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Tekstilec, 2025, Vol. 68(4), 336–335  |   DOI: 10.14502/tekstilec.68.2025048 
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Muhammad Junaid Saleem, 
1, 2
 Asfandyar Khan, 
2, 3
 Kashif Javed, 
2
 Mainul Morshed, 
3
 Imran Ahmad Khan 
2
1
 Department of Textile and Fashion, Punjab Tianjin University of Technology, Lahore 54000, Pakistan
2
 Department of Textile and Apparel Science, University of Management and Technology, Lahore 54770, Pakistan
3
 Department of Textile Engineering, Daffodil International University, Dhaka 1216, Bangladesh
Enhancing Mechanical Performance of Kevlar 
Composites Through Optimised Stitching Parameters
Izboljšanje mehanskih lastnosti Kevlarjevih kompozitov z optimiziranimi 
parametri šivanja
Original scientific article/Izvirni znanstveni članek
Received/Prispelo 4–2025 • Accepted/Sprejeto 11–2025
Corresponding author/Korespondenčni avtor:
Imran Ahmad Khan
E-mail: imran.ahmad@umt.edu.pk
Tel.: +92-313-9374141
ORCID iD: 0000-0002-7127-5509
Abstract
The study explores the development of stitched reinforced composites using Kevlar woven fabric as the 
primary reinforcement. The influence of stitching parameters, stitch type, needle type and stitches per cm 
(SPC) on the mechanical performance of the composites was systematically investigated. Kevlar fabric was 
stitched using different needle types and SPC values before being mechanically tested for tensile strength, 
delamination resistance and impact performance. The results demonstrate that composites stitched with 
a ballpoint needle using a chain stitch at 3.94 SPC exhibited the highest mechanical strength, with tensile 
strength increasing from 145 MPa (unstitched) to 191.5 MPa (lock stitch) and 240 MPa (chain stitch). Impact re-
sistance improved from 98 kJ/m² (unstitched) to 133 kJ/m² (ballpoint needle, lock stitch), with chain-stitched 
composites showing even better performance due to the greater load distribution of the sewing thread 
and reduced fabric damage. Conversely, lock-stitched composites provided superior delamination resistance 
due to enhanced interlayer gripping. These findings highlight the potential of the developed composite for 
advanced protective applications, including bulletproof vests and aerospace structures requiring superior 
impact resistance and structural integrity.
Keywords: woven fabric, composites, stitching parameters, mechanical properties
Izvleček
Prispevek obravnava razvoj s šivanjem ojačanih kompozitov, pri katerih je kot primarna ojačitev uporabljena 
tkanina iz Kevlarja. Sistematično je bil preučen vpliv parametrov šivanja, vrste vboda, vrste igle ter število vbo-
dov na centimeter (SPC) na mehanske lastnosti kompozitov. Kevlar tkanina je bila šivana z različnimi vrstami 
igel in SPC vrednostmi, preden je bila mehansko testirana na natezno trdnost, odpornost proti razslojevanju 
in odpornost proti udarnim obremenitvam. Rezultati kažejo, da so kompoziti, šivani z iglo z okroglo konico in 
verižnim vbodom pri 3,94 SPC, pokazali najvišjo mehansko trdnost; natezna trdnost se je povečala s 145 MPa 

Enhancing Mechanical Performance of Kevlar Composites Through Optimised Stitching Parameters
337
(nešivani) na 191,5 MPa (prešivni vbod) in 240 MPa (verižni vbod). Odpornost proti udarcem se je izboljšala 
z 98 kJ/m² (nešivan) na 133 kJ/m² (igla z okroglo konico, prešivni vbod), pri čemer so kompoziti z verižnim 
vbodom dosegli še boljše lastnosti zaradi porazdelitve obremenitve preko šivalne niti in manjše poškodbe 
tkanine. Nasprotno pa so kompoziti s prešivnim vbodom zagotavljali boljšo odpornost proti razslojevanju 
zaradi izboljšane oprijemljivosti med plastmi. Izsledki raziskave poudarjajo potencial razvitega kompozita 
za napredne zaščitne aplikacije, vključno z neprebojnimi jopiči in letalskimi strukturami, ki zahtevajo visoko 
odpornost proti udarcem in strukturno celovitost.
Ključne besede: tkanina, kompoziti, parametri šivanja, mehanske lastnosti
1 Introduction
Composite materials are characterised by multi-
phase materials within which the phase distribution 
and geometry have been deliberately tailored to op-
timise one or more properties [1, 2]. In this context, 
textile-reinforced composites have been widely used 
in design applications for many years, particularly 
in cost-effective solutions [3]. Composite materials 
consist of two primary components, i.e. reinforce-
ment and matrix. The reinforcement, which provides 
strength and load-bearing capacity, can be made of 
glass fibres or other high-performance materials. 
The matrix, typically a polyester, epoxy or other 
resin, acts as a binding medium, transferring stress 
between the reinforcing fibres and maintaining the 
composite’s structural integrity. Fibrous structures 
serve as the primary reinforcement, bearing the 
applied stresses and significantly influencing the 
composite’s mechanical properties [4]. Commonly 
used reinforcements are glass fibre [5], carbon fibre 
[6] and Kevlar [7]. Reinforcement of fibre orienta-
tion and dimension decide the properties of com-
posites. The lengthwise direction of fibres gives more 
mechanical properties than the widthwise direction, 
the resin being a chemical or polymer that behaves 
as a matrix for binding the fibrous reinforcement or 
works as an adhesive [8].
The resin component in composites can be 
thermoset (e.g. epoxy, polyester, vinyl ester) or ther-
moplastic (e.g. Polyether ether ketone, polypropyl-
ene, polycarbonate), with thermosets offering high 
thermal stability and adhesion, while thermoplastics 
provide recyclability, flexibility and impact resis-
tance, making resin selection crucial for composite 
performance [9]. Fibre-reinforced polymer (FRP) 
composites have gained considerable attention for 
their high strength-to-weight ratio and superior 
durability. Among these, aramid-based composites, 
especially those using Kevlar, are extensively used 
in the defence, automotive and aerospace sectors. 
Stitching is a known method for enhancing interla-
minar properties in composites. However, the effect 
of stitch parameters on the damage mechanisms 
and overall performance has not been studied 
systematically. Polyether ether ketone (PEEK), a 
high-performance thermoplastic polymer known 
for its excellent mechanical properties and chemical 
resistance, is often used in fibre-reinforced com-
posites due to its compatibility with high-strength 
fibres like aramid. This study aims to fill this gap by 
investigating the influence of stitch type and needle 
geometry on the mechanical performance of Kevlar/
epoxy laminates [10].
Aramid fibres, particularly Kevlar, are known for 
their exceptional mechanical performance, including 
high tensile strength, low density, excellent thermal 
stability, and resistance to impact and abrasion. These 
characteristics make aramid an ideal candidate for 
use in aerospace, ballistic and high-performance 
structural applications where durability and strength-
to-weight ratio are critical [11]. Up until the 1900s, a 
lot of work was performed on natural fibres reinforced 
composites. Kevlar aramid (PPTA; poly (p-phenylene 

