THE EFFECT OF FIBRE ORIENTATION ON THE MECHANICAL PROPERTIES OF SISAL ROPE COMPOSITES

Abstract

Synthetic fibre reinforced composites have been widely used in aerospace engineering, combining high mechanical performance and low densities. In the last decades, the automotive industry has been also attracted for such materials, mainly those composites made with natural fibres, being considered a sustainable alternative for many internal parts of cars. The use of natural fibres, such as sisal, have been expanding due to their low cost, moderate strength, besides being from a renewable source. In general, the use of natural fibres in industry has been hindered by the lack of a continuous fibre. In Brazil, a handmade technique has inspired the fabrication of a variety of sisal ropes for years. This work investigates the use of sisal ropes as reinforcement of epoxy polymer composites. The effect of sisal fibre orientations (0°, 45° and 90°) on the tensile strength and stiffness of cold pressed (0.26MPa) natural composites was evaluated. The elastic moduli/tensile strengths found for the levels 0°, 45° and 90° were 3.5GPa/34.92MPa, 1.86GPa/5.85MPa and 1.47GPa/5.29MPa, respectively, revealing a promising composite material for secondary structural applications.

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BCCM-3 – Brazilian Conference on Composite Materials
Gramado, RS - Brazil, August 28-31, 2016
THE EFFECT OF FIBRE ORIENTATION ON THE MECHANICAL
PROPERTIES OF SISAL ROPE COMPOSITES
Renan Borges Costa*, Luciano Machado Gomes Vieira, Júlio Cesar dos Santos, Márcio
Eduardo Silveira, Túlio Hallak Pazera
Centre for Innovation and Technology in Composite Materials - CITeC, Department of Mechanical
Engineering, Federal University of São João del Rei - UFSJ, Minas Gerais, Brazil
*Corresponding author: Renan Borges Costa
Abstract: Synthetic fibre reinforced composites have been widely used in aerospace engineering,
combining high mechanical performance and low densities. In the last decades, the automotive
industry has been also attracted for such materials, mainly those composites made with natural fibres,
being considered a sustainable alternative for many internal parts of cars. The use of natural fibres,
such as sisal, have been expanding due to their low cost, moderate strength, besides being from a
renewable source. In general, the use of natural fibres in industry has been hindered by the lack of a
continuous fibre. In Brazil, a hand-made technique has inspired the fabrication of a variety of sisal
ropes for years. This work investigates the use of sisal ropes as reinforcement of epoxy polymer
composites. The effect of sisal fibre orientations (0°, 45° and 90°) on the tensile strength and stiffness
of cold pressed (0.26MPa) natural composites was evaluated. The elastic moduli/tensile strengths
found for the levels 0°, 45° and 90° were 3.5GPa/34.92MPa, 1.86GPa/5.85MPa and
1.47GPa/5.29MPa, respectively, revealing a promising composite material for secondary structural
applications.
Keywords: Sisal, tensile properties, hand lay-up, cold-pressing, fibre orientation.
1. INTRODUCTION
The use of laminated composites nowadays, mostly, synthetic fibers in their fabrication
process, and glass and carbon fibers are still the most used ones in structural projects, since they
present low density and satisfactory mechanical performance. The high cost of synthetic fibers and its
difficult disposal to the environment are currently the main reasons to the search of new biodegradable
composites. Vegetable fibers, like sisal, coconut, curaua and jute, are among the fibers researched
nowadays. These ones are very different from each other, since they might come from different
species, leaves or fruits, providing long and short threads and different transversal sections.
Nowadays, the use of resins reinforced by natural fibers has been widely explored in engineering.
‘This occurs due to the increasing of the magnitude of its resistance and rigidity when formed by
fibers’ [1]. The sisal fiber, used as raw material in the production of threads, is extracted from the sisal
plant (genus Agave spp. L., family Agavaceae), being Brazil the biggest producer of sisal in the world,
and the state of Bahia is responsible for 80% of the entire national production of the fiber. Only 4% of
the gross mass is used in the production of the threads, being the remaining 96% disposed or being
used as compost [2].

