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(NASA-CR-135087) SYPAIN RATE EFFECTS ON N77-11116HECHANICAL PROFERTIES OF FIBE_ COHPOSITES,

PART 3 Final Report, _ Jul. 197_ - 29 Feb.

1976 (IIT Research Inst.) _9 p Unclas

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STRAIN RATE EFFECTS ON:, MECHANICAL PROPERTIES! OF FIBER COMPOSITES: 1

Final Report - Part III J

by I.M. Daniel and T. Liber

': t liT RESEARCH INSTI'rUTE

i Prer)lred for

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NATIONAL AERONAUTICS| AND 8PACE ADMINIS"TRATION

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]977004]73

https://ntrs.nasa.gov/search.jsp?R=19770004173 2020-07-11T10:19:56+00:00Z

1 Report Nn i 2 '3ocernment Access,on Nc 3 _ccJp,ent's Catain(j No!

NASA CR-135087 I4 Title and Subtl_l? 5 _eOo;t Date

Strain Rate Effects on Mechanical Pz_:pP.rties of Fiber Composites June 197(}

_. Perlof'_g Or_r_lzatlon Cooe

7 Author(s) e. Perforrn,.'xJ O;ganizat_on Repcrt No

I.M. Danieland T. I,iber D6073-IV10. Wo-k Unit No.

9. Performing Organizat,Gn Name and Address

: ]:IT Research InstituteI0W. 35 Street 11 Con:tactorGrantNo.

" Chicago,IL 60616

13. Type of Report and Period Coveted

12. Sponsoring Agency Name and Address

National Aeronautics & Space Administration Final - ]Part ffl", Washington, DC 20546 14 SponsoringAgercyCode

15. SuDolementaryNotes Project Manager, C. C. ChamisMaterials & Structures DivisionNASA Lewis Research Center

Cleveland, OH 4413516. Abst:a_

An experimental investigation was conducted _.c determine the strain rate effectsin fiber composites. Unidirectional composite specimens of boron/epoxy, graphite, /epoxy, S-glass/epoxy and Kevlar/epoxy were tested at strain rates of up to 27

_°/sec to determine longitudinal, transverse and intralaminar (in-plane) shear; properties. In the longitudinal direction the Kevlar/'epoxy shows a definite increase

-" in beth modulus and strength with strain rate. In the transverse direction, a generaltrend toward higher strength with strain rate is noticed. The intralaminar shearmoduli and sr.rengths of boron/epoxy and graphite/epoxy show a definite rise withstrain rate.

I

:[ ,7. Key Words (Suggested by Autho,(s}) 18 Distribution Statement

Strain rate, mechanica 1 properties, fiber: composites, longitudinal tension, transverse

tension, intralaminar shear, boron, graphite, Unclassified, Unlimited

i-' " i _;evlar, S-glaes, fracture stress, fracture,• -- strain

- ' f0r saleby theNat=_q,_lTechnicalInf0rn;a:i0nSe_v,ce,SplinEfleld,Virginia22151 !

NASA*( -1_ _R,', I,-71)

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1977004173-002

FOREWORD

This is the Final Report on lit Research Institute Project

No. D6073-1V, "Strain Rate Effects on Mechanical Properties of

Fiber Composites," prepared by IITRI for NASA-Lewis Research

Center, under Contract No. NAS3-16766. The work described in

this report was conducted in the period July I, 1974 to February 29,

1976. The work performed in the preceding period August I, 1972

to June 30, 1974 was reported in the First Interim Report, NASA

CR-134826 dated March 1975. Dr. C.C. Chamis was the NASA-Lewis

Project Manager. Dr. I.M. Daniel of IITRI was the principal

investigator. Additional contributions to the work reported herein

were made by Dr. T. Liber and Messrs. R. LaBedz and M. Senninger.

Respectfully submitted,

liT RESEARCH INSTITUTE

F,,- I;,_.,-- ,_'

I.M. DanielA.

