NASA TECHNICAL MEMORANDUM 100634 AVSCOM TECHNICAL MEMORANDUM 88-8-012

LOADING RATE SENSITIVITY OF OPEN HOLE COMPOSITES IN COMPRESSION

Steve Jw Lubowinski, E. G. Guynn, Wolf Elber, and Jw D. Whitcomb

August 1988

National Aeronautics and Space Administration

Langley Research Center Hampton, Virginia 23665

US ARMY AVIATION SYSTEMS COMMAND AVIATION RAT ACTIVITY

https://ntrs.nasa.gov/search.jsp?R=19880016106 2020-06-30T15:18:00+00:00Z

INTRODUCTION

The high strength-to-weight ratio of composites makes these materials

ideally suited for aerospace applications where they are used in aircraft

secondary structures and are under consideration for heavily 1 oaded primary

structures. Previous research has shown that the composite compressive

strength is reduced by local discontinuities such as holes and impact damage

[I]. A new generation of toughened polymer matrix materials have improved

the compressive strength and impact damage tolerance of composites [2,3,4].

However, the behavior of these viscoelastic polymers under long term loading

is not clearly understood, particularly when reinforced by re1 atively rigid

fibers in a composite structure.

This paper presents the experimental results of an exploratory study on

the compressive, time-dependent behavior of fiber reinforced polymer

composites. Matrix materials tested in this study were the epoxies 5208,

5245C, 1808 and 8551-7, as well as the thermoplastics Ultem and APC2 (Table

1). The effect of loading rate on open hole compressive strength was

determined for the six different material systems at 75 degrees F (21

degrees C) and 220 degrees F (104 degrees C). A new screening method is

described and used to evaluate the loading rate sensitivity of the material

systems. The slope of the strength versus elapsed-time-to-failure curve can

be used as an alternate to creep testing for ranking the viscoelastic

response of material systems. For the thermoplastic AS4/APC2, the effects

of laminate thickness and hole size were also determined.

EXPERIMENTAL PROCEDURES

Specimens

Several types of fiber/matrix combinations were studied. These

material s, ranging from brittle epoxies (thermosets) to toughened epoxies

and thermoplastics, are listed in Table 1, along with the laminate stacking

sequence and resin manufacturer. The specimens were six inches long by one

inch wide. The gage length was 1.5 inches. A hole was ultrasonically

machined on the centerline of each specimen using a diamond impregnated core

drill. Several laminate thicknesses and hole sizes were tested for the APC2

material. Except for 1808, all of the specimens were 48 ply quasi-

isotropic containing a 1/4 inch (0.64 cm) diameter hole. The 1808 was a 40

ply quasi-isotropic layup with a 1/16 inch (0.16 cm) diameter hole. The

thermoplastic interleave in the 1808 increased the thickness of the

laminate, requiring fewer plies to achieve the same thickness as a 48 ply

laminate without interleaf. Specimens were tested at both room temperature

(70 degrees F or 21 degrees C) and high temperature (approximately 220

degrees F or 104 degrees C).

Test Fixture

The compressive strength of composites is affected by both the

unsupported gage length and the fixture used. As part of the current study,

an improved compression fixture was developed to address the problem of

transverse motion of the fixture during compressive failures, as well as to

satisfy the desire to observe the micro behavior when notch effects were

introduced into the specimen [5] (Figure 1). The notable features of this

fixture are: the incorporation of linear bearings to maintain alignment and

resist lateral motion of the fixture; end loading of the specimen to prevent

s l ippage a t h igh loads; and r i g i d specimen clamping p l a t e s which r e s t r a i n

buck l ing . The ser ra ted g r i p p i n g surfaces o f t he clamping p l a t e s a l so

c o n t r i b u t e t o l oad i n t r o d u c t i o n through shear fo rces on the faces o f t he

specimen. This load ing reduces the p o s s i b i l i t y o f Poisson's deformations

(brooming) on the ends o f t he specimen. The open design o f t he f i x t u r e

a1 1 ows observat ion o f damage development du r ing compression 1 oadi ng .

