Kapoor 2016

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High strain rate compression response of woven Kevlar reinforcedpolypropylene composites

Rajat Kapoor a , Laxman Pangeni a, Aswani Kumar Bandaru a , *, Suhail Ahmad a,Naresh Bhatnagar b

a Department of Applied Mechanics, Indian Institute of Technology Delhi, New Delhi, Indiab Department of Mechanical Engineering, Indian Institute of Technology Delhi, New Delhi, India

a r t i c l e i n f o

Article history:

Received 24 August 2015

Received in revised form

26 September 2015

Accepted 20 November 2015

Available online 12 January 2016

Keywords:

A. Polymer-matrix composites

B. Delamination

A. Thermoplastic resin

D. Electron microscopy

Kevlar

a b s t r a c t

In this study, experimental investigations on Kevlar ber reinforced polypropylene (PP) woven com-

posites under high strain rate compression loading are discussed. Kevlar/PP composite laminates with 8

and 24 layers are fabricated using vacuum assisted compression molding technique. Maleic anhydride

grafted-PP (MAg-PP) is added to PP to improve the interfacial property between Kevlar ber and PP resin.

The through-thickness properties at high strain rates from 1370 to 6066 s1 are obtained using split

Hopkinson pressure bar (SHPB) setup. The behavior of PP resin is found to be different than the

commonly used thermoset resins, such as epoxy. Dynamic stressestrain relations are drawn to reveal the

mechanical properties at high strain rates and these relations appear to be rate sensitive. As a result, the

peak stress increased by three times, toughness increased by almost ten times and strain at peak stress

increased by as much as two times with an increase in the strain rate. The nal failure of the specimens is

examined by scanning electron microscopy (SEM) to explore the possible failure mechanisms such as,

delamination, ber failure and shear fracture.© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Composite materials possess high specic strength and specic

stiffness with less fatigue. Due to this advantage of composite

materials, it's been widely used in military, aerospace and other

structural applications where weight of the structure is a signicant

parameter. Composite structures undergo different loading condi-

tions, such as, static and dynamic loads during their service life.

When the composite laminate is used as a body armor material,

the armor undergoes dynamic loading when projectile impacts the

target [1]. Among all the composite materials, Kevlar nds its majorapplication in body/vehicle armors as it exhibits an improved

impact resistance with lightweight. As the necessity increases, it is

very important to understand the effect of high strain rate on the

impact performance of Kevlar composite laminates. The response

of the material under different strain rates should be clearly known

for the effective useof materials [2]. Through-the-thickness loading

is one of the crucial condition in the ballistic impact applications.

The compressive properties of composite armor materials under

high strain rate conditions are highly desirable to assess the bal-

listic impact response.

Composite materials have been extensively characterized under

quasi-static tensile, compressive and shear loading conditions [3,4].

However, understanding the mechanical behavior of these mate-

rials under dynamic loading conditions is limited due to the asso-

ciated technical hitches at high strain rates. Most widely used

technique to characterize the materials at high strain rates is a split

Hopkinson pressure bar (SHPB) [2,5e7]. Recent works on the high

strain rate behavior of polymer composites were based on the

thermoset-based laminates made from glass and carbon bers[8e15]. Few works [7,16e19] reported on the dynamic compressive

response of Kevlar composites majorly based on the thermoset-

based matrix.

Woo et al. [7] used an acoustic emission technique to charac-

terize the failure progress in Kevlar/epoxy composites under high

strain rate impact. The peak stress and toughness of the Kevlar-

woven fabric specimen were increased almost two times, with an

increase in strain rate in the range of 1182e1460 s1, whereas the

strain at peak stress decreased by approximately 16%. An experi-

mental investigation was carried out by Daniel and Liber [16] to

assess the strain rate dependence on the tensile behavior of Kevlar/* Corresponding author.

E-mail address: [email protected] (A.K. Bandaru).

