JEC COMPOSITES MAGAZINE - Issue #110 - January/February 2017 - 50

TECHNOLOGY OPLC

Fig. 2: Mode I fracture toughness for the following samples: D - not-flocked, glass
fabric composite platen press fabricated (control); DV - not-flocked, glass fabric
composites vacuum fabricated; DNL - 3 denier, 2 mm long nylon flock (low flock
density of 43 fibres/mm2); DNH - 3 denier, 2 mm long nylon flock (high flock
density of 170 fibres/mm2); DSP - 3 denier, 0.6 mm long polyester flock; DLP - 3
denier, 1.2 mm long polyester flock; DTSP - 3 denier, 0.6 mm long polyester flock
with special adhesion promoting surface treatment; Ideal not-flocked, ideal glass
fabric composite with crack propagating through the epoxy (no fabric bridging)
Tab. 1: Fracture toughness of "wet flocked" Z-axis 32 ply glass fabric laminar
composites compared to a non-flock fibre-reinforced (control) (a)

Description

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

Low flock density

5.8

0,4

1,25

High flock density

7.5

170

0,4

2,65

Control (D) - no flock,
has fibre bridging

4.5

0,6

1,25

Ideal control - no flock
or fabric bridging

0,4

0,35

(a) All flocked samples were prepared using 3 denier, 2.0 mm long nylon flock

toughness tests were carried out employing the double cantilever beam (DCB) interlaminar shear strength test (ASTM D
5528-94a). In this test, the force necessary to cleave the laminar plies of an organic polymer fibre-reinforced composite is
measured. From the force vs. delamination length data, Mode
I fracture toughness is calculated. Example results are presented in Figure 2 for various wet flocked Z-axis reinforced
composites and their controls. For these measurements, two
parameters are important: (a) the toughness when the laminate first starts to crack - DCB-First value toughness; and
(b) the initial peak toughness attained during the splitting or
cracking of the laminate - DCB-Initial peak toughness. Using this technique and these toughness parameters, fracture
toughness data for fibre-reinforced resin composite materials
were obtained for composites fabricated using the three interlaminar reinforcement processes: (1) wet flocking, (2) dry
flocking and (3) pre-flocking.
Wet flocked glass fabric laminates
The Mode I fracture toughness for various "wet" flocked samples are shown in Figure 2. It can be seen that the greatest

Fig. 3: Comparison of initiation DCB toughness values for various 16-layer jute
fabric/epoxy laminar composites - Effect of jute fabric surface treatment

toughness was achieved by the sample employing the 3 denier, 2 mm long nylon flock at a relatively high flock density
of 170 fibres/mm2.
Ideally for the non-flocked control samples, the crack will
propagate within the matrix. So the Mode I fracture toughness will remain a constant and the R-curves should be a
straight line parallel to the X axis, as shown in Figure 2, and
called "ideal" non-flocked control. However, in the actual experiments, both tested non-flocked control samples, D and
DV, showed an increasing trend. This phenomenon can be
explained by the strong interfacial bonding strength between
the glass fabric yarns and epoxy resin system. This glass fabric
had a surface finish compatible with this epoxy resin matrix
system that can cause fabric bridging as the crack propagates.
Specific data (taken from Figure 2) for the nylon flock at high
and low flock densities are shown in Table 1, along with the
ideal and control samples.
In additional experiments to further investigate the "wet"
flocking process, OPLC panels were fabricated by laying up
16 plies of jute (natural fibre) woven fabric. Plain weave and
unidirectional jute yarn fabric configurations were prepared
for this study. An amine-cured epoxy resin was used as the
composite's polymer matrix. Three individual panel samples
were prepared: (a) 16 plies of untreated (as received, raw)
jute fabric as a control; (b) 16 plies of jute fabric treated with
silane Z-6173 (Dow Corning silane coupling agent); and (c)
15 layers of silane-treated jute fabric "wet" flocked with 3 denier, 1.3 mm long nylon flock fibres, and one un-flocked silane treated "top" layer ply. This latter sample can be referred
to the Z-axis "wet" flocked reinforced laminar configuration.
After these three composite lay-ups were consolidated and
cured in a flat-press, the finished samples were cut into test
specimens.
The DCB test results are summarized in Figure 3. Here, it is
found that Z-axis flocked reinforcement fibres have a profound positive effect on enhancing the initial crack-opening
toughness of jute/epoxy resin laminar composites [15]. The
data also show that treating the composite ply interface with

jec composites magazine / No110 January-February 2017

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