Test procedure

I n a p r e l i m i n a r y study o f l ong term compressive loading, a standard

creep t e s t was performed. A l o a d equal t o 80 - 98 percent o f u l t i m a t e open

ho le s t reng th was app l ied t o the specimen i n 0.5 sec, then he ld u n t i l

f a i l u r e . The r e s u l t was t h a t some specimens f a i l e d instantaneously, w h i l e

o thers never f a i l e d . This method was abandoned due t o the e r r a t i c r e s u l t s

as w e l l as a concern over the l eng th o f t ime spent w a i t i n g f o r specimens t o

f a i l . An a l t e r n a t e method was developed which was more s u i t a b l e f o r

eval u a t i n g a 1 arge number o f specimens. I n t h i s new procedure, open-hole

specimens were loaded t o f a i l u r e a t d i f f e r e n t l o a d r a t e s us ing a servo-

hydraul i c t e s t i n g machine i n load c o n t r o l . The st rengths decreased w i t h

increas ing t ime t o f a i l u r e . The r a t e a t which the s t reng th dec l ined w i t h

t i m e - t o - f a i l u re i s a measure o f the v i scoel a s t i c response under compressive

load ing .

High temperature t e s t i n g was performed w i t h an environmental chamber

pos i t i oned around the t e s t f i x t u r e . Specimen heat ing was achieved us ing a

combination o f d i r e c t heat ing through p la tes attached t o the g r i p s and h o t

a i r t o main ta in a un i fo rm temperature on the specimen surface. I n i t i a l l y ,

the internal specimen temperature was monitored with thermocouples mounted

in shallow holes drilled into the specimen edge. The specimen temperature

equaled the temperature of the fixture within ten minutes due to the thermal

mass of the compression fixture. For convenience, the temperature of

subsequent specimens was determined from thermocouples taped to the specimen

with heat resistant tape.

A digital storage oscilloscope was used to monitor and collect data.

Ring gages were used to measure the displacement between the top and bottom

of the 1/4 inch (0.64 cm) diameter hole (Figure 2). The onset of damage

around the hole was indicated by the beginning of non-linearity of the

stress-di spl acement curve, as shown in Figure 3.

RESULTS AND DISCUSSION

Loading Rate Effects

Figures 4 and 5 are semi-logarithmic graphs of the failure stresses

versus el apsed-time-to-fai 1 ure, along with a 1 east-squares regression curve

through each set of data. The effect of loading rate can clearly be seen.

All the materials tested showed a lower failure strength for the slower

loading rates (longer el apsed-time-to-fail ure) . For a given material system

the strength at room temperature was greater than the strength at the higher

temperature (approximately 220 degrees F or 104 degrees C) as seen when

comparing the data in Figures 4 and 5. Since the compressive behavior is

dominated by the matrix properties and the polymer matrix material is weaker

a t e l eva ted temperatures compared t o room temperature, i t i s n o t s u r p r i s i n g

t h a t a t e l eva ted temperatures t h e s t r e n g t h o f t h e composite i s lower .

A d d i t i o n a l i n f o r m a t i o n was ob ta ined f rom t h e r a t e o f s t r e n g t h decrease.

The s lope o f t h e reg ress ion l i n e was used as a measure o f t h e m a t e r i a l ' s

s e n s i t i v i t y t o l o a d i n g r a t e e f f e c t s . A comparison o f t h i s s e n s i t i v i t y f o r

d i f f e r e n t m a t e r i a l s i s shown i n F igu re 6 where t h e s lopes o f t h e s t r e n g t h -

1 i f e curves f rom F igures 4 and 5 have been d i sp layed on a b a r graph. It

should be no ted t h a t t h e s lope o f t h e reg ress ion curve was used t o represen t

t h e response o f t h e m a t e r i a l systems.

A t room temperature t h e m a j o r i t y o f t h e m a t e r i a l s had r o u g h l y t h e same

s e n s i t i v i t y ( r a t e o f s t r e n g t h decrease) as shown i n F igu re 6. The l a r g e

s e n s i t i v i t y o f t h e 5245C da ta cou ld be due t o t h e l am ina te be ing made from

prepreg which exceeded t h e recommended s h e l f 1 i f e by two years. AS4/1808,

an i n t e r l e a v e d laminate, i s a l so i nc l uded i n t h i s f i g u r e t o compare t h e

t rends, a l though t h e da ta i s f o r a 40 p l y specimen w i t h a 1/16 i n c h (0.16

cm) d iameter h o l e i n s t e a d o f 48 p l y w i t h a 1/4 i n c h (0.64 cm) d iameter ho le .