Contents lists available at ScienceDirect

Composites Part B

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . co m / l o c a t e / c o m p o si t e s b

http://dx.doi.org/10.1016/j.compositesb.2015.11.044

1359-8368/©

2016 Elsevier Ltd. All rights reserved.

Composites Part B 89 (2016) 374e382

mailto:[email protected]://www.sciencedirect.com/science/journal/13598368http://www.elsevier.com/locate/compositesbhttp://dx.doi.org/10.1016/j.compositesb.2015.11.044http://dx.doi.org/10.1016/j.compositesb.2015.11.044http://dx.doi.org/10.1016/j.compositesb.2015.11.044http://dx.doi.org/10.1016/j.compositesb.2015.11.044http://dx.doi.org/10.1016/j.compositesb.2015.11.044http://dx.doi.org/10.1016/j.compositesb.2015.11.044http://www.elsevier.com/locate/compositesbhttp://www.sciencedirect.com/science/journal/13598368http://crossmark.crossref.org/dialog/?doi=10.1016/j.compositesb.2015.11.044&domain=pdfmailto:[email protected]

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epoxy at a strain rate up to 27 s1 on the modied universal testing

machine (UTM). It was observed that, with an increase in strain

rate, modulus and strength of Kevlar/epoxy increased. Dynamic

response of Kevlar/Polyester composites was investigated by Har-

ding and Welsh [17] for cylindrical projectile up to a strain rate of

400 s1. They found that the tensile modulus increased within the

strain rate range from 104 to 103 s1 and reported the non-linear

response in dynamic tension. Zhu et al. [18] carried out static and

dynamic tests on Kevlar/polyester laminates. The damage pattern

observed for dynamic loading was different than that of the cor-

responding quasi-static case. Jacob et al. [20] carried out a detailed

review of the strain rate dependence on the mechanical properties

of polymer composites and a lot of contradiction in the data,

regarding the strain rate effect was reported.

From the above literature, it is evident that the mechanical

properties of the ber reinforced composite laminates are sensitive

to strain rate. Numerous studies have been conducted to charac-

terize the mechanical properties of Kevlar bers [19e22] and

Kevlar fabrics [23]. Wang and Xia [19,21] investigated the inuence

of strain rates (104 to 103 s1) and temperature(60 to 90 C) on

Kevlar 49 ber bundles. Results show that the mechanical prop-

erties of Kevlar 49 bers are sensitive to strain rate and tempera-

ture. It was also reported that at a constant temperature, initialelastic modulus, strength, and failure strain increase with an in-

crease in the strain rate, and for a xed strain rate, the initial elastic

modulus decreases and failure strain increases with increase in test

temperature. The experimental investigation of Lim et al. [22]

included three high-performance bers (Kevlar, Kevlar 129, and

Twaron) fabricated at different times over a period of ten years.

Their experimental results show that longitudinal tensile strength

of the bers weakly dependent on the ber gage length and re-

ported the insignicant strain rate effect on tensile strengths (only

by a few percent). Tensile tests of Zhu et al. [23] on Kevlar 49 plain

weave fabric at strain rates ranging from 25 to 170 s1 reported that

the dynamic material properties in terms of Young's modulus,

tensile strength, maximum strain and toughness increase with an

increase in the strain rate.There is a lack of experimental studies on the composite laminas

made from Kevlar bers reinforced with a thermoplastic resin. The

majority of the works reported in the literature were concentrated

on the high strain rate behavior of thermoset-based composite

laminates. Limited research has been reported on the dynamic

response of thermoplastic-based Kevlar composites [24,25].

