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Low velocity impact of Kevlar and ultra high molecular weight polyethylene (UHMWPE) reinforced epoxy composites

Authors:
  • Erbil polytechnic university-technology college

Abstract

Purpose Composite laminates are considered one of the most popular damage-resistant materials when exposed to impact force in civil and military applications. In this study, a comparison of composites 12 and 20 layers of fabrics Kevlar and ultrahigh-molecular-weight poly ethylene (UHMWPE)-reinforced epoxy under low-velocity impacts represented by drop-weight impact and Izod pendulum impact has been done. During the Izod test, Kevlar-based composite showed damage at the composite center and fiber breakages. Whereas delamination was observed for UHMWPE reinforced epoxy (PE). The maximum impact strength was for Kevlar-reinforced epoxy (KE) and increases with the number of laminates. Drop-weight impact test showed the highest absorbed energy for (KE) composites. The results revealed that different behavior during the impact test for composites belongs to the impact mechanism in each test. Design/methodology/approach Aramid 1414 Kevlar 49 and UHMWPE woven fabrics were purchased from Yixing Huaheng High-Performance Fiber Textile Co. Ltd, with specifications listed in Table 1. Epoxy resin (Sikafloor-156) is supplied from Sika AG. Sikafloor-156 is a two-part, low-viscosity, solvent-free epoxy resin, with compressive strength ∼95 N/mm², flexural strength ∼30 N/mm² and shore D hardness 83 (seven days). The mixture ratio of A/B was one-third volume ratio. Two types of laminated composites with different layers 12 and 20 were prepared by hand layup: Kevlar–epoxy and UHMWPE–epoxy composites as shown in Figure 1. Mechanical pressure was applied to remove bubbles and excess resin for 24 h. The composites were left in room temperature for seven days, and then composite plates were cut for the desired dimensions. Low-velocity impact testing, drop-weight impact, drop tower impact system INSTRON CEAST 9350 (see Figure 2) was facilitated to investigate impact resistance of composites according to ASTM D7137M (Test Method for Compressive, 2005). Low-velocity impact tests have been performed at room temperature for composite with dimensions 10 × 15 cm ² utilizing a drop tower (steel indenter diameter 19.85 mm as shown in Figure 3), height (800 mm), drop mass (5 kg) and speed (3.96 m/s). Special impact equipment consisting of vertically falling impactor was used in the test. The energy is obtained from Drop tower impact systems, (2009) E = ½ mv ² (2.1). The relationship between force–time, deformation–time and energy–time and deformation was obtained. Energy–deformation and force–deformation relationships were also obtained. The depth of penetration and the radius of impactor traces were recorded. Izod pendulum impact test of plastics was applied according to ASTM D256 (Test Method for Compressive, 2005). Absorbed energy was recorded to compute the impact strength of the specimen. The specimen before the test is shown in Figure 4. Findings In order to investigate two types of impact: drop-weight impact and Izod impact on damage resistance of composites, the two tests were done. Drop-weight impact is dropping a known weight and height in a vertical direction with free fall, absorbed energy can be calculated. Izod impact measures the energy required to break a specimen by striking a specific size bar with a pendulum (Test Method for Compressive, 2005; Test Methods for Determining, 2018). The results obtained with the impact test are presented. Figure 5 shows the histogram bars of impact strength of composites. It can be noticed that Kevlar–epoxy (KE) composites give higher energy strength than UHMWPE–epoxy (PE) in 12 and 20 plies. The increasing percentage is about 18.5 and 5.7%. It can be observed in Figure 6 that samples are not destructed completely due to fiber continuity. Also, the delamination occurs obviously for UHMWPE–epoxy more than for Kevlar-based composite, which may due to weak binding between UHMWPE with an epoxy relative with Kevlar. Practical implications The force–time curves for Kevlar–epoxy (KE) and UHMWPE–epoxy (PE) composites with 12 and 20 plies are illustrated respectively in Figure 7. The contact duration between indenter and composite surface is repented by the force–time curves, so the maximum force reaches with certain displacement. It can be seen that maximum force was (13,209, 18,734.9, 23,271.07 and 19,825.38 N ) at the time (3.97, 4.43, 3.791 and 4.198 ms) for 12 KE, 12 PE, 20 KE and 20 PE, respectively. The sharp peaks of KE composite are due to the lower ductility of Kevlar compared with UHMWPE. These results agree with the results of Ahmed et al. (2016). Kevlar-based composites (KE) showed lower impact force and crack propagates in the matrix