necessary to overcome the viscous forces induced by the liquid slug.

If this pressure is not large compared to the pressure needed to overcome the viscous forces, then values

of flow rate measured will be too small.

As a check for possible errors introduced by these forces, some experiments were carried out

varying the angle of the tube, the liquid in the tube, the length of the liquid slug, and the tube size.

The results of these experiments follow.

(3)

(4)

(5)
2.4.2.1 Vertical Versus Slanted Glass Tube. It was determined early that if the glass tube

could be laid down at a slant, rather than being vertical, the testing would be much simpler. Thus a

series of tests was performed with the glass tube in a vertical position and the glass tube laid down at

an angle of 5 deg. Specimen 6B was used for all the tests and alcohol was used as the liquid for the

slug. Excellent repetition of data was obtained and typical results from a vertical versus a slant tube

are shown in figure 6. Since no differences where found, all subsequent tests were performed with the

glass tube placed on a wooden board angled at 5 deg.

0.0035

0.0030

-_-0.0025

0.0020

"_ 0.0015

0.0010

0.0005

I •n Rate (Slant) (Vertical) I

Specimen6B

[3

[]

io

0 IIIII1''=1='''1'='=1''='

0 10 20 30 40 50

Pressure (psi)

Figure 6. Flow rate results of vertical and slanted glass tube.

2.4.2.2 Type of Liquid Used as a Slug. Alcohol was typically used as the liquid for the slug,

but the use of water and alcohol with PhotoFlo ® to reduce surface tension was examined. Again,

specimen 6B was used for these tests and the blank length kept at --3 in. Results are shown in figure 7.

Since no difference was found for the liquid used as the slug, alcohol was chosen for subsequent testing

due to its ease of cleaning and fast evaporation rate.

0.005

0.004

X

_' 0.003

0.002

gC

0.001

11=

• Rate (Water) I

[] Rate (Alcohol) I +

+ Rate (Alcohol+Photo FIo)

Specimen 6B _.

#

a'"

iq_lj,,,I,,,,I.,,,I,,.,I,,,,

0 10 20 30 40 50 60

Pressure (psi)

Figure 7. Flow rates for specimen 6B using various liquids for slugs.

8
2.4.2.3 Length of Slug. It is extremelydifficult to obtainaconsistentsluglengthwhenplacing

theliquid in theglasstube.Thusa seriesof testswasperformedto seeif thesluglengthaffectedthe

measuredflow rate.Specimen5A wasusedforthesedata.Theresultsareshownin figure8. Nodifferencein measuredrateswasnoted,sosluglengthwasnotafactorandanyconvenientlengthcouldbeused.

0.0010

0.0008

0.0006

,m

_0.0004

0.0002

0

0

• 14-in. Slug

o 17-in. Slug

A 6-in. Slug

D 2.5-in. Slug

Specimen 5A

|

, *illlm,,.li**llmm|*Imm,m

5 10 15 20 25 30 35

Pressure (psi)

Figure 8. Flow rate using different lengths of liquid slugs for specimen 5A.

2.4.2.4 Size of Glass Tube. Three different sizes of glass tubes were used having inner diameters

of 0.4, 1.2, and 3 mm. For a given flow rate, the slug will obviously travel slower in the larger diameter

tube and more rapidly in the smaller diameter tube. Since the location of the slug at various times needs

to be measured, the slug cannot be travelling too fast or accurate readings cannot be achieved. Also, if the

flow rate is small, it can take a very long time to get a reading, unnecessarily slowing the data acquisition

process. To examine the affect of tube size on flow rate, three tube sizes were used to measure the flow

rate on specimen 5A. The results are presented in figure 9. Since a difference is clearly noted for the

0.4-mm tube, this test was performed on specimen 6B. The data are given in figure 10. Again the data

show a much lower measured flow rate for the 0.4-mm tube, whereas the 3-mm and 1.2-mm tubes give

excellent agreement.

0.0010

0.0008

w

0.0006

_= 0.0004

0.0002

0o

• 3-ram Tube I

,, 1.2-ram Tubem

[] 0.4-ram Tubem

Specimen 5A

[]

[]

n

[]

&

,AI_, _, , ..... m.... I.... I .... i....

5 10 15 20 25 30 35

Pressure (psi)

Figure 9. Flow rate measured on specimen 5A

using different size tubes.

0.0025

0.0020

r,J

0.0015

,a

'_ 0.0010

re,

0.0005

•

A

3-mm 1.2-mmTube TubemI

[] 0.4-mmTubel

Specimen 6B

61

[]

[]

[]

..a'

0 ,lla_^.A_ ,A.4,i4,4_ .... i .... , .... i ....