Rodriguez et al. [24] examined the effects of high strain rate on

polyethylene and aramid woven fabrics. They deduced from their

results that dynamic stressestrain curve is more linear as

compared to the static one. Dynamic compression tests and quasi-

static tests of Viswanathan et al. [25] on Kevlar 29/polyethylene

showed signicant increase in the tensile strength and decrease in

failure strain at high strain rates as compared to that of the quasi-

static tests. Carillo et al. [26] reported that the addition of poly-propylene (PP) to aramid (Kevlar 129) fabrics shows improved

impact resistance. However, low adhesion was reported between

aramid fabrics and PP matrix. There is no consistent data available

for the characterization of dynamic response of thermoplastic-

based composite laminates made from Kevlar bers and PP ma-

trix. To the best of the author's knowledge, the compressive prop-

erties of Kevlar ber reinforced thermoplastic composites in the

through-thickness direction at high strain rates have not been re-

ported in the open literature.

In the present study, the through-thickness compressive prop-

erties of thermoplastic-based composite laminates made from

Kevlar ber and maleic anhydride grafted-PP (MAg-PP) matrix are

reported at high strain rates ranging from 1370 to 6066 s1. Grafted

maleic anhydride-PP is added to PP resin to improve the adhesion

between Kevlar ber and PP resin. Kevlar/MAg-PP (K-MPP) com-

posite laminates consisting of 8 and 24 layers are fabricated using

vacuum assisted compression molding machine. High strain rate

compressive properties of the K-MPP composite laminates with

respect to through-the-thickness direction are characterized using

SHPB. The compressive stressestrain behavior, toughness,

compressivepeak stressand strain at peak stress are studied for the

fabricated composite laminates. Following the experiments, failure

mechanisms are characterized through scanning electron micro-

scopy (SEM).

2. Experiments

2.1. Fabrication of composite laminates

Kevlar 29 yarns with 1000 denier were woven into plain woven

fabric with areal density of 364 g/m2. Yarns made of thousands of

bers, were woven into this fabric. The yarn tenacity was 14.91 g/

den with 40 ends/inch in both weft and warp directions. Thermo-

plastic polymer PP was selected as matrix due to its lightweight and

density of 0.855 g/cm3. The interfacial property between Kevlar

fabric and PP was improved by adding a coupling agent called,

maleic anhydride grafted PP (MAg-PP). The main function of thiscoupling agent is linking of bers to the polymer matrix and

reducing the pull out while increasing the impact and tensile

strength. Kevlar/MAg-PP (K-MPP) composite laminates were

fabricated using vacuum assisted compression molding machine.

PP sheets were coated with grafted maleic anhydride (MAg) to

improve the interfacial property between Kevlar and PP resin. The

alternate layers of Kevlar fabric and MAg-PP sheets were stacked

and placed in the vacuum chamber containing at plate molds. The

fabric weave pattern and stacking sequence of the preform are

shown in Fig. 1. The specimens were heated at a temperature of

200 C under 10 bar pressure and cooled to room temperature. The

matrix (MAg-PP sheet) thickness was 0.05e0.1 mm and the Kevlar

fabric thickness was 0.15e0.2 mm. To avoid formation of voids

during the fabrication process, a vacuum was maintained at550 mm of Hg in a vacuum chamber. K-MPP laminates with 8 layers

(1.6e1.7 mm thick) and 24 layers (4.3e4.6 mm thick) were cut out

from a square plate of 160 160 mm, through laser cutting to

prepare the test specimen for dynamic characterization (Fig.1). The

ber volume fraction was measured through burn off test according

to ASTM-D-2584-02 and it was observed to be 50e57%.

The critical slenderness ratio (l/d) is imperative to ensure that

the results obtained from the experiment reect the desired ma-

terial properties. Inertial effects produce stress waves along the

radial and the axial directions of the specimen. If the ends of the

specimen are well lubricated, it minimizes the inertial effects. The

correction for friction depends on the l/d ratio of the specimen. The

l/d ratio of 0.3e0.5 was shown to be good criteria of SHPB specimen

design [27]. However, it was also suggested that the tests on thematerials with high ow stress and low density are less prone to

such inertial errors [28]. As the material used in the present study is

of low density, low l/d ratio was considered to avoid inertial effects.