with fast fiber breakage compared with PE composites, whereas the latter did not suffer from fabric breakage in 12 and 20 plies any more (see Figure 8). Figure 9 illustrates force–deformation curves, for 12 and 20 plies of Kevlar–epoxy (KE) and UHMWPE–epoxy (PE) composites. Curve's slop is considered the specimen's stiffness and the maximum displacement. To investigate the impact behavior of the four different composites, the comparison was made among the relative force–deformation curves. The maximum displacement was 5.119, 3.443, 1.173 and 1.17 mm for 12KE, 12 PE, 20 KE and 20 PE, respectively. It seems that UHMWPE-based composite (PE) presents lower deformation than Kevlar-based composites (KE) at a same number of laminates, although the maximum displacement is for 12 PE and 12 KE (see Figure 8). Kevlar-based composites (KE) showed more damage than UHMWPE-based composite (PE), so the maximum displacement is always higher for KE specimens with maximum indenter trace diameter (D∼11.27 mm). The onset of cracks begins along fibers on the impacted side for 20 KE and 20 PE specimens with lower indenter trace (D∼5.42 and 5.96 mm), respectively (see Table 2). These results refer to the lower stiffness of KE composites (see the slope of the curve) relative to PE composites. This result agreed with (Vieille et al. , 2013) when they found that the theoretical stiffness of laminated composite during drop-weight impact depends significantly on fiber nature (Fadhil, 2013). The matrix cracking is the first type of damage that may not change stiffness of composites overall. Material stiffness changes due to the stress concentration represented by matrix cracks, delamination and fiber breakage (Hancox, 2000). Briefly, the histogram (see Figure 10) showed that the best impact behavior was for 20 KE, highest impact force with lower deformation, indenter trace diameter and contact time. Absorbed energy–time and absorbed energy–deformation curves for composites are shown in Figures 11 and 12, respectively. The maximum absorbed energy was (36.313, 29.952, 9.783 and 6.928 J) for 12 KE, 12 PE, 20 KE and 20 PE, respectively. Test period time is only 8 ms, but the time in which composites reached maximum absorbed energy was (4.413, 3.636, 2.394 and 2.408 ms). The maximum absorbed energy was for 12 KE with lower rebound energy because part of kinetic energy transferred to potential energy kept in the composite as material damage (see Figures 3 and 4). This composite absorbs more energy as material damage which kept as potential energy. Whereas other composites 12 PE, 20 PE and 20 KE showed less damage, lower absorbed energy and higher rebound energy, which appeared in different peak behavior as the negative value of energy. Also from the absorbed energy–time curves, it had been noticed significantly the maximum contact time of indenter with composite was 4.413 ms for 12 KE, which exhibits higher deformation (5.119 mm), whereas other composites 12 PE, 20 KE and 20 PE showed less damage, contact time and deformation as (3.443, 1.173, 1.17 mm), respectively. Originality/value The main goal of the current study is to evaluate the performances of armor composite made off of Kevlar and UHMWPE fabrics reinforced epoxy thermosetting resin under the low-velocity impact. Several plates of composites were prepared by hand layup. Izod and drop-weight impact tests were facilitated to get an indication about the absorbed energy and strength of the armors.
Low velocity impact of Kevlar and
ultra high molecular weight
polyethylene (UHMWPE)
reinforced epoxy composites
Mayyadah S. Abed
Department of Materials Engineering, University of Technology - Iraq,
Baghdad, Iraq
Payman S. Ahmed
Department of Manufacturing Engineering, Koya University, Koya, Iraq
Jawad K. Oleiwi
Department of Materials Engineering, University of Technology - Iraq,
Baghdad, Iraq, and
Basim M. Fadhil
Department of Manufacturing Engineering, Koya University, Koya, Iraq
Abstract
Purpose Composite laminates are considered one of the most popular damage-resistant materials when
exposed to impact force in civil and military applications. In this study, a comparison of composites 12 and 20
layers of fabrics Kevlar and ultrahigh-molecular-weight poly ethylene (UHMWPE)-reinforced epoxy under
low-velocity impacts represented by drop-weight impact and Izod pendulum impact has been done. During the
Izod test, Kevlar-based composite showed damage at the composite center and fiber breakages. Whereas
delamination was observed for UHMWPE reinforced epoxy (PE). The maximum impact strength was for
Kevlar-reinforced epoxy (KE) and increases with the number of laminates. Drop-weight impact test showed the
highest absorbed energy for (KE) composites. The results revealed that different behavior during the impact
test for composites belongs to the impact mechanism in each test.
