Pre-impregnated material (Pre-preg)

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 16.29Front nose attached to main tub of

Mercedes Mchaven SLR with aluminium frame

Table 16.7Synthetic 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

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

560Repair 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

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 16.30First stages in building up chassis

details on the Lotus Elan (Lotus Engineering)

Figure 16.31Final 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 16.32Floor pan and outrigger attachments,

undertray and bulkheads (Lotus Engineering)

Figure 16.33Vehicle 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

door’s 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

a high-strength aluminium alloy windscreen frame, a

tubular steel scuttle beam, and steel beams in the

562Repair 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.