JEC COMPOSITES MAGAZINE - Issue #93 - December 2014 - 35

Fig. 2: The EVA's finished structure

dimension tolerances. Those would have
been necessary to build a tubular frame
structure, which previously had been
identified as the best alternative to the
CFRP structure shown in Fig. 2.

16 months of development time
Starting the development process
together with international experts in
composite design and manufacturing,
TUM CREATE discussed feasible design
solutions, part division and the timeline
of the structure in the overall project.
They determined that it was possible to
develop a composite structure for EVA
within the 16 months of available time.
Other outcomes were the number and
approximate size of parts (see specifications box), the pre-selection of design
concepts, and eventually the definition
of the manufacturing process. Using the
expertise of local partner companies and
their available facilities, the team selected
a dry fibre open mould infusion process instead of a prepreg manufacturing
process.

Material desicion
Considering the ease of layup and ability
to have higher variations in the laminate
layup, and hence ensuring higher quality
and performance, a 300 g/m² bi-axial
non-crimp fabric (NCF) was chosen as a
suitable fibre material.
The resin requirements were defined
based on the maximum temperature
exposure in a tropical climate for automotive applications (min. 100°C) and on
the mechanical properties for structural
parts, which led to the selection of a twocomponent epoxy resin. The structural
components dominated by bending loads

were designed as sandwich structures.
The core material selected for these
structures was a PET foam with a surface
coating due to its superior mechanical
properties, low cost and resin uptake. In
order to characterise the composite material and adhesive parameters, test coupons
(small pieces of composite and adhesive
material) were subjected to hot-wet tests,
in which the coupons were exposed to
high temperatures and humidity to simulate Singapore's tropical weather conditions. Additionally, material combinations
and manufacturing setups were investigated to determine the infusion parameters
for large parts with a high number of plies
as well as the groove and perforation
pattern on the sandwich core.
The design process for the structure was
an iterative process conducted in parallel
with the development of other major
components. Design for manufacturing
rules such as ply drop-off rate or minimum radii were applied to improve the
quality of manufacturing through easier
hand layup. As no crash test could be
conducted on the first prototype, a CAE
development approach was selected. Current state-of-the-art dynamic composite
simulation models require complex calibration data. Since the team was building
a prototype and simulations are sensitive
to manufacturing variations, a quasi-static
simulation approach was used.
Therefore, the structure's performance
requirements, e.g. front and side impact
scenarios, were defined. These were
converted into quasi-static equivalent
loadings and used in an inertia relief simulation to reproduce the dynamic loading
boundary conditions.

Specifications
- C-segment car
- Top speed: 111 km/h
- Acceleration (0-100 km/h): 10s
- Weight: 1,500 kg
- 4 seats
- 73 composite parts
- 32 structural parts
- 6 sandwich parts
- 4 glass fibre parts

The definition of different crash zones
using suitable materials resulted in good
crush absorbing performance, while
the integrity of the passenger cell after a
crash was maintained. Different development steps can be seen in Fig. 3.
For each model, finite element simulations were carried out to identify
the locations of stress concentrations,
ply failures and load path distribution
efficiency with the defined load cases.
The weak regions were identified and an
iterative design change was conducted
to optimise the structure. The tooling
process started with the milling of a
Styrofoam male mould using a 7-axis
CNC robot. This mould was then used to
produce a female glass fibre mould ready
for the infusion process. In total, 50
moulds were produced with the largest
one measuring 3.5 m by 2 m for the car's
underfloor. Before the 73 parts were
infused, the dry NCFs were placed in the
mould and stabilised with a binder and
tack applicator. A debulking process was
performed to stabilise the preform with a
maximum thickness of 4.8 mm, consisting of 32 plies. The parts were post-cured in an oven to ensure the required part
quality over the lifetime of the car as the
components were preconditionned

Fig. 3: Different development stages of the CFRP structure

No93 December 2014 /

jec composites magazine 35

JEC COMPOSITES MAGAZINE - Issue #93 - December 2014

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #93 - December 2014