Drop Weight Impact Studies of Woven Fibers Reinforced Modified

Leonardo Electronic Journal of Practices and Technologies

ISSN 1583-1078

Issue 24, January-June 2014

p. 97-112

97 http://lejpt.academicdirect.org

Drop Weight Impact Studies of Woven Fibers Reinforced Modified

Polyester Composites

Muhammed Tijani ISA1*, Abdulkarim Salaw AHMED1, Benjamine Olufemi ADEREMI1,

Razaina Mat TAIB2, Hazizan Md AKIL2, Ibrahim Ali MOHAMMED-DABO1

1Department of Chemical Engineering, Ahmadu Bello University, Zaria Nigeria, 810261.

2School of Materials and Mineral Resources, University of Sains Malaysia, 14300, Nibong Tebal Penang, Malaysia.

E-mails: [email protected]; [email protected] ; [email protected]; [email protected]; [email protected]; [email protected]

* Corresponding author: Phone: +2348034536374

Abstract

Low velocity impact tests were conducted on modified unsaturated polyester reinforced

with four different woven fabrics using hand-layup method to investigate the effect of

fiber type and fiber combinations. The time-load curves were analysed and scanning

electron microscopy was used to observe the surface of the impacted composite

laminates. The results indicated that all the composites had ductility index (DI) of

above two for the test conducted at impact energy of 27J with the monolithic composite

of Kevlar having the highest DI. The damage modes observed were mainly matrix

cracks and fiber breakages. Hybridization of the fibers in the matrix was observed to

minimize these damages.

Keywords

Hybrid; Fibers; Microstructures; Lay-up; Ductility Index; Impact.

Drop Weight Impact Studies of Woven Fibers Reinforced Modified Polyester Composites

Muhammed T. ISA, Abdulkarim S. AHMED, Benjamine O. ADEREMI, Razaina M. TAIB, et al.

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Introduction

Fiber reinforced composites have wide range of engineering applications in areas like

aerospace, automobile, defense and marine, because of their advantageous characteristics of

light weight , high strength, stiffness and resistance to high temperature [1- 4]. Despite these

advantages of their properties, they are still susceptible to damages caused by various factors

during manufacture and in service [5]. The damage mode caused by low impact velocity

loadings on composites consists of delamination, matrix cracking and fiber failure [4-5]. The

damage mode in high impact velocity loading is essentially the same for low impact velocity,

but with additional damage mechanisms like shear plugging [6].

Enhancement of the impact performance of composites by various methods has been

shown through several researches. Fiber treatment [7], interleaving [8], hybridization of fibers

[1, 3, 5, 8-13] and matrix modification [14] are prominent among the methods reported.

Hybridization is the combination of two or more fibers in a matrix. Hybridization has been

used by many workers because of its advantages of reducing cost by combination of cheap

fibers with expensive ones and the ability to optimize composites properties [15]. It has been

reported that the type of fibers used, fiber configuration and stacking sequence has effects on

structural and mechanical performance of hybrid composites [8, 11, 15]. It was established

that laminates with asymmetric stacking sequence outperform those with symmetric sequence

[7].

In most of the reported work, two fibers were commonly hybridized in a matrix and in

some cases short fibers are combined with woven fibers in the matrix. Carbon/epoxy

interlayer with short Kevlar 49 fiber [5], E-glass fiber in vinylester with different coupling

agent [7], aramid/polyethylene fiber in vinylester [8], polyethylene/carbon fiber in epoxy

resin [12], S2 glass/ carbon fiber in epoxy resin [9], glass/polyethylene fiber in poly(methyl

methacrylate) [16], wood fiber/talc in polyhydroxybutyrate -co- vatrate [17], bio/glass fiber in

polyester resin [18] and short glass fiber/wollastonite in polypropylene [19] are few examples

from numerous systems that have been studied. In these systems maximum of two fibers were

combined in a matrix.

Considering a wide range of materials available for combinations, there is a need to

investigate other system combinations and more importantly, a system with more than two

woven fibers in a modified matrix.

In this study the drop weight impact response of glass, Kevlar, nylon and locally


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99

woven nylon fibers composites and their hybrids (bi and ter) in modified polyester matrix

were investigated via analysis of the load-time curves and scanning electron microscopy

(SEM).

