ELSEVIER

May 1996

Materials Letters 27 (1996) 21-25

Fracture behavior and fracture toughness of particulate filled epoxy composites

Manwar Hussain, Atsushi Nakahira, Shigehiro Nishijima, Koichi Niihara

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

Received 11 October 1995; accepted 23 October 1995

Abstract

Fracture toughness for an epoxy-TiO, composite system was evaluated at room and liquid N, temperature in which both the volume and the particle size of dispersoid have been changed. Fracture toughness was significantly increased by choosing the larger particle size of the dispersed second phase and low temperature testing environment. The relationship between the fracture toughness and the fracture surface appearance of the composites has been investigated using a linear elastic fracture mechanics approach.

Keywords: Fracture toughness; Fracture behavior; Liquid N, temperature; Epoxy composite; Second phase

1. Introduction

Epoxy resins are being extensively used as matrix materials in high performance composites especially in aerospace industry, automobiles, electronics, fu- sion reactors and structural applications. These com- posites are subjected to use at both room and low

temperature. However, there are limitations in these applications because of low toughness of epoxy composites. Thus, a proper and adequate knowledge

of toughness and fracture propagation is required in order to develop materials for high performance applications.

Many researchers [ l-51 have improved the tough- ness of epoxy materials by incorporating soft materi- als and proposed some toughening mechanisms based on fracture mechanics. However, these composites exhibit a lower strength and Young’s modulus. It has been reported that second phase dispersion of a rigid filler into the matrix is a useful technique for tough- ening epoxy resins [6-81. A fracture mechanics ap-

proach is generally used as a failure criterion for

high strength materials and the plane strain fracture toughness (K,) for brittle fracture of the materials. Most of these studies were carried out incorporating glass beads, but no studies were found using micron- and nano-sized TiO, ceramic filler particles. In addi-

tion, the epoxy composites with nano-sized TiO,

possess high strength as reported in our previous

paper 191. In this paper, we investigated the effects of parti-

cle size on the fracture and toughening of cured epoxy resins at room and liquid N, temperature

using small compact tension specimens.

2. Experimental procedures

2.1. Materials

The materials used in this experiment were N.N.N’N’-tetraglycidylmethaxydiamine (TETRAD-

00167-577X/96/$12.00 Q 1996 Elsevier Science B.V. All rights reserved SSDI 00167-577X(95)00254-5

22 M. Hussain et al./Materials Letters 27 (1996) 21-25

X), Mitsubishi Gas Chemical Company, Japan and a tetra functional epoxy resin, 1,2-cyclo-hexanedi-

carboxylic anhydride (HHPA, Wako Junyaku Co> as a hardener. TiO, with a particle size of 1 km from Gojundo Chemical Co. Japan, and 20 nm from Idemitsu Chemical Co. Japan were selected as sec- ond phase particles.

2.2. Fabrication procedures

Epoxy resin was mixed with a filler using the

wet-ball milling technique [lO,ll]. Solvent was then evaporated and hardener was added. After well mix-

ing with hardener, the mixture was poured into an aluminium mold and placed in a vacuum chamber. After de-gassing, the mold was placed in a constant- temperature oven for pre-curing at 80°C for 2 h and post-cured at 180°C for 2 h. Cured samples were then cut, ground and polished for mechanical testing.

2.3. Characterization

Fracture toughness was measured by the mini-

compact tension (CT) method. Samples were ma-

chined to size, and sharp, naturally arrested cracks were introduced for each specimen by a razor blade. The sample dimension and notch are shown in Fig.

1. Fracture was performed at a cross-head rate of 2 mm/min. The fracture toughness value (K,,) was calculated based on the following equation:

KIc = ( P,/bW’12)f( a/W), (1)

where P, is the maximum load seen on the load

diagram, b is the specimen thickness, W is the width, a is the initial crack length and

f(a/W) = (2 + a/W)(0.886 + 4.64a/W

< 38mm *

Fig. 1. Specimen dimension for the compact tension test.

1.6 L

E! ---b TiOZ(n)

~ ,.4 _---&--TiOZ(m) : I’

$ 1.2 - i ,

/ P

0.4 1 I 0 2.5 5 7.5 10 12.5

Volb of filler content

Fig. 2. Fracture toughness as a function of filler content measured

at room temperature.

- 13.32a2/W2 + 14.72a/W

-5a4/6W4)/( 1 - a/W)3’2.

The corresponding crack lengths were measured af-

terward using a sliding microscope. Three different readings were taken for every crack front from three

different positions, the left (L), the right (R) edge and the tip (M) of the crack front due to the curva-

ture nature of the crack front. The readings were

averaged according to the equation:

d=(L+2M+R)/4.

In the liquid N, temperature experiments, the sample was mounted on the tension instrument through the pin-holes and immersed into a liquid N, chamber.

3. Results and discussion

Fig. 2 shows the effect of the filler volume on

fracture toughness measured at room temperature. The effect of particle size on fracture toughness at room temperature is also shown in Fig. 2, where YZ

and m refer to nano-sized TiO, and micron-sized TiO, respectively. Gradual improvement in tough- ness was observed with increase in volume fraction of the filler content for both nano- and micron-sized TiO, systems. However, a significant improvement was observed for the micron-sized TiO, dispersed system at a higher volume (10 ~01%). The improve- ment in fracture toughness with filler content and increase in particle size was explained by observing the fracture surfaces of the specimens.

