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Original article
Protective properties of warp-knitted
spacer fabrics under impact in
hemispherical form. Part II: effects of
structural parameters and lamination
Yanping Liu, Hong Hu and Wai Man Au
Abstract
This part aims to investigate the effects of structural parameters and lamination on the impact force attenuation
properties of warp-knitted spacer fabrics developed for impact protectors. A series of warp-knitted spacer fabrics
was produced on a double-needle bar Raschel machine by varying their structural parameters including spacer mono-
filament inclination and fineness, fabric thickness, and outer layer structure. The effects of fabric structural parameters,
impact energy levels, and laminated layers on the protective performance of the spacer fabrics were tested and analyzed
based on the assessment of the peak transmitted force. The results showed that all the structural parameters significantly
affect the impact force attenuation properties of the warp-knitted spacer fabrics. It was also found that lamination of the
spacer fabrics can effectively improve the force attenuation performance. Three layers of the developed warp-knitted
spacer fabrics in a total thickness of about 2.5 cm can meet the requirement of the transmitted force lower than 35kN at
an impact energy of 50 J according to the European Standard BS EN 1621-1:1998.
Keywords
Warp-knitted spacer fabric, impact protector, force attenuation, transmitted force
Impact protectors are a type of energy-absorbing
material integrated or inserted into protective clothing
or equipment for protecting the human body from
impact strokes, blows, or falls.
1
Various kinds of
impact protectors have been designed and used to pro-
tect different areas of human body such as shoulders,
elbows, hips, knees, and tibias from impact injuries in
intense sports, such as motorcycling, cycling, horse
riding, skiing, skating, rugby, and hockey.
1–3
The
wide applications have led to an increasing market
need for impact protectors.
The commonly used materials for assembling impact
protectors are polymeric foams, rubbers, gels, and plas-
tics.
1–3
However, these materials suffer from an insur-
mountable disadvantage. As a component of impact
protectors, they exhibit poor comfort properties.
Lower air permeability and moisture transmission cap-
ability of these materials cannot adequately meet the
comfort requirement of most protective clothing used
in sports and other extreme activities where sweat is
easy to be generated and should be transmitted from
the skin surface to the outer layer of the clothing,
although they are always punctured with an array of
holes to improve their permeability. Another limit is
that the protectors made of these materials are too
heavy and have low flexibility. To achieve a better bal-
ance of protection and flexibility, these materials are
often laminated in two or three layers by means of
binding or gluing. However, the shock-absorbing abil-
ity of impact protectors still stands in contradiction to
their wearing comfort and freedom of movement. It has
been found that although a variety of impact protectors
are available on the market and most athletes are aware
Institute of Textiles and Clothing, The Hong Kong Polytechnic University,
Hong Kong, China
Corresponding author:
Hong Hu, Institute of Textiles and Clothing, The Hong Kong Polytechnic
University, Hung Hom, Kowloon, Hong Kong, China.
Email: tchuhong@polyu.edu.hk
Textile Research Journal
2014, Vol 84(3) 312–322
!The Author(s) 2013
Reprints and permissions:
sagepub.co.uk/journalsPermissions.nav
DOI: 10.1177/0040517513495942
trj.sagepub.com
of their effectiveness in preventing injuries, the use of
impact protectors is still often rejected by the customers
due to their poor wearing comfort.
Warp-knitted spacer fabrics are consisting of two
separate outer fabric layers joined together but kept
apart by spacer yarns.
4
The space formed between the
two independent layers is the most important structural
feature of this kind of fabric. A combination of good
out-of-plane flexibility, excellent compressibility, high
moisture conductivity, and good thermoregulation cap-
ability makes this kind of knitted spacer fabric very
suitable for impact protection. The previous studies
showed that warp-knitted spacer fabrics can be an
ideal class of energy absorbers for cushioning applica-
tions and their energy-absorbing capacity can be tai-
lored to meet specific end-use requirements by varying
their structural parameters.
4,5
However, a systematic
experimental study on the protective performance of
warp-knitted spacer fabrics used as impact protectors
in curved shapes for human body protection is still
lacking. Therefore, there is a need to conduct
such a study in order to understand how the structural
parameters and lamination of spacer fabrics in
curved shapes affect their impact force attenuation
properties.
In Part I of this paper, the impact behavior of a
warp-knitted spacer fabric with a typical structure has
been studied in detail.