The diameter of the specimen was considered in the range of

15.5e16 mm. Length of the specimen was governed by the thick-

ness of the laminate which comes out to be approximately

1.6e1.7 mm for 8-layer specimens and 4.3e4.6 mm for 24-layer

specimens. Therefore, the l/d ratio of the present specimens was in

the range of 0.1e0.3.

2.2. High strain rate test

High strain rate experiments were performed using SHPB

apparatus available at Impact Mechanics Lab, Department of

R. Kapoor et al. / Composites Part B 89 (2016) 374e 382 375

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Applied Mechanics, IIT Delhi. The schematic representation of the

SHPB setup is shown in Fig. 2.

The specications of the SHPB setup were as follows:

Bar diameter e Striker bar, Incident bar and Transmission bar e

20 mm

Bar length e Striker bar e 300 mm; Incident bar e 1500 mm;

Transmission bar e 1500 mm

Bar material properties e Maraging steel: r e 8000 kg/m3; E e

183 GPa; Elastic wave speed (C e) of bars e 5091 m/s

Strain gage properties e Gage factor(G) e 5; Initial voltage (U ) e

5 V; and Amplication factor ( A) e 11

Detailed theory and technique involved in SHPB are well

described in the literature [29,30]. Therefore, a detailed discussion

is avoided in the present study. A dynamic stressestrain relation

can be obtained by measuring the incident, transmitted and re-

ected stress waves. The strain gage mounted on the incident bar

measures the both incident pulse (V i) and the reected pulse (V r)

while the strain gage on the output bar measures the transmitted

Striker Specimen Transmission bar Incident bar

Strain gauge 2

Signal conditioner & amplifier Signal conditioner & amplifier

Data acquisition &

storage

Strain gauge 3

Fig. 2. Schematic setup of compression SHPB.

!!!!!!

(a) (b) (c)

MAg-PP matrix(25 layers)

Kevlar fabric

24 la ers

8 layers

24 layers

Fig. 1. Schematic diagrams: (a) 2D plain weave pattern, (b) stacking sequence for 24-layer composite and (c) dynamic test specimen.

(a) (b)

0.0154 0.0156 0.0158 0.0160

-0.05

0.00

0.05

0.10

V o l t a g e ( V )

Time (sec)

8 layer 5335/s

Incident & reflective wave

Transmitted wave

0.0112 0.0114 0.0116 0.0118

-0.05

0.00

0.05

0.10

V o l t a g e ( V

)

Time (sec)

8 layer 6066/s


Transmitted wave

Fig. 3. Input and output wave signals of 8-layer K-MPP laminates in through-the-thickness compression: (a) 5335 s

1

and (b) 6066 s

1

.

R. Kapoor et al. / Composites Part B 89 (2016) 374e 382376

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pulse (V t). These values are in volts and were converted into strain

signals, including transmitted strain signal 3t and reected strain

signal 3r by using the following equations:

3r ¼ 2V rGUA

(1)

3t ¼ 2V tGUA

(2)

where G is the gage factor, U is the input voltage and A is the

amplication factor.

By using Eqs. (1) and (2), engineering stress (s), strain ( 3) andstrain rate ð _ 3Þ dened on the specimen length are measured using

the following equations [31]:

s ¼ E Ab As

3t (3)

3¼ 2C e

Ls

Z t

0

3rdt (4)

_ 3¼ 2C e

Ls 3r (5)

where Ab, As, Ls are the cross-sectional areas of pressure bar, cross-

sectional area and length of the specimen, respectively; E and C e are

Young's modulus and the wave velocity in the pressure bars,

respectively. Then the engineering stress is converted into true

stress by using the following equations [32,33]:

strue ¼ s½1 3 (6)

(a) (b)

0.0112 0.0114 0.0116 0.0118

-0.05

0.00

0.05

0.10

V o l t a g e ( V )

Time (sec)

24 layer 2538/s


Transmitted wave

0.0098 0.0100 0.0102 0.0104

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

V o l t a g e ( V )

Time (sec)

24 layer 3440/s


Transmitted wave

Fig. 4. Input and output wave signals of 24-layer K-MPP laminates in through-the-thickness compression: (a) 2538 s 1 and (b) 3440 s1.