" Science AdvisorMechanics of Materials Division

APPROVED :

t o

S.A. BortzAssistant Director

r_ _ Mechanics of Materials Division !,

i_ J

!. i|liT RESEARCH INSTITUTE

" ! "' iii

]977004]7:3-003

1 I ' ! ', 1 I ,' I i , i

STRAIN RATE EFFECTS ON MECHANICAL PROPERTIES OF FIBER COMPOSITES

ABSTRACT

An experimental investigation was conducted to determine the

strain rate effects in fiber composites. Unidirectional composite --"

specimens of boron/epoxy, graphite/epoxy, S-glass/epoxy and Kevlar/

epoxy were tested at tensile strain rates of up to 27 e/sec to

determine longitudinal, transverse and intralaminar (in-plane)

shear properties. In the longitudinal direction the Kevlar/

epoxy shows a definite increase in both modulus and strength with

strain rate. In the transverse direction a general trend toward

higher strength with strain rate is noticed. The intralaminar shear

moduli and strengths of boron/epoxy and graphite/epoxy show a

definite rise with strain rate.

L


"i iv

1977004173-004

T 1 !: J

I , J ,,

TABLE OF CONTENTS

SECTION PAGE

1.0 INTRODUCTION I

2.0 EXPERIMENTAL PROCEDURES 2 ---

3.0 RESULTS AND DISCUSSION 4

4.0 SUMMARY, CONCLUSIONS AND RECOMMENDATIONSFeq FUTURE WORK I0

REFERENCES 31

; DISTRIBUTION LIST 32

liT RESEARCH iNSTITUTE

V

1977004173-005

' I 1 I r _ '

LIST OF TABLES

TABLE PAGE

I LONGITUDINAL PROPERTIES OF UNIDIRECTIONALCOMPOSITES AT VARIOUS STRAIN RATES 5

2 TRANSVERSE PROPERTIES OF UNIDIRECTIONAL --COMPOSITES AT VARIOUS STRAIN RATES 6

3 INTRALAMINAR SHEAR PROPERTIES OF L_IDIRECTIONALCOMPOSITES AT HIGH STRAIN RATES

lit RESEARCH INSTITUTE

1977004173-006

LIST OF FIGURES

FIGURE PAGE

1 UNIDIRECTIONAL 10-DE,oR..EOFF AXIS SPECIMEN MOUNTEDIN MACHINE FOR TESTING AT HIGH STRAIN RATES 12

2 EXPERIMENTAL SETUP FOR TESTING COMPOSITE SPECIMENS

AT HIGH STRAIN RATES IN ELECTROHYDRAULIC TESTINGMACHINE 13

3 LOAD AND STF-_IN RECORDS FOR [O6] BORON/EPOXYSPECIMEN LOADED AT A STRAIN RATE OF 2.7 _/sec 14

4 LOAD AND STRAIN RECORDS FOR [06] BORON/EPOXYSPECIMENS LOADED 7N TENSION AT STRAIN RATES OF5.3 _/sec AND 10.3 _/_ec 15

5 LOAD AND STP_IN RECORDS FOR [06] GRAPHITE/EPOXYSPECIMENS LOADED IN TENSION AT STRAIN RATES OF1.9 e/see AND 5.0 e/see 16

6 LOAD AND STRAIN RECORDS FOR [06] S-GLASS/EPOXYSPECIMENS LOADED IN TENSION AT STRAIN RATES OF4.5 E/sec AND 18.8 e/sec 17

7 LOAD AND STRAIN RECORDS FOR [06] KEVLAR/EPOXYSPECIMEN LOADED AT A STRAIN RATE OF 4.0 E/see 18

8 LOAD AND STRAIN RECORDS FOR [06] KEVLAR/EPOXYSPECIMEN LOADED IN TENSION AT A STRAIN RATE OF15 e/sec 19

I, 9 LOAD AND STRAIN RECORDS FOR [908] BORON/EPOXY: SPECIMEN LOADED IN TENS'ION AT A STRAIN RATE OF

0.70 e/sec 20

i0 LOAD AND STRAIN RECORDS FOR [908] BORON/EPOXYSPECIMEN LOADED IN TENSION AT A STRAIN RATE OF19 e/see 21

ii LOAD AND STRAIN RECORDS FOR [908] GRAPHITE/EPOXYSPECIMEN LOADED IN TENSION AT A STRAIN RATE OF0.80 e/see 22 •

I

12 LOAD AND STRAIN RECORDS FOR [908] GRAPHITE/EPOXY ,SPECIMEN LOADED IN TEN_ION AT A STRAIN RATE OF

i i0 _/see " - 23 "I

liT RES_.._RCH INSTITUTE

vii ;

