FATIGUE CHARACTERISTICS OF HYBRID COMPOSITES WITH NON-WOVEN CARBON TISSUE

Masahiro DANZUKA 1 ,Yuuta AONO 1, Hiroshi NOGUCHI 1,

Seung-Hwan LEE 2 and Seong-Kyun CHEONG 3

1 Department of Mechanical Engineering Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku Fukuoka 812-8581, Japan

2 Technical Headquarters, Mitsubishi Heavy Industries, Ltd, 5-717-1 Fukahori-Machi, Nagasaki 851-0392, Japan

3 Department of Mechanical Engineering, Seoul National University of Technology, 172 Gongneung-dong, Nowon-gu Seoul 139-743, Korea

ABSTRACT Fatigue characteristics of hybrid composites with Non-Woven Carbon Tissue (NWCT) were investigated under

tension-tension cyclic loading. The hybrid laminates are made by interleaving NWCT prepreg between CFRP(Carbon Fiber Reinforced Plastics) prepreg layers. The lay-ups of laminates are [01/902]s and [01//902]s . The symbol “ // ” means that NWCT is located between CFRP prepreg layers. Static tensile tests were first performed in the two types of laminates to investigate the effect of NWCT on the tensile strength. Then NWCT didn’t directly influence the process of failure. The failure of these materials depended on the strength of 0°layers. Next, the delamination in [01/902]s was compared with the crack propagation for tensile derection in [01//902]s under tension-tension cyclic loading. Then difference of delamination growing from cracks of 90°layers was observed and NWCT seemed to be effective to check the delamination.

1. INTRODUCTION A considerable number of studies have been reported about CFRP, because it has high

mechanical characteristics, for example high specific stiffness and strength. But, in unidirectional composites stiffness and strength are lower for the direction perpendicular to the fibers. So cross-ply laminates should be employed, which increase the mechanical properties for the direction perpendicular to the fibers.

On the other hand, because unidirectional composites are considerable orthotropic materials, the problem that delamination propagates between layers occurs. To check these cracks and enhance the strength between layers, NWCT is interleaved CFRP prepreg layers.(1)(2)(3)

In the previous study, Mode� interlaminar fracture toughness (G�c) of [01//902]s improved 3 times better than that of [01/902]s

(4). In this report, the effect of NWCT in the check of delamination in cross-ply laminates is discussed through the results of replica observation with an optical microscope under tension-tension cyclic loading. In addition, we also report about the effect of NWCT on the tensile strength of cross-ply laminates under static tensile test.

2. EXPERIMENTS 2-1. Specimen

The materials used in the present study are unidirectional CFRP prepreg (SK-Chemicals, USN125 Series), thermosetting epoxy resin film (SK-Chemicals, #SKR2514) and NWCT (TFP, Optimat 203 Series, FAW: 12g/m2). Fig. 1 shows the hybrid prepreg made by overlapping NWCT prepreg and unidirectional prepreg. The hybrid composites were made by stacking hybrid prepreg. The lay-ups of laminates are [01/902]s and [01//902]s. The symbol “ // ” means that NWCT is located between the CFRP prepreg layers. The cut edges of the specimens were polished for microscopic observation. Fig. 2 shows photographs of specimen edges. Geometry of test specimen is given in Fig. 3. The test specimens were end-tabbed with E-glass tabs of 50 mm length and 1.6 mm thickness. The mechanical properties of NWCT and unidirectional CFRP laminates are given in Table 1.

On the other hand, in whether NWCT is interleaved between the CFRP prepreg layers or not, we have to care about considerable change of fiber volume fraction of UD layers. The Vf of UD

layers was determined from measurement by the Line Method, (5) Vf [01/902]s = 62% and Vf [01//902]s = 74%. Table 2 shows that Young’s modulus of 0°and 90°ply is calculated with Young’s modulus of carbon fiber and epoxy resin calculated backward with E1, E2 and Vf in Table 1. Young’s modulus of cross-ply laminates is calculated by the rule of mixtures with the values in Table 2. Fig. 4 shows the process of calculating Young’s modulus of cross-ply laminates.

T g

(a) [

NWC

Unidirectional Prepreg

Resin Film

g

“Fig. 1. Schematic of hybrid prepreg.”

01/902]s (b) [01//902]s

100µm

“Fig. 2. Photographs of specimen edges”

specimen [01/902]s [01//902]s

t 0.74 0.70

“Fig. 3. Geometry of specimen.”

