V. ARUMUGAM et al.: STUDY ON THE IN-PLANE SHEAR PERFORMANCE OF SPACER FABRICS ...
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STUDY ON THE IN-PLANE SHEAR PERFORMANCE OF SPACER
FABRICS IN COMPOSITE FORMING
[TUDIJ STRI@NIH LASTNOSTI KOMPOZITNO OBLIKOVANIH
TKANIN S SENDVI^ STRUKTURO
Veerakumar Arumugam1, Rajesh Mishra1, Maros Tunak2, Blanka Tomkova1, Jiri Militky1
1Technical University of Liberec, Faculty of Textile, Department of Materials Engineering, 461 17 Liberec, Czech Republic
2Technical University of Liberec, Faculty of Textile, Department of Textile Evaluation, 461 17 Liberec, Czech Republic
veerakumar27@gmail.com
Prejem rokopisa – received: 2017-07-11; sprejem za objavo – accepted for publication: 2017-09-21
doi:10.17222/mit.2017.115
Shear properties are important not only for standard fabrics but for textile-reinforced composites as well. Shear is one of the
most common flaws that occurs during textile-composite-reinforcement forming processes. To ensure the production of
high-quality textile-fabric-reinforced composites when forming complex shapes, the potential for an in-plane deformation of the
fabric reinforcement must be taken into consideration. This research paper will present the emerging textile fabric of this
decade, commonly known as the 3D spacer fabric, which could be used as the reinforcement for complex-shaped composites.
The objective of this research was to study the shear behavior of 3D knitted spacer fabrics using a picture-frame fixture. The
images acquired during the loading process were used for an analysis in order to obtain a full-field displacement and shear
angles at the chosen points on the surface of a test specimen. To determine its suitability for measuring the in-plane shear
properties of the 3D knitted spacer fabrics, experimental and analytical investigations of the picture-frame shear fixture were
conducted. The non-linear behavior of the shear force versus the shear angle and deformation mechanism were analyzed.
Load-displacement curves of intra-ply shear tests were also analyzed. In addition, a MATLAB program was developed, using
the Hough transform to analyze the shear angle in the real-time image taken during the displacement of the specimen at various
positions. The image-analysis results were compared with the actual experimental results. These findings are important
requirements when using 3-dimensional textile fabrics as the reinforcement in a composite formation.
Keywords: 3D spacer fabrics, shear deformation, shear stress, image analysis
Stri`ne lastnosti niso pomembne le za standardne tkanine temve~ tudi za oja~ane tekstilne kompozite. Prestrig je ena od najbolj
pogostih napak, ki nastopi med procesom oja~itve tekstilnega kompozita. Da bi zagotovili proizvodnjo visokokvalitetnega
tekstila iz oja~anih kompozitnih tkanin, je potrebno med oblikovanjem kompleksnih oblik upo{tevati deformacije oja~itve zaradi
ravninskega striga. V tem ~lanku avtorji opisujejo raziskave novih tekstilnih tkanin, ki se v zadnjem ~asu imenujejo
3D-prostorske tkanine in bi lahko bile uporabne za oja~anje kompleksno oblikovanih kompozitov. Predmet te {tudije je stri`no
obna{anje prostorsko pletene tkanine z uporabo PFF-tehnologije (angl.: Picture Frame Fixture). Zato, da bi dobili odmike po
celotnem polju in stri`ne kote v izbrani to~ki na povr{ini testnega vzorca, so avtorji prispevka slike, zajete med procesom
nalaganja, uporabili za analizo. Izvedli so prakti~ne in analiti~ne preiskave, da bi dolo~ili primernost merjenja v ravnini striga
pletenih 3D-prostorskih tkanin. Analizirali so nelinearno obna{anje stri`nih sil v odvisnosti od stri`nega kota in mehanizem
deformacije. Prav tako so analizirali krivulje odvisnosti obremenitev-odmik, dobljene med stri`nimi preizkusi. Razvili so
program na osnovi MATLAB, z uporabo Houghove transformacije, da bi analizirali stri`ni kot v realnem ~asu odjema slike med
odmikom vzorca v razli~nih polo`ajih. Rezultate analize slike so primerjali z dejanskimi eksperimentalnimi rezultati.