6
The study has shown that the
force attenuation of the spacer fabric is determined by
its energy absorption ability and the relevant destruc-
tion modes under different kinetic energies, including
plastic deformation, filament breakages, and thermo-
plastic transformation. According to the frenquency
domain analysis, these destruction modes correspond
to the specific frequency bands. The study also indi-
cated that the spacer fabric in the hemispherical shape
has a lower efficiency in force attenuation than that in
the planar shape because the spacer monofilaments of
the fabric in the hemispherical shape do not effectively
resist the impact loading. In addition, a single layer of
the spacer fabric cannot offer sufficient protection
according to the European Standard BS EN 1621-
1:1998 for limb protection.
7
While the study in Part I
focused on the energy absorption and force attenuation
properties of a typical spacer fabric in a single layer, a
further study on the effects of structural parameters and
lamination of spacer fabrics is the main objective of
current paper. It is expected that this study can provide
useful information in the optimized structural design of
warp-knitted spacer fabrics for human body protection
against impact in curved forms.
Experimental details
Spacer fabric samples
Twelve warp-knitted spacer fabrics (S1–S12) produced
with different fabric thicknesses, outer layer structures,
spacer monofilament diameters, and inclinations were
used for analyzing the effects of structural parameters
on their impact behavior. These fabrics are the same
ones that were used for the static and dynamic com-
pression tests in the planar form in our previous stu-
dies
4,5
and were all knitted on a GE296 (RD6) E18 high
speed double-needle bar Raschel warp knitting
machine. Whereas polyester multifilaments of 300
denier (96 filaments) were used to create the binding
of the structure in the knitting process through GB1,
GB2 for the top outer layer and GB5, GB6 for the
bottom outer layer, polyester monofilaments of
0.2 mm in diameter (400 denier) for samples S1–S11
and of 0.16 mm in diameter (250 denier) for sample
S12 were used as spacer yarns to connect the two
outer layers together through GB3 and GB4. Four dif-
ferent structures, i.e. locknit, chain plus inlay, small-size
rhombic mesh, and large-size hexagonal mesh
were used for knitting the outer layers (Table 1).
Three different spacer monofilament inclinations cor-
responding to underlapping one, two, or three needles
between the front- and back-needle bars were used for
linking the two outer layers together, respectively
(Table 2). The details of each fabric sample can be
found in Table 3, in which the fabric thickness was
the mean value of ten results measured via a digital
thickness tester (SDL Atlas M034e) with the standard
deviation.
Table 1. Chain notation of yarn guide bars for outer layers
4
Structure GB1/GB6 GB2/GB5 Threading
Locknit (L) 1-0 0-0/3-2 3-3// 2-1 1-1/ 1-0 0-0// full
Chain plus
Inlay (CI) 0-0 0-0/ 5-5 5-5// 1-0 0-0/ 1-0 0-0// full
Rhombic Mesh (RM) 1-0 0-0/1-2 2-2/2-3 3-3/2-1 1-1// 2-3 3-3/2-1 1-1/1-0 0-0/ 1-2 2-2// 1 full, 1 empty
Hexagonal Mesh (HM) (1-1 1-0/ 3-3 3-2) 3/(4-4 5-4 /3-3 3-2) 3// (4-4 5-4/3-3 3-2) 3/(1-1 1-0/3-3 3-2) 3// 2 full, 2 empty
Liu et al. 313
Impact tests
The spacer fabrics laminated in single layer, double
layer, and tripple layers were tested with kinetic energies
of 5, 10, 20, 30, 40, and 50J, respectively, following the
method indicated by the European Standard BS EN
1621-1:1998 ‘‘Motorcyclists’ protective clothing against
mechanical impact. Requirements and test methods for
impact protectors’’.
7
The testing device and conditions
are kept the same as used in Part I.
6
Ten specimens were
tested for each kind of spacer fabric sample and lamin-
ation method to obtain accurate results. As the trans-
mitted force is required by the standard to assess the
impact force attenuation properties of an impact pro-
tector, only the results of transmitted forces are adopted
for discussion of the effects of structural parameters and
laminated layers. All the transmitted force–time curves
presented are the most representative curves, and all the
peak transmitted forces presented are the mean values of
ten test results with standard error.