“Table 1. Mechanical properties of the CFRP.”

Hybrid Prepre

NWCT Prepre

N CT layer

W

100µm

“Table 2. Young’s modulus in the case of different Vf.”

“Fig. 4. The process of calculating Yo g’s modulus of cross-ply laminates.”

-2. Experimental procedures

T ts (6) were performed using an Autograph 5000A SHIMADZU at a cross head

tests were performed using a servo-hydraulic testing machine (SHIMADZU se

V f E 1 [GPa] E 2 [GPa]62% 140 8.774% 170 13

UD CFRP

0� ‹ 90� ‹Young's modulus

E [GPa] 146 9.4 14

Tensile StrengthY [MPa] 2600 70 215

Poisson's ratioυ

0.3 0.02 0.41

Shear modulusG [GPa] 4.9 4.9 5.0

UD (V f = 65%) NWCT(V f = 11%)

UD Vf = 65 %

Experiment E1 , E2

Ef , Em[01/902]s UD Vf = 62 %

[01//902]s UD Vf = 74 %

Cross ply E1 , E2

The rule of mixtures

The rule of mixtures

Measurement

UD Vf = 65 %

Experiment E1 , E2

Ef , Em[01/902]s UD Vf = 62 %

[01//902]s UD Vf = 74 %

Cross ply E1 , E2

The rule of mixtures

The rule of mixtures

Measurement

The numerical subscripts 1: 0° direction 2: 90°direction

un

2

Tensile test he tensile tes

speed of 0.5 [mm/min]. The strain was measured using an extensometer with a gauge length of 50 mm. The tensile test is respectively performed in the two specimens for [0 /90 ] and [0 //90 ] . For one specimen the tests were respectively stopped at 1.0% and 1.3% and the specimens were unloaded to observe the edges, then loaded again. For another specimen the tensile tests were performed without stopping to failure.

Fatigue test (7)

1 2 s

1 2 s

The fatigue rvopulser EHF-FB1 control system). Tension-tension fatigue tests were performed under a

load-controlled mode with a sinusoidal waveform at a frequency of 10Hz and a constant stress ratio of 0.1(R = σmin /σmax). Tests were continued to N = 107 in order to establish the S-N curves.

All tests were performed at room temperature. The fracture surfaces and the edge surfaces of sp

. RESULTS & DISCUSSION

stress-strain curves of the two types of specimens. This Figure also shows el

T

“Table 3. The result of experiment”

ig. 6 shows the replica photographs of [01/902]s and [01//902]s specimen edges at 1.0% strain. F

men edges at 1.3%.At

crack density

ecimens during the tensile or the fatigue tests were observed using replica and the failure mechanism was discussed.

3Tensile test Fig. 5 showsasticity calculated by the rule of mixtures for cross-ply laminates with the values of Table 2.

The each line calculated by the rule of mixtures overlapped with the value of experiment. From this figure it is found that failure strains are the same (about 1.5%) in [01/902]s and [01//902]s specimens. So, the improvement of strain at fracture by the effect of NWCT was not observed.

Table 3 shows tensile strength, strain at fracture and the transverse crack density. From this able, the crack density of [01//902]s is 1.5 times as large as that of [01/902]s. This is reason why

90°Young’s modulus of [01//902]s is larger than that of [01/902]s or stress concentration is larger between fibers because of higher Vf.

0 0.5 1 1.50

200

400

600

800

1000

strain ε [%]

stre

ss σ

[MPa

]

experiment the rule of mixtures in elasticity[01/902]s [01//902]s

“Fig. 5. Stress-strain curves.”

specimenσ B

[MPa]

ε B

[%](e = 1.3%)[1/mm]

average676 1.43 -722 1.5 0.62736 1.43 -793 1.5 1.08

[01/902]s

[01//902]s

For [01/902]s the transverse crack propagated to 0°ply, but for [01//902]s it didn’t and it stopped

at short fiber in NWCT layer. This was also observed in another point. Next, Fig. 7 shows the photographs of [01/902]s and [01//902]s speci

1.

Loading Direction Loading Direction

(a) [01/902]s, ε = 1.0 % (b) [01//902]s, ε = 1.0 %

“Fi ransverse crack.”