Ugotovitve avtorjev so pomembne za oblikovanje zahtev, potrebnih za uporabo 3D-tekstilnih tkanin, za oja~anje v kompozitnih
tvorbah.
Klju~ne besede: 3D prostorske tkanine, stri`na deformacija, stri`ne napetosti, analiza slike
1 INTRODUCTION
The shearing behavior of a fabric determines its per-
formance properties when subjected to a wide variety of
complex deformations in use. This property enables the
fabric to undergo complex deformations and to conform
to the shape of the body. Shear properties are important
not only for fabrics but for textile composites as well.
Automated manufacturing of textile, composite shell-like
products typically requires draping of dry or pre-impreg-
nated textile sheets. Large local deformations occur in a
textile sheet in order for it to adapt to the curved shape.
These deformations affect the local fibre directions,
volume fractions, and thickness, i.e., the factors that,
together with the consolidation level and the occurrence
of flaws (e.g., wrinkling and tearing), determine the pro-
duct quality.1 Three-dimensional (3D) textile structural
composites provide excellent through-thickness strength,
outstanding damage tolerance and favorable impact and
fatigue resistance.2 As one type of the 3D textile struc-
tural reinforcements for composites, the 3D spacer fabric
has been widely used in the engineering field owing to
its easy and efficient processing in warp and weft
knitting. In addition, the most attractive advantage of the
spacer fabric is the three-dimensional shape-forming
capacity to manufacture composites. 3D-spacer-fabric
preforms have excellent mechanical properties and good
formability.3 With the development of the preforming
technology, complex shapes and different sizes of the
structural parts can be produced. In the structural
Materiali in tehnologije / Materials and technology 52 (2018) 1, 47–50 47
UDK 620.176.2:621.397.3:677.017.3 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(1)47(2018)
integrated manufacture of composite products, the 3D
spacer fabric is preformed according to the shape of the
final composite product that can be complex.4
The in-plane behavior and the interlaminar behavior
of 3D spacer fabrics are the most important deforma-
tions, and the shear behavior predominates the deforma-
tion mode of a material.5 It is valuable to study the
inter/intra-ply shear behavior of 3D fabrics because of
their wide application in the production, especially in the
case of a forming process. Lin et al. established a finite-
element model based on the geometry of a 2D fabric to
simulate an in-plane shear deformation, and the simula-
tion results were identical with the experiments. Chen et
al. developed a FEM model to predict in-plane and
interlaminar shear properties of laminates.6 However, the
in-plane shear behavior of a spacer fabric has been rarely
reported. A. Charmetant et al.7 built a hyper-elastic
model to simulate the formability of a 3D fabric.
In this paper, a picture-frame shear fixture is pre-
sented and a careful study of its performance and the
in-plane-shear behaviors of 3D warp-knitted spacer fab-
rics with different densities, thicknesses and structures is
reported on. The shear stresses versus shear-angle curves
and the wrinkle position of the in-plane shear test are
recorded and load-displacement curves of the in-plane
shear test are also analyzed. In addition, a program was
developed in MATLAB using the Hough transform to
analyze the shear angle in the image. These image-anal-
ysis results are compared with the actual experimental
results. Overall, it is expected that a clear picture of tail-
oring a knitted spacer fabric with a promising shear
property is created for composite-forming applications.
2 EXPERIMENTAL PART
Six different spacer fabrics made of a polyester fila-
ment yarn were knitted, using a Raschel warp-knitting
machine with a gauge of E22 and 6 guide bars. The
sample classification is clearly presented in Table 1. The
structure and knit pattern of both the lock knit and
hexagonal net-warp-knit structures are given in Figure 1.
The fabric characteristics such as areal density, stitch
density, structure and thickness, are presented in Table 2.
All the experiments were carried out under the standard
ambient condition and as per standard.
Table 1: Description of warp-knitted spacer fabrics
S. No. Struc-ture
Fibre
composi-
tion (%)
Face
layer
(dtex)
Middle
layer
(spacer)
(dtex)
Back
layer
(dtex)
Spacer-
yarn
dia.