Design/methodology/approach Aramid 1414 Kevlar 49 and UHMWPE woven fabrics were purchased
from Yixing Huaheng High-Performance Fiber Textile Co. Ltd, with specifications listed in Table 1. Epoxy
resin (Sikafloor-156) is supplied from Sika AG. Sikafloor-156 is a two-part, low-viscosity, solvent-free epoxy
resin, with compressive strength 95 N/mm
2
, flexural strength 30 N/mm
2
and shore D hardness 83 (seven
days). The mixture ratio of A/B was one-third volume ratio. Two types of laminated composites with different
layers 12 and 20 were prepared by hand layup: Kevlarepoxy and UHMWPEepoxy composites as shown in
Figure 1. Mechanical pressure was applied to remove bubbles and excess resin for 24 h. The composites were
left in room temperature for seven days, and then composite plates were cut for the desired dimensions. Low-
velocity impact testing, drop-weight impact, drop tower impact system INSTRON CEAST 9350 (see Figure 2)
was facilitated to investigate impact resistance of composites according to ASTM D7137M (Test Method for
Compressive, 2005). Low-velocity impact tests have been performed at room temperature for composite with
dimensions 10 315 cm
2
utilizing a drop tower (steel indenter diameter 19.85 mm as shown in Figure 3), height
(800 mm), drop mass (5 kg) and speed (3.96 m/s). Special impact equipment consisting of vertically falling
impactor was used in the test. The energy is obtained from Drop tower impact systems, (2009) E 5½mv
2
(2.1).
The relationship between forcetime, deformationtime and energytime and deformation was obtained.
Energydeformation and forcedeformation relationships were also obtained. The depth of penetration and
the radius of impactor traces were recorded. Izod pendulum impact test of plastics was applied according to
Impact of
Kevlar and
UHMWPE
Conflict of Interest: This work was not funded by any organization, and the authors have no conflicts of
interest.
The authors would like to thank and appreciate all valuable efforts of all colleagues in the
Department of Materials Engineering, University of Technology-Iraq, and the Department of
Manufacturing Engineering, Koya University, Koya, Iraq, who have provided a benefit and helpful
support which led to the achievement and improvement of this work.
The current issue and full text archive of this journal is available on Emerald Insight at:
https://www.emerald.com/insight/1573-6105.htm
Received 9 September 2019
Revised 14 December 2019
Accepted 11 March 2020
Multidiscipline Modeling in
Materials and Structures
© Emerald Publishing Limited
1573-6105
DOI 10.1108/MMMS-09-2019-0164
Citation
Abed, M.S., Ahmed, P.S., Oleiwi, J.K. and Fadhil, B.M. (2020), "Low velocity impact of Kevlar and ultra high molecular
weight polyethylene (UHMWPE) reinforced epoxy composites", Multidiscipline Modeling in Materials and Structures,
Vol. 16 No. 6, pp. 1617-1630. https://doi.org/10.1108/MMMS-09-2019-0164
ASTM D256 (Test Method for Compressive, 2005). Absorbed energy was recorded to compute the impact
strength of the specimen. The specimen before the test is shown in Figure 4.
Findings In order to investigate two types of impact: drop-weight impact and Izod impact on damage
resistance of composites, the two tests were done. Drop-weight impact is dropping a known weight and height
in a vertical direction with free fall, absorbed energy can be calculated. Izod impact measures the energy
required to break a specimen by striking a specific size bar with a pendulum (Test Method for Compressive,
2005; Test Methods for Determining, 2018). The results obtained with the impact test are presented. Figure 5
shows the histogram bars of impact strength of composites. It can be noticed that Kevlarepoxy (KE)
composites give higher energy strength than UHMWPEepoxy (PE) in 12 and 20 plies. The increasing
percentage is about 18.5 and 5.7%. It can be observed in Figure 6 that samples are not destructed completely
due to fiber continuity. Also, the delamination occurs obviously for UHMWPEepoxy more than for Kevlar-
based composite, which may due to weak binding between UHMWPE with an epoxy relative with Kevlar.