Even w i t h t h e t he rmop las t i c i n t e r l e a v e , t h e AS4/1808 specimens behaved much

1 i k e a t y p i c a l b r i t t l e thermoset (e.g. 5208) i n a l l regards.

A t t h e h i g h e r temperature Ultem and 8551-7 showed more s e n s i t i v i t y t o

l o a d i n g r a t e than t h e o t h e r t h r e e systems (F igu re 6 ) . Ultem, an amorphous

thermop las t i c , showed t h e g r e a t e s t s e n s i t i v i t y which can be expected

cons ide r i ng t h e n o n c r y s t a l l i n e na tu re o f t h e m a t r i x m a t e r i a l . 8551-7, a

toughened m a t r i x , d i sp layed a s i m i l a r s e n s i t i v i t y . 8551-7 achieves i t s

toughness by second-phase rubber p r e c i p i t a t e s a t t he p l y i n t e r f a c e ; these

rubber p a r t i c l e s may be responsib le f o r the increased l oad ing r a t e

s e n s i t i v i t y a t t he h igher temperature. The remaining th ree mater i a1 s had

s i m i l a r slopes i n d i c a t i n g the same s e n s i t i v i t y a t t h e h igher temperature.

From a design v iewpoint , t he s e n s i t i v i t y t o t ime under l o a d cou ld r e s u l t

i n a lower a l lowab le s t ress f o r app l i ca t i ons w i t h susta ined load ing . F igure

7 i s a p r o j e c t i o n o f t h i s s t reng th decrease fo r a 1,000,000 second (12 day)

t e s t compared t o a more t y p i c a l t e s t of 60 seconds. The regress ion 1 ines i n

f i g u r e 4 and 5 were used t o ex t rapo la te the s t rengths . The r e s u l t a n t

rank ing o f t he ma te r ia l s i s s i m i l a r t o those obta ined us ing the s lope o f t he

s t r e n g t h - l i f e curve. A t room temperature most ma te r i a l s showed a s t reng th

decrease o f 11 t o 12 percent . The APC2 was s l i g h t l y lower (7 percent) w h i l e

t he 5245C was almost double a t 20 percent. A t t he h igher temperature t h e

s t reng th decrease o f Ultem and 8551-7 (17 t o 21 percent) was about tw i ce

those o f t he remaining ma te r ia l s . This p o t e n t i a l f o r lower compressive

s t rengths p o i n t s ou t a need f o r a b e t t e r c h a r a c t e r i z a t i o n o f t h e l o n g term

compressive creep behavior o f composite ma te r i a l s, p a r t i c u l a r l y t he newer

generat i o n mat r ices a t h igh temperatures.

E f f e c t o f Specimen Geometry

The e f f e c t o f specimen geometry on the t ime dependent behavior was

explored by t e s t i n g 24, 32 and 48 p l y APC2 specimens w i t h 1/16 i n c h (0.16

cm) and 1/4 i nch (0.64 cm) holes. F igures 8, 9 and 10 are semi - logar i thmic

graphs o f t he f a i l u r e s t resses versus e l apsed- t ime- to - fa i l ure. The s o l i d

l i n e s are the least-squares regression curve f o r each se t o f da ta a t room

temperature, while the broken lines are the regression curve for the high

temperature tests. For all configurations, the strength is less for the

slower loading rates (longer time under load). At room temperature net

compressive strength increases with both increasing number of plys and

decreasing hole size. At elevated temperatures the same effect occurs with

one exception; the strength of the 24 ply material is higher than the 32 ply

for the 1/4 inch (0 .64 cm) diameter hole. The reason may be data scatter,

since the coefficient of variation is large for this particular specimen

geometry. Scatter in the APC2 data was greater than that for the other

materials, emphasizing the need for more specimens in follow up studies.