1977004173-007

' r ' T [ I; ] I ! --

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LIST OF FIGURES (Co::t'd_

FIGURE PAGE

13 LOAD AND _T.R._INRECORDS FOR [908] S-GLASS/EPOXYSPECIMEN LOADED IN TENSION AT A STRAIN RATE OF0.50 _/sec 24

= 14 LOAD AND STRAIN RECORDS FOR [908] S-GLASS/EPOXY _.SPECIMEN LOADED IN TENSION AT A STP_EIN RATE OF

" 6.3 _/sec 25

15 LOAD AND STRAIN RECORDS FOR [908] S-GLASS/EPOXYSPECIMEN LOADED IN TENSIOF AT A STRAIN RATE OF18 _/sec 20

" 16 LOAD AND STRAIN RECORDS FOR [106] BORON/EPOXYOFF-AXIS SPECIMEN LOADED IN TENSION AT A STP_INRATE OF 0.57 e/sec 27

• 17 LOAD AND STRAIN RECORDS FOR [106] BORON/EPOXY OFF- .AXIS SPECIMEN LOADED IN TENSION AT A STRAIN RATEOF 6.3 _/sec 28

18 LOAD ._]D STRAIN RECORDS FOR [106] GRAPHITE/EPOXYOFF-AXIS SPECIMEN LOADED IN TENSION AT A STRAINRATE OF 0.41 _/sec 29

19 LOAD AND STRAIN RECORDS FOR [106] GRAPHITE/EPOXYOFF-AXIS SPECIMEN LOADED IN TENSION AT A STRAINRATE OF 4 e/sec 30

111 RESEARCH INSI :TIITE

viii

1977004173-008

IITR! Research Project No. D6073-1V

STRAIN ._TE EFFECTS ON _CHANICAL PROPERTIES OF FIBER COMPOSITES "

1.0 INTRODUCTION

Most composite materials of interest to structural

applications have been fully characterized under stdtic loading

conditions. Extensive data were presented in prior reports. 1'2 --.

Very little work on composite properties at high rates of loading

has been reported. The need for characterization at high loading

rates arises from the fact that in problems of projectile impact

and foreign object damage (FOD) the material experiences very

high strain rates. Strain rates in the range of 250-650 /sec

were measured in impscted uniaxial and angle-ply boron/epoxy and

graphi'.e/epoxy laminates in the preceding task. One would expect

that z:iffnesses and ultimate values, especially those governed by

the viscoelastic matrix, would be time and rate dependent. The

objective of this task was to measure the stiffness, strength

and ultimate strain of unidirectional composites subjected to high

rates of loading. The materials investiBated were the same ones

used in the two previous tasks, i.e., boron/epoxy, graphite/epoxy,

S_glass/epoxy and Kevlar 49/epoxy.

lit RESEARCH INSTITUTE

" j

1977004173-009

2.0 EXPERIMENTAL PROCEDUkES

Longitudinal (0-degree) tensile properties were obtained

using 6-ply coupons 1.27 cm (0.50 in.) wide with a 5.08 cm (2 in.)

gage length and tabbed with fiberglass tabs. Transverse (90-

degree) tensile properties were obtained _._._r6 8-ply coupons,

1.27 cm (0.50 in.) wide with a 7.62 cm (3 in.) gage length. These

specimens were instrumented with a two-gage rosette on each side --

of the specimen. Tn-plane shear properties were obtained by

using i0-degree off-axis coupons, 6-ply thick, 1.27 cm (0.50 in.)

•_ide with a 7.62 cm (3 in.) gage length. They were instrtur_ented

_ with a th,_ee-gage rosette on each side of the specimen. Boron/

epoxy, graphite/epoxy, S-glass/epoxy and Kevlar 49/epoxy specimens

for uniaxial tensile testing at 0-, 90- and 10-degrees to the

fibers were prepared and instrumented.