Practical implications The forcetime curves for Kevlarepoxy (KE) and UHMWPEepoxy (PE) composites
with 12 and 20 plies are illustrated respectively in Figure 7. The contact duration between indenter and composite
surface is repented by the forcetime curves, so the maximum force reaches with certain displacement. It can be
seen thatmaximum force was (13,209, 18,734.9, 23,271.07and 19,825.38N) at the time (3.97,4.43, 3.791 and4.198 ms)
for 12 KE, 12 PE, 20 KE and 20 PE, respectively. The sharp peaks of KE composite are due to the lower ductility of
Kevlar compared with UHMWPE. These results agree with the results of Ahmed et al. (2016). Kevlar-based
composites (KE)showed lower impact force and crackpropagates inthe matrix with fast fiber breakagecompared
with PE composites, whereas the latter did not suffer from fabric breakage in 12 and 20 plies any more (see
Figure 8). Figure 9 illustrates forcedeformation curves, for 12 and 2 0 plies of Kevlarepoxy (KE) and UHMWPE
epoxy (PE) composites. Curves slop is considered the specimens stiffness and the maximum displacement. To
investigate the impact behavior of the four different composites, the comparison was made among the relative
forcedeformation curves. Themaximum displacementwas 5.119, 3.443, 1.173and 1.17 mm for12KE, 12 PE, 20 KE
and 20 PE, respectively. It seems that UHMWPE-based composite (PE) presents lower deformation than Kevlar-
basedcomposites(KE) at a same numberof laminates,althoughthe maximum displacement is for12 PE and 12 KE
(see Figure 8). Kevlar-based composites (KE)showed more damage than UHMWPE-based composite (PE), so the
maximum displacement isalways higherfor KE specimenswith maximum indentertrace diameter(D11.27 mm).
The onset of cracks begins along fibers on the impacted side for 20 KE and 20 PE specimens with lower indenter
trace (D5.42 and 5.96 mm), respectively (see Table 2). These results refer to the lower stiffness ofKE composites
(see the slope of the curve) relative to PE composites. This result agreed with (Vieille et al., 2013)when they found
that the theoretical stiffness of laminated composite during drop-weight impact depends significantly on fiber
nature (Fadhil, 2013). Thematrix cracking is the firsttype of damage that may not change stiffness of composites
overall. Materialstiffness changes due to the stress concentration represented by matrix cracks, delamination and
fiber breakage (Hancox, 2000). Briefly, the histogram (see Figure 10)showed that the best impact behavior wasfor
20 KE, highest impact force with lower deformation, indenter trace diameter and contact time. Absorbedenergy
time and absorbed energydeformation curves for composites are shown in Figures 11 and 12, respectively. The
maximum absorbed energy was (36.313,29.952, 9.783 and 6.928 J) for 12 KE, 12 PE, 20 KE and 20 PE, respectively.
Test period time is only 8 ms, but the time in which composites reached maximum absorbed energy was (4.413,
3.636,2.394 and 2.408ms). The maximumabsorbed energy was for 12KE with lower rebound energy because part
of kinetic energy transferred to potential energy kept in the composite as material damage (see Figures 3 and 4).
This composite absorbs more energy as material damage which kept as potential energy. Whereas other
composites 12 PE, 20 PE and 20K E showed less damage, lower absorbed energy and higher reboundenergy, which
appeared in different peakbehavior as thenegative value of energy. Also fromthe absorbedenergytimecurves, it
had beennoticed significantly themaximum contact timeof indenterwith compositewas 4.413 msfor 12 KE, which
exhibits higher deformation (5.119 mm), whereas other composites 12 PE, 20 KE and 20 PE showed less damage,
contact time and deformation as (3.443, 1.173, 1.17 mm), respectively.
Originality/value The main goal of the current study is to evaluate the performances of armor composite
made off of Kevlar and UHMWPE fabrics reinforced epoxy thermosetting resin under the low-velocity impact.
Several plates of composites were prepared by hand layup. Izod and drop-weight impact tests were facilitated
to get an indication about the absorbed energy and strength of the armors.
Keywords Kevlar, UHMWPE, Laminated composites, Epoxy, Lzod impact, Drop weight impact
Paper type Research paper
Introduction
Foreign bodies may strike a structure during its life when its manufactured, serves or
maintenance steps. For example, striking stones during takeoff and landing of the airplane
with very high velocity by the tires. Also dropped tools during manufacturing or maintenance
could cause impact damage. Laminated composite structures are more susceptible to impact
damage when compared with the same metallic structure (Randjbaran et al., 2014).
MMMS
Composites are vastly utilized in security industries and defense applications such as
helmets, body armor and vehicle shielding (Rodr
ıguez Mill
an et al.,2015;Oleiwi et al.,2015),
because of their desirable laminar strengths, damage resistance, cost-effective and multipurpose
design capability (Li et al., 2019;Oleiwi et al.,2010). Composites have excellent behavior-to-weight
ratio, better damping characteristics, good fatigue resistance, high resistance to corrosion when
compared to other engineering materials (Bozkurt et al., 2017). Aerospace, spacecraft, marine
industries utilize polymer matrix composites in high percentages. These composites, through
their service life, undergo impact events (low, high or hyper velocity) commonly (Rawat and
Kalyan, 2017), for that composite materials require a comprehensive analysis when composite
materials are under dynamic loading to obtain more safety (Rodr
ıguez Mill
an et al., 2015).