Figure 11 is a bar graph of the slopes of the strength-life curves of

Figures 8, 9, and 10 showing the relative sensitivity to loading rate

effects due to specimen geometry. At room temperature the trend is

decreasing sensitivity for both increasing number of pl ies and increasing

hole size. At the higher temperature the same trend is evident, with the 48

ply material with a 1/4 inch (0 .64 cm) diameter hole being the only

exception. For both hole sizes there is less sensitivity at the higher

temperature compared to room temperature. This may be a consequence of the

lower failure strength in general for polymer composites at high

temperatures.

Damage Characterization

In these compression tests, a damage zone, similar to a fatigue crack in

metals, initiates at the edges of the hole and propagates across the width

of the specimens resulting in final failure. The damage zone is virtually

symmetric about the hole (ignoring asymmetries due to load introduction and

imperfections in the specimen.) The damage is initiated by local fiber

buck1 ing at the edges of the hole and was not detected until loads exceeded

88 percent of the failure load. The length or size of the damage zone grows

with increasing compressive load. Figure 12 shows the initiation and

propagation of this damage zone across the specimen's width. This damage

zone was observed only in the ductile materials systems (APC2, ULTEM and

8551-7). Growth of the damage zone in the brittle systems occurs quickly

and is difficult to observe prior to catastrophic failure. The ring gage

measurements for the 32 and 48 ply APC2 showed the displacement across the

1/4 inch (0.64 cm) hole averaged 6000 to 8000 microinches (0.15 to 0.20 mm)

prior to catastrophic failure.

Figures 13 through 17 show edge views of several failed specimens. For

these figures, specimen (a) was tested at room temperature, while specimen

(b) was tested at the higher temperature. The T300/5208 specimen (Figure

13) exhibited failures typical of a brittle epoxy at both temperatures w i t h

extensive longitudinal delamination. AS4/APC2 (Figure 14), a toughened

thermoplastic, had similar brittle behavior at room temperature; at the

higher temperature the failure mode changed to shear crippling with few

signs of longitudinal delamination. C12000/ULTEM (Figure 15), an amorphous

thermoset, failed predominately due to shear crippl ing accompanied by small

amounts of longitudinal delamination at room temperature. At the higher

temperature the failure mode was exclusively shear crippl ing. IM7/8551-7

(Figure 16), an epoxy toughened with rubber precipitates, had areas of shear

crippling in the failures at both temperatures, however fewer longitudinal

de l aminat ions occurred a t the h igher temperature. The i n t e r l e a v e d ma te r ia l

AS4/1808 (F igure 17) i s noted f o r improved impact res i s tance [3,4], however

. the f a i l u r e sur face o f t he open ho le compression specimen was almost

i d e n t i c a l t o t h a t o f t he T300/5208 i n F igure 13, a t bo th room and h igh

temperatures.

For a l l b u t t he most b r i t t l e ma te r i a l s , t he f a i l u r e mode a t t h e h igher

temperature conta ined shear c r i p p l i n g and fewer i n t e r p l y delaminat ions. ( A

d iscuss ion o f the r o l e o f f i b e r k i n k i n g and shear c r i p p l i n g i s conta ined i n

re fe rence 6 ) . A t room temperature t h e amount o f shear c r i p p l i n g was r e l a t e d

t o the amount o f toughening employed t o improve the m a t r i x ma te r i a l . The

toughening r e s u l t s i n a lower m a t r i x s t i f f n e s s which g ives l e s s support t o

the f i b e r and increases the occurrence o f shear c r i p p l ing . A d e t a i l e d

examination o f t he through-the- th ickness damage o f open ho le compressive

f a i l u r e s can be found i n a separate a r t i c l e by t h e second author i n

reference 7.

The damage zone growth and f a i l u r e modes f o r these specimens c l o s e l y

resembles the f a i l u r e s repor ted by Wi l l iams [8] us ing l a r g e r 5 by 10 i nch

panels from reference 9 . Thus the new compression t e s t f i x t u r e repor ted i n

t h i s paper has the add i t i ona l advantage o f r e q u i r i n g l e s s ma te r ia l f o r

compression and notch e f f e c t s tudies.