0.00000 0.00005 0.00010 0.00015 0.00020 0.00025

0

100

200

300

400

500

600

S t r e s s ( M P a )

Time (sec)

24 layer K-MPP

2005/s

3239/s

0.00000 0.00005 0.00010 0.00015 0.00020

0

4

8

12

16

S t r a i n ( % )

Time (sec)

24 layer K-MPP

2005/s

3239/s

Fig. 5. Variation of stress and strain with time at different strain rates.


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3true ¼ ln½1 3 (7)

3. Results and discussion

3.1. Dynamic stressestrain behavior

High strain rate tests are performed on SHPB for 8-layer and 24-

layer K-MPP laminates at strain rates from 1370 to 6066 s1. The

strain waves sensed by strain gages mounted on the incident and

transmission bars at strain rates of 5335 and 6066 s1

for 8-layerlaminate and 2538 and 3440 s1 for 24-layer laminate are shown in

Figs. 3 and 4.

The transient data for each specimen tested under high strain

rate is recorded and saved. The incident wave is a function of

impact velocity and the data is initiated as the wave reaches the

location of strain gage on the incident bar. The reected wave is not

uniform for a long time. The reected wave reaches a maximum

value and oscillates at a constant value and approaches zero. Strain

rate versus time and stress versus time response are saved, to plot

dynamic stressestrain behavior. The time at which transmitting

pulse starts deviating from zero, is selected as the starting time and

when it drops tozerois selectedas the end time. The portions of the

reected pulse are taken to the equivalent time range and incor-

porated to obtain strain versus stress data using Eqs. (3)e(5) at each

strain rate. The stress versus time and strain rate versus time plots

for high strain rate compressive behavior of 24-layer K-MPP lami-

nate is shown in Fig. 5.

The dynamic true stress-true strain behavior of 24-layer and 8-

layer K-MPP laminates through-the-thickness direction at high

strain rates is shown in Fig. 6.

0 3 6 9 12 15 18

0

100

200

300

400

500

600

T r u e S t r e s

s ( M P a )

True Strain (%)

8 layer Kevlar/MAg-PP

@ 5335/s strain rate


0 2 4 6 8 10 12

0.0

2.0x108

4.0x108

6.0x108

24 layer Kevlar/MAg-PP

@ 2538/s strarin rate


T r u e S t r e s s

( M P a )

True Strain (%)

(a) (b)

Fig. 6. Dynamic true stresse

true strain response of K-MPP (a) 8 layers and (b) 24 layers.

Fig. 7. Dynamic true stresse

true strain behavior of 24-layer K-MPP laminate.


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It is observed that for 8-layer K-MPP laminates lots of un-

dulations and inconsistencies were recorded in the experimental

results (Fig. 6a). These inconsistencies could be attributed to the

low l/d ratio of the test specimen. Further, a series of initial ex-

periments were conducted on the specimens with different diam-

eter values and consistent results were obtained for 24-layer

specimens with an l/d ratio of 0.3 (Fig. 6b). Hence, further discus-

sions are emphasized for 24-layer composites. From the stresse-

strain behavior of laminates under various strain rates, the effect of

strain rate on the compressive dynamic properties of K-MPP lam-

inates are studied in terms of compressive peak stress, strain at

peak stress, compression modulus and toughness.

Fig. 7 shows the dynamic true stressetrue strain curves of plain

woven 24-layer K-MPP laminate in the through-thickness direction

subjected to different strain rates. As the matrix is dominant in

through-the-thickness behavior of composites, the compressive

stress vs strain behavior obtained from the present tests are qual-

itatively comparable with the dynamic stress vs strain behavior of

PP [34] (Fig. 7b). Due to the presence of Kevlar bers, exact and

quantitative comparison of the results cannot be made. The high

strain rate compressive properties measured by SHPB tests for 8-

layer and 24-layer K-MPP laminates, are listed in Table 1.