These specimens were tested in uniaxial tension at various _

strain rates. An MTS electro-hydraulic closed-loop system capable _,

of delivering a wide range of input pulses at velocities up to

i 5.1 ms -I (12,000 in/min) was used. Special fixtures wer_ designed •

' and built for each type of test. One tabbed end of the specimen

was clamped between two metal grips and connected through a pin to

a clevis link attached to the upper crosshead of the loading frame.

At the bottom end two additional fiberglass tabs extending beyond

the original tabs were bonded on. These secondary tabs were

connected through a pin to a load link which in turn was connected

to the bottom clevis link attached tu the moving ram. The fixture

allowed the ram to accelerate for approximately 6.3 nm (1/4 in.)

before transmitting load to the specimen. This allows for a more

uniform rate of loading. A i0-degree specimen with the loading

fixture mounted in the machine is shown in Fig. I. The dynamic

load initially was measl,red wi_h a piezoelectric crystal load%

cell, however, it was found _._ at the higher strain rates the

load signals were perturbed b_'resonant oscillations. Subsequently, "

r

i liT RESEARCH INSTITUTE

• i 2

1977004173-010

I l I [,%

load measurement was done by means of an aluminum li_'_ _L_ rumented

; with strain gages and connected in series w_th the _pec]me_ Strain

gages from the test specimen _,d the load cell were recorded on

oscilloscopes. The overall experlmenaal setup is showT_ _n Fig. 2.

In all cases above a few quasistatic tests were conducted

to provide a reference at very slow rstes for specimens of

identical geometry. These quasistatic results were used to1.2

supplement previously obtained similar results.

%

lit IIESEAItCH iNSTITUTE

' 3i

197700417:3-011

3.0 RESULTS AND DISCUSSION

,¢

Typical lead and strain oscilloscope signals for the

[06 ] boron/epoxy, graphite/epoxy, S-glass/epoxy and Kevlar

&9/epoxy specimens are shown in Figs. 3 through 8. Resul_s

from these tests as well as corresponding static tests are

tabulated in Table i. Results for boron/epoxy show no signi-

ficant changes in modulus but some increase in strength at the

higher strain rates The ultimate strain ,emains relatively

constant, with one exception at the 5.3 e/sec strain rate where

the limit strain is 8.4 x 10 -3 . In the case of graptite/epoxy

there is a slight increase in modulus with strain rate but no

significant changes in strength or limit strain. Tn the case of

S-glass/epoxy there are no significant changes in modulus or

strength but some trend towards higher ultimate strain at the

higher rates of loading. Kevlar 49/epoxy shows a definite

increase in modulus with strain rate and some _ncrease in strength.

The ultimate strain at the higher rates of loading are slightly

4 : lower than for static loading. Ir this series of tests where

" the properties of the composites are dominated by the fibecsJ

trends of properties with increasing strain rate are barely

detectable. This is due p_imarily to the fact that the properties

of Ehe fiber materials, with "he possible oxception of Kevlar, are

not very rate dependent, and due to the small number of specimens.

Unidirectional eight-ply 90-deRree specimens of the

four materials above were tested in tension to failure statically

and at various high strain rates. A more sensitive load cell

: link was designed and used in these tests. Strain rates ranged

from quasistatic to 27 e/sec. Typical load and strain records

are shown in Figs. 9 through 15. Results from these tests '.

as well as corresponding static tests are labul_ted It_ Table 2 ,{

lit RESEAICH INSTITUTE

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In the boron/epoxy the modulus and strength are Benerally lower

than corresponding static values, except for the highest strain

rates, 19 and 27 g/sec, where the strength is higher than the

static one. The ultimate strain at the high strain rates is, with

one exception, much higher than the static values. There is no

plausible explanation for these results. In the graphite/epoxy

there is some increase in modulus and strength at the higher rates

of loading accompanied by an increase in limit strain. In ther

S-glass/epoxy there _eems to be an unlikely reduction in modulus

with strain rate, a fact which may not be significant. There is

some trend towmrd higher strength with increasing strain

, rate. No significant trends in limit strain are apparent. The

Kevlar 49/epoxy specimens were too fragile to handle ia the

electrohydraulic machine and failed at very low loads. The results

" above do not show any drastic changes of properties with strain

rate although these properties are governed by the matrix, which

should be more rate dependent.