Results and analysis
Effect of the spacer monofilament inclination
A group of three fabrics (S2, S3, and S4) with the same
outer layer structure (chain plus inlay) and the same
spacer monofilament yarn but with different spacer
monofilament inclinations (underlapping one needle,
two needles, and three needles between the front- and
back-needle bars) is used to analyze the effect of the
spacer inclination on the impact force attenuation
properties of warp-knitted spacer fabrics. The fabric
thickness and stitch density of the outer layers are
kept nearly the same. The number of the needles under-
lapped determines the spacer monofilament inclination
and length. The higher the number of the needles
underlapped, the longer and more inclined the spacer
monofilaments are.
As shown in Figure 1, the transmitted force–time
curves of these fabrics in single layer under impact at
a kinetic energy of 5 J are used as an example for dis-
cussing the effect of the spacer monofilament inclin-
ation with the same impact energy. It can be seen that
while the duration from the beginning point where the
striker contacts the fabric upper surface to the peak
point where the transmitted force reaches the maximal
value increases as the spacer monofilament inclination
increases; the peak transmitted force decreases as the
spacer yarn inclination increases. This means that the
spacer fabric with a higher spacer monofilament inclin-
ation and a longer spacer monofilament length more
effectively resists the impact due to a lower peak trans-
mitted force. This result is quite different from the flat-
wise impact test result obtained in our previous work,
5
which has demonstrated that the fabric with moderate
inclination spacer monofilaments (two needles under-
lapped) has the lowest peak transmitted force. Under
the flatwise impact, the fabric with too vertical spacer
yarns (one needle underlapped) becomes less stable and
shear can easily occur between the two outer layers. On
the other hand, too inclined spacer monofilaments
Table 3. Details of spacer fabric samples
4,5
Sample no.
To p l a y e r
structure
Bottom layer
structure
Spacer
monofilament
inclination
Fabric
thickness
(mm)
Areal
density
(g/m
2
)
Stitch
density
(stitches/ cm
2
)
S1 L L II 7.52 0.06 1008.29 10.68 41.15
S2 CI CI I 7.57 0.08 900.11 9.01 37.95
S3 CI CI II 7.59 0.10 901.75 14.58 37.26
S4 CI CI III 7.40 0.06 923.20 8.44 37.95
S5 CI CI II 5.64 0.03 790.63 14.51 34.98
S6 CI CI II 8.45 0.09 1022.08 13.38 43.50
S7 CI CI II 10.62 0.10 1010.42 8.83 37.95
S8 RM CI II 7.20 0.05 830.05 11.53 39.33
S9 RM RM II 7.76 0.06 907.24 17.07 51.10
S10 HM CI II 7.56 0.08 812.70 6.61 37.95
S11 HM HM II 7.62 0.06 724.82 8.34 38.86
S12 CI CI III 7.06 0.09 746.53 6.81 39.44
Table 2. Chain notation of yarn guide bars for spacer yarns
4
Lapping GB3 GB4 Threading
I 1-0 2-1/2-1 1-0// 2-1 1-0/1-0 2-1// 1 full, 1 empty
II 1-0 3-2/3-2 1-0// 3-2 1-0/1-0 3-2// 1 full, 1 empty
III 1-0 4-3/4-3 1-0// 4-3 1-0/1-0 4-3// 1 full, 1 empty
314 Textile Research Journal 84(3)
(three needles underlapped) in a fabric have a longer
length and are less oriented to the impact compression
direction. According to the theory of elastic stability, a
longer elastic rod which is less oriented to the direction
of the compression has a lower critical load. Therefore,
the fabric with moderate inclination spacer monofila-
ments (two needles underlapped) has the best compres-
sion resistance to the flatwise impact, which makes it
absorb more impact energy at the plateau stage. As a
result, the peak transmitted force is the lowest.
However, under impact in the hemispherical shape,
the situation becomes complicated. When a planar
spacer fabric is placed on the hemispherical surface of
the anvil, its upper outer layer will be extended and the
bottom layer will be contracted. This special boundary
condition makes the spacer monofilaments easier to
shear rather than to buckle under impact. In this
case, the stability of the spacer monofilaments becomes
a critical point for the spacer fabric to resist the impact,
because the shear movements will decrease the com-
pression resistance of the spacer yarns to the impact
load, which leads to a high peak transmitted force.
Since the fabric with higher inclination spacer mono-
filaments has a higher shear resistance, it will have a
lower peak transmitted force. Therefore, among these
three fabrics, the force attenuation performance of the
spacer fabrics increases as the length and inclination of
spacer yarns increase under impact in the hemispherical
shape.
The peak transmitted forces of fabric samples S2, S3,
and S4 under impact with different kinetic energies and
laminated layers are listed in Table 4. The similar trend,
in which the peak transmitted force decreases as the
spacer monofilament inclination increases, is obtained
for other levels of impact energies and laminated layers.