CONCLUSIONS

The e f f e c t o f l o a d i n g r a t e on open h o l e compressive s t r e n g t h was

determined f o r t h e epox ies 5208, 5245C, 1808 and 8551-7 and t h e

t he rmop las t i c s Ul tem and APC2 a t 75 degrees F (21 degrees C) and 220 degrees

F (104 degrees C). For t h e AS4/APC2 m a t e r i a l system t h e e f f e c t s o f lamina te

t h i ckness and h o l e s i z e were determined by t e s t i n g 24, 32 and 48 p l y

specimens w i t h 1/16 i n c h (0.16 cm) and 1/4 i n c h (0.64 cm) d iameter ho les. A

new sc reen ing method was descr ibed and used t o eva lua te t h e l o a d i n g r a t e

s e n s i t i v i t y o f t h e m a t e r i a l systems.

A l l t h e m a t e r i a l s e x h i b i t e d l o a d i n g r a t e e f f e c t s a t bo th 70 and 220

degrees F (21 and 104 degrees C). A l l t h e polymer m a t r i x m a t e r i a l s t e s t e d

had g r e a t e r s t r e n g t h a t room temperature than t h e same m a t e r i a l a t t h e

h i ghe r temperature. S t reng th f o r a 1,000,000 second (12 day) t e s t was

compared t o a more t y p i c a l 60 second t e s t t o p r o j e c t t h e l o n g term des ign

i m p l i c a t i o n s . A t room temperature t h e s t r e n g t h decrease due t o decreased

l o a d i n g r a t e was 11 t o 12 percent . The decrease f o r APC2 was s l i g h t l y l owe r

( 7 pe rcen t ) w h i l e t h a t f o r t h e 5245C was almost double (20 pe rcen t ) . A t t h e

h i g h e r temperature t h e s t r e n g t h decrease f o r Ul tem and 8551-7 was about

t w i c e t h a t o f t h e o t h e r m a t e r i a l s (17 t o 21 percen t compared t o 9 percen t ) .

The s lope o f t h e s t r e n g t h versus e l apsed - t ime - to - f a i 1 u r e curve was used

t o r ank t h e l o a d i n g r a t e s e n s i t i v i t y o f t h e s i x m a t e r i a l s . A t room

temperature t h e l o a d i n g r a t e s e n s i t i v i t y was about t h e same f o r t h e m a j o r i t y

of t h e m a t e r i a l s . The l o a d i n g r a t e s e n s i t i v i t y was l e s s f o r 220 degrees F

(104 degrees C) than f o r 70 degrees F (21 degrees C). However, ULTEM and

IM7/8551-7 were notably more sensitive to loading rate than the other

materials at the higher temperature.

AS4/APC2 specimens in several laminate thicknesses and hole sizes were

evaluated. The trend for both room and elevated temperature was higher net

compressive strength with both increasing number of plys and decreasing hole

size. For loading rate sensitivity, the trend is lower sensitivity with

both increasing number of plys and increasing hole size. Loading rate

sensitivity was less at the higher temperature compared to room temperature

for the same specimen configuration. Scatter in the APC2 data was greater

than that for the other materials.

During compressive loading, a damage zone, similar to a fatigue crack in

metals, initiates at the edges of the hole and propagates across the width

of the specimens, resulting in specimen failure. Failed specimens contained

both longitudinal splitting and shear crippling, with the amount of each

related to both the test temperature and the toughening employed in the

matrix.

The compressive loading rate effects identified in this study reflect

viscoelastic behavior of the resins, particularly the toughened resins.

Long term stability of composites loaded in compression must be considered,

especi a1 ly at elevated temperatures.

REFERENCES

[l] Starnes, J. H., Jr.; Rhodes, M. D.; and Wi l l i ams, J. G.: " E f f e c t o f

Impact Damage and Holes on t h e Compressive S t reng th o f a Graphite/Epoxy

Laminate, " Nondes t ruc t i ve Eva1 u a t i o n and Flaw C r i t i c a l i t y f o r Composite

M a t e r i a l s , ASTM STP 696, R. B. Pipes, Ed., American Soc ie t y f o r T e s t i n g and

M a t e r i a l s , 1979, pp. 145-171.

[2 ] W i l l i ams , J. G.; and Rhodes, M. D.: " E f f e c t o f Resin on Impact Damage

To1 erance o f Graphi te/Epoxy Laminates," Composite M a t e r i a1 s: T e s t i n g and

Design ( S i x t h Conference), ASTM STP 787, I. M. Dan ie l , Ed., American Soc ie t y

f o r T e s t i n g and M a t e r i a l s , 1982, pp. 450-480.