UnidirectionaT. six-ply specimens of the four materials

above were tested in uniaxial tension at 10-degrees to the fiber

_ direction statically and at various h_gh strai_ rates. These

i specimens were instrumented with a three-gage rosette on each

side. Signals from the load cell and from the longitudinal_

transverse and 45-degree gages we_°e recorded on fou_ oscilloscopes.

- Axial strain rates ranged from quasistatic to 7.7 _/sec. Typical

load and strain records are shown in Figs. 16 throu_,h 19.

Results from these tests as well as corresponding static tests are

tabulated in Table 3. In the boron/eDoxy bo_h the in-nlane

shear modulus and shear strength show an _ncrease with strain rate.

The limit shear strain at the high rates of loading, however, is

lower than the corresponding s_at_u one. The trends in graphite/

epoxy are similar to those in boron/epoxy, with shear Todulus and

shear strength increasing with strain rate. No significant trends


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were noticed in the S-glass/epoxy and Kevlar/epoxy materials.

Properties obtained at high strain rates were comparable with

those obtained statically for the same batch of specimens.

However, these properties are lower than those obtained initially2

for the same materials , indicating aging and deterioration of

the prepreg materials used. It was not possible to obtain a

fresh batch of these materials as the matrix resin ERLA 4617 was

no longer available.


9

1977004173-017

4.0 SUM_tARY, CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK

An electrohydraulic system was set up for testing uni-

directional _9m_osite cGupons in tension at high strain rates.

Four materials were investigated, bolon/epoxy, graphite/epoxy,

S-glass/epoxy and Kevlar 49/epoxy. Longitudinal, transverse

and in-plane shear properties, including modulus, Poisson's ratio,

strength and ultimate strain, were determined by testing 0-, --"

90- and 10-degree unidirectional coupons. Strains were measured

by means of strain gages bonded on the coupons and loads were

measured by means of a strain gage load cell mounted in series

with the specimen. Signals from all strain gages were recorded

on oscilloscopes.

In the case of 0-degree properties which are governed by

the fibers, variation of properties with strain rate are barely

detectable. The boron/epoxy shows some increase in strength at

.he higher strain rates but no significant changes in modulus or

ultimate strain. In the graphite/epoxy there appears to be a

slight increase in modulus with strain tate but no significant

changes in strength or ultimate strain. In the S-glass/epoxy

there are no significant changes in modulus or strength but some

trend toward higher ultimate strains at th_ higher rates of loading.

T_e Kevlar 49/epoxy shows a definite increase in modulus and

strength with strain rate because the fibers in this case have

more rate dependent properties.

_. One would expect that the 90-degree properties, which are

governed by the properties of the matrix materials, would be

much more rate dependent. In the boron/epoxy there is an unexplain-

, able trend toward lower values in modulus, however, the strength

and limit strain in general increase slightly with strain rate.

_ i In the graphite/epoxy there is a general slight increase in modulus,i

strength and ultimate strain In the S-glass/epoxy there appear to

be two contrary trends toward lower modulus and higher strength

_ I liT RESEARCH INSTITUTE,I

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1977004173-018

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with increasing strain rate, however, these _y not be significant.

The in-plane shear properties are also governed by those

of the matrix material and as such they should be rate dependent.

Both the boron/epoxy and graphite/epoxy show definite trends of

increasing shear modulus and shear strength with strain rate.

The increase in strength at the high strain rates over the static

values is approximately 15 percent. No significant trends were

detected in the S-glass/epoxy and Kevlar/epoxy materials. There --"

is some q1_estion on the validity of results from these two materials

as the specimens were prepared from an aged batch of prepreg

material.

In general, properties of unidirectional composites such

as modulus and strength tend to increase with s_rain rate.

more systematic series of tests with more replications per test

is necessary to establish these trends in a reliable quantitative

manner. The rate of testing must be increased further by at

least an order of n_agntiude to achieve conditions comparable to

those existing in impacted laminates. It is anticipated that

strain rate effects will be more pronounced at higher strain rates.

