Charpy Impact Test of Epoxy Matrix Composites Reinforced with Buriti Fibers

CHARPY IMPACT TEST OF EPOXY MATRIX COMPOSITES REINFORCED WITH BURITI FIBERS

Anderson de Paula Barbosa1.a, Michel Picanço Oliveira2.b, Giulio Rodrigues Altoé1.c, Frederico Muylaert Margem1.d,

Sergio Neves Monteiro2.e 1 UENF: State University of the Northern Rio de Janeiro, Advanced Materials Laboratory, Av.

Alberto Lamego, 2000, 28013-602, Campos dos Goytacazes, Brazil. 2 IME: Military Institute of Engineering, Department of Materials Science, Praça General Tibúrcio,

80, 22290-270, Rio de Janeiro, Brazil.

[email protected], [email protected], [email protected], [email protected], sergio.neves@ig,com.bre

Keywords: Charpy test, buriti fibers, epoxy composites, impact energy.

Abstract. The buriti (Muritia flexuosa) fiber are among the lignocellulosic fibers with apotential to

be used as reinforcement of polymer composites. In recent years, the buriti fiber has been

characterized for its properties as an engineering natural material. The toughness of buriti

composites remains to be a evaluated. Therefore, the present work evaluated the toughness of epoxy

composites reinforced with different amounts of buriti fibers by means of Charpy impact tests. It

was found a significant increase in the impact resistance with the volume fraction of buriti fibers.

Fracture observations by scanning electron microscopy revealed the mechanism responsible for this

toughness behavior.

Introduction

Engineering fibers reinforced composites belong to a well known class of material, which has been

steadily growing since the past century [1,2]. Synthetic fibers such as glass and carbon constitute

the main reason of successful reinforcement of polymer matrix in composites currently applied in

practically all technological fields, from sport gears to aerospace components [3]. In recent decades,

however, environmental issues concerning long term pollution and climate changes [4] associated

with the stable structure of synthetic fibers, especially the glass fiber which also has an energy

intensive processing, is motivating the use of natural fibers [5-10]. These vegetable-based fibers,

here denoted as lignocellulosic fibers, are not only environmentally friendly but of economical,

social and technical advantages [10]. In Brazil lignocellulosic are abundant and several of them

native of proper region in the country, such as the curaua, piassava and sponge-gourd [11].

Another less-known lignocellulosic fiber with potential as composite reinforcement is that extracted

from the petiole of the buriti palm tree (Mauritia flexuosa), illustrated in Fig 1. In recent years, the

buriti fiber has been studied as a possible engineering material in association with polymer

composites [12-19]. In spite of these research efforts, the impact resistance of these buriti fiber

composites still needs investigation. Thus, the objective of the present work was to investigate the

notch-toughness behavior of epoxy composites reinforced with different volume fractions, up to

30% of continuous and aligned buriti fibers by means of Charpy impact test.

Materials Science Forum Vols. 775-776 (2014) pp 296-301© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.775-776.296

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 177.11.125.12-28/12/13,14:53:18)

Fig 1: Buriti palm tree (a), piece of petiole (b) and buriti fibers cut from the petiole (c).

Materials and Methods

Two basic materials were used in this work. A commercially available, Dow Chemical, type

diglycidyl ether of the bisphenol-A (DGEBA) epoxy, 187.3 g/equiv, resin hardened with

stoichiometric, phr 13, trietylene tetramine (TETA), was applied as matrix. Petiole buriti fibers,

here referred as buriti fibers for short, were kindly supplied by Prof. Nubia S.S. Santos from the

University of Para, North of Brazil. Based on a statistical analysis presented elsewhere [18] six

intervals from 0.1 to 0.8 mm of equivalent diameter were considered. An average diameter of 0.58

mm was calculated. The as-received buriti fibers were cleaned and dried before setting inside

rectangular shaped steel molds with 55x12.7x10 mm dimensions for Charpy specimens.

Continuous and align fibers in volume fraction of 0, 10, 20 and 30% were aligned along the 55 mm

dimension of the mold. Still fluid DGEBA/TETA epoxy was poured onto the fibers in each mold.

The so prepared composites were then allowed to cure for 24 hours at room temperature before

unmolding. Ten composite specimens were fabricated for a given volume fraction of buriti fibers. A

V-notch was simultaneously machined in all specimens according to the ASTM D 6110 norm. The

specimens were impact tested in a Pantec pendulum illustrated in Fig. 2 and set in the Charpy

configuration corresponding to a specimen schematically shown in Fig 2(b).

Fig. 2: Charpy pendulum and specimen schematic.

The fracture surfaces of impact tested specimens were analyzed by scanning electron microscopy

(SEM) in a model SSX-500 Shimadzu microscope. Gold sputtered samples of fracture surfaces

were observed with secondary electrons accelerated at 15 KV.

45°

2.54

63

12.7

10

Direction of fibers alignment

55

Materials Science Forum Vols. 775-776 297

Results and Discussion

Figure 3 shows the macroscopic aspect of ruptured Charpy specimens. In this figure the typical

epoxy composite specimens corresponding to a given amount of buriti fibers, including 0% (neat

epoxy), are completely separated in two parts. One should observe that the neat epoxy specimen has

a uniform traversal fracture. With increasing amount of fiber, the fracture becomes non-uniform

with a tendency towards longitudinal ruptured regions along the buriti fibers, which is clearly

shown for the 30% specimens in Fig 3.

