Chapter 5. Fabrication procedures for proposed composite post 139
Chapter 5
Fabrication Procedures of the Proposed Post in
Composite Materials
The following chapter is a summary of the most common types of processes used in the
fabrication of the composite structures. There are two fundamental processes required for the
fabrication of composite materials: the first is the stacking or lay-up of the laminas made up of
fibre and matrix components, the second process is the polymerisation of the resin and is
achieved by means of curing. Curing requires elevated temperatures or pressure application or
a combination of two to ensure that optimum properties of the piece are achieved. Heat causes
molecular linking in the resin (thermoset resins) to form chains or polymers that solidifies the
structure. Pressure and vacuum are applied in the curing process so as to minimise defections
and maintain the shape of the final piece.
The chapter describes the different types of lay-up, curing methods and combinations of the
two. Advantages and disadvantages of the processes are made in terms of price, availability of
devices required, and whether they are compatible with the proposed structure. Related
systems including the clean room and its preparation requirements are outlined as well as
storing conditions of composite materials.
Finally, the most suitable fabrication process for the proposed post structure is defined with a
step by step summary which includes a list of the principal and auxiliary materials required.
5.1 Constituent Materials of Composite
This fabrication process, which is common to all, can be divided into two phases: lay-up
(laminate configuration) and the curing cycle. With regards to the first phase, the fibres and
matrix material can be obtained commercially, either individually or combined together to form
a lamina in an uncured state. Fibres made available separately from the matrix are done so in
groups of bundled, but not twisted, or also spun onto spools. Fibres saturated with resin such
as epoxy, which subsequently becomes the matrix component, are known as preimpregnated
fibres and are often referred to as prepregs. These prepregs are available in tape form where in
the unidirectional fibre prepreg the fibre runs parallel to the tape’s length. The fibres are held in
Chapter 5. Fabrication procedures for proposed composite post 140
place by the matrix and the tape is wound into a roll. Teflon removable-backing prevents the
matrix from sticking together in the roll. Prepreg fabrics, in which two sets of fibres are
perpendicular to each other, are also available and are fabricated initially interweaving the two
sets of fibres and subsequently impregnating them with resin.
The curing phase is the polymerisation process of the matrix component in which forms
permanent bonds between the fibres and matrix within the lamina and between laminas of the
laminate. The process occurs naturally or accelerated energetically by means of heat and
pressure application [5], [3].
5.2 Lay-up
There are four principal lay-up or stacking processes for laminated fibre-reinforced composite
materials. These include
• Manual/automated lay-up
• Filament Winding
• Moulding
• Spray-up
The choice of lay-up process depends on profile and size of the structure, cost, time, familiarity
with techniques, and processes available. Of the four, the process most appropriate in the
proposed composite structure of this project is the manual lay-up of prepreg plies. Automated
lay-up of the plate laminates would also be appropriate if available, however it would not be
well suited to lay-up of the beam components. Of the four, the following only describes the
manual lay-up process as it is the most relevant lay-up type to the project.
The manual lay-up or hand lay-up process consists of two variations: prepreg fibre lay-up, and
separate fibre and matrix parts lay-up. In relation to the first, the preimpregnated fibre comes
in roll form which is unwound and cut to the desired geometry depending on the individual
orientation of each ply. Laying-up can be done by hand or automated. The pre-cut layers are
laid-up in the appropriate desired configuration beginning at lamina 1 and finishing at lamina
1+x. Each prepreg ply’s protective backing is maintained in place during lay-up and is only
removed after manual light compaction with a spatula before the next layer is laid up. This
removable backing also serves for the manual handling of the ply, i.e. it gives the ply more
rigidity during stacking and configuring on the laminate while keeping the prepreg free from
contamination of hand and countertop substances such as small particles, liquids and greases.
Because the removable backing is of a teflon based material, manual light compaction is
achievable between each layer using a rounded-head spatula. If the spatula is not made of
teflon itself, the removable back prevents the spatula sticking to the epoxy and also reduces the
risk of fibre separation during this type of compaction. It is important to note that plies cannot
Chapter 5. Fabrication procedures for proposed composite post 141
be jointly laid-up together end to end through the direction of the fibre as this creates a
discontinuity and stresses are not transferred through the entire layer. Plies however can be
laid-up together at their edges parallel to the fibre direction with care taken so as avoid gaps
and overlaps between the two plies. Figure 5.1 shows the removable back being stripped from
an already laid-up ply in a laminate [9].