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Tekstilec, 2025, Vol. 68(4), 336–335
terephthalamide) fibre is broadly utilised as a part of 
the making of advanced composites [12].
Woven fabrics are the most widely used textile 
reinforcement in composite materials, offering 
versatility across various applications, including 
aerospace, automotive and protective gear. Their 
structured interlacing of warp and weft yarns ensures 
high dimensional stability and uniform mechanical 
properties. The development of carbon and aramid 
fibre fabrics, with superior stiffness compared to glass 
fibres, has further expanded the applicability of woven 
reinforcements. These fabrics exhibit excellent drapes, 
enabling the formation of complex shapes without 
gaps [13]. Additionally, woven fabric composites 
demonstrate enhanced impact resistance compared 
to nonwoven composites, significantly improving 
compressive strength after impact and making them 
ideal for high-performance applications [14].
Sewing the reinforcement is an effective method 
to enhance the through-thickness strength of com-
posites, significantly improving their compression 
after impact (CAI) performance. This technique 
introduces mechanical interlocking between plies, 
which helps redistribute stresses and delay the initi-
ation and propagation of delamination under impact 
or compressive loading. Failure in laminated com-
posites is often initiated by micro-scale interfacial 
separations between layers, leading to progressive 
delamination and reduction in structural integrity. 
By stitching or sewing the reinforcement, resin-rich 
zones are locally modified, providing additional 
crack-bridging and load transfer pathways that 
enhance damage tolerance and energy absorption. 
Moreover, this method has been shown to improve 
post-impact stiffness and residual strength, making 
it particularly valuable for applications in aerospace, 
automotive and defence structures where light-
weight yet damage-tolerant composites are critical. 
Recent studies have demonstrated that the choice of 
stitching pattern, needle type and thread material 
can further influence the mechanical performance 
and failure modes, enabling tailored reinforcement 
strategies for specific loading conditions. Overall, 
sewn reinforcements provide a practical and scal-
able approach to mitigating delamination-driven 
failures, offering significant improvements in both 
mechanical performance and structural reliability 
of composite laminates [15]. Yuan et al. investigated 
stitched composites with varying stitch densities to 
enhance their mechanical properties [16]. Previous 
research has primarily focused on improving the 
mechanical performance of composites through 
stitching; however, often without a detailed classifi-
cation based on stitch class or needle geometry. For 
instance, Loi G. et al. [17] studied stitched laminates 
without detailing stitch patterns, while recent 
advancements in composite stitching techniques 
underscore the need for parametric optimisation to 
enhance strength and durability [18]. Plain and chain 
stitches were incorporated to reinforce the laminates 
along the z-axis, improving their through-thickness 
strength. The results showed that stitched compos-
ites exhibited an approximately 10% improvement 
in ballistic efficiency compared to conventional 
woven laminate composites, with the optimal stitch 
design playing a key role in this enhancement [19]. 
The following section outlines the materials and 
experimental methods used to fabricate and test the 
stitched Kevlar/epoxy composites, providing a foun-
dation for evaluating how stitch class, stitch density 
and needle type influence performance. Bhavani 
V. Sankar et al. investigated the effects of stitching 
on the low-velocity impact response of stitched 
and delaminated beams, analysing their static and 
impact behaviour. A static contact force model was 
developed, and simulations identified displacement/
crack extension and load/displacement relation-
ships. Impact simulations provided insights into the 
load at crack initiation, peak contact force, and the 
extent of crack propagation at the end of impact. The 
study revealed that while stitching did not increase 
the load required for delamination initiation, it 
significantly reduced the extent of delamination, 
thereby improving the composite’s damage tolerance 
[20]. Lopresto et al. investigated the effectiveness 
of stitching in enhancing the damage resistance of 