5 10 15 20 25 30 35 40

Pressure (psi)

Figure 10. Flow rate measured on specimen

6B using different size tubes.

9
Usingactualdata,thepressureneededto overcometheviscousforcesof theslugcanbefound

fromequation(5).Theresultsfor thethreetubesizesusedareshowngraphicallyin figure 11asa

functionof flow ratev. A slug length of I 0 in. was assumed. The liquid was assumed to be isopropyl

alcohol at room temperature. This plot shows just how critical the size of the tube can be when it comes

to viscous forces. Not only does the smaller tube give rise to much higher viscous forces for a given flow

rate, but for a given specimen the flow rate will be highest in the smallest tube, further increasing the

viscous forces that need to be overcome.

0.6

0.5

_. 0.4

.=

_ O.3

O.

.-_ 0.2

0.1

0

0

I • 3-mmViscousPressure |

I [] 1.2-mmViscousPressure| j

I A O.4-mmViscousPressureU

/ 1.2-mmTube ._____--_

/ [ub.e.

0.2 0.4 0.6 0.8 .0

FlowRate(in./sec)

Figure 1 I. Plot of viscous pressure on slug as a function of flow rate.

From the ideal gas law

where

p m

nRT

V

(6)

p = pressure in the system

n = number of moles of gas in the system

R = universal gas constant

T = absolute temperature

V = volume of system.

At the start of the test, p = 14.7 psi (plus a small amount to keep the slug from falling back down the

tube) and V is =1 in. 3. Using these values as the starting point for the tests, the original number of moles

of gas in the system turns out to be =6.8185 × 10-4 moles. As more moles of gas are added (as the

helium leaks in through the specimen), the volume of the system expands and/or the pressure increases.

For a given flow rate, the increase in volume is given by

10
vi = ftzcr 2 , (7)

where

f=

t =

increase in volume

flow rate (in./sec)

time of flow (sec)

radius of tube (in.).

The increase in the number of moles of gas in the volume is given by:

f

II i

1,367 in3 / tool

(8)

where

ni

f

1,367 in.3/mol

= increase in moles of gas in the volume

= flow rate (in./sec)

= volume of one mole of an ideal gas.

Thus the increase in pressure is given by equation (6) with the new volume and number of moles added in

73.58 in.- lb )(293K)

Pi = (6.8185 X 10 4 tool + n i ) tool - K

(lin. 3 +vi)

- 14.7 psi (9)

A plot of increase in pressure as a function of flow rate for the three tube sizes is given in

figure 12. This was based on an elapsed test time of 60 sec and assumes the beginning volume was I in. 3.

As mentioned earlier, the pressure generated by the increase in the number of moles of helium

entering the volume must be greater than the viscous pressures or error will be introduced. As a check,

data from figures 11 and 12 are superimposed for each size tube and presented in figures 13-15. The

increase in pressure due to the added helium is termed the "driving" pressure and the pressure due to

viscous forces is termed the "retarding" pressure.

In figure 13, it can be seen that at any flow rate the driving pressure will always be much greater

than the viscous pressures, so very little error is introduced if the pressures due to slug viscosity are

ignored. In figure 14, the driving pressure is higher than the viscous pressure, so little error should be

measured when using this size tube; although, the difference here is not as great as that for the 3-mm tube.

Figure 15 shows that the viscous pressures are always much larger than the driving pressures, thus error

will be introduced into the flow rate measurements (they will be low). This is clearly demonstrated in the

data from figures 9 and 10. Thus the 0.4-ram tube cannot be used with the specified parameters in this

study.

11
0.5

0.4

0.3

p-

_ 0.2

O0.1

Figure 12.

3-mm Tube I

-i1.2-ram Tube I

"_'- 0'4-ram Tube I j

t= 60 sec

/

0.2 0.4 0.6 0.8 1.0

Flow Rate (in./se¢)

Increase in pressure as a function of

flow rate.

0.5

0.4

_0.3

=

_, 0.2

r4

0.1

Figure 13.

3-mm Viscous Pressure |

-o- Pressure Increase (3-mm Tube)P

t= 60 see .,_

Slug Length= 10 in.

_r

/ _ . Retarding Pressure

i i , | , • • I • A . I . I i i '

0.2 0.4 0.6 0.8 1.0

Flow Rate (in./se¢)

Comparison of driving and retarding

(viscous) pressures for a 3-mm tube.

0.12

0.10

1.2-ram Viscous Pressure II

Pressure Increase (l.2-mm Tube)|/,I

i

" t = 60 see ,._

._0.08o. "- Slug Length = 10 in._

Drivin

" 0.04

0.02 ____ RetaldngPrelsure

0 0.2 0.4 0.6 0.8 1.0

Flow Rate (in./sec)

Figure 14. Comparison of driving and retarding

(viscous) pressures for a 1.2-mm tube.