Material and Method

The materials employed in this investigation were general purpose polyester resin

manufactured by ADD resins and chemicals (pty) Ltd., South Africa. The glass fiber was

woven roving E-glass fiber of denier 10,820, tightness of weave 7.65 cm2, manufactured by

Jiaxing Sunlong Industrial and Trading Co., Ltd., China. The Kevlar 49 fiber was a plain

weave mat with denier 1500, tightness of weave 46.24 cm2, manufactured by Carr

Reinforcement Limited UK. The nylon fiber was plain weave with denier 1320, tightness of

weave 183.40cm2, manufactured by Tar Erh, Co. LTD China. The hand woven nylon fiber

had denier 2360, tightness of weave 117.81 cm2. Dicotyl phthalate (DOP) manufactured by

Zhenzhou p and b Chemical Co. Ltd, China. The fiber mats were used as purchased, while the

polyester resin was modified by 5 wt% dicotyl phthalate (DOP).

Production of Composite samples

5 wt% of dioctyl phthalate (DOP) [20-21] was added to General purpose polyester

resin and mixed for 10 minutes, after which 2 wt% of methyl ethyl ketone was added, mixed

for 3 minutes and 2 wt% cobalt accelerator was then added and mixed for another 2 minutes.

Previously, a metallic mould of dimension 21x16x4 cm was cleaned and coated with release

agent.

Nine layers of the glass fiber were weighed and then hand layup in the mould using 1

inch pure bristles brush to apply the polyester mix one after the other. The mould was covered

and transferred to a Carver laboratory hydraulic press, (model M, serial number, 23505 - 208)

for compression at a pressure of 1013.40 kN/m2 [22]. The mould was removed from the press

after 6 h and was left to stay for another 18 h after which the content, glass fiber reinforced

polyester (GFRP) was removed and post cured in the oven at 60oC for 3 h. The same

procedure was adopted to produce other samples: Kevlar composite (KFRP), nylon composite

(NFRP) and locally woven nylon composite (LNFRP). Asymmetric stacking sequence was



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used to produce hybrid of samples of Kevlar and glass composite (KGFRP), glass and nylon

composite (GNFRP) and glass and local nylon composite (GLNFRP) using 5 layers of glass

with 4 layers of the other fiber respectively with the same procedure as earlier described. Ter

hybrid composites of the fibers were produced using asymmetric stacking sequence with 3

layers each of the fibers. Some basic features of the laminates and their compositions are

presented in Table 1 and 2. Figure 1 shows sample schematic diagrams of composite

laminates. Figure 1 A serves as sample for NFRP, KFRP and LNFRP, Figure 1 B serves as

sample for GNFRP and GLNFRP but with the 5 layers of glass fiber on top of the 4 layers of

nylon fiber and local nylon fiber respectively and Figure 1 C serves as sample for the

KGLNFRP.

(A) (B)

(C)

Figure 1. Sample diagrams of composite laminates: (A) laminate of 9 layers of glass fiber, (B) hybrid laminate of 4 layers of Kevlar fibers and 5 layers of glass fiber (C) ter hybrid of 3

layers each of Kevlar, glass and nylon fibers [23]

9 layers of glass fiber

4 layers of Kevlar fiber

5 layers of glass fiber

3 layers Kevlar fiber

3 layers glass fiber

3 layers nylon fiber


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Table 1. Basic Features of Composites Laminate Used Material Thickness (mm) Density (g/cm3) Geometry (mm x mm)

GFRP 4.5 1.887 80 x 80 KFRP 4.0 1.146 80 x 80 NFRP 4.0 1.145 80 x 80

LNFRP 9.0 1.161 80 x 80 KGFRP 4.0 1.518 80 x 80 GNFRP 5.0 1.522 80 x 80

GLNFRP 7.5 1.364 80 x 80 KGNFRP 3.5 1.373 80 x 80

KGLNFRP 7.0 1.200 80 x 80

Drop Weight Impact Test

The samples were cut into approximately size of 80 mm x 80 mm and one sample at a

time was held on a beam with double sided tape, on the sample holder of a drop weight

impact tester that was set up with sensitivity of – 4.019, fast on trigger mode, pre-time and

post-time of 1000 seconds respectively. A weight of 2 kg was added to a load cell which

already weighed 2.5 kg with hemi-spherical impactor of length 60 mm and diameter of 12.24

mm. The load cell was then released from a distance of 0.1m which is equivalent to 4.5 J to

impact on the sample. The load – time response was captured through Dewsoft, software for

capturing load-time response data. The same procedure was followed to test other samples at

impact distance of 0.4 m and 0.6 m corresponding to impact energy of 18 J and 27 J

respectively. The drop weight impact test was performed for all composite materials

produced.