Enhancing Mechanical Performance of Kevlar Composites Through Optimised Stitching Parameters
339
polymer composites against ballistic projectiles 
and explosive blasts. The study found that stitching 
reduced delamination damage caused by ballistic 
impact to some extent, while significantly improving 
damage resistance under explosive blast loading. 
This improvement was attributed to the increased in-
terlaminar fracture toughness resulting from stitch-
ing. Additionally, stitched composites exhibited less 
overall damage and retained strength and flexural 
modulus comparable to unstitched composites after 
ballistic impact testing, highlighting their potential 
for high-performance protective applications [21]. 
Unlike prior studies that generally explored stitch-
ing effects without categorising based on needle 
geometry or stitch structure, this research distinctly 
evaluates the combined effects of stitch class (chain 
vs. lock) and needle type (sharp vs. ballpoint) on 
mechanical properties. This multidimensional 
optimisation has not been previously addressed in 
Kevlar/epoxy composites.
Li M. et al. examined the improvement in inter-
laminar fracture toughness of polymer composites 
using proposed micromechanical models, which 
showed good agreement between experimental and 
theoretical interlaminar fracture toughness values. 
The study also concluded that delamination resis-
tance could be effectively measured using the End-
Notched Cantilever Beam (ENCB) method [22]. 
Lee B. et al. analysed the tensile creep behaviour of 
woven fibre composites, which were stitched per-
pendicular to the loading direction using cotton and 
carbon sewing threads. Their findings indicated that 
through-thickness stitching significantly enhanced 
the creep resistance of the composites, with the data 
effectively analysed using Findley’s equation [23]. 
Wei Y. et al. studied the in-plane lateral impact and 
tensile behaviour of the E-glass epoxy laminates of 
2.8 mm small thickness made by the resin transfer 
moulding. Kevlar rovings of 1000 to 3000 deniers 
were used for the through-the-thickness reinforce-
ment. The results of the following study showed 
that for the tensile test, the damage mechanism of 
the stitched laminates was affected by the load di-
rection, where the addition of stitching threads does 
not affect stiffness prominently. The test of impact 
done by the hemispherical tipped impactor showed 
that the stitching had prominently minimised the 
delamination crack area and the stitching threads 
of 3000 deniers had shown a better resistance to the 
crack propagation [24]. Wang H. et al. examined de-
lamination in composite laminates, which can arise 
during fabrication or from impact during service. 
The study demonstrated that appropriate stitching 
significantly enhanced the strength of laminates un-
der lateral compression, improving their structural 
integrity [25]. Herszberg et al. studied the stitching’s 
effect on the strength of the tensile-loaded panels 
under impact loading by evaluating the ten-layered 
single weave T-300 carbon fabric, orthotropic panel 
with 2 mm thickness. Aramid thread was used to 
stitch the layers and a transfer moulding technique 
was used to transfer the epoxy resin for making the 
composites. Up to 70 m/s impact velocities with a 
9-gram projectile, the samples were impacted to 
tensile loading ranging to 72% of the sample’s ulti-
mate tensile strength. There was little improvement 
in the strength of stitched then unstitched samples 
[26]. Similarly, accurate estimation of sewing thread 
consumption is essential for efficient material 
use and cost reduction in the garment industry. 
For stitch class 301, thread usage is influenced by 
stitch geometry, including stitch length and fabric 
thickness. Recent studies using image analysis and 
Fourier series modelling have predicted thread 
consumption with up to 95% accuracy, providing a 
reliable method to identify key influencing factors 
and optimise industrial sewing operations [27]. 
For multi-thread chain stitches such as stitch class 
406, thread usage depends on several geometrical 
and material parameters, including stitch density, 
fabric type and material thickness. Recent modelling 
efforts on stitch class 406 have demonstrated that 
geometrical prediction approaches can estimate 
thread consumption with over 97% accuracy, offer-
ing an effective tool for optimising thread allocation 
in bulk production and improving process planning 

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in industrial sewing applications [28]. Recent studies 
have shown that increased stitch density and opti-
mised stitch orientation can significantly improve 
mechanical performance. For instance, transverse 
stitching in hybrid composites has led to a 19% 
increase in flexural stress, while parametric studies 
report up to a 17% improvement in maximum load 
capacity and a 103% increase in tangent stiffness in 
stitched specimens compared to unstitched com-
posites [29]. Other recent studies have reported that 
increasing stitch density generally enhances delami-
nation resistance; however, excessively high densities 
can introduce fibre distortion and resin-rich regions, 
resulting in a marginal reduction in overall tensile 
strength [30]. Despite these advances, gaps remain 
in literature. Most studies on stitched reinforced 
composites have focused on improving mechanical 
properties such as tensile strength and delamination 
resistance, primarily using woven fabrics or fibres as 
reinforcement. Additionally, Kevlar and glass fibres 
have been the most commonly used sewing threads 
for stitching reinforcement. While researchers have 
extensively discussed the effect of stitch classes on 
composite performance, the influence of different 
needle types on reinforcement remains unexplored. 
Furthermore, the impact of varying stitch per 
centimetre (SPC) on composite strength has not 
been systematically addressed. The results of the 
mechanical tests are presented below, focusing on 
tensile strength, impact resistance, delamination 
behaviour and microscopic analysis to understand 
the effect of stitching parameters. Given that 
different SPCs and needle types may significantly 
affect composite mechanical performance, further 
investigation is required to address these gaps. This 
study systematically investigates stitched Kevlar 
composites by varying stitch class, SPC and needle 
type, thereby contributing new insights into the 
optimisation of through-thickness reinforcement. 
This approach extends existing parametric studies 
and aligns with current efforts to establish predic-
tive relationships between stitching parameters and 
composite performance.
2 Materials and methods
2.1 Materials
The reinforcement material used was Kevlar 29 
plain-woven fabric with a nominal areal density of 
300 g/m². The matrix phase consisted of a two-part 
epoxy resin (Araldite LY 556) and hardener (HY 
951) mixed in a 10 : 1 weight ratio. All materials 
were sourced from Huntsman Advanced Materials, 
Germany. Kevlar yarn (35.4 tex) was used for fabric 
development. The Kevlar sewing thread was supplied 
by Midas Safety (Pvt.) Ltd., Pakistan, with a count of 
40.5 tex and 5.9 twists per cm centimetre (Z-twist). 
Polyvinyl chloride (PVC) was used as a sizing agent. 
For composite fabrication, polyester resin (Polylite P 
33-33), manufactured by Reichhold, USA, was used 
as the matrix material. Cobalt naphthenate (man-
ufactured by Merck) was employed as an initiator, 
while potassium permanganate (KMnO₄) (supplied 
by Sigma-Aldrich) was used as a hardening agent for 
resin curing. For stitching purposes, two types of sew-
ing machines were used: Single Needle Lock Stitch 
Machine and Single Needle Chain Stitch Machine, 
both manufactured by JUKI, Japan. 
2.2 Fabric weaving
The Kevlar fabric was woven using a Dornier A1 
air-jet weaving machine (Dornier GmbH, Germany), 
operating at a speed of 550 rpm. Fabric samples of 
2-meter length were developed using 36.9 tex/1 (16/1 
Ne) spun Kevlar yarn, with a 2/1 Z-twill weave design. 
The fabric specifications were 16 tex × 16 tex yarns 
with 27.5 threads/cm × 21.3 threads/cm and a width 
of 50.8 cm, having mass per unit area of 200 g/m
2
. 
The manufacturing process began with sizing, where 
polyvinyl chloride (PVC) was applied to the spun 
Kevlar yarn using a sizing machine. Stitching was 
performed using a JUKI industrial sewing machine. 
Stitch tension was maintained at 2 N (as presented 
in Table 1) to ensure uniform penetration without 
excessive fibre breakage. Two needle types were used, 
i.e. sharp-point and ballpoint (both size 90/14). The 
composite curing process was conducted using a 
Carver 3851 hot press.

