Woven material is available pre-impregnated
with resin. It is referred to as Pre-preg. This
means that the material has exactly the right
amount of resin applied to it. The resin is fully
coating the material so that there are no dry
spots which could lead to component failure.
Pre-preg is therefore quicker to use and the resin
density is accurate.
Pre-preg has a limited shelf life which is compounded
by the fact that it must be stored at
_18 C. A deep freeze cabinet is therefore needed
for storage. The pre-preg can not be unrolled nor
cut when it is in the frozen state, so it must
be removed from the freezer and brought up to
Reinforced composite materials 559
normal room temperature. It is only possible to
freeze and de-frost the pre-preg a limited number
of times so the material must be managed carefully.
The usual way to do this is by means of a
control card. The dates and times of defrosting are
recorded as is the amount of material taken off the
roll. That way the life of the roll and the amount of
material left can be seen without removing the roll
from the freezer.
Figure 16.29 Front nose attached to main tub of
Mercedes Mchaven SLR with aluminium frame
Table 16.7 Synthetic polymers and their characteristics for use in foam making
Foam material Abbreviation Characteristics
Polyvinyl chloride PVC Good resistance to water; available in plain sheet or grid scored
Polystyrene PS Very light; low mechanical strength
Polyurethane PU Good for thermal and acoustic insulation; can be used at up to 150 C
Polymethyl methacrylamide Acrylic Very strong, but expensive
Polyetherimide PEI Good fire resistance, used for interior trim on public carrying vehicles
Styreneacrylonitrile SAN High impact strength
should any stress be applied. To speed up the
hardening process and to add extra strength to the
component it is normal to use an oven. The oven
may be a simple box with an heating element, or
an autoclave which is a cylindrical shaped oven
that can be pressurized or evacuated inside. The
normal procedure is to place the newly made
component in the oven, or autoclave, then rack up
the temperature gently, over a period of about 30
minutes. Maintain the temperature typically at
150 C for about 5 hours, then gradually lower
the temperature, again over about a 30-minute
period. The best way to do this is with a computer
control system.
Core materials
Engineering theory tells us in most cases that the
stiffness of a panel is proportional to the cube of its
thickness. That is, the further apart that we can
keep the outer fibres the stiffer the panel will be.
Putting a low density core between two layers of
composite material will add stiffness with minimum
weight and at reasonable cost.
Foam
A variety of materials are used, one of the most
common is foam. Foam can be made from a variety
of synthetic polymers (Table 16.7) Densities
of foam can vary between 30 and 300 kg/m3 and
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thicknesses available are from 5 to 50 mm.
Honeycomb
Honeycombs are made from a variety of materials,
including extruded thermoplastic ABS, polycarbonate,
polypropylene and polyethylene bonded
paper, aluminium alloy and for fire resistant parts,
Nomex. Nomex is a paper-like material based on
Kevlar fibres.
Curing
The resin, whether it is by wet lay-up or pre-preg
needs time and heat to dry it out and make it hard.
When the hardener is added to the resin it will
generate heat chemically. Be careful, this heat can
cause fire and other damage. However, at normal
temperature, 20 C, it will take about 5 days for
the resin to become fully hard. During this time
period the component should not be moved nor
560 Repair of Vehicle Bodies
Heat
A point to be noted is that most carbon fibre materials
are affected by heat. Thermal expansion can
lead to micro-cracking. A carbon fibre panel which
is painted black will absorb a lot of heat if left in
the sun for a long period. This can cause the panel
to expand which could lead to micro-cracks in the
panel and cracks in the paint work. This will then
allow in moisture which will cause further deterioration
of the panel.
16.8 Body production in reinforced
composite plastic (Lotus)
At present composite reinforced plastic finds its
use in road transport applications, where in some
cases complete cabs and bodies are manufactured
using the material. It is also on the increase in the
manufacture of public service vehicles, luxury
coaches and caravans. The manufacture of car bodies
in this material is still somewhat limited,
although some of the British car manufacturers,
particularly Lotus Cars Limited, are developing the
use of this material in their fibre reinforced composite
constructed bodies.
Shapes and forms which are acceptable in steel
vehicle bodies can also be produced in composite
materials. These materials, such as Kevlar, carbon
fibre, glass fibre, non-woven, unidirectional, diagonal
and bidirectional forms, can produce moulded
structures with a variety of properties. The performance
of body panels can be changed whilst
retaining the panel thickness, simply by altering
the type of reinforcement used within any given
panel thickness. The ability to create these effects
in composite vehicle design depends on the skill of
the designer. Advanced structures are made by
incorporating premoulded rigid foam and metallic
inserts in the fibre reinforced resin during the
injection moulding process, ensuring flexibility
that in turn allows reinforcement properties to be
accurately tailored to a specific design requirement.
This can be done without compromising the
original design concept.
Lotus chassis design and construction
Lotus has modified its practice of using a pure
backbone chassis for the Elan and opted for a
unique, composite platform and backbone type of
construction. A major factor behind this decision
was the engineering requirements to manufacture a
very taut, rigid open sports car.