Lay-up technicians generally can lay composite fabrics more efficiently than unidirectional
tapes. This is because the fabric material is more pliable or workable. Composite tapes tend to
fold or separate when laid-up over curvatures or complex profiles making the manual lay-up a
more demanding process. However, unidirectional composites (as well as fabric composites)
are well suited to the automated lay-up process. The advantages of the automation process, if
available, is that facilitates optimum strength to weight ratio designs, better process
consistency, and shortened process time [30].
Figure 5.1: Manual lay-up of unidirectional composite
In relation to the second type of manual lay-up where the fibres and matrix are laid-up
separately, the process consists of firstly applying a resin layer onto the mould and secondly
applying the fibrous layer above. A new layer of resin is applied between every fibrous layer by
using a roller until the desired amount of layers is reached. The resin is allowed to cure
naturally in air or in an oven depending on the resin requirements. Exact fibre orientation is
difficult to achieve and this process is therefore suited to structures with reduced structural
responsibility. This process has been successfully utilised in fibre glass reinforced composite
structures. However while it is a cost effective, this type of lay-up is not considered in the
proposed structure.
Chapter 5. Fabrication procedures for proposed composite post 142
5.3 Curing Processes of Composite Materials
The following processes of curing require that the composite materials are cured through heat
or pressure, or a combination of the two. The devices outlined in this section include the
autoclave, hot-press plate, high temperature chamber, and the quickstep process.
5.3.1 Autoclave
The autoclave process is normally used for curing structures made up of prepreg plies and it is
for that reason the most ideal form of curing for the composite structure proposed in this
project. The autoclave can be best described as being similar to a large version of an ordinary
pressure cooker. The main attributes of the autoclave is its ability to cure composite structures
by means of elevated isostatic pressure and temperature. The elevated pressure and
temperature that forms the curing cycle depend on the type of matrix material (whether it is
thermoset or thermoplastic matrix) and fibre type. In relation to the proposed post’s material,
the matrix is epoxy which is a thermoset resin. Heat acts as a catalyst by speeding the natural
chemical reaction of polymerisation. For epoxies, volatile gases are given off during curing as a
result of the heating and evaporation of the solvents used to retard solidifying prior to the
curing cycle [3]. The heat also causes the resin to flow more easily obtaining uniform
distribution. The pressure part of the curing process consolidates the fibre and matrix
components together by removing any air trapped between the layers and excess resin. An
image of an autoclave is shown in figure 5.2.
Figure 5.2: Autoclave device
After lay-up, the pre-cured laminate is placed within a vacuum bag where its fabrication process
is described in detail in section 5.5.3. The bag helps in compaction of the laminas and protects
the resin from being burnt during the curing process. The bag is sealed using a high
temperature chromate adhesive sealant tape. Teflon sheets are placed on either face of the
Chapter 5. Fabrication procedures for proposed composite post 143
laminate so as to prevent sticking to neighbouring surfaces. Natural cork in tape form is used at
the panel’s boundaries so as to prevent resin from escaping, and the vacuum is uniformly
distributed within the bag by means of a breather membrane (airwave). Figure 5.3 shows the
complete vacuum bag and the pre-cured laminate contained within. Also shown are two valves
and two copper thermocouples connected from the vacuum assembly to the autoclave. Each
valve serves a different purpose: one is to maintain and control vacuum in the bag, and the
other is to measure the vacuum within the bag. The thermocouples are required to measure
and correct accordingly the temperature during the curing cycle. One thermocouple is
connected to the panel itself and the other is connected to the mould or tool on which the
laminate and vacuum bag are fixed. The process is monitored by a computer which relates the
real-time data including temperatures, pressure and the performance of the vacuum bags, i.e.
quality of their seal.