3.2. In uence of strain rate on the dynamic compressive properties

The inuence of strain rate on the dynamic compressive

response of 24-layer K-MPP specimens is dened in terms of peak

stress, strain at peak stress, toughness and compressive stiffness.

Fig. 8 shows the effect of strain rate on the compressive peak

stress of K-MPP (24 layers) laminate at various strain rates. In this

case the peak stress, increased from 220.32 MPa to 652.91 MPa at

strain rates of 1370 s1 and 4264 s1, respectively, with an increase

of 196.35% over the given range of strain rates.

The relation between the compressive peak stresses (smax) with

strain rate can be tted through a linear curve with the following

equation:

smax ¼ 0:165_ 3 21:362 (8)

Fig. 9 depicts the effect of strain rate on the failure strain at

different strain rates for 24-layer K-MPP laminates. The strain at

peak stress increases linearly with strain rate and it is evident fromFig. 7a. This can be attributed to the ductile failure of the specimen,

as a result of which the damage propagation in the material be-

comes slow with an increase in the strain rate, which indicates

higher impact resistance of the material. The strain at peak stress,

increased approximately by 133.8% over the given range of strain

rates.

The linear relationship between the strains at peak stress ( 3f )

and strain rate can be tted as:

3f ¼ 0:003_ 3þ 2:686 (9)

In the case of Kevlar/epoxy laminates [7], strain at peak stress

decreased as the strain rate increased. But in the present study, for

K-MPP laminates, strain at peak stress, increases with the strainrate due to the ductile behavior of Kevlar ber and PP matrix. This

phenomenon makes the present material desirable for armor ap-

plications due to increased energy absorption.

Fig. 10 illustrates the toughness of 24-layer K-MPP laminates at

various strain rates. It can be observed that the toughness increases

with the strain rate and the trend of increase is polynomial in na-

ture. The increase in toughness is 808.8%.

Table 1

Dynamic properties of K-MPP laminates.

No. of layers Strain rate (s1) Young's modulus (GPa) Toughness (MPa) Peak stress (MPa) Failure strain

8 5335 102 60.62 407.98 0.170

6066 143 66.18 544.00 0.146

24 1370 38 11.66 220.32 0.065

2005 69 21.37 264.40 0.086

2538 81 33.80 405.00 0.100

3239 105 54.22 515.81 0.1143440 111 62.55 591.34 0.117

4264 131 105.97 652.91 0.152

1000 1500 2000 2500 3000 3500 4000 4500

100

200

300

400

500

600

700

800

P e a k s t r e s s ( M P a )

Strain rate (s-1)

Experimemt

Linear Fit of Peak stress

Fig. 8. In

uence of strain rate on compressive peak stress (24 layers). Fig. 9. In

uence of strain rate on failure strain at peak stress (24 layers).


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The polynomial relationship between toughness and strain rate

can be tted as following equation:

E t ¼ 0:000009_ 32 0:017_ 3þ 20:013 (10)

Fig. 11 shows the variation of compression stiffness at various

strain rates. As the strain rate increases the compressive stiffness

increases. The increase of compression stiffness is almost linear and

can be tted with the following equation:

E C ¼ 0:032_ 3 0:062 (11)