Impact behavior mechanism is affected sensitively on the composite structure, volume
fraction of fiber, composite dimension and the number of composite laminates. The utilization
of high-performance fiber significantly affects the impact and compression characteristics (Li
et al., 2019). Multilayered composites formed by impregnated high-modulus, high-strength
polymeric fibers in the form of woven fabrics in resin matrix form distinct type of armor fiber-
reinforced composites. Kevlar is the best influential and vastly utilized fiber, which helps as a
protective shield in different forms for the defense equipment such as a helicopter, tanks and
body armor. Its essential duty in this use is to hinder perforation of the objects or fragments
through the protected surface (Nayak et al., 2017).
Armor, in general, can be defined as a protective covering developed to supply safety from
a physical attack. According to the designing purposes, the armor can be designed as body
armor (personal), light armor (for craft and vehicle) and heavy armor (for tank) (Sorrentino
et al., 2015). Kevlar is made through the polymerization process. Kevlar is an ideal material for
use in body vest because it is five times stronger than steel, has high strength-to-weight ratio,
high tensile strength and capability to energy absorption relative with another material
(Stopforth and Adali, 2019). Ultrahigh-molecular-weight polyethylene (UHMWPE) fiber-
reinforced composites have obtained considerable importance in the last two decades for
structural and armor applications because of their high specific strength and high energy
absorption under dynamic load (Reddy et al., 2017).
Bandaru and Ahmed (2017) studied the ballistic characteristics of Kevlar/polypropylene
composites. The investigations were executed depending on experimental and numerical
studies. Kevlar/polypropylene composites showed better response compared with Kevlar/
Vinylester composites mentioned in the literature. It is noticed that the failure mechanism is
controlled by shearing in Kevlar/polypropylene composite (Bandaru and Ahmad, 2017). Braga
et al. (2018) performed ballistic tests on multilayered body armor composed of different
materials such as ceramics, composites and metals against high-energy projectiles.
Multilayered armor systems (MASs) provide efficient protection by making use of lighter
and more efficient materials. A typical armor composed of three layers: a front ceramic then
compositeand finally backed by a ductilematerials like a metal. The second layer was polyester
resins incorporated with 10, 20 and 30 vol. % of sisal fibers (Braga et al.,2018
). Khatiwada et al.
(2013) studied the hypervelocity impact characteristics of UHMWPE fabrics with single-walled
carbon nanotubes embedded in epoxy. The behavior of the nanocomposites was compared
against that of the UHMWPE/epoxy composites without CNTs and aluminum plates having a
similar density to the composites. It had been noted that the nanocomposites and the neat
composites showed better performance than the Al plates, but lesser bumper shields
(Khatiwada et al., 2013). Fadhil et al. (2016) utilized weight percentages (0, 0.2, 0.4, 0.6, 0.8, 1)
Wt.% of multi-walled carbon nanotubes (MWCNTs) to reinforce epoxy resin. Mechanical
properties of the composites were assessed by tensile and drop-weight impact tests. The results
revealed that 0.2 Wt.% of MWCNTs improve tensile properties, whereas 0.6 Wt.% of
MWCNTs improve impact characteristics (Fadhil et al.,2016). Ahmed (2016) used different
designs of the impactor in drop-weight impact test (bullet, cone and hemispherical) and
Impact of
Kevlar and
UHMWPE
investigated this effect on the impact properties of epoxy composites reinforced with
unidirectional glass, unidirectional carbon, woven glass and hybrid woven (glass þcarbon)
fibers. Results showed that changing the impactor design had no effect on the impact
characteristics of composites reinforced with woven fiber, whereas it has a huge effect on
unidirectional fiber-reinforced composites. Indentation and perforation were observed as
impact damage behavior of woven composites while matrix cracking and splitting along the
fiber direction and the fracture were for unidirectional fiber-reinforced composites (Ahmed,
2016). Others studied hybrid armor made of UHMWPE and soft structure panel to increase
energy absorption (Liu et al., 2018). Also shear thickeningfluid was utilized to increase theyarn-
to-yarn friction of Kevlar fabric by growing ZnO nanorod on it chemically (Dixit et al.,2019).
This work investigates the impact performance at low velocity of two types of composites:
Kevlar-reinforced epoxy and UHMWPE-reinforced epoxy at different numbers of ply (12 and
20). Most researches used commercial or laboratorial epoxy for such purposes, while in the
current work, epoxy type (Sikafloor-156) had been utilized, which has unique characteristics
and antidamage applications.
