Fatigue behaviour of carbon fibre reinforced plastic at cryogenic temperatures

Fatigue behaviour of carbon fibre reinforced plastic at cryogenic temperatures* K. A h l b o r n Institut fiJr Material- und Festk6rperforschung IV, Kernforschungszentrum Karlsruhe GmbH, 7500 Karlsruhe, FRG The fatigue behaviour of carbon fibre reinforced plastic (CFRP) composites has been investigated. Three laboratory-type CFRPs and t w o structural elements are described and compared.

Keywords: composites; mechanical properties; low temperature studies

Nomenclature P E Vf N

Fracture probability Initial Young's modulus Fibre volume fraction Number of load cycles

Greek letters

c Strain q~ Angle relative to the direction of P ~r Stress

Carbon fibre reinforced composites (CFRPs) have a high specific strength and stiffness, and excellent fatigue behaviour. Thus, they are used as substitutes for metals in many applications. At low temperatures the thermal conductivity of CFRPs is very low which is an important feature in solving thermal insulation problems. Mainly unidirectional (UD) reinforced composites have been tested at low temperatures. Only a few fatigue experiments have been done on multidirectionally reinforced composites. In this Paper, the fatigue behaviour of composites from structural elements and three laboratory-type composites will be described and compared with the fatigue behaviour of a pure matrix material. It is significant for fibre reinforced plastics that the material properties can be widely varied by the fibre content, Vf, fibre orientation and matrix properties. Therefore, a comparison becomes more difficult when composites with different fibre arrangements are in-

*Paper presented at the ICMC conference Non-Metallic Materials and Composites at Low Temperatures IV, Heidelberg, FRG, 28-29 July 1986

0011-2275/88/040267 4]6 $03.00 ~ 1988 Butterworth & Co (Publishers) Ltd

Indices

U R T D f m c s

Ultimate, upper Residual Tensile After 107 load cycles Fibre longitudinal Matrix Composite Semiflexible

volved. A simple measure of the mechanical behaviour is the in-plane Young's modulus, E c. The Young's modulus under uniaxial load can be estimated easily using a simple equation. Assuming a parallel connection of fibre and matrix, the strain is equal in both components, and E c ~ EliIVfr/o+ Em(1 -- Vf)

(1)

where Ell I is the longitudinal Young's modulus of the fibre and E m is the Young's modulus of the matrix*. The factor ~/o takes into account the orientation of the single UD layers relative to the load direction ~/0 =

n=l Vff

c°s4q~,

(la)

Vf, is the fibre content of layer n. The orientation factor, r/o, is 1 for UD composites, 0.5 for symmetrically crossplied composites, and 0.375 for quasiplanar composites (0°/--F 45°/90°)sy m. With the assumption that the matrix fracture strain, tThis equation is valid only for balanced composites loaded in one

fibre direction

Cryogenics 1988 Vol 28 April

267

Carbon fibre reinforced plastic: K. Ahlborn em, is much higher than the fibre fracture strain •f

(2)

~., >> el,

3,9 OPa carbonfibres

the composite will fracture at the fibre strain (3)

'Sc ~' gfl]

CO

and the composite fracture strength can be roughly estimated by

II /'

(4)

2,2

In Figure 1 the stress-strain behaviour at 77 K of CFRP composites (shaded area) and their components is given. In multidirectional composites the ideal fracture strain of the fibres is not reached

2,0

o"e ~ Ee~:fl I

77K I

t

UD,tough matrix

1,4

/ ~ (~UD, rigidmatrix

composites

/;c
therefore, the strengths are lower as calculated O'e,UT ~--- E c g

1,0

crossply

e < ~rc

Some of the reasons for this behaviour are as follows: 1 as the coefficients of thermal expansion differ between the fibre and the matrix, the matrix is pre-strained in the composite when cooled down 1°. For rigid matrix systems, for example, highly cross-linked epoxy resins, the residual strain is of the order of the fibre strain and the thermal pre-strain has to be included in Equation

(2)

~ quasiplanar O

~

0

I

I

1,0 1,6

I

3,2

_..Pc cg.5%1 i

/.,5 -----'- £,%

Figure 1 Schematic stress-strain behaviour of uni- and multidirectional CFRPs and their components at 77 K. (Numbers refer to the index given in the tables)

8m - - /;m,pre ~'~/;flJ

Then the propagation of primary cracks in the transverse layers of multidirectional composites is enhancedZ,3; 2 pores and inhomogeneities act as local stress risers and, therefore, enhance matrix cracking; and 3 in structural elements and in test specimens further stress concentrations occur.