The peak stress values obtained for K-MPP laminates are

compared with Kevlar/epoxy reported in Ref. [7]. But the length of

the specimens in Woo and Kim [7] was 8 mm while in the present

study it is 4 mm. According to Eqs. (3) and (6), the dynamic stress is

independent of the length and depends primarily on the cross

section area of the bar and the specimen. However, for the purposeof comparison, analytical calculations were performed in MATLAB

by doubling the length of the present specimen, to obtain an esti-

mate of the peak stress value. The response obtained from the

present study for 8 mm length was not exact, but it is assumed that

the results remain in the similar range. The strain rate values of K-

MPP laminate, which are in close range with the Kevlar/epoxy are

considered for the comparison. Peak stress values of Ref. [7] are

376 MPa and 161 MPa for strain rates of 1460 s1 and 1182 s1,

respectively. Assuming that the stress value remains unaffected by

the length of the specimen, the obtained peak stress from the

present study is 437 MPa and 190 MPa for strain rates of 1452 s 1

and 1173 s1, respectively. Hence, it indicates that the compressive

properties of K-MPP laminateshas better impact response than that

of the Kevlar/epoxy because the failure behavior of Kevlar/epoxy is

brittle and for K-MPP it is ductile in nature.

3.3. Fractography

The morphologies of the composite specimens are investigated

by SEM. The SEM micrographs of 24-layer K-MPP laminates at

different strain rates are studied. Since the loading is in through-

the-thickness direction, the failure initiates from the surface edge

of the specimen (Fig.12), which is in line with the loading direction.

As the strain rate increases the edge failure increases and inter

laminar shear stresses generate which are expected to be the

probable cause for the occurrence delamination. The progress of

the delamination at different strain rates can be seen in Fig. 12.

The failure modes of the samples in through-the-thickness di-rection at strain rates of 2005 s1 and 3440 s1 are illustrated in

Fig. 13a and b respectively, through SEM micrographs. The SEM

micrographs choose from selected locations on the samples to

highlight the dominant failure modes. These micrographs illustrate

clearly the failure modes through delamination, ber deformation,

shear fracture, ber failure, ber buckling and matrix cracking.

Fig. 13(a) shows the shear fracture, delamination, deformation

of bers and ber crush on the impact face. The crush is induced by

the compressive loading. It is observed that, after the impact, the

specimen is marginally damaged and is characterized by ber crush

and the shedding of bers (Fig.13(b)). It indicates that the specimen

undergoes a signicant compressive stress during impact, severe

shedding of bers and damage due to surcial friction. While

Fig. 10. Inuence of strain rate on toughness (24 layers).

Fig. 11. Inuence of strain rate on compressive stiffness (24 layers).

Fig. 12. SEM micrographs of the edge failure at strain rates of 2005, 3239 and 4349 s

1

.


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comparing Fig. 13(a) to (b) at higher strain rates, the material

showed more delamination and several ber breakages which may

possibly due to the changes in local boundary bonding.

4. Conclusions

In the present study, experimental investigations were carried

out on 8- and 24-layer Kevlar/MAg-PP (K-MPP) composite lami-

nates in the through-thickness direction under high strain rate

loading. Interfacial behavior between Kevlar and PP was improved

by adding grafted maleic anhydride PP to the matrix. The high

strain rate tests were conducted using split Hopkinson pressure bar

(SHPB) apparatus. The samples were tested in the strain rate range

of 1370 s1 to 6066 s1. The dynamic true stressetrue strain

behavior of composite specimens wasobtained and the inuence of

strain rate on the through-thickness compression properties was

studied. At high strain loading, the though-thickness compressive

properties were rate sensitive. Especially, the strain at peak stress

for K-MPP increased with the strain rate, whereas for Kevlar/epoxylaminates it decreased. This behavior can be attributed to the

ductile behavior of Kevlar fabric and PP matrix. Fractography

through SEM revealed various failure modes such as, matrix

cracking, shear fracture, delamination, ber failure, ber defor-

mation, friction between bers and the shedding of bers. The

ductile response of K-MPP laminates observed in the present study

can be adopted in the design of composite laminates under high

strain rate loading conditions, such as ballistic impact and impul-

sive loading.

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Fig. 13. SEM micrographs of K-MPP specimen at strain rates of (a) 2005 s1 and (b) 3239 s1.


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