SAMPE Journal - March/April 2023 - 18

FEATURE / HELICOID FIBER ARCHITECTURES
advanced fiber-reinforced materials with different
constituents. Therefore, this greatly expand the
range of potential applications that can benefit
from this bio-inspired fiber architecture.
Prepreg and Autoclave - Helicoid
vs
Conventional QI Hybrid Laminates
As reported in the previous section discussion,
while Helicoid laminates achieved higher
performance than monolithic NFRP conventional
QI laminates, these still underperformed
monolithic GFRP conventional QI laminates (GQI).
This is mainly related to the lower interfacial fiber/
matrix strength and toughness of flax-epoxy, along
with the lower fiber strength of flax fibers compared
to glass-epoxy laminates5,6
. As a consequence,
the best performing NFRP Helicoid (FH7.5) had a
perforation energy which was about 72% of GQI.
We have therefore investigated different grades
of hybridization to improve the performances of
the Helicoid NFRP laminates while decreasing
the carbon footprint of the raw material used
compared to a monolithic GFRP.
Table 2 and Figure 4B show that for Helicoid
hybrids, by increasing the amount of GFRP by
mass in the laminate the impact performances
continuously increase up to values approaching
the monolithic Helicoid GFRP solution (GH). To
note that by adding 21% by mass of GFRP, it was
possible to achieve similar performance to GQI
both in terms of perforation energy as well as in
terms of energy dissipation before the peak load
(hence before catastrophic failure). Furthermore,
with 30% by mass of GFRP, HY30% achieved a 17%
higher perforation energy than GQI while with 50%
by mass of GFRP, HY50% achieved a 32% higher
perforation energy, a similar peak load, a 105%
higher energy dissipation before the peak load and
a 60% delayed catastrophic failure. Photographs
taken at the back face of the HY laminates show
the highly diffused sub-critical damage pattern
reported also for the monolithic Helicoid laminates
(see Figure 2). Hybrid conventional QI laminates
show that, similarly to the HY laminates, increasing
the percentage of GFRP by mass increases the
overall performance of the laminate. However,
it should be noted that for similar hybridization
strategies, Helicoid hybrids always outperformed
in all metrics the QIY laminates. This indicates the
capability of Helicoid technology to integrating
hybridization in the performance enhancement
design process to deliver better performing
structures. Furthermore, photographs of the QIY
samples, clearly show how via increasing the
amount of GFRP, the damage tends to localize at
the impact location with extensive fiber failure.
This is due to the increasing number of GFRP/GFRP
interfaces which are characterized by a higher
strength and toughness than the GFRP/NFRP or
NFRP/NFRP interfaces. This in turn constrains
the propagation of matrix damage, favoring the
occurrence of localized fiber failure.
Overall, the Helicoid fiber architectures
demonstrated a superior damage resistance,
energy absorption capability, load-bearing
capability and structural integrity compared to
conventional monolithic and hybrid QI lamination
sequences. Therefore, Helicoid offers a versatile
solution which paves the way for a more extensive
use of low carbon footprint materials, such as
natural-fiber reinforcements in load-bearing and
high-performance applications, where impact
toughness is a key design requirement. Applications
such as EV battery enclosures, protective domes of
hydrogen pressure vessels, impact resistant skins
of wind rotor blades and, in general, high piercethrough
resistant composite structures would
greatly benefit form Helicoid fiber architectures.
For instance, the driving mechanical
requirements of EV battery enclosures include
resistance to perforation, limited deflection upon
impact and avoidance of fiber shuttering at the back
face of the protective enclosure. Such components
are produced in mass with volumes of more than
3 ktons/year for a single passenger vehicle with
overall volumes for multiple vehicles projected to
>10 ktons/year. Currently, EV enclosures are made
mostly of Glass-fiber reinforced composites. Each
part weighs 20-30kg. 1.7-2.5 tonnes CO2-eq are
produced per tonne of glass fibre22
. Since natural
fibres only have a carbon footprint ~0.35-0.55
tonnes CO2-eq per tonne of fibre22
, replacing 50%
by mass of glass fiber with natural fiber for the same
battery enclosures/underbody panels application
would lead to a 7-9.7 kton of CO2 saved.
NCF and VA-RTM - Helicoid vs Conventional
QI Monolithic Laminates
While the work conducted on NCF and VA-RTM
is still in progress and hybrid configurations have
not yet been investigated, preliminary results
performed on monolithic NCF-GFRP, and NCFNFRP
show similar trends to the one obtained for
the prepreg/autoclave materials. Table 2 and Figure
4d show that NCF-FH9 achieved a 14% higher peak
load, 38% delayed catastrophic failure and a 74%
higher
energy
dissipation
before
catastrophic
failure (peak load) than NCF-FQI. Similarly to the
observations made for the prepreg and autoclave
laminates, the images of the back face of a
18 | SAMPE JOURNAL | MARCH APRIL 2023
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