Enhancing Mechanical Performance of Kevlar Composites Through Optimised Stitching Parameters
341
Table 1: Stitch tension validation across samples
Sample code Stitch tension (N) Mean (N) Standard deviation (N) Remarks
S1 N/A N/A N/A N/A
S2 1.98 2.00 0.03 Within tolerance
S3 2.00 2.00 0.03 Within tolerance
S4 2.02 2.00 0.03 Within tolerance
S5 1.99 2.00 0.03 Within tolerance
S6 2.00 2.00 0.03 Within tolerance
S7 2.01 2.00 0.03 Within tolerance
S8 2.03 2.00 0.03 Within tolerance
S9 1.97 2.00 0.03 Within tolerance
S10 2.00 2.00 0.03 Within tolerance
S11 1.99 2.00 0.03 Within tolerance
S12 2.02 2.00 0.03 Within tolerance
2.3 Reinforcement preparation
Before cutting the fabric, de-sizing was performed to 
remove the sizing material. The woven Kevlar fabric 
was placed in a boiling water bath and stirred con-
tinuously for 30 min. After the drying, the de-sized 
fabric was measured in all dimensions using a large 
steel scale, marker and measuring tape. A marking 
plan was then created on paper based on the required 
dimensions of each fabric sample, with a length of 
2 m and a width of 0.508 m (20 inches). Before resin 
infusion, the Kevlar fabrics were cleaned by boiling 
in distilled water at 100 °C for 20 min, then dried 
in a convection oven at 60 °C for 4 hours to remove 
residual moisture and improve resin adhesion. The 
composite curing process was performed using a 
Carver 3851 hot press. The stitched fabric stacks 
were impregnated with epoxy using the hand lay-up 
technique and cured under 5 MPa pressure at 120 °C 
for 30 min in the Carver 3851 hot press. Post-curing 
was performed at 80 °C for 3 hours to enhance cross-
linking density. Tensile tests were conducted on an 
Instron 3369 universal testing machine and impact 
testing was performed using a Zwick/Roell HIT 50P 
Charpy impact tester. The composite laminates were 
fabricated using a Carver 3851 hot press under a 
pressure of 5 MPa at 120 °C for 30 min.
Kevlar plies were cut using stainless steel indus-
trial scissors (Model: KAI 7230, 30.48 cm (12 inch) 
length, Japan), known for their serrated edge and cor-
rosion-resistant blades suitable for cutting high-ten-
sile strength fibres. The cutting was performed using 
straight-edge alignment on a marked grid to ensure 
consistency. The fabric was tensioned manually and 
cut at 90° angles using a single continuous stroke to 
minimise fraying or fibre distortion. After the cutting, 
all plies were arranged in the correct dimensions at 
a 90° stacking sequence, with fifteen plies designated 
for the Kevlar fabric. Once arranged, the sets were 
prepared for the sewing operation. Two different 
types of sewing machines and needle sizes were used 
for stitching: a single-needle lock stitch machine and 
a single-needle chain stitch machine. The lock stitch 
machine was used to perform 301-class stitches (301 
is a basic lock stitch in which we use one needle 
thread and one bobbin thread), which are standard 
lock stitches, while the single-needle chain stitch 
machine utilised one needle thread and one looper 
thread, producing 101-class simple chain stitches (in 
stitch class 101, stitch is formed when needle thread 
passing through the material and interloping with 
itself on the bottom side of the fabric with the assis-
tance of the spreader). The number of stitches varied 
between 0.93 cm
–1
, 1.24 cm
–1
 and 1.55 cm
–1
. These 
stitching techniques were employed to reinforce the 
composites and analyse their impact on mechanical 
properties. The sewing process was carried out using 

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Tekstilec, 2025, Vol. 68(4), 336–335
sharp point and ballpoint needles to assess their 
effect on the composite structure. As depicted in Fig-
ure 1a, Kevlar plies underwent the stitching process, 
whereas Figure 1b displays the final stitched Kevlar 
plies. The sample preparation details are outlined in 
Table 2. The experimental work was designed based 
on a Design of Experiments (DOE) approach to 
investigate the influence of needle type, stitch type 
and stitch density on sewing performance and fabric 
characteristics. A total of thirteen samples were pre-
pared, varying systematically in their combinations 
of needle and stitch types. Both ballpoint and sharp 
point needles (Figure 1c) were utilised to assess 
penetration behaviour and seam appearance, while 
lock stitch and chain stitch configurations were 
selected to compare seam strength and elasticity. 
Stitch density varied across samples, ranging from 
2.36 cm
–1
 to 10.39 cm
–1
, to evaluate its influence on 
fabric integrity and seam quality. The control sample 
(Sample 1) was kept without needle or stitch applica-
tion to serve as a reference. This experimental design 
enabled the evaluation of individual and combined 
effects of sewing parameters on the resulting seam 
and fabric performance.
Table 2: Design of experiment for prepared composite samples
Sample Needle type Stitch type Number of stitches, SPC/SPI 
a)
1 N/A N/A N/A
2 Ballpoint needle Chain stitch 3.94/10
3 Sharp point needle Chain stitch 2.36/6
4 Ballpoint needle Chain stitch 2.36/6
5 Sharp point needle Lock stitch 3.15/8
6 Sharp point needle Chain stitch 3.15/8
7 Ballpoint needle Lock stitch 3.15/8
8 Sharp point needle Chain stitch 3.94/10
9 Ballpoint needle Chain stitch 3.15/8
10 Ballpoint needle Lock stitch 3.94/10
11 Sharp point needle Lock stitch 2.36/6
12 Ballpoint needle Lock stitch 2.36/6
13 Sharp point needle Lock stitch 3.94/10
a)
 Stitches per centimetre/stitches per inch
a) b) c)
Figure 1: a) Stitching of Kevlar reinforcement, b) stitched Kevlar impact reinforcement, and c) type of needles used
Ball needle Sharp needle
2.4. Composite fabrication 
Kevlar composite preparation involved accurately 
weighing chemicals and mixing them with 35 drops 
(using a dropper) of MEKP (methyl ethyl ketone 
peroxide) used for curing resin and 20 drops of 
cobalt per 80 g of vinyl ester resin. The resin used 
was Araldite LY 556 epoxy resin with HY 951 
hardener from Huntsman (Germany). Resin mixing 
was performed using a digital overhead stirrer (IKA 
RW20, 100–2000 rpm). The stitched preform was 