Table 2. Composition in Weight Percent of Fibers in Composites, wt % = Weight of

Reinforcement/total composite weight x 100 [23] Reinforcements (wt %)

Composites Glass fiber Kevlar 49 fiber Nylon fiber Local nylon fiber GFRP 59.2 (9 layers) 0 0 0 KFRP 0 40.8 (9 layers) 0 0 NFRP 0 0 50.0 (9layers) 0

LNFRP 47.5 (9 layers) KGFRP 43.01 (5 layers) 11.89 (4 layers) 0 0 GNFRP 41.0 (5 layers) 0 16.0 (4 layers) 0

GLNFRP 26.9 (5 layers) 0 0 21.5 (4 layers) KGNFRP 25.4 (3 layers) 8.9 (3 layers) 12.5 (3 layers) 0

KGLNFRP 21.9 (3 layers) 7.6 (3 layers) 0 21.9 (3 layers)

Microscopy Analysis

The samples were prepared for SEM analysis by cutting 2mm x 2mm of the impacted

surface of the samples and placed in Bio Rad scanning electron microscope (SEM) coating



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system for the samples to be thinly gold coated. The gold coated samples were transferred to

the SEM machine, ZEISS Supra, 35 VP, where they were observed and images captured at

different magnifications.

Results

Samples of the load – time response curves obtained from the drop weight analysis are

presented as Figure 2.

Figure 2. Impact load-time response of composites used in this study: (A) GFRP, (B)

KGNFRP, (C) NFRP

(A)

(B)

(

C


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Properties such as the peak loads and ductility index obtained from the load-time

response are presented as Figures 3- 5.

Figures 6-14 present the scanning electron microscope (SEM) micrograph of the front

view of the impacted samples.

0

1

2

3

4

5

6

GFRPKFRP

NFRP

LNFRP

GLNFRP

GNFRPKGRP

KGNFRP

KGLNFRP

Composite materials used

Peak

load

(kN

)4.5 J 18 J 27 J

Figure 3. Effect of fiber type and combination on peak load of composites at different impact

energies

00.20.40.60.8

11.21.4

GFRPKFRP

NFRP

LNFRP

GLNFRP

GNFRPKGRP

KGNFRP

KGLNFRP


Nor

mal

ized

pea

k lo

ad (k

N/m

m)

4.5 J 18 J

27 J

Figure 4. Comparison of effects of fiber type and combination on peak load of composites at

different impact energies using concept of normalization



104

0

2

4

6

8

10

12

14

16

18

20

GFRPKFRP

NFRP

LNFRP

GLNFRP

GNFRPKGRP

KGNFRP

KGLNFRP


Duc

tility

inde

x

4.5 J 18 J 27 J

Figure 5. Ductility index of composites at different impact energies

Figure 6. SEM Micrograph of Glass Fiber Reinforced Polyester(GFRP)

Impacted at Energy of 27 J


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Figure 7. SEM Micrograph of Kevlar Fiber Reinforced Polyester (KFRP)

Impacted at Energy of 27J

Figure 8. SEM Micrograph of Nylon Fiber Reinforced Polyester (NFRP)


Figure 9. SEM Micrograph of Local Nylon Fiber Reinforced Polyester (LNFRP)




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Figure 10. SEM Micrograph of Kevlar-Glass Fiber Reinforced Polyester (KGFRP)


Figure 11. SEM Micrograph of Glass-Nylon Fiber Reinforced Polyester (GNFRP)



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Figure 12. SEM Micrograph of Glass- Local Nylon Fiber Reinforced Polyester (GLNFRP) Impacted at Energy of 27J

Figure 13. SEM Micrograph of Kevlar-Glass-Nylon Fiber Reinforced Polyester (KGNFRP)


Figure 14. SEM Micrograph of Kevlar-Glass- Local Nylon Fiber Reinforced Polyester

(KGLNFRP) Impacted at Energy of 27J

Discussion

The shape of the load-time response curves’ indicated that all the samples tested at

impact energy of 4.5 J exhibited load-time response that is symmetrical with equal loading

and unloading except KGLNFRP and KGNFRP that showed slight tilt from the position of

symmetry. This is an indication that samples tested at this impact energy were essentially

undamaged [9]. At most, little dent at the point of impact as a result of impact force was only

observed in some of the samples. As the impact was increased to 18 J, there was change in



108

slope of the load-time response curve for most of the samples. The load-time response of

LNFRP was still symmetrical, while those of some samples were asymmetrical. The

KGLNFRP showed very little deviation from symmetry. At impact energy of 27 J, all the

load-time responses were unsymmetrical in shape, although to lesser extent for the LNRFP,

GLNFRP and KGLNFRP. The extent of damage was more at this impact energy level for all

the samples when compared to impact energy of 18 J. These observations indicated that

increase in impact energy results into more matrix cracking and fiber breakages and splits

along the fiber direction [5]. However, there was no penetration or perforation of any of the

samples at these impact energies.