However, the values of the peak transmitted force are
dramatically reduced as the number of fabric layers
increases. In a single layer, although the peak trans-
mitted force decreases with the increase of the spacer
monofilament inclination, the effect is not evident.
However, by increasing the fabric layers, the effect of
the spacer monofilament inclination becomes very sig-
nificant. This is because a single layer of the spacer
fabrics is not strong enough to resist the impact with
a high kinetic energy and is easier to be compressed into
a high densification stage. In a single layer, all three
fabrics were compressed into their high densification
stages. In these cases, the striker compressed the mono-
filament material rather than making the spacer mono-
filaments buckle. As explained in Part I of this paper,
the transmitted force depends on the contact stiffness of
the compressed yarn material. Since all the fabrics were
made of polyester monofilaments, the difference in peak
transmitted force among fabric samples S2, S3, and S4
is not significant. However, as the fabric layers increase,
the energy absorption of the laminated fabrics increases
and therefore they will not be compressed into a high
densification stage. In this case, the post-buckling of
spacer monofilaments plays the key role in resisting
the impact loading and the spacer monofilament inclin-
ation effect works. Therefore, a high difference in the
value of the peak transmitted force can be observed for
01234567
–2
0
2
4
6
8
10
12
Transmitted Force (kN)
Time (ms)
S2: underlapped one needle
S3: underlapped two needles
S4: underlapped three needles
Figure 1. Effect of spacer monofilament inclination on transmitted force–time curves (fabric layer: 1; impact energy: 5 J).
Liu et al. 315
the specimens with more layers. Another point that
needs to be noted is that by increasing the fabric
layers, better impact force attenuation of spacer fabrics
can be obtained when the impact energy is at a lower
level. For instance, under impacts with the kinetic ener-
gies of 5 J and 10 J, adding one layer of the fabric can
nearly reduce the peak transmitted force by half.
Effect of the spacer monofilament fineness
Two fabrics (S4 and S12) with the same spacer mono-
filament inclination (underlapping three needles) and
the same outer layer structure (chain plus inlay) but
with two different spacer monofilament diameters
(0.2 and 0.16 mm) are chosen to study the effect of
the spacer monofilament fineness on the impact force
attenuation properties of warp-knitted spacer fabrics.
The thickness and outer layer stitch density of the
two fabrics are also kept nearly the same.
As shown in Figure 2, the transmitted force–time
curves of these two fabrics in a single layer under
impact at a kinetic energy of 5 J are used as an example
to analyze the effect of the spacer monofilament fine-
ness with the same impact energy. It can be seen that
the spacer fabric with the coarser spacer monofilament
has a lower peak transmitted force and a longer time to
the peak point and, therefore, has a better impact force
attenuation property. This result is consistent with that
012345678
–4
–2
0
2
4
6
8
10
12
14
Transmitted Force (kN)
Time (ms)
S4: 0.2mm
S12: 0.16mm
Figure 2. Effect of spacer monofilament diameter on transmitted force–time curves (fabric layer: 1; impact energy: 5 J).
Table 4. Effect of spacer monofilament inclination on peak transmitted force
Lamination Sample
Peak transmitted force (kN)
5 J 10 J 20 J 30 J 40 J 50 J
Single layer S2 11.38 0.051 20.61 0.046 31.66 0.064 37.10 0.000 39.66 0.060 41.18 0.049
S3 11.35 0.048 20.53 0.065 31.27 0.037 37.08 0.037 39.6 0.032 41.14 0.051
S4 11.06 0.027 20.22 0.051 31.68 0.063 36.94 0.024 39.46 0.040 40.9 0.063
Double layers S2 4.98 0.128 12.98 0.162 25.18 0.086 35.45 0.052 38.52 0.037 39.74 0.051
S3 4.36 0.040 11.74 0.060 24.66 0.075 34.82 0.057 38.42 0.049 39.30 0.045
S4 3.90 0.032 11.70 0.071 23.30 0.032 34.46 0.083 38.38 0.037 39.60 0.032
Three layers S2 2.52 0.124 7.76 0.051 18.68 0.080 28.12 0.136 35.46 0.201 38.28 0.058
S3 1.22 0.037 5.70 0.055 16.58 0.116 25.90 0.192 34.30 0.055 37.86 0.112
S4 1.06 0.040 5.50 0.055 15.56 0.040 25.00 0.152 33.24 0.098 37.08 0.049
316 Textile Research Journal 84(3)
obtained from the flatwise impact test, although the
boundary condition is changed to the hemispherical
form. This is normal because the fabric with coarser
spacer monofilaments has higher compression resist-
ance which can decelerate the striker more quickly
and make the striker experience a longer time to
reach the peak transmitted force point than the fabric
with finer spacer monofilaments. Since the duration of
deceleration is essential to the impact protection, the
impact process should be as long as possible to
absorb more energy in order to reduce the peak trans-
mitted force. As shown in Table 5, this result is also
valid for other impact energy levels and fabric lami-
nated layers. Similar to the flatwise impact, by increas-
ing the spacer monofilament diameter, the force
attenuation properties of spacer fabrics in the hemi-
spherical shape can be improved considerably.
