JEC COMPOSITES MAGAZINE - Issue #90 - June/July 2014 - 46

toughening
testing. Even from the mechanical point of view alone, delamination causes substantial weakening of the structure under
compressive loading. A well-established procedure used in
aircraft construction is the compression after impact (CAI) test
according to AITM 1-0010.
The extent of the damage caused by a low-velocity impact to
fibre-reinforced materials can be reduced by suitably designing
the layer structure and by modifying the resin system used for
this purpose. The energy absorption potential can be increased
in a similar way.

Conventional approaches to solving the problem
The fracture toughness of the resin system is usually modified
using soft elastomers or else with hard filling materials such as
silicates or nanoparticles. However, all of these measures increase the resin's viscosity so substantially that it can no longer
be used in the RTM process. Furthermore, such additives are
filtered by the fibre structure itself, causing an inhomogeneous
distribution of the particles. On the other hand, using such
additives in the production of prepregs presents few problems
because in this case only one or at most a few fibre layers are
soaked in the process of impregnation with the melted or dissolved matrix material.

A new approach for modifying
the fracture toughness of epoxy resins
The novelty of this approach consists in integrating the additives for improving fracture toughness not into the resin, but
into the semi-fabricated textile material. This has two advantages: not only can a low-viscosity unmodified resin system be
used, but also building the modifier into the textile structure automatically ensures its uniform distribution in the component;
and these cancel the main disadvantages of resin modification.
In practice, a thermoplastic polyamide 12 yarn from EMS
Chemie AG in the form of a separate multiaxial laid scrim is
introduced in-between the individual fibre layers (Figure 2).
This toughens the resin-rich interlaminar layer, significantly
reducing the extent of delamination and hence of the invisible
damage caused by the impact loading. In addition, the viscoelastic behaviour of the thermoplastic material reduces crack
propagation and so further reduces the extent of the damage. It
is precisely the discrete spacing of the thermoplastic that diverts

Fig. 2: The non-crimp carbon fabric with the thermoplastic laid scrim

46 jec composites magazine / No90 June - July 2014

the cracking, thus serving as a stopper.
Besides improving fracture toughness, this new approach using
discrete thermoplastic laid scrims has four additional advantages for the process and for the resulting properties of the finished
component:
- The thermoplastic yarn can be applied even in the preforming process, as a binding agent. During the pre-warming of the
semi-finished textile, the thermoplastic yarn becomes sticky,
helping to shape the preform: on the one hand it helps stabilise
the shape, while on the other it helps debulking for aircraft applications.
- Flow testing has demonstrated that the in-plane permeability,
taken as a measure of the semi-fabricated fabric's wettability, can be boosted by as much as 300%, the thermoplastic
semi-finished textile acting as a flow media. The impregnation
behaviour can also be decisively improved in the direction
of the thickness, which is vital for the special compression
RTM process, for vacuum assisted processing (VAP) (Airbus,
EADS) as well as for resin transfer infusion (RTI) (Bombardier
Aerospace), bringing considerable process advantages in the
production of thick-walled structural parts.
- The electrical conductivity of the fibre-reinforced component
in the direction of the thickness is not impaired - as it is with
non-woven fabrics (e.g. fleece) - thanks to the discrete form of
the laid scrim.
- The thermoplastic yarn can also serve as a carrier for other
additives with the object of adding further properties to the
component.

Results of experimental investigations
All the test samples were produced using RTI (resin transfer infusion), a process in which the preform is saturated with epoxy
resin by infusion and then cured in an autoclave. A quasi-isotropic layup was chosen [0/+45/-45/90]2S, consisting of sixteen layers of carbon fibre (298 g/m2) with the polyamide 12
laid scrim (10 g/m²) laid progressively between the consecutive
fibre layers. The resins Hexflow RTM6 from Hexcel and Cycom
890 from Cytec were used as matrix.

Fig. 3: The damaged surface area after impact testing without (left) and with
(right) thermoplastic laid scrim

JEC COMPOSITES MAGAZINE - Issue #90 - June/July 2014

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