Materials Table 1 lists the composition and lay-up of the seven composites tested. For reinforcement, two types of high tensile fibre with a Young's modulus of about 240 GPa and a fracture strain of about 1.6 % were chosen. Different rigid epoxy resins (EP), a semiflexible epoxy resin (EPs) and a thermoplastic polymer (PC) were selected as the matrix system. The multidirectional composites were Table 1

tested on specimens 'necked' in the width. The UD composites, by contrast, were tested on specimens 'necked' in the thickness. Specimens of composites 1, la, lb, 3 and 4 (see Table 1) were cut out of plates. The specimens of the composite 5 were cut out of a thick-walled torus structure made of woven fabrics7. Composite 2 consists of unidirectional CFRP loops as used for the ISO-satellite (European Space Agency); they were tested as delivered. The fibre content of all composites was about 60 vol %. In addition, the polycarbonate, semiflexible epoxy and rigid epoxy matrix resins were tested on cylindrical, 'necked' specimens.

Experiments The static properties were determined under tensile load. The fatigue strength was determined by tensile fatigue

Description of the tested CFRPs and components

Indexa

Lay-up

Fibre type

Matrix

Reference

I 1a 1b 2

UD UD UD UD Structural element Cross-ply (0°/90°/0°/90°/0°)s Quasiplanar ( 0 ° / + 45°/90°)s Quasiplanar (0 °, 9 0 ° / + 4 5 ° ) n Structural element

T300 T300 HTA7 T300

EP - rigid EPs - semiflexible PC - thermoplastic EP - rigid

4 4 5

HTA7

PC - thermoplastic

-

T300

EP - rigid

4

HTA7

EP - rigid

6

3 4 5

alndex numbers refer to numbers in figures

268

Cryogenics1988Vol 28 April

Carbon fibre reinforced plastic. K. Ahlborn tests (R ,,~ 0.1). Servohydraulic test apparatus (30 Hz) and resonance testing equipment (80-120Hz) were used tt. The residual strength of run-outs was determined in tensile tests after more than 10 6 cycles. The load was measured with a piezorestrictive ring. The strain was measured with strain gauges. All devices were calibrated at low temperature and intercalibrated during the tests with devices placed outside the cryostat. The specimens were cooled with LHe or LN 2. Energy dissipation during the fatigue experiment causes a small temperature rise inside the specimen. The temperature difference, AT, between the interior and the surface of the structural part (composite 2, 5 x 6 mm 2 cross-section) was measured with very small silicium-diode sensors. The difference, AT, at a frequency of 120 Hz was about 5 K in LHe and about 10 K in evaporated gaseous He.

a 3,0e,------.___ ~-o.L

2,0'o

l

1.0.

01 b

0,8

..........0

0,5

Results and discussion --

::::-_.

=========

0,1-

Static properties at different temperatures

/.,2

The longitudin~.l properties of carbon fibres were assumed to be independent of temperature because their transition temperature is very high (1700 K). However, the properties of the matrices are highly dependent on temperature. As shown in Figure 2, the matrices get brittle at low temperatures and the fracture strain decreases significantly. As an exception, polycarbonate (PC) at 77 K has a fracture strain more than twice as high as rigid epoxy resins (EP). At 4.2 K it still has a fracture strain of 3.5 %. Therefore, PC was suggested as a matrix polymer for low temperature applicationsL The matrix strain influences the composite behaviour. Composites with tough matrices ( la, lb and 3) show an increase in fracture strain when cooled down, whereas the fracture strain of composites 1 and 5 with rigid matrices decreases. In Figure 3 the measured fracture strengths of the composites and matrices considered are given as a

P% 100 .s /

50

S d

9,5\%

3,5% 100 ""< 2 % 1,5

-- : =~

7'7 ~

293 T,K

Figure 3 Ultimate tensile strength of the three matrices and the tested composites. (Numbers refer to the index given in the tables.) See Figure 2 caption for key to symbols

function of temperature. All matrix systems show a significant increase from room temperature (RT) to 4.2 K*. Again, only the systems with tough matrices (la, lb and 3) show an increase in fracture strength with decreasing temperatures whereas the fracture strength of composites with rigid matrices decreases. Over the whole temperature range, structural elements 2 and 5 show a lower fracture strength than the specimens taken from ideally manufactured plates. The temperature dependence of the Young's modulus can roughly be calculated from Equation (1). As only the Young's modulus of the matrix depends on temperature, the following relationship can be applied AEc(T) ~ AEm(TX1 - V~)< 2.5 GPa

(5)

Accordingly, the increase in Young's modulus of the composites tested should lie between 2 and 7 %. In fact, it is somewhat higher, namely between 4 and 10%.