M. Hussain et al. /Materials L.etters 27 (1996) 21-25 23

Fig. 3 shows the fracture surface of the matrix and specimens containing 1 pm TiO, filler tested at room temperature. The characteristic features of un-

filled epoxy materials, in longitudinal lines region [9] are smooth fracture surfaces, with occasional river patterns. In addition, there are regions with tracks and features (Fig. 3a). For low volume fraction of

Fig. 4. SEM micrographs showing the arrest lines on fracture

surface morphology: (a) no filler, (b) 2.5% nano-sized TiO 2 and

(c) 2.5% micro-sized TiO, filler.

Fig. 3. SEM micrographs showing the influence of tiller on

fracture surface morphology: (a) no tiller, (b) 2.5% TiO, and (c)

10 ~01% of TiO, filler.

the filler (2.5 vol%), features were observed in the epoxy matrix which was aligned in the direction of crack propagation. River patterns and the branches became prominent in this system. Crack propagation was also found to be deviated by filler particles and bowing. However, for a higher volume fraction of filler content, the fracture surface morphology was

24 M. Hussain et al. /Materials Letters 27 (1996) 21-25

dominated by the high dense filler particles. Surfaces were rough and crack direction was changed due to the presence of filler particles as shown in Fig. 3c. An increase in crack branching, crack blunting or bowing and the plastic deformation around the parti-

cles were responsible for higher fracture toughness. Fig. 4 shows the arrest lines of composite with

nano-and micron-sized TiO,. The arrow indicates the direction of crack growth. At the beginning of

the test the fracture surface shows the many steps, i.e. stick slip behavior and each crack jumped and

arrested. This arrest line has a ridged appearance. In case of an unfilled matrix, the crack arrest area is smaller compared to TiO, dispersed epoxy compos- ites. Another feature which can be noticed on nano- sized dispersed TiO, composite surfaces is the oc- currence of secondary arrest lines. Two different shades separated by an arrest line suggest that there

is an abrupt change of the plane angle as seen in larger-particle dispersed TiO, composites in Fig. 4c.

Changes in fracture plane associated with high frac-

ture energy contribute high fracture toughness. Fig. 5 shows the temperature dependence of

toughness for both nano- and micron-sized TiO, sys- tems. It was observed clearly that fracture toughness of both systems increases with filler content at liquid

N, temperature. Similar results were observed at room temperature. However, this value was two-fold at liquid N, temperature compared to the room tem- perature value. Relatively little improvement was observed at the low volume fraction region of TiO,. Micron-sized TiO, showed a significant improve-

ment in toughness at a higher volume fraction of the

2.4 I s + TiOt(n) E 2.2 - --+-Ti02(m) A

g 2-

#@’ :

8’ 1.8 - .’

Fig. 5. Fracture toughness as a function of filler content measured

at liquid N, temperature.

Fig. 6. SEM micrographs of the fracture surface morphology

tested at liquid N, temperature: (a) no filler, (b) 10% nano-sized

TiO, and Cc) 10% micro-sized TiO, filler.

filler content compared to the nano-sized filler dis- persed system.

The effect of testing temperature upon crack prop- agation in the resin and particulate filled resin was also studied. At a particular cross-head speed, differ- ent fracture surfaces were observed as shown in Fig. 6. Crack arrest marking was observed clearly for all

M. Hussain et al./Materials Otters 27 (1996) 21-25 25

the specimens. Crack jumping occurs and the jumps become larger at low temperature as compared to room temperature tests. However, micron-sized TiO, dispersed epoxy composites exhibit the steps of frac- ture, well defined arrest lines and fine markings. High fracture toughness of micron-sized TiO, dis- persed epoxy at low temperature was suggested due to stepwise crack jump, releasing large amount of excess energy which is dissipated as crack branching or plastic deformation. The density of river markings

is higher possibly due to more secondary cracks.

4. Conclusion

Fracture toughness of particulate filled epoxy

composites has been investigated varying the TiO, filler volume fraction, particle size and the test tem- perature. The increase in particle size exhibits higher toughness with volulme fraction. A significant im-

provement in toughness was observed when tested at liquid N, temperature and was explained in terms of

the fracture mechanics approach by observing the

fracture surfaces by SEM.

References

[ll C.V. Bucknall, Advan. Polym. Sci. 27 (1978) 121.

[2] A.J. Kinloch, S.J. Shaw and D.L. Hunston, Polymer 24

(1983) 1341.

131 R.A. Pearson and A.F. Yee, J. Mater. Sci. 21 (1986) 2475.

[4] R.A. Pearson and A.F. Yee, J. Mater. Sci. 24 (1989) 2571.

[51 A.F. Yee and R.A. Pearson, J. Mater. Sci. 21 (1986) 2462.

161 A.C. Maloney, H.H. Kausch and H.R. Stieger, J. Mater. Sci.

18 (1983) 208.

171 AC. Moloney, H.H. Kausch and H.R. Stieger, J. Mater. Sci.

19 (1984) 1125.

[8] A.C. Maloney, H.H. Kausch and T. Kaiser and H.R. Beer, J.

Mater. Sci. 22 (1987) 381.

[9] M. Hussain, A. Nakahira, S. Nishijima and K. Niihara,

Mater. Letters 00 (1996) 000.

[lo] M. Hussain, Y. Oku A. Nakahira and K. Niihara, Mater.

Letters OO(1996) 000.

[ll] B.W. Cherry and K.W. Thomson, J. Mater. Sci. 16 (1981)

1925.