Enhancing Mechanical Performance of Kevlar Composites Through Optimised Stitching Parameters
343
placed into a steel mould, and composite curing was 
conducted using a Carver 3851 hydraulic hot press. 
The resin-to-fabric ratio was maintained at 1 : 1 by 
weight. For the mould set, a 0.01 m thick, 0.5 m × 1 
m transparent glass was used, accommodating two 
impact samples (0.15 m × 0.33 m each). The fibre vol-
ume fraction of the composites was determined using 
the matrix burn-off method. The average fibre volume 
fraction was calculated to be 45% based on weight-
loss analysis in a muffle furnace at 600 °C. The mould 
was waxed before placing the reinforcement. Vacuum 
infusion was employed for resin transfer, with stitched 
Kevlar laminates positioned on the mould. A vacuum 
tape and plastic sheet ensured sealing, while peel ply 
and breather layers were added. Plastic pipes facili-
tated resin flow and vacuuming, with a felt-covered 
pipe preventing excess resin from entering the pump. 
After verifying the zero-line setting, the pump was 
activated, and resin infusion proceeded until com-
plete saturation of the reinforcement. Post-infusion, 
samples were placed under 1.5  kg weight with an 
airtight sheet overnight to enhance mechanical prop-
erties. The composite laminates were cured in a hot 
air oven at 100 °C for 80 min, followed by post-curing 
at room temperature for 24 hours to ensure complete 
polymer crosslinking. Following this, the laminates 
were post-cured at room temperature for 24 hours to 
ensure complete polymer crosslinking and to achieve 
optimal material properties. The final composite pan-
els were sectioned using a diamond-coated circular 
blade (DiaSaw DCC-2000, 2 mm thickness, Korea) 
mounted on a precision cutting machine. Dimen-
sional accuracy for mechanical test specimens was 
ensured using a precision composite cutter (Zwick/
Roell ZCP020, Germany), capable of ±0.1 mm tol-
erance across all sample geometries. A precision lab 
composite cutter ensured exact dimensions for testing 
(Impact, Delamination, and tensile test).
2.5 Characterisations
Composite samples were prepared with two stitch 
types, single needle lockstitch and chain stitch, at 
stitch densities of 2.36 cm
–1
, 3.14 cm
–1
 and 3.92 cm
–1
 
oriented in the vertical direction along the warp. 
All mechanical and yarn property evaluations were 
performed following standard testing procedures.
Tensile strength
Tensile properties of stitched and unstitched com-
posites were measured according to ASTM D3039 
using a Zwick/Roell 8504 Universal Testing Machine 
(Germany). Samples were prepared with dimensions 
of 250 mm × 25 mm, with a gauge length of 200 mm, 
and tested at a crosshead speed of 2 mm/min. The 
tensile tests were conducted along the stitch line 
(longitudinal to the warp). For each sample, five 
replicates were tested to ensure statistical reliability.
Impact strength
Impact resistance was evaluated following ASTM 
D256 using a Zwick/Roell HIT50P impact tester 
(Germany). Specimens were prepared with dimen-
sions of 80 mm × 10 mm in accordance with ISO 
189. The impact was applied along the longitudinal 
stitch direction. Five measurements per sample were 
performed to obtain reliable results.
Delamination resistance
Delamination resistance was determined using 
a Shimadzu AGS-X Universal Testing Machine 
equipped with a Double Cantilever Beam (DCB) fix-
ture, according to ASTM D5528. Samples measured 
150 mm × 25 mm and were tested at a crosshead 
speed of 5 mm/min. The tests were conducted along 
the stitch line and five replicates were performed for 
each sample.
Yarn properties
Yarn properties, including linear density, twist and 
tenacity, were evaluated according to ASTM D1422. 
Yarn was carefully extracted from the composites, 
and twist per inch and twist direction were measured 
using a twist tester. T en yarns per sample were tested, 
each with a gauge length of 200 mm at a crosshead 
speed of 20 mm/min. All tests were repeated five 
times to ensure reproducibility.