To properly evaluate results of d_amic loading of

composites the basic material properties over pertinent time-

sca'.es, i.e., high strain rates, must be determined. The need

exists for continuing and extending the work conducted under the

task reported here. A systematic series of tests with more

replications per test is needed. Compressive properties and

properties of angle-ply laminates must be added. The present

electrohydraulic system must be modified to increase the rate of

loading by at least one order of magnitude, to achieve rates

comprable to those encountered under impact loading. It is

anticipated that strain rate effects will be more pronounced

at higher strain rates.


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LOAD AND STRAIN RECNRDS FOR [06] BORON/Fig. 4EPOXY S_ECIMENS LOADED IN TENSION AT

STRAIN RATES OF 5.3 _/sec and 10.3 c/sec.

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STRAIN RATES OF 1.9 e/see and 5.0 E/see.

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TENSION AT A STRAIN RATE OF 0.70 elsec.

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AT A S7RAIN RATE OF 0.80 E/see.

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1977004173-030

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Fig. 12 LOAD AND STRAIN RECOP,DS FOR [90B]GRAPHITE/EPOXY SPECIMEN LOADED IN

TENSION AT A STRAIN RATE OF I0 e/sec.

23

1977004173-031

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LOAD AND STRAIN RECORDS FOR [908]Fig. 13

S-C_,AS%/EPOXY SPECIMEN LOADED IN

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TENSION AT A STRAIN RATE OF 6.3 e/see.

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1977004173-033

! r'I

,--4 .; / "

:> ---I-- ' l•,-I C'

_,_ f--I _'_ _ • .....

f_

/

I

100 :_s / d.!v

Fig, 15 LOAD AND STRAIN RECORDS FOR [908]

S-GLASS/EPOXY SPECIMEN LOADED _._

: TENSION AT A STRAIN RATE OF 18 c/see.

26

.i

1977004173-034

Fig. 16 LOAD AND STRAIN RECORDS FOR [106] BORON/EPOXY OFF-AXIS SPECIMEN LOADED IN TENSION

AT A STRAIN RATE OF 0.57 E/see.

i

1977004173-035

--,---,r-- .....-m-'-- .... r........ -] .... -"r..............f ...... --r---- "r ,)

500 _s/div

Fig. 17 LOAD AND STRAIN RECORDS FOR [106] BORON/EPOXYOFF-AXIS SPECIMEN LOADED IN TENSION AT A

STRAIN RATE OF 6.3 a/sec.

2_

i

1977004173-036

o - .......;..... ?,_. -,. ......_Di , ,m

---_ ...... r---_ I, - [ -'-r --"T--'- r--'-'-r, ...."..........T -

'2 ,.s/div

Fig. 18 LOAD AND STRAIN RECORDS FOR [106] GRAPHITE/EPOXY OFF-AXIS SPECIMEN LOADED IN TENSION

AT A STRAIN RATE OF 0.41 _/sec,.'_

• i

1977004173-037

........ _ .... "r"-'--r--- ............_'r......................T..........."----"------- ...............- '

",_ _ ,

J

500 ;,sldiv

Y

Fig. 19 LOAD AND STRAIN RECORDS FOR [106] GRAPHITE/EPOXY OFF-AXIS SPECIMEN LOADED IN TENSION

AT A STRAIN RATE OF 4 _/sec.

"_C_

A

,r

1977004173-038

.................r........;.............7-7--7.......r i

I

REFERENCES

i. Daniel, I.M. and Liber, T., "i,a_nination Residual Stressesin Fiber Composites," IITRI Report D6073-I, for NASA-Lewis Research Center; NASA CR-i3_826, March 1975.

2. Daniel, I.M. and Liber, T., "Lamlnation Residual Stressesin Hybrid Composites," IIYRI Report D6073-II, for NASA- "Lewis Research Center; NASA CR-135085, June 1976.

111' RESEARCH INSTITUTE

3 l

1977004173-039

i i i - nasa...i i i! i i i) nasa cft-13e087 j iitr_ r..:6073-1v (nasa-cr-135087) sypain rate...

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