It should be noted that although increasing the
spacer monofilament diameter can considerably
improve the force attenuation capacity of the spacer
fabric in the hemispherical shape, increasing the
spacer monofilament diameter can increase the fabric
stiffness and, thus, decrease the comfort property of the
fabric. In this regard, the balance between the comfort
and protective performance should be taken into con-
sideration by selecting a suitable spacer monofilament
fineness for a specific protective application.
Effect of the fabric thickness
A group of four fabric samples (S5, S3, S6, and S7)
produced with the same spacer monofilament diameter
(0.20 mm), the same spacer monofilament inclination
(underlapping two needles), and the same outer layer
structure (chain plus inlay) but with different thick-
nesses (5.64, 7.59, 8.45, and 10.62 mm) is used to ana-
lyze the effect of the fabric thickness on the impact
force attenuation properties of warp-knitted spacer fab-
rics in the hemispherical form.
As shown in Figure 3, the transmitted force–time
curves of these fabrics in a single layer under impact
at a kinetic energy of 5 J are selected as an example to
analyze the effect of the fabric thickness with the same
impact energy. It can be found that the peak trans-
mitted force decreases and the duration from the start-
ing point to the peak transmitted force point increases
as the fabric thickness increases. This phenomenon can
be explained as follows. On the one hand, more time is
required to compress a thicker fabric to its densification
stage at a larger displacement. Since the increase of the
compression time and displacement allows a thicker
fabric to absorb more impact energy, a lower peak
transmitted force can be obtained for a thicker fabric.
On the other hand, as indicated in Part I of this paper,
the contact area of the fabric with the striker rapidly
increases as the displacement of the striker increases.
A thicker fabric can be compressed into a larger dis-
placement and therefore more spacer monofilaments
are involved in resisting impact loading. Therefore,
the thicker fabric has a lower peak transmitted force
under impact in the hemispherical shape.
The peak transmitted forces for different impact
energies and laminated layers are listed in Table 6.
The similar trend, in which the peak transmitted force
decreases as the fabric thickness increases is also
obtained for other impact energy levels and laminated
layers when all the fabric samples have the same
destruction modes under impact. However, two excep-
tions are found for a single layer of the fabrics under
impact with 20 J and double layers of the fabrics under
impact with 50 J. In these two special cases, the thickest
fabrics do not have the lowest peak transmitted forces
due to the different destruction modes compared with
other fabric samples. Under impact at a kinetic energy
of 20 J, the obvious damage of multifilaments in the top
outer layers of fabric samples S5, S3, and S6 in single
layer can be observed, but no obvious damage can be
observed in S7 which is the thickest. The same phenom-
enon is obtained in double layers of the fabrics under
impact with 50 J. Since fabric sample S7 cannot absorb
additional impact energy because of the damage of the
fabric structure in these two cases, its peak transmitted
force can be higher than that of a thinner fabric. The
above analysis demonstrates that the fabric thickness
Table 5. Effect of spacer monofilament fineness on peak transmitted force
Lamination Sample
Peak transmitted force (kN)
5 J 10 J 20 J 30 J 40 J 50 J
Single layer S4 11.06 0.027 20.22 0.051 31.68 0.063 36.94 0.024 39.46 0.040 40.9 0.063
S12 12.72 0.057 21.77 0.063 33.16 0.058 37.08 0.020 40.23 0.033 42.27 0.088
Double layers S4 3.90 0.032 11.70 0.071 23.30 0.032 34.46 0.083 38.38 0.037 39.60 0.032
S12 6.12 0.049 15.92 0.086 27.48 0.066 36.12 0.044 38.86 0.040 39.82 0.049
Three layers S4 1.06 0.040 5.50 0.055 15.56 0.040 25.00 0.152 33.24 0.098 37.08 0.049
S12 2.84 0.040 8.38 0.049 20.08 0.136 30.32 0.139 37.30 0.071 38.88 0.058
Liu et al. 317
should be carefully selected according to the destruc-
tion modes when a spacer fabric will be subjected to
impact with higher levels of kinetic energies during use.