Costa, R. B., Vieira, L. M. G., dos Santos, J. C.
The main advantages of the use of vegetable fibers, like sisal, includes the biodegradability,
renewable and its availability may be considered unlimited, it requires lower amount of energy in their
production; low cost production when compared to other fibers currently used in the market and
generate incomings, development and contributes in the local economy [3]. Besides the advantage of
being a product of easy disposal, the sisal fiber may be driven to the directions of the strain the body
will be subjected to, allowing a wide range of options, not only when it comes to performance, but
also in the types of production processes.
The mechanical behavior of a composite may be affected from several parameters, such as the
fibers angle, adhesion of the fiber/resin (interface), chemical treatment of the fibers, variation in the
resin used, among others. ‘The vegetable fibers have the characteristic of non-uniformity along their
threads, which makes the use of the ropes made from these fibers a great precursor of laminates and
making necessary the use of long ropes to produce them. Sisal has fibers that vary from 90 to 120cm
length, which makes it a great option of work in the production of ropes or wefts’ [4].
The rope has as main advantage its unlimited extension, because it is made of the union of
numberless threads twisted in spiral among each other, allowing, in its production, the choice of length
and diameter of the thread, which makes it a raw material desirable by the industries, since it is
possible to produce a rope with no size limitations, something that does not apply to the unidirectional
rope, which is restricted by the length of the leaf from where it is extracted.
According to Silva [5] the addiction of sisal fibers in hybrid composite provided not only
higher modulus of elasticity and mechanical strength, but also decrease density values using low level
fraction of sisal fibers (30%). Vieira [6] noted that interaction fiber/resin is crucial to loading
distribution on sisal fiber, and this distribution can be improved increasing the superficial area
between fiber and resin. Thus, the use of correct angles promote a better loading distribution along
rope, benefiting proprieties as modulus of elasticity and mechanical strength.
The present work aims to study the mechanical performance of a composite that uses the sisal
rope as reinforcement, and observe the variations in the stresses due to the change of the disposition of
the ropes, with orientations of 0º, 45º and 90º. The analysis of variance (ANOVA) at 5 % of
significance level (α) was conducted to identify differences between means on the response-variables,
tensile modulus and strength. Equivalence between mean values of forces was assumed as null
hypothesis (H0) and nonequivalence as alternative hypothesis (H1). A P-value equal or lower than 0.05
implies rejecting H0, which reveals the factor significantly affect the response. Kolmogorov-smirnov
testing, which evaluates the distribution of the data and verifies the homogeneity of variances between
fibers alignments, were used to validate the ANOVA. This testing considers a normality distribution as
a null hypothesis and non-normality as an alternative hypothesis. A P-value superior to the
significance level 0.05 implies an acceptance of H0 and rejecting it otherwise.
2. MATERIAL AND METHODS
It was used ANOVA One-Way to evaluate the orientation factors. Each condition makes up a
different population, being that theoretically oriented fibers in 0º must have superior resistance
modulus values.
The process to produce the models can be divided in three steps: production of the mold,
preparation of the fibers and lamination. The mold was made with two steel plane plates and an area
proper for lamination of 60 x 32cm was obtained. To hold a good surface finishing on the laminate, a
treatment was performed on the surface of the plates with two sandpapers of high granulometry and
later covered with armalon, in order to reduce the variations that could still occur on the surfaces of
the plates and to facilitate the draining of the resin inside the mold, thus, allowing a uniformity on the
distribution of the resin inside the mold (Figure 1a).

BCCM-3 – Brazilian Conference on Composite Materials
Gramado, RS - Brazil, August 28-31, 2016
The fibers used on the lamination came from a sisal rug, which the fibers with bad appearance
and with thickness different from the average were removed, followed by the removal by hand of
almost every thread of the fabric. Then, the fibers were aligned one by one, occupying all the empty
spaces in the mold, which provides a minimum space between the ropes and also higher amount of
fiber per unit of area (Figure 1b).
After the uniform distribution of the fibers over the whole mold, the epoxy resin was inserted
over the entire extension of the fibers, which were rubbed by hand, aiming to move the spaces
between the ropes of the thread, allowing a better insertion of the resin into the thread, thus generating
a wider resin/fiber contact area and a better interface between both materials. To promote the
dissipation of occasional air bubbles in the composite which would generate concentration points of
stress, the cold press process was adopted, with a 0.26MPa compression.
The platen used in the process was of the brand Macon, with 15 tons capacity, and the
pressure applied by it was aimed at the center of the mold, thus, promoting a uniform pressure above
all the extension of the laminate. The mold was also sealed with adhesive tape to prevent possible loss
of resin and, consequently, variation in the matrix/reinforcement proportion, which was 50/50. The
mold was under pressure for 15 hours before being removed, period settled by the manufacturer of the
resin (Figure 1c).
Figure 1-(a) Production of the mold, (b) selection of the fibers, (c) lamination.
After the removal of the laminate of the mold, 7 days had been waited before it was handled.
This waiting time is taken in order to promote the total heal of the basic compounds that creates the
resin. With the laminate composite in working conditions, it was cut according with the orientations of
the fibers, of which the stresses will be studied in the directions 0º, 45º and 90º. The samples were
tested in the Shimadzo AG-X plus machine, with 100kN capacity and controlled room temperature of
23ºC, with head displacement to the movement of traction of 2mm per minute. Their dimensions were
based on the traction rule in composites ASTM D3039.