The Elan body platform is a one-piece 3 mm nominal
thickness vacuum assisted resin injection (VARI)
moulding which is riveted and bonded to the welded
steel reinforcing outriggers comprising: inner sill, toe
board, heel board, A-post and B-post. When bolted
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to the backbone chassis this results in high torsional
stiffness which gives the car exceptional handling
characteristics (Figures 16.30 and 16.31). The floor
pan is manufactured from isophthalic polyester
Figure 16.30 First stages in building up chassis
details on the Lotus Elan (Lotus Engineering)
Figure 16.31 Final stages of building up chassis
details on the Lotus Elan (Lotus Engineering)
resin continuous filament glass fibre with additional
local reinforcements in high-load areas such as the
body to chassis attachment points and the fuel tank
mounting area. The outrigger and A- and B-posts are
manufactured from 18 gauge steel, E coated and wax
Reinforced composite materials 561
injected for maximum corrosion resistance prior to
assembly. Elastomeric polyurethane adhesives are
used throughout the construction. These steel components
not only contribute to the bending and torsional
stiffness of the vehicle but also provide rigid
attachment points for seat runners, lower seatbelt
mountings and door hinges. Additional structural
rigidity and side impact protection is provided by
steel cross-braces between the A-posts at the front
and the B-posts at the rear (Figure 16.32). The backbone
chassis extends rearwards from the front bulkdoors
and B-posts. Most composite exterior panels
are bonded to this structure using a flexible
polyurethane adhesive, but the frontal panels
are secured by threaded fasteners for ease of service
access and collision repair. The front bumper/spoiler
and rear bumper valance are flexible reinforced
polyurethane mouldings resistant to damage from
minor knocks. Composite structures have the ability
to absorb high impact loads by progressive collapse,
with impact damage being localized. In accidents
this feature protects the occupants from injurious
shock loads and greatly reduces the danger of entrapment
by deformation of body panels. This behaviour
also facilitates repair by replacing the damaged bolton
or bonded panels using recognized approved
methods.
All the outer body panels are a nominal 2 mm as
they are cosmetic and not load bearing. However,
there are some exceptions: the undertray, bulkheads,
bumper armatures and door inners are thicker to contribute
to the structural performance (Figure 16.33).
Figure 16.32 Floor pan and outrigger attachments,
undertray and bulkheads (Lotus Engineering)
Figure 16.33 Vehicle undergoing interior trim (Lotus
Engineering)
All Elans have RRIM bumpers front and rear to
comply with US federal regulations, and energy
absorbing front bumper construction is used. The
doors outer panel shape does not allow conventional
hinges: a unique design allows the door to
swing in an arc outside, instead of the more traditional
inside, of the front A panel. At the latch
end of the door a tapered interlock bar has been
designed so that during side impact the load path
of the low mounted side intrusion beam is through
the latch and hence into the main vehicle structure.
head and incorporates the rear suspension pick-up
points, while the front longeron/underframe assembly
bolts on to the front of the backbone frame. This
incorporates the front suspension pick-up points,
engine mountings and front energy absorbing structures.
The complete subframe assembly, including
the power train, is detachable to ease both manufacturing
and service. High-strength cast aluminium is
used for the windscreen pillars, which bolt directly
on to the top of the A-posts and are joined by an
extruded and formed aluminium header rail.
Lotus body design and construction
The body structure comprises a moulded composite
floor pan reinforced with steel in key areas to form
stiff box sections. The floor pan is bolted at 16 points
to the box section steel back-bone chassis, with further
rigidity and occupational protection provided by
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a high-strength aluminium alloy windscreen frame, a
tubular steel scuttle beam, and steel beams in the
562 Repair of Vehicle Bodies
The body panels are produced from composite
materials, which include a low-profile non-shrink
polyester polymer system which has been developed
to suit the Lotus VARI process requirements. A
patent fibreform process has been developed by
Lotus to provide preformed fibre reinforcement
which is self-locating inside the VARI tools during
the moulding process. An added sophistication is
that the production moulds have an electroplated
nickel shell surface, which not only extends tool life
but also gives a high standard of finish to the body
panels, allowing minimal preparation for the painting
process (Figure 16.34).
strand mat and activate three times this weight of
resin. If a large repair is being attempted, do not prepare
more than half a pound (0.2 kg) of resin at a time
to avoid the mix curing before it can be used.
The first essential when considering the repair of
a reinforced composite moulding is to ensure that
the area to be repaired is clean and dry, including
the rough edge of any torn portion. In many cases a
vehicle with minor damage will be driven back
from the site of the accident and road moisture and
dirt is deposited on the damaged area. The dirt must
be washed off and the area dried using some convenient
form of heating, but care must be taken that
the moulding is not further damaged by too high a
temperature. The area of the damage must then be
checked and marked for cutting. It is usually found
that any break in a moulding is surrounded by an
area of bruising where the resin is crushed, and this
must be removed. It is unusual for this to extend
more than about 8 cm from a break, and the easiest
way of checking the exact limits is to shine a powerful
lamp through the laminate. The bruising will
then show up as a dark or light patch depending on
the colour and content of the laminate, and can be
marked out for removal.
After all damaged areas have been removed, the
cut edges should be feathered on the non-weathering
side and then the surface is roughened for about
two inches back from the cut. A single layer of
chopped strand mat and polyester resin is then
laminated into the roughened area. While the area
is still wet, a sheet of cellophane large enough to
cover this material and the hole is applied and
pressed into the uncured resin. The cellophane is then
supported by sheet metal, hardboard or card which
can be fastened to the moulding with adhesive tape
and struts fixed to hold it to contour. The hollow is
then almost filled by applying polyester resin and
chopped strand mat, continuing to build it up until it
is nearly flush with the original surface. The laminate
is allowed to cure and then finished off flush and
smooth with a standard two-compound gap filler.