0.6

0.5

•--_0.4

O.

._ 0.3

m 0.2

0.1

iii/

-o- 0.4-ram Viscous Pressure I1

"'l -0- Pressure Increase (0.4-mm Tube)II/I'-

" t=eOsec /l

SlugLength= 10 in.

J Driving Pressure

, , , m • - - _ . El . . . P . . j

0 0.2 0.4 0.6 0.8 1.0

Flow Rate (in./sec)

Figure 15. Comparison of driving and retarding

(viscous) pressures for a 0.4-mm tube.

12
3. RESULTS

This section will present results of the impact, leak check, and permeability testing.

3.1 Impact Testing

A total of 10 specimens was impacted, 2 at each of five energy levels. The resulting visual

surface damage is presented in figure 16. Duplicates are not presented since the visual damage was

nearly identical in every case. Damage of some form can be noted on all specimens, even those

impacted at the smallest impact energy of 0.84 ft-lb.

(a) Front, specimen 2, 2.51 ft-lb (b) Back, specimen 2, 2.51 ft-lb

(c) Front, specimen 3, 2.09 ft-lb (d) Back, specimen 3, 2.09 ft-lb

Figure 16. Surface views of impacted specimens.

13
(g) Specimen 5A, i.26 ft-lb (h) Specimen 5B, 1.26 ft-lb

(i) Specimen 6A, 0.84 ft-lb (j) Specimen 6B, 0.84 ft-lb

Figure 17. Leak check images (continued).

All samples, with the exception of 6A, showed leakage. The larger the bubbles are in the pictures, the higher the leak rate. Specimen 2B was difficult to photograph due to the extremely large

bubbles that were forming. Specimen 2A can be seen to rapidly expel the leak detect solution and form

relatively large bubbles. As the impact damage becomes less severe, the bubbles become smaller. In fact,

in specimens 5A and 6B, the leak rate is such that the leak detection fluid forms a fine "foam" that

emanates from the impacted area.

3.3 Permeability Testing
0.025 0.035

Specimen 3A

Impact Energy = 2.09 ft-lb p

0.020 /

Rate = 4.92e-5 P

0.005

0 J J I I -'-_l I I l/L-X"'"Irl I I _ _ _ I I _ _ _ I I I I I

0 5 10 15 20 25 30

Pressure (psi)

Figure 18. Flow rate (permeability) versus applied

pressure for specimen 3A.

A

_" 0.015

._

" 0.010

0.007

" Specimen 4A

0.006 - Impact Energy = 1.67 ft-lb /4

/

Rate = 7.79e-6 p2-1.41e-5 P- 6.45e-5 /

0.002

0.001 i

0 , , _ _gP-.r_-, _ J I , , , , I , , , , I , _ , _ I _ , , , I _ _ , ,

0 5 10 15 20 25 30 35

Pressure (psi)

Figure 20. Flow rate (permeability) versus applied

pressure for specimen 4A.

0.005

_. 0.004

c

0.003

0.0010

0.0008

0.0006

0.0004

Specimen 5A

Impact Energy = 1.26 ft-lb

aeo

0.0002

0 i i I J i i I i i i J i i | i | i i | i I i i i i I | i i i

0 5 10 15 20 25 30 35

Pressure (psi)

Figure 22. Flow rate (permeability) versus applied

pressure for specimen 5A.

A

o,

0.030

0.025

0.020

0.0151

0.010

0.005

0

0

Figure

Specimen 3B

Impact Energy = 2.09 ft-lb

/

Rat

I ' ' ' I , , , , I , . , , I , , . , I . . i i I , • , ,

5 10 15 20 25 30

Pressure (psi)

19. Flow rate (permeability) versus applied

pressure for specimen 3B.

0.0035

0.0030

0.0025

0.0020

0.0015

0.0010

0,0005

Specimen 4B

Impact Energy = 1.67 ft-lb )

/

Rate = 1.84e-6_

0 .... I .... I .... I .... | ....

5 10 15 20

Pressure (psi)

Figure 21. Flow rate (permeability) versus applied

pressure for specimen 4B.

25

9 10-5

Specimen 5B

8 10-5

3 10-5

2

2 10-5 _ Rate =-1.79e-9 p2 + 1.91e-6 P-2.42e-5

110-5 i i i i I i i i i I i i i J I J i i i I J i i i

20 30 40 50 60

Pressure (psi)

Figure 23. Flow rate (permeability) versus applied

pressure for specimen 5B.