(mm)
WAS 1 Lockknit
100%
Polyester 83f36 33f1 83f36 0.055
WAS 2 Lockknit
100%
Polyester 83f36 33f1 83f36 0.055
WAS 3 Lockknit
100%
Polyester 83f36 33f1 83f36 0.055
WAS 4 Lockknit
100%
Polyester 83f36 108f1 83f36 0.1
WAS 5 Lockknit
100%
Polyester 83f36 108f1 83f36 0.1
WAS 6 Lockknit
100%
Polyester 83f36 108f1 83f36 0.1
In-plane shear behavior – a picture-frame test
A picture-frame test, as shown in Figure 2, is an
effective way of characterizing the in-plane shear
property of fabrics. The picture-frame test is preferred by
many researchers for shear testing since it generates a
pure state of the strain that can be imposed on the test
specimen. Shearing is induced by restraining the textile
reinforcement in a rhomboid deformation frame with the
fibers constrained to move parallel to the frame edges.
The frame is extended at diagonally opposing corners
using simple tensile-testing equipment. The 3D spacer
V. ARUMUGAM et al.: STUDY ON THE IN-PLANE SHEAR PERFORMANCE OF SPACER FABRICS ...
48 Materiali in tehnologije / Materials and technology 52 (2018) 1, 47–50
Figure 2: Picture-frame shear fixture and deformation kinematicsFigure 1: Structure and knit pattern of warp-knitted spacer fabrics
fabrics for the shear tests were prepared according to the
size of the picture frame and deformation kinematics is
shown in Figure 2. The complete displacement of the
specimens during the loading process was recorded using
image-analysis methods. The images were captured at
certain regular intervals using a digital camera (Fig-
ure 3). A special program was developed in MATLAB
7.10 (R 2010a) using the Hough transform to find the
angle between the lines on the specimens.
A direct measurement of the axial load and shear
angle is possible with the equations below. The shear
force (Fs) is determined by the axial force (Fx), frame-rig
length (L) and frame angle (). Meanwhile, the frame
angle can be determined directly from the crosshead dis-
placement (d). The shear angle (
) can be obtained from
the frame angle using Equations (2) and (3):
F
Fx
s = 2 cos 
(1)
 =
+⎡
⎣⎢
⎤
⎦⎥
−cos 1
2
2
L d
L
(2)

 = −
π
2
2 (3)
3 RESULTS AND DISCUSSIONS
3.1 Influence of structural characteristics on the
in-plane shear behavior
In Figure 4, the stress-strain curve reveals that the
shear resistance decreases with an increase in the thick-
ness. It can be seen that the samples have similar be-
haviors as they behave linearly in the surface-extension
stage due to the same surface structure and density. In
the next stage of the shear deformation, the compressive
stress also plays a vital role, making the fabrics resist the
shear force. It is also observed that the thicker fabric has
the ability to undergo a larger deformation in both shear
and compression conditions. The shear stress-strain
behavior of spacer fabrics appears to change signifi-
cantly with an appreciable decrease in the spacer thick-
ness.