Effect of the outer layer structure
A group of six fabrics (S1, S3, S8, S9, S10, and S11)
with the same underlapped needles for the spacer yarns
(two needles) and nearly the same thickness but with
different outer layer structures is chosen to investigate
the effect of the outer layer structure on the impact
force attenuation properties of warp-knitted spacer
fabrics. The details of these fabrics can be found in
Table 3.
As shown in Figure 4, the transmitted force–time
curves of these fabrics in a single layer under impact
at a kinetic energy of 5 J are used as an example for
analyzing the effect of the outer layer structure on the
impact force attenuation properties. It is found that the
outer layer structures have an obvious effect. While
0123456789
-2
0
2
4
6
8
10
12
14
S5: 5.64mm
S3: 7.59mm
S6: 8.45mm
S7: 10.62mm
Transmitted Force (kN)
Time (ms)
Figure 3. Effect of the fabric thickness on transmitted force–time curves (fabric layer: 1; impact energy: 5 J).
Table 6. Effect of the fabric thickness on peak transmitted force
Lamination Sample
Peak transmitted force (kN)
5 J 10 J 20 J 30 J 40 J 50 J
Single layer S5 12.27 0.075 21.11 0.043 33.12 0.039 36.86 0.081 40.30 0.100 41.45 0.050
S3 11.35 0.048 20.53 0.065 31.27 0.037 37.08 0.037 39.6 0.032 41.14 0.051
S6 10.18 0.061 19.42 0.036 30.89 0.046 36.88 0.037 39.58 0.020 40.84 0.040
S7 9.74 0.069 19.10 0.026 32.06 0.033 36.54 0.040 39.30 0.000 40.50 0.000
Double layers S5 5.68 0.020 14.60 0.126 27.78 0.037 35.83 0.047 39.00 0.032 40.38 0.020
S3 4.36 0.040 11.74 0.060 24.66 0.075 34.82 0.057 38.42 0.049 39.30 0.045
S6 3.34 0.068 10.08 0.139 22.78 0.146 32.95 0.086 37.88 0.058 39.00 0.045
S7 2.86 0.075 8.72 0.066 21.46 0.144 31.45 0.192 37.94 0.051 39.74 0.051
Three layers S5 1.98 0.020 7.74 0.068 18.50 0.109 29.24 0.136 36.84 0.081 38.90 0.063
S3 1.22 0.037 5.70 0.055 16.58 0.116 25.90 0.192 34.30 0.055 37.86 0.112
S6 0.96 0.040 4.78 0.080 14.54 0.108 24.08 0.198 31.86 0.216 35.02 0.066
S7 1.00 0.063 4.42 0.102 13.42 0.166 22.04 0.144 29.08 0.132 33.44 0.098
318 Textile Research Journal 84(3)
fabric sample S11 knitted with large-size hexagonal
meshes for both outer layers has the highest peak trans-
mitted force and the shortest duration from the starting
point to the peak transmitted force point, fabric sample
S3 knitted with a chain plus inlay structure for both
outer layers has the lowest peak transmitted force and
the longest impact duration. The peak transmitted
forces of the fabrics with other outer layer structures
(S1, S8, S9, and S10) are between the values for these
two fabric samples. As listed in Table 7, similar results
Table 7. Effect of the outer layer structure on peak transmitted force
Lamination Sample
Peak transmitted force (kN)
5 J 10 J 20 J 30 J 40 J 50 J
Single layer S1 11.88 0.084 20.77 0.054 31.06 0.045 36.92 0.058 39.68 0.037 41.14 0.024
S3 11.35 0.048 20.53 0.065 31.27 0.037 37.08 0.037 39.6 0.032 41.14 0.051
S8 11.81 0.049 20.82 0.053 31.72 0.074 36.76 0.024 39.28 0.049 40.96 0.068
S9 11.87 0.062 20.57 0.067 31.50 0.068 37.06 0.024 39.62 0.073 41.32 0.037
S10 11.45 0.040 19.80 0.071 31.03 0.042 36.92 0.037 39.56 0.040 41.16 0.051
S11 12.98 0.077 21.50 0.056 31.29 0.144 37.28 0.073 40.27 0.133 41.65 0.050
Double layers S1 4.44 0.075 12.04 0.133 26.18 0.080 35.12 0.039 38.52 0.037 39.48 0.020
S3 4.36 0.040 11.74 0.060 24.66 0.075 34.82 0.057 38.42 0.049 39.30 0.045
S8 4.54 0.040 12.72 0.124 26.34 0.060 35.53 0.058 38.74 0.024 39.74 0.051
S9 5.38 0.080 12.84 0.068 26.68 0.097 35.63 0.030 38.72 0.037 39.64 0.051
S10 5.08 0.097 13.46 0.223 25.34 0.198 35.17 0.184 38.88 0.037 39.98 0.049
S11 6.22 0.020 15.44 0.098 28.18 0.171 36.10 0.122 39.30 0.000 40.92 0.049