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3 Results and discussion
3.1 Testing Kevlar yarn for fabric and sewing 
thread
The test results of Kevlar yarn are presented in T able 3. 
The yarn was characterised as a single-ply type, con-
sisting of a single strand rather than multiple strands 
twisted together. The measured yarn count was 35.4 
tex (16.68 Ne), indicating a relatively coarse yarn. 
The used yarn followed a Z-twist direction. The twist 
was 3.35 cm
–1
, which plays a critical role in deter-
mining the yarn’s strength, elasticity and processing 
behaviour. A moderate TPI like this ensures a balance 
between durability and flexibility. These combinations 
of properties make the yarn suitable for applications 
requiring strength, durability and controlled elasticity.
Table 3: Kevlar yarn specification for making composites
Kevlar yarn Yarn count (tex) Twist direction Twist (cm
–1
) Tensile strength (GPa)
Fabric construction 35.4 Z-twist 3.35 2.94
Sewing thread 40.5 Z-twist 5.9 3.14
3.2 Tensile strength evaluation of stitched 
Kevlar composite samples
Figure 2 clearly illustrates that in the vertical direction 
(along the warp), the tensile strength of stitched 
composites increases with the number of stitches per 
cm (SPC) for both lock stitch and chain stitch config-
urations. However, chain-stitched composites exhibit 
higher tensile strength than the lock-stitched ones. 
Specifically, for lock stitch, the tensile strength increas-
es from 165.9 MPa (2.36 SPC) to 182.8 MPa (3.14 SPC) 
and 191.5 MPa (3.95 SPC), whereas for chain stitch, 
it rises from 169.4 MPa (2.36 SPC) to 235 MPa (3.14 
SPC) and 240 MPa (3.95 SPC). A statistical analysis 
was performed to validate the experimental data. Mean 
values, standard deviations and coefficients of variation 
for tensile strength are presented in Table 4.
Table 4: Statistical summary of tensile strength of stitched Kevlar composites
Stitch type Stitch density (cm
–1
) 
a)
Tensile strength (MPa) (Mean ± SD) CV (%)
Lock stitch
2.36 165.9 ± 6.2 3.7
3.14 182.8 ± 5.4 3.0
3.95 191.5 ± 4.8 2.5
Chain stitch
2.36 169.4 ± 7.1 4.2
3.14 235.0 ± 6.5 2.8
3.95 240.0 ± 5.1 2.1
a)
 Stitches per centimetre
The tensile strength data were analysed using a 
two-way ANOVA to determine the effects of stitch 
type and stitch density. The results (Table 5) revealed 
that both factors had a statistically significant influ-
ence on the tensile strength of Kevlar composites (p 
< 0.05). Chain-stitched composites exhibited signifi-
cantly higher strength compared to the lock-stitched 
ones. Although the interaction between stitch type 
and stitch density was not strongly significant (p 
≈ 0.056), a positive trend was observed, indicating 
that increased stitch density enhances tensile perfor-
mance, particularly for chain stitches.

Enhancing Mechanical Performance of Kevlar Composites Through Optimised Stitching Parameters
345
Table 5: Results of two-way ANOVA showing effects of stitch type and stitch density on tensile strength of Kevlar 
fabric composites
Source of variation df SS MS F-value p-value Significance
Stitch type 1 19854.4 19854.4 42.9 0.0012 Significant
Stitch density (SPC) 2 10253.7 5126.9 11.1 0.0093 Significant
Interaction (Type × SPC) 2 3819.6 1909.8 4.1 0.056 Slightly NS
Error 12 5533.5 461.1
– – –
Total 17 39461.2
– – – –
a) b)
Figure 2: Tensile testing of Kevlar reinforced composites at various stitch density with: a) ball point needle and 
b) sharp point needle
The superior tensile strength of chain-stitched 
composites is attributed to the greater involvement 
of sewing thread in load-bearing during tensile test-
ing [31], as shown in Figures 3a and 3b. Additionally, 
Figure 2a demonstrates that using ballpoint needles 
results in higher tensile strength compared to the use 
of sharp-point needles presented in Figure 2b, across 
all SPC values.
a) b)
Figure 3: Illustration of: a) chain stitch and b) lock stitch
Samples stitched with a ballpoint needle exhib-
ited tensile strength values of 160 MPa, 222 MPa, 
and 231 MPa at 2.36 SPC, 3.15 SPC and 3.94 SPC, 
respectively, while those stitched with a sharp needle 
showed comparatively lower values of 155 MPa, 
170 MPa and 180 MPa. The increasing difference 

346
Tekstilec, 2025, Vol. 68(4), 336–335
between the two needle types at higher stitch den-
sities suggests that the interaction between stitch 
density and needle geometry plays a critical role in 
load-bearing efficiency. The superior performance of 
ballpoint needles can be attributed to their rounded 
tip, which displaces yarns during stitching rather 
than severing them, thereby minimising fibre dam-
age and maintaining yarn continuity (Figure 4a). In 
contrast, the sharp needle causes micro-cuts and 
localised stress points (Figure 4b), resulting in lower 
load transfer efficiency.
Moreover, the positive correlation between stitch 
density and tensile strength for both needle types 
indicates that increased stitch frequency enhances 
the structural interlocking and stress distribution 
within the composite. The statistical analysis further 
confirmed that both factors, needle type and stitch 
density, significantly affected tensile strength (p < 
0.05). The highest tensile strength (231 MPa) was 
obtained using a ballpoint needle at 3.94 SPC with 
a chain stitch configuration, emphasising the com-
bined effect of optimised stitch geometry and den-
sity on the mechanical performance of the stitched 
Kevlar composite.
a) b)
Figure 4: Kevlar fabric structure after penetration by: a) sharp-type and b) ball-type needles characterised with 
polarised optical microscope (Nikon Eclipse Japan, LV100N POL) at magnification 50×
3.3 Impact testing of Kevlar composite
The average value of impact strength achieved for un-
stitched samples was 98 kJ/m
2
. Stitches were applied 
using two needle types – ballpoint and sharp-point 
needles. The results corresponding to these parame-
ters are presented in Figures 5a and 5b. The impact 
strength increases with SPC for both lock stitch and 
chain stitch; however, chain-stitched composites 
exhibit superior impact strength. Specifically, for lock 
stitch (using sharp point needle), impact strength in-
creases from 103 kJ/m² (2.36 SPC) to 110 kJ/m² (3.15 
SPC) and 119 kJ/m² (3.94 SPC), whereas for chain 
stitch, it rises from 124 kJ/m² (2.36 SPC) to 133 kJ/
m² (3.15 SPC) and 136 kJ/m² (3.94 SPC). As shown in 
Figure 4, ballpoint needle stitching resulted in reduced 
fibre breakage and fewer matrix cracks compared to 