It is observed that the shear stress is high for the
fabrics with a coarser spacer yarn. During the surface
elongation and compression (the 1st stage), a higher shear
force is observed for the fabrics with a coarser spacer
yarn as compared to the fabrics made of finer spacer
yarns. It might be that due to the large diameter and
linear density, the coarse spacer yarns have the ability to
resist more compressive stress than the fabrics made of a
finer spacer yarn. In the 2nd stage, pre-buckling is ob-
served after 20 % of the shear strain because the surface
layers come in contact with each other, leading to a lock-
ing effect. The fabrics have almost identical stress-strain
curves, with the same thickness of the lock-knit spacers
in the 1st stage region. However, a significant difference
in the shear stress-strain behavior is observed in the
densification stage. This is due to the fact that the
Materiali in tehnologije / Materials and technology 52 (2018) 1, 47–50 49
V. ARUMUGAM et al.: STUDY ON THE IN-PLANE SHEAR PERFORMANCE OF SPACER FABRICS ...
Figure 3: In-plane shear angles of spacer fabrics using image-analysis
method
Table 2: Structural characteristics of warp-knitted spacer fabrics
Warp-spacer
samples
Stitch density (stitches/cm2) GSM(g m–2) Thickness (mm)
Density
(kg m–3)
Mean ME LL UL Mean ME LL UL Mean ME LL UL Mean
WAS 1 120.4 1.1 119.30 121.50 87.33 0.72 261.62 263.06 1.50 0.03 1.47 1.53 174.89
WAS 2 121 1.4 119.60 122.40 89.73 0.77 353.55 355.09 2.50 0.04 2.46 2.54 141.73
WAS 3 120.3 1.3 119.00 121.60 90.77 3.10 442.88 449.08 3.50 0.03 3.47 3.53 127.42
WAS 4 119 1.2 117.80 120.20 72.31 0.68 572.41 573.77 1.50 0.02 1.48 1.52 382.06
WAS 5 120 1.1 118.90 121.10 74.73 0.62 871.30 872.54 2.50 0.01 2.49 2.51 348.77
WAS 6 120.5 0.9 119.60 121.40 75.69 0.69 1173.4 1174.8 3.50 0.01 3.49 3.51 335.47
ME – margin of errors, LL – lower limit, UL – upper limit
Figure 4: Influence of structural characteristics on the in-plane shear
behavior
internal surface and spacer layer experience a closer
compaction, which depends on the thickness, the surface
structure and spacer-yarn properties.
3.2 Determination of the shear angle of warp-knit
spacers using the image-analysis method
The spacer-fabric samples (WAS 1–WAS 6) exhibit
smooth and linear increases in the shear angle until a
25-mm displacement. The irregular trend in the shear
angle observed after the 25-mm displacement may be
due to the fact that the spacer fabric undergoes a com-
paction, which leads to buckling. In a buckled fabric, it is
extremely complicated to find the same shear angle at
different points, using the Hough-transformation image-
analysis method. The image-analysis shear angle is
plotted against the shear strain for the lock-knit struc-
tures such as the lock knit (WAS 1–WAS 6) as given in
Figure 5. Also, a linear model fit was presented in the
figures with the shear strain as an independent variable
and the shear angle as a dependent variable. The re-
gression equation for all the samples is given in Table 3,
with the correlation coefficient (R2) to find the degree of
linear fit.
Table 3: Prediction of the shear angle using image analysis as a func-
tion of the shear strain
Equation y = a + b*x
DF Model – 1, Error – 7, Total – 8
Sample Nos. Value Standarderror
Residual
sum of
squares
F value Prob>F
WAS 1
Intercept -2.091 1.668
51.552 245.259 0.000
Slope 1.097 0.070
WAS 2
Intercept -1.347 1.474
40.269 280.577 0.000
Slope 1.037 0.062
WAS 3
Intercept -0.071 1.097
22.283 414.594 0.000
Slope 0.938 0.046
WAS 4
Intercept -2.736 2.229
92.088 144.907 0.000
Slope 1.127 0.094
WAS 5
Intercept -2.884 1.859
64.069 218.124 0.000
Slope 1.154 0.078
WAS 6
Intercept -2.516 1.561
45.129 300.433 0.000
Slope 1.136 0.066
4 CONCLUSIONS
The in-plane shear behavior of spacer fabrics is
greatly influenced by their surface structures, type of the
spacer yarn and the fabric stitch density. The outer-layer
structure affects the monofilament-yarn inclination,
binding condition with the surface, distribution and
multifilament stitches in the surface layers. The
non-linearity of the shear deformation increases after the
limiting locking angle, which initiates the buckling of a
sample. Significant differences between the shear
stress-strain curves for the samples were obtained in the
densification stage. They were due to the fact that the
internal surface and spacer layer experience a closer
compaction, which depends on the thickness, surface
structure and spacer-yarn properties. The differences
between the image analysis and the calculated shear
angle, using the sample length at the chosen points, are
relatively small. There is no significant difference until
pre-buckling occurs, but a significant difference occurs
after a 20-mm displacement.
Acknowledgment
This work was supported by the Department of
Materials Engineering, Faculty of Textile, Technical
University of Liberec, Czech Republic and project No.
21195 within the student-grant scheme, Czech Republic.
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V. ARUMUGAM et al.: STUDY ON THE IN-PLANE SHEAR PERFORMANCE OF SPACER FABRICS ...
50 Materiali in tehnologije / Materials and technology 52 (2018) 1, 47–50
Figure 5: Determination of the shear angle of warp-knit spacers using
image-analysis method