Three layers S1 1.66 0.060 6.28 0.116 17.02 0.132 27.23 0.085 35.24 0.196 38.12 0.066
S3 1.22 0.037 5.70 0.055 16.58 0.116 25.90 0.192 34.30 0.055 37.86 0.112
S8 1.48 0.020 6.20 0.000 17.46 0.051 27.84 0.060 35.40 0.105 37.80 0.089
S9 1.44 0.024 6.42 0.058 17.46 0.117 28.10 0.114 35.98 0.080 38.10 0.077
S10 1.62 0.020 6.54 0.075 18.08 0.162 27.50 0.147 34.48 0.213 37.74 0.246
S11 2.02 0.080 7.68 0.080 21.34 0.196 31.60 0.192 37.36 0.150 38.88 0.058
01234567
–4
–2
0
2
4
6
8
10
12
14
Transmitted Force (kN)
Time (ms)
S1: L+L
S3: CI+CI
S8: RM+CI
S9: RM+RM
S10: HM+CI
S11: HM+HM
Figure 4. Effect of the outer layer structure on transmitted force–time curves (fabric layer: 1; impact energy: 5 J).
Liu et al. 319
are also obtained for other levels of impact energies and
laminated layers. These differences mainly come from
different geometric features of outer layer structures
which lead to different geometric arrangements of
multifilaments and different inclinations and binding
conditions of spacer monofilaments. As indicated in
our previous study under flatwise impact,
5
the fabric
knitted with large-size meshes exhibits the poorest
impact protective performance due to highly buckled
and inclined spacer monofilaments, and the fabric
knitted with small-size meshes demonstrates the best
impact protection ability due to tight binding condi-
tions. The fabrics knitted with a close structure have
moderate impact protection performance due to the
combined effects of loose binding structures and lowly
buckled and inclined spacer monofilaments. Under
impact in the hemispherical shape, apart from the
above stated factors, the deformation of the outer
layer structures to fit the shape of the anvil also plays
a significant role in affecting the impact force attenu-
ation properties of the fabrics.
There is no doubt that a close structure is more
stable and stiffer than an open or mesh structure. For
the close structures, the chain plus inlay structure is
more stable out-of-plane than the locknit structure,
because the chain loops and the inlay yarns are crossed
in a perpendicular manner which are more difficult to
extend and shear than the locknit stitches (Figure 5).
When placed onto the hemispherical surface of the
anvil, a fabric with a more stable outer layer structure
can be less extended and sheared than a fabric with a
less stable outer layer structure. Therefore, the number
of spacer monofilaments which can resist the impact
cannot be significantly reduced due to a lower deform-
ation of the stable outer layer structure. On the other
hand, the stable outer layers make the fabric stiffer,
which is helpful to disperse the stress wave to a larger
area and absorb more energy. As a result, the fabric
knitted with the most stable outer layer structure (chain
plus inlay) has the best force attenuation performance.
The combined effects of low stable outer layer structure
and highly buckled and inclined spacer monofilaments
make the fabric knitted with large-size hexagonal
meshes have the poorest impact force attenuation.
The above analysis shows that the effects of outer
layer structure on the force attenuation properties
under impact in the planar and hemispherical forms
are different. The comfort and formability of fabrics
stand in contradiction to the impact force attenuation
properties. The fabric knitted with a stable and stiff
outer layer structure has better force attenuation ability
than the fabric with a flexible structure. However,
the stiff outer layer structure makes the fabric uncom-
fortable and difficult to fit a curved shape. Therefore,
the balance between the comfort and protective per-
formance requires further investigations to select a suit-
able outer layer structure for a specific protective
application.