The main goal of the current study is to evaluate the performances of impact damage-
resistant composite made of Kevlar and UHMWPE fabrics reinforced epoxy thermosetting
resin under the low-velocity impact. Several plates of composites were prepared by hand
layup. Izod and drop-weight impact tests were facilitated to get an indication about the
absorbed energy and strength of the armors.
2. Experimental work
2.1 Materials
Aramid 1414 Kevlar 49 and UHMWPE woven fabrics were purchased from Yixing Huaheng
High-Performance Fiber Textile Co. Ltd, China, with specifications listed in Table 1. Epoxy
resin (Sikafloor-156) is supplied from Sika AG., Turkey, Sikafloor-156 is a two-part, low-
viscosity, solvent-free epoxy resin, with compressive strength 95 N/mm
2
, flexural
strength 30 N/mm
2
and shore D hardness 83 (seven days). The mixture ratio of A (resin)/
B (hardener) was one-third volume ratio. Two types of composites were prepared: Kevlar
epoxy and UHMWPEepoxy composites. Laminated composites with different layers 12 and
20 were prepared by hand layup for each type of composite as shown in Figure 1.
Mechanical pressure was fixed by jaws and optimized using dial gauge to 2 MPa to
remove bubbles and excess resin for 24 h. The composites were left in room temperature for
seven days, and then composite plates were cut for the desired dimensions.
Properties Aramid1414 Kevlar 49 UHMWPE fiber
Density (g/cm
3
) 1.45 0.970.98
Tensile strength (cN/tex) 200 285.6408
Tensile modulus (cN/tex) 8,300 928214,280
Elongation at break (%) 2.5 3.53.7
Temperature range (8C) 204 80
Decomposition temperature (8C) 400 145160
3008C 100 h strength retention (%) 6065 6870
Moisture absorption (%) 4.5 0.6
Wear resistance General Good
Solvent resistance Good Good
Acid resistance Bad Good
Alkali resistance Good Good
UV resistance Bad Good
Table 1.
Properties of
Aramid1414 Kevlar
and UHMWPE fiber
MMMS
2.2 Low-velocity impact testing
2.2.1 Drop-weight impact. Drop tower impact system INSTRON CEAST 9350 (see Figure 2)
was facilitated to investigate impact resistance of composites according to ASTM
D7137M (Test Method for Compressive, 2005). Low-velocity impact tests have been
performed at room temperature for composite with dimensions 10 315 cm
2
utilizing a
drop tower (steel indenter diameter 19.85 mm as shown in Figure 3), height (800 mm), drop
mass (5 kg) and speed (3.96 m/s). Special impact equipment consisting of vertically falling
impactor was used in the test. The energy is obtained from Drop tower impact systems
(2005)
E¼1
2mv2(1)
The relationship between forcetime, deformationtime and energytime and deformation
was obtained. Energydeformation and forcedeformation relationships were also obtained
as well. The depth of penetration and the radius of impactor traces were recorded.
2.2.2 Izod impact. Izod pendulum impact test of plastics was applied according to ASTM
D256 using Izod impactor; model XJU-22, TIME Group Inc. (Test Method for Compressive,
2005). The unnotched sample design was used with dimensions (1 cm width 310 cm
long 3sample thickness). Absorbed energy was recorded to compute the impact strength of
the specimen. The specimen before the test is shown in Figure 4
3. Results and discussion
In order to investigate two types of impact: drop-weight impact and Izod impact on damage
resistance of composites. Drop-weight impact is dropping a known weight and height in a
vertical direction with free fall, absorbed energy can be calculated. Izod impact measures the
energy required to break a specimen by striking a specific size bar with a pendulum (Test
Method for Compressive, 2005;Test Methods for Determining, 2018). The results obtained
with the impact test are presented. Figure 5 shows the histogram bars of impact strength of
composites. It can be noticed that Kevlarepoxy (KE) composites give higher energy strength
than UHMWPEepoxy (PE) in 12 and 20 ply due to the unique mechanical properties of
Kevlar woven fa bric Kevlar-epoxy composite
Hand lay- out
UHMWPE woven fabric UHMWPE-epoxy composite
Hand lay-out
Figure 1.
Hand layout of
prepared composites
Impact of
Kevlar and
UHMWPE
Kevlar under shear stresses and impact forces. The increasing percentage is about 18.5 and
5.7%. It can be observed in Figure 6 that samples are not destructed completely due to fiber
continuity. Also, the delamination occurs obviously for UHMWPEepoxy more than for
Kevlar-based composite, which may due to weak binding between UHMWPE with an epoxy
relative with Kevlar.