[ 3 ] H i rschbueh le r , Kev in R.: "An Improved 270 Degree F Performance

I n t e r l e a f System Having Ext remely High Impact Resistance," SAMPE Q u a r t e r l y ,

Volume 17, No. 1, October 1985, pp 46-49.

[4 ] Masters, John E.: "Cha rac te r i za t i on o f Impact Damage Development i n

Graphite/Epoxy Laminates," Presented a t ASTM Symposium on Fractography o f

Modern Eng ineer ing M a t e r i a l s , Nashv i l l e , Tennessee, Nov. 1985.

[ 5 ] Gardner, Micky R., "Continuous L inea r A1 ignment T e s t i n g Gr ips , " NASA Tech B r i e f

LAR-13493, May 1986.

[ 6 ] Evans, A. G. ; Ad le r , W.F.; "K ink ing as a Mode o f S t r u c t u r a l Degradat ion

i n Carbon F i b e r Composites," Acta M e t a l l u r g i c a , Volume 26,1977, pp 725-738.

[ 7 ] Guynn, E. G. ; Brad ley, W. H.; and E lber , W . : "Micromechanics o f

Compression F a i l u r e s i n Open Hole Composite Laminates," Presented a t ASTM

Second Symposium on Composite M a t e r i a l s : Fa t i gue and Frac tu re , C i n c i n n a t i ,

Ohio, A p r i l 1987.

[ 8 ] Starnes J. H. Jr. ; and W i l l i ans , 3 . G. : " F a i l u r e C h a r a c t e r i s t i c s o f

Graphite-Epoxy Structural Components Loaded in Compression," Proceedings of

1st IUTAM Symposium on Mechanics of Composite Materials, 1982, pp. 283-306.

[9] "Standard Tests for Toughened Resin Composites," Revised Edition, NASA

RP-1092, July 1983.

.End stop

Figure 1 - Compression test figure.

RING GAGE MOUNTED IN CENTER HOLE

Figure 2 - Ring gages installed in 1/4 in. (0.64 cm) diameter center hole.

Net Stress,

MPa -200

I

- - Specimen P80-21 - - 70° F (21 O C) - - - - - - Onset of damage - - - - - Specimen P80-27 - 220 O F (104 O C) - - - - P - - - - -

1/4 in. (0.64 cm) dia. hole P

0 -0.002 -0.004 -0.006 -0.008 inches

Displacement, mm

Figure 3 - Typical clip gage displacement versus net stress for 1000 second compression test. This data is for two 48 ply AS4/APC2 specimens with a 1/4 in. (0.64 cm) diameter hole.

Net failure

strength MPa

-75

-6 5

ksi

-5 5

-4 5

-35 .I 10 1000 100000

Time to failure, sec

- - - - 0 - 48 P ~ Y

1/4 in. (0.64 cm) dia. hole - 70°F (2I0C)

- - - - - - - - - - - - - - Note: 18081 is 40 ply interleaf with - - 1/16 in. (0.16 cm) dia. hole - - - - I I I I I I

- LEGEND -

Figure 4 - Room temperature compressive strength versus el apsed-time-to- failure for 48 ply specimens.

Net ks i failure -400

strength, M Pa -5 5

10 1000 100000

Time to failure, sec

- - - 48 P ~ Y - - 1/4 in. (0.64 cm) dia. hole - - 220° F (104O C) - - - - - - - Note: 18081 is 40 ply interleaf with - - 1/16 in. (0.16 cm) dia. hole - - -

- - - - - - - - - - - - I I I I I I

- LEGEND -

Figure 5 - High temperature compressive strength versus elapsed-time-to- failure for 48 ply specimens.

Slope of -3 strength

VS log life

(ksi/ log sec) -2

1 in. (2.5 cml width

18081 is 40 ply interleaf with 1/16 in. (0.16 cm) dia. hole

MPa/ log sec

Figure 6 - Slope of the strength-life curve from figure 5 showing the relative sensitivity to loading rate.

