JEC COMPOSITES MAGAZINE - Issue #110 - January/February 2017 - 51

Flock fibre-reinforced laminar composites for improved Mode I fracture toughness

Tab. 2: Fracture toughness of "dry flocked" Z-axis 12 ply glass mat laminar
composites compared to a non-flocked control (a)

Description (b)

Estimated Crack opening Initial peak
Thickness
flock density toughness toughness
(mm)
(fibres/mm2)
(kJ/m2)
(kJ/m2)

VAF

4.2

0.63

1.96

BBA

3.6

0.38

1.36

VAF/ BBA

5.3

0.52

1.61

CONTROL (no flock)

3.7

0.19

1.08

Tab. 4: DCB fracture toughness of "pre-flocked" Z-axis 20-ply, glass fabric laminar composites compared to a non-flocked fibre-reinforced (control) (a)

(a) Flocked samples prepared using 15 denier, 3.8 mm long nylon flock
(b) Glass mat samples were 12-ply composite assemblies

Description

Tab. 3: DCB fracture toughness of "dry flocked" Z-axis glass fabric laminar composites compared to a dry-flocked (with no assist), no flock control (a)

Description (b)

Fig. 4:
Cross-section
photomicrograph of
"pre-flocked" glass
fabric: 3 denier,
1.2 mm long nylon on
0.2 mm thick glass
fabric

Estimated Crack opening Initial peak
Thickness
flock density toughness toughness
(mm)
(fibres/mm2)
(kJ/m2)
(kJ/m2)

VAF - 12 ply

1.8

0.17

1.72

BBA - 12 ply

1.6

0.07

0.91

VAF/ BBA - 20 ply

3.7

0.60

1.31

CONTROL (no flock, no
assist) - 12 ply

2.9

0.37

1.49

Estimated Crack opening Initial peak
Thickness
flock density toughness toughness
(mm)
(fibres/mm2)
(kJ/m2)
(kJ/m2)

Low-density sizing

4.1

1.20

2.6

Intermediate-density sizing

5.8

103

1.18

4.14

High-density sizing

6.2

151

0.77

1.28

Control, no flock

2.9

0.37

1.49

(a) All flocked samples were prepared using 3 denier, 1.2 mm long nylon flock

(a) All flocked samples were prepared using 15 denier, 3.8 mm long nylon flock

Z-axis oriented flock fibre is far superior to treating the jute
yarn surface with a silane coupling agent. Compared to the
raw control, only a limited improvement in fracture toughness was observed with the silane-treated jute fabric. For the
plain weave jute fabric laminate, the addition of a Z-axis flock
fibre interfacial reinforcement increased the crack opening
toughness by 81% compared to the raw jute fabric laminate.
This dramatic increase is the result of the added Z-direction
flock fibre reinforcement at each of the jute ply-layer interfaces. Be reminded that a jute fabric's open, rough texture is
perhaps why jute/epoxy resin composites are found to be so
well adapted to the Z-axis flock reinforcement technology.
The flock fibres can more readily penetrate into the fabric's
interstices and inter-yarn spaces.
In support of this, in photo-micrographic studies of these
composites, flock fibre to polymer matrix pull-out was observed to be the dominating mode of interlaminar failure of
this sample. Full flock fibre pull-out requires an additional
amount of energy to overcome crack opening and growth,
thereby increasing fracture toughness. As noted in Figure 3,
Z-axis reinforcement has a much greater interface toughening
effect for plain weave jute/epoxy resin composites than for
comparable jute yarn unidirectional composites. The reason
for this is that plain weave fabrics will have a higher inter-ply
layer contact factor than the unidirectional fabrics. Plain
weave fabrics have full-thickness yarns in both major axes
(weave and warp directions). Unidirectional fabrics have jute
yarns along only one axis; the weave direction. Therefore, the
plain weave laminates are capable of possessing a more highly supported interfacial contact area which would be more

adaptable to supporting a well embedded Z-axis flock fibre
reinforcement. In conclusion, Z-axis flock reinforcement of
jute composites appears to be an acceptable way of enhancing
the crack initiation toughness of these natural fibre composite materials [16, 17].
Dry flocked glass mat and glass fabric laminates
Glass mat and glass fabric reinforcement layers were "dry"
flocked, fabricated into a laminar composite and DCB tested. The "dry" flocking was performed using two methods.
One method, called vacuum-assisted flocking (VAF), used
an added air suction force applied during flocking. Applying
a vacuum under the porous fibrous material during flocking
enabled the flock to be more deeply embedded in the "bare"
fibrous structure of the substrate. In the second method,
called beater bar assisted (BBA) flocking, the flocked-upon
sample was placed on a vibrating platform and vibrated during flocking. This "beater bar" causes the platform and the
mounted fibrous material sample to vibrate in the normal
direction as flock fibre is electrostatically applied to the ply.
DCB results showing a higher fracture toughness of the
VAF-prepared epoxy matrix laminar composites (compared
to the control) are presented in Tables 2 and 3 [18]. The vacuum-assisted samples had a higher toughness (1.96 and 1.72
kJ/m2 respectively) for both the glass mat and fabric compared to the beater bar (1.36 and 0.91 kJ/m2). The combined
VAF/BBA process showed 1.61 and 1.31 kJ/m2; the no-flock
control toughness was 1.08 and 1.49 kJ/m2 for the glass mat
and glass fabric respectively. Comparing Tables 2 and 3, we
conclude that the BBA and the combined VAF/BBA processes enhanced the toughness properties of the glass mat laminate, they were detrimental for the glass fabric material. It
appears then that VAF is the process of choice for preparing

No110 January-February 2017 /

jec composites magazine

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