Loading Direction Loading Direction

(a) [01/902]s, ε = 1.3 %

“ Repl rack d delamination.”

atigue test

ws the relation between total length of crack propagation for tensile direction ob

3%, the transverse crack propagated to 0°ply for [01//902]s and the delamination occurred in both specimens regardless of behavior had occurred in NWCT layer before and the specimens extended to failure. Therefore, in the tensile test NWCT doesn’t directly influence the process of failure and the failure of these materials depend on the strength of 0°layers.

g. 6. Replica photogr of taphs

(b) [01//902]s, ε = 1.3 %

100µm 100µm

100µm 100µm

Fig. 7. ica photogra of tr sverse c anphs an

FFig. 8 shoserved at the edge and the number of cycles for the two types of specimens at σmax = 450

[MPa]. Fig. 9 shows the definition of crack propagation for tensile direction L [mm]. L in [01//902]s gradually increased as a number of cycles increased. Onset of delamination in [01/902]s was later than onset of crack propagation within NWCT layer in [01//902]s, but propagation was faster. So it is said that by interleaving NWCT between CFRP prepreg layers the propagation of delamination is checked. Fig. 10 (a) and (b) show the photographs of delamination and crack propagation within NWCT layer. For [01/902]s delamination straightly propagates along between layers, but for [01//902]s the crack propagates within NWCT layer like a wave. From this photograph, the appearance that NWCT checks crack propagation between 0°and 90°layer is observed.

“Fig. 9. Definition of crac r tensile direction L.”

Loading Direction Loading Direction

1 2 s (b) [01//902]s

“Fig. 10. thin NWCT.” Next Fig. 11 with NWCT

be

etween stress state of each

“Fig. 8. Relation between total length of delamination and number of cycles.”

k propagation fo

(a) [0 /90 ]

∑= iLL

100 µm

104 1050

50

100

Number of cycles N

Tota

l len

gth

of c

rack

pro

paga

tion

fo

r ten

sile

dire

ctio

n L

[mm

]

σmax = 450MPa [01 / 902]s [01 // 902]s

crack propagation

100 µm

NWCT layer

delamination

90� ‹ply

0� ‹ply Li

90� ‹ply

0� ‹ply Li

Photographs of delamination and crack propagation wi shows the S-N curves. The fatigue limit reduced by interleaving

tween CFRP prepreg layers. This seems that the stress of each layer in [01//902]s is larger than that in [01/902]s because of larger Vf or thickness of NWCT layer.

Therefore, as the assignment after this the quantitative relation b

la

. CONCLUSIONS sts of hybrid composites with non-woven carbon tissue were performed,

2 were

eferences ., Noguchi, H., and Cheong, S. K. “Fatigue behavior characteristics of hybrid composites with

2. of hybrid composites with non-woven

3.

4. n carbon tissue on interlaminar

5.posite

yer and the propagation or the onset of delamination is suggested.

104 105 106 107

300

400

500

600

Number of cycles to failure [N]

Max

imum

stre

ss [M

Pa]

[01 / 902]s [01 // 902]s

R = 0.1

“Fig. 11. S-N curves.”

41 The static tensile te

with the result that NWCT was not effective on improvement of the strain at failure. The tension-tension fatigue tests of hybrid composites with non-woven carbon tissue performed, with the result that NWCT had a function of checking propagation of delamination.

R1. Lee, S. H

non-woven carbon tissue.”, J Compos Mater., 37/3 (2003), 253-268. Lee, S. H., Noguchi, H., and Kim, Y. B. “Damage mechanism

carbon tissue subjected to quasi –static indentation loads. ” J Compos Mater, 37/4 (2003), 333-349. Lee, S. H., Noguchi, H., and Cheong, S. K. “Static behavior characteristics of hybrid composites with

non-woven carbon tissue”, J Compos Mater, 37/3 (2003), 233-252. Lee, S. H., Noguchi, H., and Kim, Y. B. “Effect of interleaved non-wove

fracture toughness of laminated composites. �. Mode�”, J Compos Mater, 36/18 (2002), 2153-2168. L. A. Carlsson , R. B. Pipes Experimental Characterization of Advanced Composite Materials.

om6. ASTM D3039/3039M-95 Standard Test Method for Tensile Properties of Polymer Matrix CMaterials (1995).

7. ASTM D3479/D3479M-96 Standard Test Method for Tension-Tension Fatigue Matrix Composite Materials (1996).