Figure 5. Microscopic pictures of outer layers: (a) locknit; (b) chain plus inlay; (c) rhombic mesh and (d) hexagonal mesh.
4,5
320 Textile Research Journal 84(3)
Discussion
The influences of the structural parameters, including
spacer monofilament inclination and fineness, fabric
thickness, and outer layer structure as well as fabric
lamination on the impact force attenuation properties
of the warp-knitted spacer fabrics have been analyzed.
The results have shown that these structural parameters
have significant effects on the resultant peak trans-
mitted force. Hence, a warp-knitted spacer fabric can
be designable by maximizing its force attenuation cap-
acity and meanwhile minimizing its density and thick-
ness for wearing comfort. The results have also shown
that the structural parameters do not determine the
peak transmitted force independently. The peak trans-
mitted force is also affected by the predefined impact
kinetic energy. Therefore, no best spacer fabric exists
for all impact kinetic energy levels. To optimize the
structure of spacer fabrics, the specific application
with an impact kinetic energy level should be identified
first. Then, suitable spacer fabrics can be tailored by
varying their structural parameters quantitatively to
achieve a specific peak transmitted force under a par-
ticular impact kinetic energy.
The human body can be subjected to different levels
of energy impacts in different circumstances or sports,
and various types of impact protectors have been avail-
able on the market for human body protection.
To date, the use of the European Standard BS EN
1621-1:1998
7
to assess motorcycle protective clothing
has been widely accepted. In order to evaluate the feasi-
bility of replacing commonly used polymeric foams
with the developed spacer fabrics, the protective per-
formance of these fabrics are compared with the
requirement of the standard. According to the stand-
ard, the peak transmitted force of limb protectors for
motorcyclists shall not exceed 35 kN, and no single
value shall exceed 50 kN under impact at a kinetic
energy of 50 J. Figure 6 shows the peak transmitted
forces of all the developed spacer fabrics laminated
with three layers. It can be found that only fabric sam-
ples S6 and S7 comply with the standard. These two
spacer fabrics are knitted with the same outer layer
structure (chain plus inlay), but with different thick-
nesses and different stitch densities. Although sample
S6 has a smaller thickness than sample S7, three layers
of S6 still have a thickness of 25.35 mm, which is too
thick for use in protective clothing. Since a fabric
knitted with coarser and more inclined spacer mono-
filaments has a better force attenuation capacity, the
thickness can be decreased by increasing the diameter
or inclination of the spacer monofilaments. However,
increasing the spacer monofilament diameter will make
the fabric stiffer, resulting in a reduction in comfort.
Further design and optimization of spacer fabrics by
manipulating the structural parameters is required to
achieve a good balance between the force attenuation
capacity and comfort.
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12
0
5
10
15
20
25
30
35
40
45
Peak transmitted force (kN)
Figure 6. Peak transmitted forces of spacer fabrics laminated with three layers.
Liu et al. 321
Conclusions
The transmitted forces of twelve warp-knitted spacer
fabrics specially designed and manufactured for
protective application were measured. The effects of
different structural parameters including spacer mono-
filament inclination and fineness, fabric thickness, and
outer layer structure as well as fabric lamination on the
peak transmitted forces were discussed and analyzed.
According to the experimental results and analysis, the
conclusions can be drawn as follows:
1. The structural parameters of a spacer fabric signifi-
cantly affect its protective performance. Among a
group of spacer fabrics, the spacer fabric knitted
with higher inclination and coarser spacer monofila-
ments, a bigger fabric thickness, and a more stable
outer layer structure will have a better force attenu-
ation capacity, if its destruction modes under differ-
ent levels of impact energies are not different from
others.
2. The boundary condition in the hemispherical shape
can change the effects of spacer monofilament inclin-
ation and outer layer fabric structure on the force
attenuation properties of the spacer fabrics.
3. The lamination of spacer fabrics can effectively
improve the force attenuation properties. Three
layers of the spacer fabrics knitted with a chain
plus inlay structure for both outer layers in a total
thickness of about 2.5 cm can comply with the
European Standard BS EN 1621-1:1998.
7
Funding
The work was supported by a grant from the Innovation and
Technology Commission of the Government of the Hong
Kong Special Administrative Region, China, in the form of
an Innovation and Technology Fund project (Project No.
GHP/063/09TP).
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322 Textile Research Journal 84(3)