The forcetime curves for Kevlarepoxy (KE) and UHMWPEepoxy (PE) composites
with 12 and 20 ply are illustrated respectively in Figure 7. The contact duration between the
indenter and composite surface is represented by the forcetime curves. It can clearly
determine the maximum force reached at a certain time for each sample. It can be seen that
maximum force was (13.209, 18.734.9, 23.271.07 and 19.825.38 N) at the time (3.97, 4.43, 3.791
and 4.198 ms) for 12KE, 12 PE, 20 KE and 20 PE, respectively. The sharp peaks of KE
Figure 3.
Drop impactor tip
Figure 2.
Drop tower impact
system
MMMS
composite are due to the lower ductility of Kevlar compared with UHMWPE. These results
agree with the results of Ahmed et al. (2016). Kevlar-based composites (KE) showed lower
impact force and crack propagates in the matrix with fast fiber breakage compared with PE
composites, whereas the latter did not suffer from fabric breakage in 12 and 20 ply any more
(see Figure 8).
Figure 9 illustrates impact forcedeformation curves, for 12 and 20 ply of Kevlarepoxy
(KE) and UHMWPEepoxy (PE) composites. Curves slop is considered the specimens
stiffness and the maximum displacement. To investigate the impact behavior of the four
different composites, the comparison was made among the relative forcedeformation
curves. The maximum displacement was 5.119, 3.443, 1.173 and 1.17 mm for 12KE, 12 PE,
20 KE and 20 PE, respectively. It seems that UHMWPE-based composite (PE) presents
lower deformation than Kevlar-based composites (KE) at a same number of laminates,
although the maximum displacement is for 12 PE and 12 KE. Kevlar-based composites (KE)
showed more damage than UHMWPE-based composite (PE), so the maximum
displacement is always higher for KE specimens with maximum indenter trace diameter
(D11.27 mm). The onset of cracks begins along fibers on the impacted side for 20 KE and
20 PE specimens with lower indenter trace (D5.42 and 5.96 mm), respectively (see
Table 2). These results refer to the lower stiffness of KE composites (see the slope of the
92
135
75
127
0
20
40
60
80
100
120
140
160
12 KE 20 KE 12 PE 20 PE
Impact strength(kJ/m^2)
Izod Impact
Figure 5.
Impact strength of
composite calculated
from Izod impact test
Figure 4.
Impact specimens for
Izod impact test
Impact of
Kevlar and
UHMWPE
curve) relative to PE composites. This result agreed with (Vieille et al., 2013) when they
found that the theoretical stiffness of laminated composite during drop-weight impact
depends significantly on fiber nature (Fadhil, 2013). The matrix cracking is considered the
damage initiation in a composite, so that it may not affect obviously the stiffness of
composites in the first stage of damage. Material stiffness changes due to the stress
concentration represented by matrix cracks, delamination and fiber breakage
(Hancox, 2000).
Figure 6.
Impact specimens for
Izod impact test; (a) and
(b) front and side view
of impacted 12 ply KE,
(c) and (d) front and
side view of impacted
20 ply KE, (e) and (f)
front and side view of
impacted 12 ply PE, (g)
and (h) front and side
view of impacted 20
ply PE
MMMS
and20plyanymore(seeFig. 8).
56.21
13209 N
0
5000
10000
15000
20000
25000
30000
0510
Impact Force [N]
Time [ms]
12 ply KE
0
19825 N
0
5000
10000
15000
20000
25000
30000
0510
Impact Force [N]
Time [ms]
163.01
23271 N
0
5000
10000
15000
20000
25000
30000
0510
Impact Force [N]
Time [ms]
61.831
18734 N
0
5000
10000
15000
20000
25000
30000
0510
Impact Force [N]
Time [ms]
20 ply KE
12 ply PE
12 ply PE
Figure 7.
Impact forcetime
curves of prepared
composites
Figure 8.
Impacted samples by
drop-weight impact
Impact of
Kevlar and
UHMWPE
0
10000
20000
30000
–40 –20 0 20
Impact Force [N]
DeformaƟon [mm]
0
10000
20000
30000
–40 –20 0 20
Impact Force [N]
Deformaion (mm)
0
10000
20000
30000
–40 –20 0 20
Impact Force [N]
DeformaƟon [mm]
0
10000
20000
30000
–40 –20 0 20
Impact Force [N]
DeformaƟon [mm]
20 ply KE
20 ply PE
Sample
Sample
thickness
(mm)
Impact
force (N)
Absorbed
energy (J)
Deformation
(mm)
Time
(ms)
Indenter
trace
diameter
(mm) Perforation
12KE 5.32 13.2 36.313 5.119 4.413 11.27 non
12PE 8.25 18.7 29.952 3.443 3.636 6.89 non
20KE 8.09 23.3 9.783 1.173 2.394 5.42 non
20PE 10.93 19.8 6.928 1.17 2.408 5.96 non
0
5
10
15
20
25
12KE 12PE 20KE 20PE
Impact force ( N)
Deformaon (mm)
Time (ms)
Identor trace diameter
(mm)
Figure 9.