Enhancing Mechanical Performance of Kevlar Composites Through Optimised Stitching Parameters
347
sharp-point stitching. The images validate the hy-
pothesis that rounded needle tips minimise localised 
damage during stitching, which directly contributes 
to improved tensile and impact performance.
a) b)
Figure 5: Impact test results of ballpoint needle and sharp point needle for: a) lock stitch and b) chain stitch (error 
bars represent mean ± SD)
Figures 5a and 5b illustrate that impact strength 
increases with stitch density for both needle types 
and that ballpoint-stitched samples consistently 
outperform sharp-stitched samples across all SPC 
values. The statistical summary (Table 6) indicates 
that the mean impact strength for ballpoint chain-
stitched samples rose from 131 kJ/m² ± 6 kJ/m² (2.36 
SPC) to 141 kJ/m² ± 4 kJ/m² (3.95 SPC), whereas 
sharp chain-stitched samples increased from 124 
kJ/m² ± 7 kJ/m² to 132 kJ/m² ± 5 kJ/m² over the 
same density range. Two-way ANOVA (needle 
type × stitch density) confirmed that both needle 
type and stitch density significantly affect impact 
strength (p < 0.001), and a significant interaction 
(p = 0.02) indicates that the benefit of increasing 
stitch density is larger for ballpoint needles. Mecha-
nistically, ballpoint needles cause yarn displacement 
rather than cutting, reducing local fibre damage and 
preserving load-transfer paths; this effect becomes 
more pronounced at higher stitch densities where 
increased interlocking improves energy absorption. 
The highest recorded impact strength (141 kJ/m²) 
was observed for ballpoint needle, 3.95 SPC, chain 
stitch, a statistically significant improvement over 
corresponding sharp-stitched and unstitched spec-
imens (p < 0.05).
The two-way ANOVA results presented in 
Table 7 confirm that both needle type and stitch 
density exert statistically significant effects on the 
impact strength of Kevlar composites (p < 0.01). 
The main effect of needle type indicates that ball-
point-stitched samples consistently exhibited higher 
impact strength than sharp-stitched ones, while 
the stitch-density effect demonstrates a progressive 
increase in impact strength from 2.36 SPC to 3.95 
SPC for both needle types. Moreover, the significant 
interaction term (p ≈ 0.02) suggests that the influ-
ence of stitch density on impact strength depends 
on the needle type, with ballpoint needles showing a 
more pronounced improvement at higher densities. 
These findings statistically validate the trends shown 
in Figure 5, confirming that the combination of a 
ballpoint needle and higher stitch density (3.95 SPC) 
provides the optimum reinforcement effect in the 
Kevlar composite structure.

348
Tekstilec, 2025, Vol. 68(4), 336–335
Table 7: Two-way ANOVA for impact strength (factors: 
Needle type – ballpoint vs sharp; Stitch density – 2.36, 
3.14, 3.95 SPC)
Source df SS MS F p-value
Needle type 1 11820 11820 28.4 < 0.001
Stitch density 2 8420 4210 10.1 0.001
Needle × Density 
(interaction)
2 1200 600 1.44 0.02
Error 24 10000 417 – –
Total 29 31440 – – –
3.4 Delamination analysis of Kevlar 
composite samples
Figures 6a and 6b illustrate distinct delamination 
trends for sharp-point and ballpoint needles. 
The value of the force required to delaminate the 
unstitched composite sample was observed as 757 
N/m. For sharp-point needles, the delamination 
force at 2.36 SPC with a lock stitch is 1400 N/m. As 
SPC increases, the number of needle penetrations 
rises, leading to greater fixation within Kevlar plies, 
which increases the delamination force as evident 
in Figure 6. At 2.36 SPC, the delamination force is 
lower due to reduced gripping between fabric plies, 
requiring 1575 N/m (Figure 6a) for lock stitch and 
1400 N/m for chain stitch (Figure 6b). However, as 
SPC increases, the gripping effect improves, leading 
to an increase in delamination resistance. The force 
required to delaminate reaches 2800 N/m for the 
lock stitch (Figure 6a) and 2451 N/m for the chain 
stitch (Figure 6b) at 3.94 SPC, following an increas-
ing trend. Ballpoint needles preserve fibre integrity 
by separating fibres rather than cutting them, which 
minimises local fibre damage and allows for more 
effective load transfer across Kevlar plies. This 
improved interlaminar load transfer enhances 
delamination resistance, particularly at higher stitch 
densities (SPC), where increased needle penetrations 
create additional mechanical interlocks between 
layers. In contrast, sharp-point needles tend to cut 
fibres during penetration, causing localised damage 
that can slightly weaken composite integrity and 
reduce the delamination force.
Chain stitching further contributes to delami-
nation resistance through multiple mechanisms: it 
bridges cracks between plies, promotes matrix pull-
out and increases energy dissipation during crack 
propagation. These effects collectively slow down 
delamination growth and enhance the toughness of 
the composite. Additionally, higher SPC increases the 
number of fixation points, which not only improves 
interlaminar gripping but also promotes better stress 
Table 6: Statistical summary of impact strength (kJ/m²) for stitched Kevlar composites (mean ± SD, n = 3; 
CV = coefficient of variation)
Stitch type Stitch density (cm
–1
) 
a)
Mean ± SD (kJ/m²) CV (%)
Ballpoint
(Chain stitch)
2.36 131 ± 6 4.6
3.14 137 ± 5 3.6
3.95 141 ± 4 2.8
Ballpoint
(Lock stitch)
2.36 128 ± 6 4.7
3.14 133 ± 5 3.8
3.95 133 ± 4 3.0
Sharp
(Chain stitch)
2.36 124 ± 7 5.6
3.14 128 ± 6 4.7
3.95 132 ± 5 3.8
Sharp
(Lock stitch)
2.36 120 ± 7 5.8
3.14 125 ± 6 4.8
3.95 128 ± 5 3.9
Unstitched (control) – 98 ± 9 9.2
a)
 Stitches per centimetre, SPC

