JEC COMPOSITES MAGAZINE - Issue #110 - January/February 2017 - 49

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

forced resin laminar composite materials [10-14]. The principle of this interlaminar strength improvement technique
involves the electrostatic pre-placement of perpendicularly-oriented short fibres between the laminar plies during a
composite's fabrication/lay-up stage. Short fibre placement
is accomplished by electrostatically propelling short fibre elements at a fibrous ply layer of the composite coated with
an uncured polymer (fluid) matrix. This electrostatic fibre
coating process is called "flocking". During the course of this
UMD research, three modes of Z-axis flock fibre application
were researched and developed. These methods are referred
to as (1) "wet" flocked, (2) "dry" flocked and (3) "preflocked". These are briefly described in the following section.

Z-axis flock fibre interlaminar reinforcement methodology
Wet flocking
In the original variation of the process, flock fibres were applied to an uncured, liquid resin impregnated fabric ply-layer
surface before the composite lay-up was assembled and consolidated. This process is referred to as Z-axis reinforcement
"wet flocking" process because the flock is applied to a "wet"
adhesive (matrix impregnated) fibrous ply layer. When these
ply-layers are assembled and finally consolidated, the composite is rendered Z-axis reinforced (see Figure 1). It was
found that these Z-axis short fibre reinforcements result in
an OPLC having greatly improved interlaminar strength. Generically, what is formed here is a "hybrid composite" made
up by continuously alternating two types of fibrous composite layer material configurations: (a) a relatively thin, Z-oriented short fibre-reinforced resin ply; and, (b) a woven fabric
(or random fibrous mat) structural layer component.
Dry flocking
In later studies, it was discovered that Z-axis oriented short
fibres could first be applied directly to non-resin containing
"bare" fibrous mats or fabric. These flock fibre-containing
layers could then be assembled, impregnated with a matrix
resin and consolidated to form a finished laminate. Specifically, the process involved: (1) flock fibres being electrostatically applied to the surfaces of the individual laminar ("dry"
as received) fibrous plies; (2) these so-treated laminar plies
would then be, by very careful handling, stacked together to
form the desired structural laminate configuration; and (3)
the combined laminar configuration would then be impregnated (throughout) with a liquid matrix resin (epoxy, vinyl
ester or unsaturated polyester); and finally, (4) the stack of

Fig. 1: Schematic diagram of glass fabric/Z-oriented short fibre Z-axis
interlayer OPLC

Z-axis fibre-reinforced fibrous plies would be consolidated
by a vacuum bagging or flat-press curing process to form the
final laminar structure. This technique is referred to as the
Z-axis reinforcement "dry flocking" process.
Pre-flocked composite reinforcing layers
In a more recent development, a more "consumer-friendly"
approach to this Z-axis laminar ply reinforcement technology
was devised whereby flock fibres are tacitly applied to individual "dry" fibrous laminar ply layers to form a "stand-alone"
composite ply layer material entity. This is accomplished by
first applying a very thin sizing resin (wet and uncured) to a
fibrous reinforcing (fabric or mat) ply just before flocking.
Allowing this thin resin sizing layer to cure in place serves
to loosely secure the upright-oriented flock fibres in place.
These flocked composite reinforcement layers are now ready
for subsequent composite ply material assembly manipulation; this includes gentle material handling, long-term
storage, materials packaging and manipulation as it is to be
subsequently used to fabricate a laminate. Since this process
involves applying the upright-orienting flock to ordinary,
standard off-the-shelf available composite reinforcement materials, this process is referred to as the "pre-flocking" technique.
The following presents the results of experimental studies
conducted to compare the interlaminar shear strength of
OPLCs prepared using the three Z-axis flock composite reinforcement techniques.

Experimental materials and laminating procedures
All the materials used in this study were obtained from Fibre-Glast Developments Corp., Brookville, Ohio.
Glass fabric: 203 g/sq. metre 18 x 18 plain weave, 0.2 mm
thick, epoxy resin compatible.
Glass fabric mat: 457 grams/sq. metre chopped strand mat,
1.1 mm thick, binder used to hold the chopped fibre mat together.
Epoxy matrix resin: System 2000 epoxy resin with 2120 epoxy
(amine) hardener.
All flock-treated laminar ply panels were laid-up in the desired assembly and impregnated, as needed ("dry" flock and
"pre-flocked"), with a fluid epoxy matrix resin. The samples
were wrapped in suitable release films and consolidated in a
flat press at 250 psi; they were all cured in the press overnight
at room temperature. The laminate's release sheets were then
removed and the panels were cured for 2 hours in an 80oC
oven. The "dry" and "pre-flocked" laminar assemblies were
impregnated and saturated with an epoxy matrix resin using a
pressure-roller technique. Subsequent DCB tests showed that
these samples were completely impregnated with the matrix
resin.

Experimental methodology and results
To compare the efficacy of these three methods, fracture
No110 January-February 2017 /

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