Costa, R. B., Vieira, L. M. G., dos Santos, J. C.
3. RESULTS
Table 1 shows the values of MOE and tensile strength according to angle variation of Sisal
fibers. The elastic modulus varied from 1.47 GPa to 3.68GPa. The tensile strength varied from
5.29MPa to 39.23MPa. Table 2 shows the ANOVA One-Way analysis for the composites. One-way
ANOVA was conducted to verify the influence of fiber alignment on the MOE and strength responses.
The P-values lower than 0.05 (Table 2) reveals the fiber alignment significantly affect the MOE and
tensile strength responses.
Kolmogorov-smirnov test was conducted based on the ANOVA residues. P-value obtained
was higher than 0.05 for all responses, ranging between 0.079 and 0.099 (Table 2), thereby indicating
homogeneity of data. Tuckey test, see
Table 3, indicates whether the experimental levels are significantly different. If a mean value
does not share a letter group (i.e., A, B, C), it means it is significantly different from the others.
Table 1. Means of elastic modulus and ultimate strength
Fiber Orientation
Young’s Modulus
(GPa)
Ultimate Strength
(MPa)
0°
3.68 ±0.11
39.23 ± 1.36
45°
1.86 ±0.072
5.60 ± 0.21
90°
1.47 ± 0.062
5.29±0.41
Table 2. ANOVA One way
Factor
p-value
Modulus
Strength
Fiber orientation
0.000
0.000
R² (adj)
99.33%
99.75%
Normality test (kolmogorov-smirnov)
0.075
0.099
Table 3. Tukey Test
Fiber orientation
Modulus
Strength
Group
Group
0°
A
A
45°
B
B
90°
C
B
The results were consistent with other experiments already performed. Navin Chand [7]
describes in his study that such fact is resultant of the flexibility that the sisal fiber has in its thread and
of the variation of the fiber/resin contact area, which varies according to the orientation of the fibers,
allowing, thus, an interchange of stress and deformities more uniform between matrix and
reinforcement, which reduces the fragile fracture of the epoxy matrix and causes the increase of the
resistance to the shearing.

BCCM-3 – Brazilian Conference on Composite Materials
Gramado, RS - Brazil, August 28-31, 2016
According Table 2 and Table 3 the composites samples machined in 45º direction exhibited
tensile strength nearly the same as those in the transverse direction (90º) and significantly lower than
the longitudinal direction (0°). The 45º direction samples indicated a lower ductility, see Figure 1, as
compared with longitudinal direction, explained by strain limiting effects of fibers and more
pronounced matrix shear band formation in this direction.
Figure 1. Stress - Strain curves of composites under alignment of 0º, 45º and 90º.
Therefore, it was observed a resistance to maximum traction in the longitudinal fibers almost
two times higher than in the other ones, where the resistance is practically the resistance provided by
the iteration fiber/matrix, because the contact area to promote the distribution and adjustment of the
sisal thread is much lower, causing a crack in the interface of both materials. These facts that did not
occur in the longitudinal fibers, which adjust themselves, firstly, to the stress applied to them and then
distribute this stress equally among all ropes that form them.
After that, occurred the beginning of the crack of the resin and a decrease of the stress over the
model. This decrease is a result of the adjustment of the sisal rope due to its flexibility and consequent
new distribution of force on the fibers of the model, successively, until the fibers of the rope do not
support the stress applied to them and the cracking of the fibers is initiated, causing the collapse of the
sample. Besides the parameters listed, it is worth mentioning other parameters that influence in the
stress found in vegetable fibers. Fernandes [4], in his studies with sisal fibers as reinforcement in
wooden structures, uses the variation of the winding of the threads, amount of fiber per unit of area
and chemical treatments on the surface or the rope as parameter of study. It was possible to notice a
significant variation in the stresses found, which creates the possibility of studies about the iteration of
the variation of all these parameters together.
4. CONCLUSIONS
0
5
10
15
20
25
30
35
40
0 0,005 0,01 0,015 0,02 0,025 0,03
Stress (MPa)
Strain (%)
0º
45º
90º

Costa, R. B., Vieira, L. M. G., dos Santos, J. C.
Given the theoretical and practical results, the rope made of sisal used as reinforcements in
structural composites is technically and financially practicable Besides being a very low cost raw
material when compared to synthetic fibers, the sisal fiber is biodegradable, which makes it a great
alternative to reduce the impact to the environment in different industrial sectors. The stresses
supported by the fiber may vary from 5.29MPa to 35.92MPa in average, making the substitution
of synthetic fiber for the vegetable fiber possible in different applications, due to the variability of
stresses observed with the changes in the orientation of the fibers, allowing to model them
according to the stresses that the material will take.
ACKNOWLEDGEMENTS
The authors would like to thank the Brazilian Research Agencies, CNPq, CAPES and
FAPEMIG for the financial support provided.
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