Impact force-
deformation curves of
prepared composites
Table 2.
illustrates the
indentation results of
drop-weight impact
Figure 10.
Impact behavior
histogram of prepared
composites
MMMS
Briefly, the histogram (see Figure 10) showed that the best impact behavior was for 20 KE,
highest impact force with lower deformation, indenter trace diameter and contact time.
Absorbed energytime and absorbed energydeformation curves for composites are
shown in Figures 11 and 12 respectively. The maximum absorbed energy was (36.31, 29.95,
9.78 and 6.92 Joul) for 12 KE, 12 PE, 20 KE and 20 PE, respectively. Test period time is only
8 ms but the time in which composites reached maximum absorbed energy was (4.413, 3.636,
2.394 and 2.408 ms). The maximum absorbed energy was for 12 KE with lower rebound
energy because part of kinetic energy transferred to potential energy kept in the composite as
material damage (see Figure 8). This composite suffer from more damage and absorb more
energy which kept as potential energy. Whereas other composites 12 PE, 20 PE and 20 KE
showed less damage, lower absorbed energy and higher rebound energy, which appeared in
different peak behavior as the negative value of energy.
Also from the absorbed energytime curves, it had been noticed significantly the
maximum contact time of indenter with composite was 4.413 ms for 12 KE, which exhibits
–400
–300
–200
–100
0
100
0246810
Impact Energy [J]
Time [ms]
–400
–300
–200
–100
0
100
0246810
Impact Energy [J]
Time [ms]
–400
–300
–200
–100
0
100
0246810
Imact Energy [J]
Time [ms]
12 ply KE
20 ply KE
12 ply PE
–400
–300
–200
–100
0
100
0246810
Impact Energy [J]
Time [ms]
20 ply PE
0
10
20
30
40
0246
Impact Energy [J]
DeformaƟon [mm]
0
10
20
30
40
0246
Impact Energy [J]
DeformaƟon
[
mm
]
0
10
20
30
40
0246
Impact Energy [J]
DeformaƟon [mm]
0
10
20
30
40
0246
Impact Energy [ J]
DeformaƟon
[mm]
20 ply PE
20 ply KE
12 ply PE
12 ply KE
Figure 11.
Absorbed energytime
curves of prepared
composites
Figure 12.
Absorbed energy
deformation curves of
prepared composites
Impact of
Kevlar and
UHMWPE
higher deformation (5.119 mm), whereas other composites 12PE, 20KE and 20 PE showed less
damage, contact time and deformation as (3.443,1.173, 1.17 mm), respectively.
4. Conclusions
In summary, the conclusion of this study involves two parts according to the impact
mechanism:
(1) The results of the Izod pendulum impact test revealed that Kevlar-reinforced epoxy
composites have higher impact strength than UHMWPE-reinforced epoxy, also this
strength increases with the number of layers. UHMWPE-based composites exhibit
delamination failure, whereas Kevlar-based showed matrix and fiber damage.
(2) During drop-weight impact test, no specimen was perforated in spite of existence of
indentation trace. The highest deformation was for Kevlar-reinforced epoxy, which
depends significantly on fabric stiffness. Also, the deformation decreases with the
increasing of laminated layers and the total thickness of the composite plate. It
seems that UHMWPE-based composite (PE) presents lower deformation than
Kevlar-based composites (KE) at the same number of laminates. The maximum
absorbed energy was for Kevlar-based composites (KE) at the same number of
laminates.
(3) The maximum impact strength was 92 and 135 (kJ/m
2
) for (KE) at 12 and 20 ply,
respectively, whereas it was 75 and 127 (kJ/m
2
) for (PE) at 12 and 20 ply, respectively.
The enhancement in impact strength increases with the number of laminates from 12
to 20 ply about 46 and 69% for KE and PE, respectively. Drop-weight impact test
showed the highest absorbed energy for (KE) composites about (17.5% for 12 ply and
29.1% for 20 ply) than 12 and 20 ply of (PE) composites, respectively.
It is suggested to perform numerical analysis and bulletproof test to the same composites in
the future work.
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Corresponding author
Mayyadah S. Abed can be contacted at: 11038@uotechnology.edu.iq
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MMMS
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