Figure 5.3: Laminate in vacuum bag prepared for curing
The cycle duration for the proposed structure would be approximately 8 to 9 hours, climaxing
at a temperature and pressure of 180oC and 9 bars, respectively. The cycle begins with a
gradual temperature increase or ramp stage under vacuum conditions so the volatiles and
water are removed. Following that, the temperature is further increased to the maximum
curing temperature which is held for a couple of hours at maximum pressure also to activate
linking and solidification of the resin, and consolidation of the laminate. At this the stage of high
temperature, the resin becomes solidified however, the resin remains soft with a low stiffness
as a result of the elevated temperatures. The temperature is then gradually decreased to room
temperature over a period of approximately 60 to 90 minutes so as to avoid thermal shock [3],
[9].
The advantage of the autoclave is that practically all types of geometry are achievable in the
curing process. This is due to the isostatic pressure applied on the piece being cured, i.e. the
Chapter 5. Fabrication procedures for proposed composite post 144
pressure on all surfaces of the piece is uniform and equal. The disadvantage of this type of
process is that it is quite time consuming and expends large amounts of energy making it a
costly process.
5.3.2 Hot-Press Plates
The principles of the autoclave process apply to a large extent to those of the hot-press plates.
As with the autoclave, the process is used for prepreg composites, the curing process applies
heat and pressure onto the piece, and the lay-up process (Section 5.2) is the same. The
laminate is covered by a teflon film which is sealed with high temperature resistant tape, taking
into account that the borders need to be sealed sufficiently to prevent resin from escaping.
The hot-press plate device is shown in figure 5.4. It consists of two planar plates that are heated
and are piston driven so as to subject loading onto the laminate. The temperature of each plate
can be controlled separately. Increased compaction of the piece may be achieved by applying a
vacuum through an external device. While the temperature requires a ramp-up and ramp-down
stage, the pressure on the piece remains constant at its prescribed maximum, when possible.
Figure 5.4: Hot-press plate [9]
The advantage of this process over the autoclave is that pieces are cheap to fabricate. However,
only planar pieces of relatively small sizes can be fabricated and furthermore, need to be
mechanised at their borders where the high-temperature resistant tape is situated during
curing as it causes a reduction in thickness of the piece. The pieces fabricated by this process
are ideal for specimen testing for laboratories [9].
• Oven or High-Temperature Chamber
• Quickstep
• Combined Lay-up and Curing Processes
• Electron-Beam Process
• Pultrusion
Chapter 5. Fabrication procedures for proposed composite post 145
5.4 Related Systems for Fabrication of Composite Materials
5.4.1 Clean Room and Storage Conditions of Composite Material
After its production, the clean room is the unique area in which the uncured composite
material is permitted to be exposed. The uncured material has a certain life duration which is
due to the natural polymerisation or solidification of the resin and is accelerated by increase in
temperature. The process of polymerisation is delayed by storing the material in a freezer
chamber. To prevent damage or liquid freezing on the material surface, the material is
contained in an airtight plastic bag.
It is therefore important that composite materials are exposed to precise conditions of
temperature and humidity ensuring the best possible properties for that material. The clean
room contains a double door system for entry and exit. This prevents direct access to the
exterior and interior creating a space that can have its cleanliness controlled and inspected
more easily. The room is airtight and is served by a climatisation-ventilation system that
controls ambient parameters which are altered with respect to the conditions required by the
material. This system limits the quantity of airborne particles by constant air circulation thereby
controlling and reducing contamination of the exposed uncured composite material.
In relation to the clean room conditions required for the epoxy resin used in this project (Hexcel
M21E), its storage life, under freezing conditions of -18oC, is exactly six months from the day of
its production. Its exposure to clean room conditions reduces its life duration or tack life to a
total of 10 days or 240 hours. The most optimum ambient conditions for this material are a
temperature of 23oC and a relative humidity of 50%. The principal data provided with the roll of
the unidirectional material from the manufacturer is evidently, the material type, roll length,
batch number, section of batch, date of production, and finally a visual check of the entire role
length with irregularities, if any, highlighted and their corresponding length locations.