~ _--

-

_ .

.

.

.

.

.

.

.

.

.

_Ee.

Fatigue behaviour at low temperatures The stress-life (S-N) curves of the materials under tensile fatigue were determined by a non-linear regression analysis applying the following equation s

""~ 3 . 2 %

1,0

au(N) = OD+

O'UT -- O"D

~

(6)

0,5 ®

-

Z,,2

-

r

T

7'7

293 - - - - - T,K

Figure 2 Ultimate tensile strain of three matrices and the tested composites, x , Matrices PC, EPs and EP; O, unidirectional, tough matrix; A , multidirectional, tough matrix; Q, unidirectional, rigid matrix; A , multidirectional, rigid matrix. (Numbers refer to the index given in the table,,J)

where e and # are shape parameters. In Figure 4 the calculated S-N curves of five composites and a semiflexible matrix resin are given. All curves correspond to a 50% probability of fracture. If available, the fatigue *ffUT at RT = 50-80 MPa; 6rUT at 4.2 K = 150-180 MPa. Note that the fracture strength of shear loaded matrices is assumed to increase on the same order

Cryogenics 1988 Vol 28 April

269

Carbon fibre reinforced plastic. K. Ahlborn

1,250 •

~

UD(~)

(_~

Io,9o,oi9o,o]s

o.7~.o 0.732

@

6o x , ~ [0,90/_+45]n

"UD, E P ( ~ I

~

E019010190103s PC@

0,/.87 0

r

I

I

J

I

I

I

1

2

3

4

5

6

7

0,370

8

-~, log N Figure 5 Stress-life curves of the CFRPs tested and of the semiflexible epoxy resin at low temperatures normalized to the static tensile strength. (Numbers refer to index given in tables)

0,172 '-x- ~" Eps "~'~x ~ x~ ~--x~X~. ""

x_

x _

I

100

101

I

10z

I

I

10~

104

r

105

I

10 6

_

I

107

_

I

108

N Figure 4 Calculatedstress-lifecurvesof the CFRPstestedand of a semiflexible epoxy resin at low temperatures (fracture probability 50%, run-outs indicated by arrows), (Numbers refer to the index given in the tables.) See Figure 2 caption for key to symbols

strength after 107 load cycles, aD, for a fracture probability of 10% is given in Table 2 together with other characteristic values from the experiments. UD composites show the highest fatigue strengths and excellent fatigue behaviour. The fatigue strengths of the multidirectional composites are a little lower. The strength of the semiflexible epoxy resin gives the lowest limit 4'5. Obviously, fatigue strength depends on the fibre orientation. In Figure 5 the stress-life curves are given normalized to static values. UD composites again have the highest

relative fatigue strengths, namely 80 %. For multidirectional composites the ratios are lower. The fatigue strength is much smaller for the matrix (stress amplitude about 77 MPa, strain amplitude about 0.7 %)4 than for the fibre (stress amplitude about 3.4 GPa, strain amplitude about 1.4 % in UD). In multidirectional composites, the matrix and all transverse layers fracture at an early stage of fatigue life. Transverse cracks are generated which cut fibres locally in the load bearing UD layer 2. Therefore, the normalized fatigue endurance limit of the angle ply composite 4 is lower (only 70 %). In addition, inadequate craftmanship enhances degradation by fatigue. Angle plies 5 and 3 show relative fatigue endurance limits of 50 and 37 %, respectively. The quasiplanar composite 5 contains a high proportion of voids and misoriented fibres. Cross-ply 3 is poorly bonded and delaminates already after a small number of fatigue cycles. The influence of temperature on S-N curves is low because the fatigue behaviour is (primarily) dominated by the fibres. In Figure 6 the stress-life curves of the unidirectionally reinforced loop (composite 2) are given for three temperatures.

Table 2. Stress ratio, R, frequency, f, static fracture strength and fatigue strengths of the tested CFR Ps and EPs at low temperatures (fracture probability 50 and 1 0 % ) O"D for P = 5 0 %

~D for P = 1 0 %

Index a

Lay-up

R

f(Hz)

~uT(MPa)

(%)

(MPa)

(%)

(MPa)

1 2 3 4 5 Matrix

UD UD Cross- ply Quasiplanar Quasiplanar -

0.1 0.2 0.1 0.1 0.1 0.01

82 120 88 92 87 30

1250 732 740 486 370 172

88 82 ~ 37 ~ 70 ~ 50 57

1100 600 ,~ 274 ~ 340 ~ 185 98

~82 ~ 60 ~43 45

~ 1030 ~ 440 ,~ 159 77

alndex numbers refer to numbers on figures

270

Cryogenics 1988 Vol 28 April

Reference 4 5 4 6 8

Carbon fibre reinforced plastic, K. Ahlborn ],0-

¢ UT 293 K

(_%

1

cR,m

u"

