INTERNAL FRICTION AND YOUNG'S MODULUS IN COMPOSITE MATERIALS

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INTERNAL FRICTION AND YOUNG’S MODULUS

IN COMPOSITE MATERIALS

K. Matsushita, S. Nishijima, T. Okada, T. Okamoto

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C10, supplbment au n012, Tome 46, decembre 1985 page C10-569

INTERNAL FRICTION AND YOUNG'S MODULUS IN COMPOSITE MATERIALS

K. MATSUSHITA, S. NISHIJIMA, T. OKADA AND T. OKAMOTO

The Institute of Scientific and Industrial Research, Osaka

University, 8-1 Mihogaoka, Ibaraki, Osaka 567, Japan

Abstract- Internal friction and young's modulus have been studied on glass cloth reinforced plastics in the temperature range of 130 to 350K. The temperature dependence of internal friction shows difference between the matrices. The strain amplitude dependences are also measured on variously fatigued specimens. The degree of fracture in interlaminar can closely relate to the strain amplitude dependences.

Introduction- The mechanical characteristics of composite materials are affected considerably by not only the mechanical behavior of each component but also the interface characteristics. The interface failures degradate the over-all mechanical properties of composite materials even though the reinforcements were not fractured. The interface failure process, therefore, should be clarified in studying the fracture mechanisms of composite materials. In this work the interlaminar fa5lure of Glass Fiber Reinforced Plastics (GFRP) is revealed in terms of internal friction and young's modulus.

Experimental- Two types of commercially available GFRP with epoxy matrix were employed, one was Lamiverre-A supplied by Nitto Electric Industry Co.Ltd. and the other was Hoxan from Hoxan Co.Ltd., which

were defined as EIF45P-E/76 and EIF42P-E/52, respectively (I). They

are reinforced by woven glass cloths. The specimens with the dimen- sions of 100mmx10mmx2mm were cut from a sheet of 2mm thick for a

flexural fatigue test. The specimen axis was parallel to the warp

direction. The fatigued specimens of Hoxan, which was translucent, were also used for the measurements of laser attenuation.

The flexural fatigue test was performed with a frequency of 30Hz

at 294.5 K to introduce an interlaminar failure. The fatigue test was

made at a stress level of 70% of the static breaking stress and the cycles of repeated loading were changed to vary the degree of the interface failure.

The flexural vibration and the free-decay methods were used for measurements of Young's modulus and internal friction, respectively.

C10-570 JOURNAL DE PHYSIQUE

The measurements were carried out within the strain amplitude range of 10-9 to 7x10-7 at 294.5 K and with a fixed amplitude of approxi- mately 7x10-8 down to 130K. The changing rate of the temperdture was 3~/min. The resonant fraquency of the specimens ranged from 600 to 800Hz.

The laser transmittance coefficient,X was measured using He-Ne laser. It is defined by the following formula;

I = I, exp ( - A d )

where I is the intensity of the transmitted light, I, the initial intensity of light, and d the thickness of the specimen.

Results discussion- The fatigue cycle dependence of Young's

modulus are presented in Fig.1 which were measured at the 10-9

strain amplitude on Hoxan. Young's modulus decrease with increasing fatigue cycle. The strain amplitude dependences of internal friction

of Hoxan are presented in Fig.2. Inthese figure the increase of the

specimen numnber means the progress of the fatigue degree. The virgin specimen No.1 show low internal friction, high Young's modulus, and no amplitude dependence. As the fatigue progresses, internal friction begins to increase with smaller strain amplitude. The same features are also obtained in Lamiverre-A (2).

Figure 3 shows the laser attenuation coefficient for Hoxan, The abscissa shows the position of the specimen (i.e. the zero position means the center of the specimen). An increase in fatigue degree makes the attenuation coefficient higher, because of interlaminar

Strain Amplitude Fig.1 Fatigue cycle dependence of

Young's modulus. The applied stress Fig.2 strain anplitude dependence

was 0.7%. of internal friction and Young's

f a i l u r e c a u s e s randam r e f l e c t i o n of l i g h t .

A model t o e x p l a i n t h e s t r a i n a m p l i t u d e d e p e n d e n c e s of Young's modulus and i n t e r n a l f r i c t i o n i s p r e s e n t e d i n Fig.4. The s t r e s s - s t r a i n c u r v e s of f a t i g u e d s p e c i m e n s would bend due t o i n t e r l a m i n a r f a i l u r e . The i n t e r n a l f r i c t i o n i s r e l a t e d t o t h e a r e a of a h y s t e r e s i s l o o p of t h e s t r e s s - s t r a i n c u r v e s and t h e dynamic Young's modulus is

d e t e r m i n e d from t h e a v e r a g e s l o p e of t h e s t r e s s - s t r a i n c u r v e s ( t h e s l o p e of t h e b r o k e n l i n e ) . An i n c r e a s e i n s t r a i n a m p l i t u d e b r i n g s t h e l a r g e r b e n d i n g of s t r e s s - s t r a i n c u r v e s and r e s u l t s i n a l a r g e r

h y s t e r e s i s l o o p and s m a l l e r Young's modulus. The d r a s t i c c h a n g e s a r e b r o u g h t when t h e s t r e s s - s t r a i n c u r v e s d e v i a t e from t h e i n i t i a l s t r a i g h t l i n e and t h i s s h o u l d c o r r e s p o n d t o t h e b e g i n n i n g of s l i p p i n g a t t h e i n t e r f a c e . A more s e v e r e l y f a t i g u e d specimen would bend more markedly i n s t r e s s - s t r a i n c u r v e s and h e n c e e v e n a t t h e i d e n t i c a l

s t r a i n a m p l i t u d e , a more s e v e r e l y damaged specimen shows l a r g e r i n t e r n a l f r i c t i o n and s m a l l e r Young's modulus.

The t e m p e r a t u r e d e p e n d e n c e s of i n t e r n a l f r i c t i o n and Young's modulus i n b o t h v i r g i n s p e c i m e n s (Lamiverre-A: oped c i r c l e s , Hoxan:

I

-

-10 -5 0 5 10

POSITION fmm)

Fig.3 Laser attenuation i n fatigued specimens from Hoxan.

ClO-572

JOURNAL

D E

PHYSIQUE

closed circles) are presented in Fig.5. Both GFRP have

almost same young's modulus

-

9

fraction of glass fiber. n 0

-

The temperature dependence of V)

internal friction is different

from each other as demonstrated

i

Y,

in Fig.5. The difference is

2

$

could be distinguished in terms

of temperature dependence of I I

b

internal friction. 2 0 0 300

TEMPERATURE ( K )

Fig.5 Temperature dependences Conclusion- AS fatigue progresses, of young's n-cdulus and internal

friction in GETU?.

Young's modulus decreases, internal

friction increases. The results agree well to the laser attenuation and shows the degree of interlaminar failure. A large increase in internal friction and a decrease in Young's modulus against strain amplitude have close connection with slipping of the interfaces. The epoxy matrices could be distinguished from the temperature dependence of internal friction.

Acknowledgements

The authors would like to thank Associated Professor T.Hagihara, at Osaka Kyoiku University for his help in laser attenuation measure- ment

.

References

1. M.B.Kasen, "Nonmetallic Materials and Composites at Low Temperatures 2", G.Harteig and D.Evans, eds., Plenum Press, New York (1 982), P.327.

2. T.Okada, S.Nishijima, K.Matsushita, T.Okamoto, H.Yamaoka et al., Advances in Cryogenic Engineering, =(1984), 9.

3. T.Hirai and D.E.Kline, J.Appl.Polym.Sci., =(1972), 3145.