Irregularities of the material include fibre accumulation, inclusion of fibre debris, overlapping,
blisters at the surface, and resin depletion causing a reduction in thickness of the tape. A
timeline is also provided with the roll that obligates personnel handling the roll outside freezing
conditions to subtract the amount of time (in hours) from the initial 240 hours. As well as the
clean room, the material is exposed intermittently during its transportation and must be
factored into the timeline once arriving to its final destination, if not done so already before
[31].
Composite structures of high structural responsibility such as in aeronautics are required to
conform to such criteria. However, if the material is not cured within the life duration, a
possible reclassification of the material by the manufacturer can extend its life longevity and
permit its usage in such structures of high structural responsibility. If the expired material is not
reclassified, it may still be laid-up and cured as pieces but of lesser structural responsibility.
Chapter 5. Fabrication procedures for proposed composite post 146
5.4.2 Automated Cutting
Before lay-up, plies must be cut into suitably sized patterns so as to minimise waste of the
material. While composite materials can be cut to size manually, automated cutting is utilised
for prepreg tapes and fabrics on an industrial scale. The device shown in figure 10 contains a
spool from which the roll is hung and allowed to unwind over a conveyor-type table that moves
the cut patterns onto a receiving desk. Over the conveyor is a portal structure with a moveable
cutter head that cuts the material to the prescribed measurements inputted to a computer
with geometrical CAD software which can optimise the amount of material cut in a given
length. The cutter is capable of moving about two axes which include movement along the
length and over the width of the unrolled material. The automated device has the ability to cut
large quantities of material and reduces the amount of material cut to waste. Precision is
increased as there is no deviation of the angles cut in relation to the fibre direction [9].
Figure 5.5: Automated cutting device [9]
5.5 Process Most Suitable for Proposed Post in Composite Materials
The following section describes the fabrication steps felt by the author most suited in
completing the proposed post structure in composite materials. The overall process in general
consists of cutting of prepreg, lay-up, curing, mechanisation, bonding, and post-curing. Both the
cutting and lay-up steps can be carried out manually or automated and depends on the
availability of the automated devices. In any case, both types of production methods are
described in the relevant steps. Within this general description of the process are a number of
steps that must be completed including preparation of the clean room and utilities,
intermediate and finish vacuum bag, mechanisation of the structure, and finally bonding and
post-curing of subcomponents. As previously outlined, the structure is made up of four
subcomponents: two plates with mechanised holes and two UPN beams. The plate
Chapter 5. Fabrication procedures for proposed composite post 147
subcomponents of the structure are fabricated as flat laminates with no requirements for
curvatures and therefore can be placed on the flat steel moulds during the autoclave curing
process. However, the beam components require a rectangular mould preferably made of
aluminium as it is easier to mechanise, and sized according to the beams internal dimensions.
The mould’s edges are slightly rounded so as to give the finished composite beam a slight
internal curvature at the meeting of their flange and web parts.
The materials required to complete the structure includes primary materials that are actual
components of the structure, and auxiliary materials that are utilised in its fabrication but not
components of the final structure. Primary materials include:
• Preimpregnated fibre composites
o Unidirectional fibre/epoxy (268 g/m2)
o Woven fibre/epoxy (300 g/m2)
• Adhesive
o Film adhesive: FM300M (epoxy based), (150 g/m2)
Auxiliary materials include:
• Vacuum bag assembly
o Teflon fabric and film
o Breather membrane
o Plastic bagging film
o Cork tape
o High temperature resistant tape
o Chromate sealant tape
• Peel-ply: EA9895
5.5.1 Preparation
As already mentioned, the first step of the fabrication process is preparation. This includes
setting the ambient parameters such as temperature and humidity to required levels as
prescribed by the composite material used in fabrication. If the material stored in a roll, it must
be removed from the freezer chamber, but kept in its airtight bag, 24 hours before
cutting/lamination can be carried out. This time allows the entire roll to acclimatise with the
ambient temperature increasing the tack or stickiness of the material. The clean room must be
inspected and cleaned if necessary. Also, utilities such as the moulds and plates used in the
curing process need to be completely clean, i.e. no cured resin, chromate sealant tape, or high
temperature resistant tape resulting from previous usage in curing cycles. After each cycle the
plate must be cleaned by using an organic solvent such as acetone so as not to generate
Chapter 5. Fabrication procedures for proposed composite post 148
imperfections on the surface of the cured laminate which could induce stress concentrations
over these areas when loading is applied. If automated cutting and lay-up are to be used, the
relevant CAD data should be compiled and completed beforehand, so as not to waste time
between steps. If manual cutting is to be used, patterns for cutting must be completed before
this step can commence. Manual lay-up requires that the ply configuration be listed in hard-
copy or written out so that during lay-up when each ply is placed onto the mould, it is struck off
the configuration list, maintaining a clear order of lay-up for the technician. Finally, an
intermediate vacuum bag is prepared according to the dimensions of the laminate to be
fabricated.