7 10-5

_. 610 -5

510-5

4 10-5

7O

17
0.0040

0.0035

A0.0030 .E

E

0.0025

.m

0.0020

0.0015

0.0010

0.0005

0

0

Specimen 6B /8

Impact Energy = 0.84 ft-lb

R

z _ i I I i i i ' ' ' I , , J , I , , , , I , , , ,

10 20 30 40 50 60

Pressure (psi)

Figure 24. Flow rate (permeability) versus applied

pressure for specimen 6B.

In general, the larger the impact energy the higher the flow rate for a given applied pressure,

which is expected. A noticeable exception is specimen 5B, which showed a very low flow rate, even

though it was hit harder than specimen 6B. The amount of nonlinearity in flow rate versus applied

pressure also varied between samples; however, most of the nonlinearity is observed at the lower pressures and as the applied pressure increased, the linearity of the flow rate versus pressure increased.

18
4. CONCLUSIONS

A simple test apparatus can be used to measure permeability of impact damaged composite

specimens, as long as the flow rate through the sample is not too great. Using the apparatus presented in

this study, it was demonstrated that tube size is critical in obtaining proper flow rate readings. Too small

of a tube can give rise to unacceptable levels of viscous forces on the liquid "slug" inside the tube,

resulting in flow rate measurements that are too small. The type of liquid, length of the slug, and angle

that the tube makes with the horizontal were all shown to have little or no affect on the flow rate readings.

For permeability-after-impact testing, the residual flow rate usually has a nonlinear dependence on

the applied pressure, increasing more rapidly as a higher pressure is applied. Thus the permeability cannot

be stated as a constant per unit of applied pressure.

The four-ply laminates tested in this study showed leakage after impact, even when visible damage

could only be detected with magnifying techniques.

The qualitative measurement of leakage with the bubble-type leak detector solution corresponded

with the qualitative permeability measurements.

19

REFERENCES

° Nettles, A.T.: "Impact Damage Resistance of Carbon/Epoxy Composite Tubes for the DC-XA

Liquid Hydrogen Feedline," NASAFFP--3583, September 1995.

. Stokes, E.H.: "Permeability of Rayon Based Polymer Composites," Proceedings of the First

Symposium on Computational Mechanics of Porous Materials," Tucson, AZ, pp. 45-51,

April 28-May 8, 1992.

. Brydges, W.T.; Gulati, S.T.; and Baum, G.: "Permeability of Glass Ribbon-Reinforced

Composites," Journal of Materials Science, Vol. i 0, pp. 2044-2049, December ! 975.

. Murry, C.E; et al.: "Development of Composite Pressure Vessels With Nonmetallic Liners," The

28th AIAA, SAE, ASME, and ASEE Joint Propulsion Conference and Exhibit, Nashville, TN,

pp. 14-21, July 6-8, 1992.

21
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1. AGENCY USE ONLY (Leave Blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

February 2001 Technical Memorandum

4. TITLEANDSUBTITLE 5. FUNDING NUMBERS

Permeability Testing of Impacted Composite Laminates for Use

on Reusable Launch Vehicles

6. AUTHORS

A.T. Nettles

7. PERFORMINGORGANIZATIONNAMES(S) ANDADDRESS(ES)

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

9. SPONSORING/MONITORING AGENCYNAME(S)ANDADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

8. PERFORMING ORGANIZATION

REPORT NUMBER

M-1001

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA/TM--2001-210799

11. SUPPLEMENTARY NOTES

Prepared by Materials, Manufacturing, and Processes Depa_ment, Engineering Directorate

12a. DISTRIBUTIONIAVAILABlUTY STATEMENT

Unclassified-Unlimited

Subject Category 24

Nonstandard Distribution

12b. DISTRIBUTION CODE

13. ABSTRACT (Max/mum 200 words)

Since composite laminates are beginning to be identified for use in reusable launch vehicle

propulsion systems, an understanding of their permeance is needed. A foreign object impact

event can cause a localized area of permeability (leakage) in a polymer matrix composite, and it

is the aim of this study to assess a method of quantifying permeability-after-impact results. A

simple test apparatus is presented, and variables that could affect the measured values of

permeability-after-impact were assessed. Once it was determined that valid numbers were being

measured, a fiber/resin system was impacted at various impact levels and the resulting

permeability measured, first with a leak check solution (qualitative) then using the new apparatus

(quantitative). The results showed that as the impact level increased, so did the measured

leakage. As the pressure to the specimen was increased, the leak rate was seen to increase in a

nonlinear fashion for almost all the specimens tested.

14. SUBJECT TERMS

composite materials, impact, permeability

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