Fig. 3: Typical Charpy impact tested epoxy composites specimens with different amounts of buriti

fibers.

Based on the results shown in Fig 3, it is proposed that the fracture of the neat epoxy occurs by

nucleation of a single crack at the specimen notch, see Fig 2(b). This crack propagates transversally

throughout the brittle epoxy matrix until total rupture. A similar fracture behavior occurs for the

10% buriti fibers composite in which the main crack, nucleated at the notch, propagates most of the

time through the brittle epoxy matrix. Eventually, the relatively few buriti fibers act as barrier for

the main cracks and, as a consequence, other cracks are nucleated at the fiber/matrix interface.

These interfacial cracks promote a decohesion along the fiber surface. Nonetheless, only a relatively

small percentage of the fracture is associated with the rupture of few buriti fibers sticking out of the

mostly transversal fracture of the 10 % specimens, as shown in Fig 2.

For the 20 % specimen in Fig. 2, a large number of ruptured fibers and some longitudinal fracture

can be observed. With 30% of buriti fibers, Fig 2, the fracture surface is irregular and nom-uniform.

A greater participation of longitudinal fracture indicates that rupture is predominantly occurring

along the fibers/matrix interface, with many pointing buriti fibers.

Table 1 presents the results of Charpy impact energy of epoxy matrix composites reinforced with

different volume fraction of aligned buriti fibers. In this table it is important to notice that the

impact energy increases with the amount of buriti fibers. Based on the results presented in Table 1

the variation of Charpy impact energy with the volume fraction of continuous and aligned buriti

fibers is shown in Fig 4. An almost mathematical linear relationship can adjust the points in Fig 4,

where CIE in the Charpy impact energy and V(%) the fibers volume fraction, as indicated in the

insert in Fig. 4. Similar impact energy behavior was reported for distinct lignocellulosic fibers

polymer composites, such as coir [20], piassava [21] and curaua [22] fibers.

298 20th Brazilian Conference on Materials Science and Engineering

Table 1: Charpy impact energy of epoxy composites with different amounts of continuous and

aligned buriti fibers.

Volume fraction of buriti fiber(%) Energy (J/m)

0 13.77 ± 2.07

10 58.26 ± 14.93

20 94.88 ± 15.01

30 114.17 ± 14.37

0 10 20 300

20

40

60

80

100

120

140

C

h

a

r

p

y

i

m

p

a

c

t

e

n

e

r

g

y

(

J

/

m

)

Volume fraction of buriti fiber(%)

CIE = 3.61V% + 13.93

Fig. 4: Variation of the charpy impact energy with the volume fraction of continuous and aligned

buriti fibers.

Regarding the possible mechanism responsible for the significant improvement in the Charpy

impact energy with incorporation of continuous and aligned buriti fibers into epoxy composites,

Fig. 5, shows the fracture surface of a 30% fibers composite. In this figure the SEM image with

lower magnification, Fig. 5(a), reveals buriti fibers (white arrow) adhered to the epoxy matrix but

projecting away from the surface. Empty holes (black arrow) indicate that buriti fibers were also

pulled out from the epoxy matrix. Although part of the fracture corresponds to transversal rupture

through the epoxy matrix, see Fig. 3, of an initial crack nucleated at the specimen notch, another

significant part is associated with longitudinal cracks propagating in the fibers/matrix interface.

This preferential interfacial decohesion is a consequence of a relatively weak interface between the

buriti fibers and the epoxy matrix [23]. Thus, a comparatively larger fracture surface is formed

owing to the presence of the buriti fibers. This results in higher energy, Fig. 4, to debond a

corresponding larger longitudinal surface area in between the fiber and the matrix as compared to

the transversal fracture area in the neat polymer [24].

With higher magnification, Fig. 5(b), the river pattern (black arrow) is an indication that the

transversal crack propagating in the brittle epoxy matrix is arrested by the buriti fiber (white arrow).

Therefore, the combined mechanism of interfacial longitudinal cracks and transversal crack arrest

by the buriti fibers is responsible for the significant increase in the impact resistance, Table 1 and

Fig 4, observed in epoxy composites incorporated with continuous and aligned buriti fibers.

Materials Science Forum Vols. 775-776 299

(a) (b)

Fig. 5: SEM fractographs of a 30 vol% continuous and aligned buriti fiber incorporated composite

(a) 27 X and (b) 100 X.

Conclusions

• DGEBA/TETA epoxy composites incorporated with up to 30 vol% of continuous and aligned

buriti fibers display a significant increase in the notch toughness measured by the Charpy

impact energy. This increase in the Charpy impact energy follows an almost linear relationship

with increasing amount of buriti fibers in the epoxy matrix.

• The fracture surface of the Charpy tested composite specimens, tends to become irregular and

non-uniform with the amount of incorporated buriti fiber, which causes longitudinal rupture in

addition to the transversal rupture through the brittle epoxy matrix.

• The greater longitudinal rupture area associated with fiber/matrix interfacial debonding

together with the arrest of transversal cracks by the buriti fibers are the mechanisms responsible

for the improvement in the toughness of these buriti fibers composites.

Acknowledgements

The authors tank the support by Brazilian agencies CNPq, CAPES and FAPERJ.

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