Strength decrease

%

- Legend - 48 P ~ Y 1/4 in. (0.64 cm) dia. hole 1 in. (2.5 cm) width

Note: 18081 is 40 ply interleaf with 1/16 in. (0.16 cm) dia. hole

F igure 7 - Decrease i n compressive s t rength f o r a 1,000,000 second t e s t compared t o a 60 second t e s t .

Net ks i failure

strength, MPa -60

IU w

-50

- - - - - AS4/APC2 - - - 24 P ~ Y - 1 in. (2.5 cm) width - - - - - - - - 70°F (2I0C) - - - - 220' F (104 O C)

- - - - -

- - - - - - - - I I I I I I 10 1000 100000

Time t o failure, sec

Hole diameter in. (cm)

Figure 8 - Compressive strength versus elapsed-time-to-failure for 24 ply AS4/APC2 specimens.

Net failure

strength MPa

AS4/APC2 32 P ~ Y 1 inl2.5 cm) width

10 1000 100000

Time to failure, sec

Hole diameter in. (cm)

0.250 (0.64)

0 0.063 (0.16)

Figure 9 - Compressive strength versus elapsed-time-to-failure for 32 ply AS4/APC2 specimens.

Net ksi failure

strength, MPa

rU -400 1 -60

W

- - - 70' F (2I0C) AS4/APC2 - - - - 48 P ~ Y 2200 (Io4 O 1 in(2.5 cm) width

- - 0 -

- - - - - - - - 0 - - - - - - - ---, A - A - - - - - - - - - A - - - - _ - - & - - - - - A - - - - - - - - - -

A - - - - - _ z*- - - - - + - + - - - - - - - ,- - - -+ - , - - - -+-----+a- + - - + - - - - - - - - - - _ - - - - I I I I I 1

Hole diameter in. (cm)

Time to failure, sec

Figure 10 - Compressive strength versus el apsed-time-to-fail ure for 48 ply AS4/APC2 specimens.

AS4/APC2 1 in (2.5 cm) width

1/16 in. (0.16 cm) dia. hole

1/4 in. (0.64 cm) dia. hole

Slope of strength

v S log life MPa/

log sec -1 5 &si/

log sec)

24 ply 32 ply 48 ply 24 ply 32 ply 48 ply

Figure 11 - Slope of the strength-life curve from figures 8, 9, and 10 showing the re1 ative sensitivity to loading rate.

Figure 12 - I n i t i a t i o n and propagation o f damage zone p r i o r t o compressive f a i l u r e i n an AC4/APC2 specimen. Hole diameter i s 1/16 I n . (0.16 cm) . Damage zone lengths are: ( a ) 0.020 i n . (0.05 cm), (b) 0.030 i n . (0.08 cm), (c) 0.050 i n . (0.13 cm) and 0.120 i n . (0.30 cm), (d) 0.140 i n . (0.36 cm) and 0.140 i n . (0.36 cm).

Figure 13 - Edge view o f f a i l e d T300/5208 compression specimen. (a) 70 degrees F ( 21 degrees C ) . (b) 220 degrees F (104 degrees C ) .

Figure 14 - Edge view o f fa i led AS4/APC2 compression specimen. ( a ) 70 degrees F ( 21 degrees C ) . (b) 220 degrees F (104 degrees C ) .

Figure 15 - Edge view o f f a i l e d C12000/ULTEM compression specimen. (a) 70 degrees F ( 21 degrees C ) . (b) 220 degrees F (104 degrees C ) . I

Figure 16 - Edge view o f failed IM7/8551-7 compression specimen. (a ) 70 degrees F ( 21 degrees C). I

(b) 220 degrees F (104 degrees 0). I

Figure 17 - Edge view o f f a i l e d AS4/18081 compression specimen. (a) 70 degrees F ( 21 degrees C). (b) 220 degrees F (104 degrees C) .

r

NASA tL,I.',,l I".,.,N,l,-.,,,,l a ,I . I ,. l",l.lrr'.ll 111..

Report Documentation Page

1 fl~!port No.

NASA TM- 100634 AVSCOM TM 88-B-012

- - -. -. -.