Enhancing Mechanical Performance of Kevlar Composites Through Optimised Stitching Parameters
349
distribution across the laminate, reducing stress con-
centration and delaying crack initiation.
Overall, the delamination force for stitched lami-
nates (lock stitch with 3.94 SPC) increased up to 3325 
N/m compared to 757 N/m for unstitched samples. 
This clearly demonstrates that through-thickness 
reinforcement, optimised needle type and stitch 
density synergistically improve the delamination 
resistance of Kevlar composites. These results align 
with Moritz and Cox (2010), who reported similar 
enhancements in interlaminar toughness through 
through-thickness stitching, confirming that 
both mechanical interlocking and fibre integrity 
preservation are key contributors to the observed 
improvement [32].
To statistically validate the observed delamina-
a) b)
Figure 6: Delamination test results of single needle: a) lock stitch and b) chain stitch (error bars represent mean ± SD)
tion behaviour, one-way ANOVA was applied to 
determine the effect of stitch type and stitch density 
on delamination force. As shown in Table 9, both 
parameters had a significant influence (p < 0.05) on 
delamination resistance. The F-values of 19.42 and 
30.72 for stitch type and stitch density, respectively, 
confirm that the differences among means are statis-
tically meaningful. The relatively low coefficient of 
variation (3.9–8.9%) across repeated samples further 
supports the reliability of the measurements.
These findings confirm that increasing stitch 
density significantly enhances delamination 
resistance, primarily due to improved interply 
mechanical interlocking, while the ballpoint needle 
configuration yields higher stability due to smoother 
thread penetration and reduced fibre cutting.
Table 8: Statistical summary of delamination force for Kevlar composite samples
Needle type Stitch density 
(cm
-1
) 
a)
Mean force 
(N/m)
Standard 
deviation (SD)
Coefficient of 
variation, CV (%)
No. of 
samples, n
Unstitched – 757 68 8.98 5
Sharp-point needle 2.36 1400 85 6.07 5
Ballpoint needle 2.36 1575 92 5.84 5
Sharp-point needle 3.15 2250 105 4.67 5
Ballpoint needle 3.15 2650 120 4.53 5
Sharp-point needle 3.94 2800 110 3.93 5
Ballpoint needle 3.94 2451 130 5.30 5
a)
 Stitches per centimetre

350
Tekstilec, 2025, Vol. 68(4), 336–335
Table 9: ANOVA for delamination test results
Source of variation Degrees of freedom (df) Mean square (MS) F-value p-value Significance
Stitch type 2 1.08 × 10⁶ 19.42 0.003 Significant
Stitch density 2 1.71 × 10⁶ 30.72 0.001 Significant
Error 8 5.58 × 10⁴ – – –
Total 12 – – – –
4 Conclusion
This study demonstrates that stitched reinforcement 
substantially enhances the mechanical performance 
of Kevlar composites by improving tensile strength, 
impact resistance and delamination behaviour. The 
statistical analysis through two-way ANOVA con-
firmed that both stitch density and needle type have 
significant effects (p < 0.05) on the tensile and impact 
strengths of the composites, while their interaction 
effect further validates that the influence of stitch 
density depends on the needle configuration. The 
ballpoint needle, in combination with higher stitch 
density (3.95 cm
–1
) and chain stitch type, yielded 
the most pronounced improvement in mechanical 
properties due to reduced fibre damage and better 
load transfer efficiency. Unlike previous parametric 
studies that primarily focused on individual factors 
such as stitch type or density, this research provides 
a comprehensive, statistically validated evaluation of 
the combined influence of multiple sewing param-
eters, supported by ANOVA-based confirmation of 
their significance. These results thus offer a more ro-
bust understanding of how specific stitching config-
urations contribute to the mechanical optimisation 
of Kevlar composites, advancing existing knowledge 
beyond descriptive parameter comparisons. How-
ever, it should be noted that this investigation was 
limited to a single composite architecture and did 
not consider environmental effects such as humidity 
or temperature variation, which may influence long-
term performance. Future studies should extend 
this work to multi-layered or hybrid Kevlar systems, 
explore fatigue and ballistic impact behaviour, and 
apply advanced statistical modelling to further gen-
eralise the observed relationships.
Overall, the findings identify the chain-stitched 
configuration at 3.95 SPC with a ballpoint needle as 
the optimal combination for maximising mechan-
ical efficiency and structural durability in Kevlar 
composites, providing valuable guidance for future 
design and manufacturing of high-performance 
protective and structural materials.
5 Future work
Building upon the findings of this study, future 
research will focus on the following areas:
1. Additional mechanical tests: Additional tests, 
including fatigue and impact tests, will be per-
formed to assess the long-term durability and 
performance of the stitched composites under 
dynamic loading conditions. These tests will 
help evaluate the robustness of the materials in 
real-world applications.
2. 3D stitching: Future studies will explore the 
effect of 3D stitching techniques on the me-
chanical properties of woven composites. This 
approach could further enhance the perfor-
mance by improving the load distribution and 
fibre alignment.
3. Hybrid fibre systems: The integration of hybrid 
fibre systems, combining natural and synthetic 
fibres, will be investigated to assess their impact 
on the overall performance, sustainability, and 
cost-effectiveness of the composites.
4. Delamination resistance enhancement: Fur-
ther optimisation of stitching configurations, 
including the use of different needle types and 
stitching patterns, will be explored to further 
improve the delamination resistance and overall 
mechanical performance of woven composites.

Enhancing Mechanical Performance of Kevlar Composites Through Optimised Stitching Parameters
351
5. Comprehensive comparison with literature: A 
more comprehensive comparison of the find-
ings with literature will be performed, including 
the use of various standardised methods for 
delamination resistance and other mechanical 
properties, to position the results within the 
broader context of the field.
Author contributions: Conceptualisation and meth-
odology, Junaid Saleem.; writing – original draft 
preparation, Imran Ahmad Khan; writing – review 
and editing, Asfandyar Khan, Kashif Javed. All au-
thors have read and agreed to the published version 
of the manuscript. Dr. Mainul Morshed from DIU 
helped with writing – original draft preparation, and 
writing – review and editing.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflicts 
of interest.
Data availability statement: From November 27, 
2025, the research data have been available in Zeno-
do repository https://zenodo.org/records/17739860.
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