As expected, the peak load (Figure 3) which is the maximum load that a laminate can

sustain before major damage [7] was found to increase with increase in impact energy except

for KFRP, GNFRP and KGFRP, where the peak load at 18 J was higher than that of impact

energy of 27 J. At impact energies of 18 J and 27 J, the GLNFRP had the highest peak load.

This was an indication that it was stiffest of all the samples tested and therefore, absorbed and

sustained more load. Stiffer materials deform less and carry higher load [5]. This observation

could be attributed to the compactness of the local nylon fiber that was combined with the

glass fiber. The NFRP composite exhibited the least peak load; this was an indication that

probably it was the most flexible of the composite samples. The KGFRP, GLNFRP and

GNFRP composites showed peak load higher than their plain composites. This could be due

to hybridization effect with the glass fiber introduced into the component contributing to the

load bearing effect. The ter hybrid of Kevlar-glass-nylon (KGN) showed the least peak load

among the hybrids, but the difference was not too evident when compared to the plain

composites of nylon and Kevlar. The KGLNFRP exhibited tremendous level of load bearing

capacity, in terms of stiffness when compared to other hybrids, nevertheless, with exception

of glass- local nylon (GLN) hybrid and plain LNFRP composites.

Sample thickness has effect on load bearing and energy absorption capability of a

material. Despite the fact that same ply of samples were prepared, differences in thickness

were obtained because of the fiber nature. The fibers have different numbers of strands

making up the weave, therefore different fiber thicknesses resulting to different composite

sample thicknesses. Due to this, the concept of normalization (Figure 4) was adopted where

the ratio of peak load to the thickness of each sample was taken. It was observed that there

was tremendous difference between the normalized peak load and the original peak load.


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The ductility index which is the ratio of the area under the post peak to the pre-peak

load in the load-time response [5] showed that the KFRP showed highest ductility index at

impact energy of 27 J (Figure 5). This indicated that most of the load was sustained at post

peak. Therefore, the KFRP could sustain load after considerable damage without leading to

catastrophic failure on impact. Low DI indicates that a larger amount of energy is required to

initiate damage, but it also indicates that if damage does occur, little additional energy is

required to cause total failure [24]. Despite other samples having ductility index (DI) above

two at the impact energy of 27 J, the hybrid composite of KGNFRP had DI higher than plain

glass fiber composites and other composites.

Matrix cracks, matrix debonding and fiber breakage were observed in GFRP sample

(Figure 6). Some of these failure modes were also observed on the other samples, but the level

differed from sample to sample. The NFRP (Figure 8) showed highest extensive matrix crack

and debonding when compared to other composite samples, indicating that there was

probably no proper fiber - matrix adhesion and resin penetration of the fiber mat because of

its high tightness of weave. Effect of hybridization is evident in Figures. 10-14 when

compared to Figures6 and 8 respectively. The level of matrix cracks and matrix debonding are

lower when compared to some of the monolithic composites samples. The fiber combination

resulting from the use of the highest strain rate to failure material at the back face would have

resulted to this observation, less damage at the front face and especially at the back face of the

hybrid composites.

Conclusions

Drop weight impact response of composites laminates, fabricated from four different

fibers and their combinations was studied. The peak load and the ductility index DI were

analysed. The SEM micrographs of the impacted surfaces were also analysed. The following

observations were made from the study:

Peak load generally increased with increase in test impact energy with GFRP having

the highest on normalization and the hybrids KGFRP, GLNFRP, KGNFRP showing

appreciable peak load.



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70% of the composites showed equal area on loading and unloading after peak load at

test impact energy of 4.5 J, indicating little or no damage at that test energy. It is evident that

most of the laminates will not fail catastrophically under the impact energy investigated

because they have ductility index above 1.

Hybridization of the fiber mats had effect on the impact response of the materials as it

reduced the level of matrix crack, debonding and fiber crack in the laminates.

Acknowledgements

Authors would like to acknowledge Universiti Sains Malaysia (USM) for the use of

their equipment in conducting the research. The authors also wish to acknowledge Education

Trust Fund, Science and Technology Education Post-Basic (STEP-B) Project 2010 Innovators

of Tomorrow (IOT) and Ahmadu Bello University Board of Research for contributing funds

for the project.

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Documents

Drop Weight Impact Studies of Woven Fibers Reinforced Modified