2. Government Accession NO. 3. Recipient's Catalog No.

i- .11tlv ;I~III Sot)title

LOADING RATE SENSITIVITY OF OPEN HOLE COMPOSITES I ri COMPRESSION

- - -

7 Author (~)

Steve J. Lubowinsk i , E . G. Guynn, Wol f E l be r , and J. D. Whitcomb

.. - ~ -- 9. Porformlng Organization Narne and Address

- -

Aeros t r u c tures D i r e c t o r a t e USAARTA- AVS COM Lanqley Research Center Hampton, VA 23665-5225 .. -. .- 12. Sponsoring Agency Name and Address

Ua t i ona l Aeronaut ics and Space A d m i n i s t r a t i o n dashington, DC 20546 and U. S . Army A v i a t i o n Sys tems Command S t . Lou is , MO 63120-1798 .---- 15. Supplementary Notes

5 Report Date

August 1988 -

6. Performing Organization Code

8. Performing Organization Report No.

lo . work Unit NO.

505-63-01-05 11. Contract or Grant No.

13. Type of Report and Period Covered

Techn ica l Memorandum 14. Sponsoring egency Code

Army P r o j e c t No: lL161102AHS5C

Steve J. Lubowinski , PRC Kentron, Hampton, VA; E. G. Guynn, Texas A&M U n i v e r s i t y , I o l l e g e S ta t i on , TX; Wolf E lber , U.S. A rmy Ae ros t r uc tu res D i r e c t o r a t e , USAARTA - qVSCOM, L a n g l e ~ Hesearch Center, Hampton, VA; and J. D. Whitcomb, Langley Research :enter, Hampton, VA -- -- - --- -- 16. Abstract I h i s pdper r e p o r t s t he r e s u l t s o f an exper imenta l s tudy on t h e compressive, t ime- rlependent behavi o r o f y raph i t e f i b e r r e i n f o r c e d polymer composi te lami na tes w i t h open ho les . The e f f e c t o f l oad iny r a t e on compressive s t r e n y t h was determined f o r s i x n d t e r i d l sytems rany iny frorn b r i t t l e epoxies t o t he rmop las t i c s a t b o t h 75O and 220". Specimens were loaded t o f a i l u r e us i ng d i f f e r e n t l oad ing ra tes . The s l ope o f t h e s t r e n y t h versus e lapsed t i m e - t o - f a i l u r e curve was used t o rank t h e m a t e r i a l s ' l o a d i n g r a t e s e n s i t i v i t y . A l l of t h e m a t e r i a l s had g r e a t e r s t r e n g t h a t 75" compared t o t h e sarne m d t e r i a l a t Z2U°F. A l l of t h e m a t e r i a l s showed l o a d i n g r a t e e f f e c t s i n t h e f o rm of reduced f a i l u r e s t r e n y t h f o r l onger e lapsed- t ime- to - fa i l u r e . Loading r a t e sens i - t i v i t y was l e s s d t 220" than t h e same m a t e r i a l a t 7U0F. However, C12000/ULTEM and 1M7/8SS1-7 were more s e n s i t i v e t o l oad iny r a t e than t h e o t h e r m a t e r i a l s a t t h e 220°F \S4/HPCZ lamina tes w i t h 24, 32, and 48 p l i e s and 1/16 and 114 i n c h d iamete r ho les dere tes ted . The s e n s i t i v i t y t o l o a d i n g r a t e was l e s s f o r e i t h e r i n c r e a s i n g number 3 f p l i e s o r l a r y e r h o l e s i ze . The f a i l u r e of t h e specimens made f rom b r i t t l e r e s i n s das dccompanied by ex tens i ve de lamina t ions w h i l e t h e f a i l u r e of t h e toughened systems das predorni n d n t l y by shear c r i p p l i n g . Fewer de lamina t ion f a i l u r e s were observed a t :tie h i yher temperature. .

17. Key Words (Suggested by Author(~) )

Polymer Composites Open- h o l e Loadi ng - ra te H i qh- temq_er_hture

18. Distribution Statement

U n c l a s s i f i e d - U n l i m i t e d

Sub jec t Category 24

22. Price

A0 3 i NASA FORM 1828 OCT 86

21. No. of pages

3 1

19. Socurity CIass~f. (of Ihls report)

U n c l a s s i f i e d

20. Security Classlf. (of thls pagel

U n c l a s s i f i e d