1./,0,5

~

7o ~.__~//~I0

o oX°~ ~ ° ~

o

1.0-

0.6"

0.1 0

0

I

2

3

L,

5

6

7 log N

Figure 6 S-N curve of structural composite 2 at I O, 77 and 293 K (data taken from Reference 6)

0.2-

I0

100

800600-

~UT

'R,T

~

~//~I00 O/o 91%

t,00-

200-

10

4

10 0

>106 ----N

Figure 7 Residual fracture strength at 77 K of cross-ply 3 preloaded with 106 cycles (specimen number and standard deviation indicated)

Properties after fatigue cycling The residual strength and strain of run-outs with a fatigue life of more than 106 cycles were determined in tensile tests. The tensile strength decreased by about 9 % after pre-cycling at stress amplitudes near the fatigue strength, whereas the fracture strain increased by about 4%, as shown in Figures 7 and 8.

Summary It has been found that the fracture strain of the matrix influences the ultimate tensile strain and strength of uni- and multidirectional carbon fibre reinforced plastics at low temperatures. For example, the unidirectional composite with a tough, semiflexible epoxy matrix shows an excellent ultimate tensile strain of 1.5 % and a strength of 2.2 G P a at 77 K.

4

>106 ----- N

Figure 8 Fracture strain at 77 K of cross-ply 3 pre.-Ioaded with 106 cycles (specimen number and standard deviation indicated)

Carbon fibre composites with tough matrices are advantageous for low temperature applications. At low temperatures, for undirectional composites, an excellent tensile fatigue behaviour is observed. For multidirectional composites, the fatigue behaviour is slightly worse. This depends on the proportion of fibres in the load direction and on the influence of cracks generated in transverse layers. After a fatigue life of more than ! 06 cycles, carbon fibre reinforced plastics take more than 90% of their initial strength in a tensile test at low temperature. Because of defects in manufacture and stress concentrations, the strengths of structural elements are in some cases below 60 % of the strengths of laboratory specimens.

Acknowledgements The author thanks the Bundesministerium ftir Forschung und Technologie for funding parts of this work. The investigations described have been performed over the last decade at the Karlsruhe Nuclear Research Centre. Thanks are due to co-workers, especially to Dr G. Hartwig. The useful help of MBB-Ottobrunn and SKTGiessen is also gratefully acknowledged.

References 1 Hartwig, G. Low temperature ductile matrices for advanced fibre composites, in Non-metallic Materials and Composites at Low Temperatures 3 (Eds. Hartwig, G. and Evans, D.) Plenum Press, New York, USA (1986) 153-160 2 Jamison, R.D., Schuite, K., Reifsnider, K.C. and Stinchcomb, W.W. Characterization and analysis of damage mechanisms in fatigue of graphite/epoxy laminates ~4STM-STP 836 (Ed Arber, A.) Philadelphia, USA (1984) 3 Hartwig, G. and Knaak, S. Fibre-epoxy composites at low temperatures Cryogenics (1984) 24 639~47 4 Weiss, W. Low temperature properties of carbon fibre reinforced epoxide resins, in Non-metallic Materials and Composites at Low Temperatures 3 (Eds Hartwig, G. and Evans, D.) Plenum Press, New York, USA (1982) 5 Weiss, W. Tieflemperatureigenschaften yon hochfesten und hoch-

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Carbon fibre reinforced plastic: K. Ahlborn moduligen Kohlen stoffaserverbunden, DGLR-Lecture No. 82-009, Stuttgart, FRG (1982) 6 Schedler, A. Fibre composites in satellites Cryogenics (1988) 28 220 7 Ahlborn, K. and Knaak, S. Cryogenic mechanical behaviour of a thick-walled carbon fibre reinforced plastic structure Cryogenics (1988) 28 273 8 Gecks, M. and Och, F. Ermittlung dynamischer Festigkeitskenn-

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Cryogenics 1988 Vol 28 April

linien durch nichtlineare Regressionsanalyse, Sonderheft DFVLR Strukturmechaniktagung, Ottobrunn (1977) 9 Weiss, W. Ermiidungsverhalten yon Epoxidharzen bei tiefen Temperaturen Progr Colloid Polym Sci (1978) 64 68 10 Rohwer, K. and Yie Ming Jiu Micromechanical curing stresses in CFRP Comp Sci Technol (1986) 25 169 11 Hartwig, G. and Wiichner, F. Rev Sci lnstrum (1975) 46 481