5.5.2 Cutting and Lay-up
As highlighted already, the lamination system may be automated or manual. As with
automated cutting, the automated lay-up system contains a spool or drum device on which the
prepreg roll is hung. The material is unwrapped from the supply spool and fed down onto the
mould by the head device while its protective teflon backing is removed from the prepreg
directly after contact is made between the newly unwrapped prepreg and the surface. The
removed backing is rolled up onto a second spool called the take-up spool. The system contains
an automated cutter capable of movement through multiple axes. The layer is orientated and
cut to the appropriate dimensions, and laid-up onto the mould, adhered and compacted to
certain degree by a soft roller that applies pressure onto the lamina as shown below in figure
5.6. The cutting mechanism is situated close to the compaction roller which is capable of
cutting the prepreg without cutting the protective backing [30].
Figure 5.6: Automated lay-up process [30]
Chapter 5. Fabrication procedures for proposed composite post 149
Manual lamination requires that the axial directions for fibre orientation are clearly defined
beforehand so as to prevent confusion of the configuration during stacking. To prevent resin
sticking to the plate/mould surface, the area on which the prepreg is to be applied is either
covered with a teflon film or treated with a demoulding agent. For complex geometries the
demoulding agent is best suited while the flat mould for the used for the plate components is
more efficiently covered with teflon film. As previously mentioned, the laminate configuration
sequence is written out and each ply marked-off accordingly throughout lay-up so as to avoid
confusion. Each layer after lay-up is compacted manually by a spatula made preferably of teflon
material. The process involves applying pressure onto the prepreg commencing always at the
centre of the lamina and moving outwards in the same direction of the fibre so as to prevent
separation from the matrix. This process is repeated until no visual pockets of air remain
between the layers.
The pre-cut ply patterns can be made automatically or manually and, before they are laid-up,
should be stored in separate groups relevant to their orientations in the laminate configuration.
The cutting of the beam plies is slightly more complex in that the change in geometry over the
laminated surface needs to be factored into the cutting of the plies, i.e. for every layer applied
to the lay-up, the radius at each corner of the laminate increases and as a result, the next ply
requires a slight increase in its width to fully cover the previously laid-up ply. Intermediate
vacuum is applied at lay-up intervals of four layers and maintained for duration of 10 minutes,
this process furthermore compacts the plies by removing the remaining interlaminar air
pockets.
5.5.3 Vacuum Bag Assembly
On completing the lay-up process, the laminate needs to be prepared for curing in the
autoclave. This preparation involves fabricating a vacuum bag in which the piece is contained.
During the curing cycle, the rectangular moulds are supported on a flat tool or mould, similar to
those on which the plate components are laid-up, so as to facilitate transfer from the clean
room to the autoclave and mounting of the vacuum assembly. In relation to the vacuum
assembly, the sequence of materials from the mould upwards includes teflon fabric or film,
piece, teflon film, breather, plastic bagging film, all of which are shown schematically in figure
5.7. Other components include high temperature sealant tape, natural cork tape, high
temperature resistance tape, vacuum valve and vacuum line.
Chapter 5. Fabrication procedures for proposed composite post 150
Figure 5.7: Sectional view of vacuum bag assembly
A complete step by step process of the vacuum bag assembly is described below. The images
accompanied with the steps are of a small composite laminate piece which is not an actual
component of the proposed structure but is only representative of the process, which would be
exactly the same for the proposed plate component of the composite post structure
differentiating only dimensionally.
1. Teflon film is placed perfectly flat onto the tool and
above that is the pre-cured laminate. The film is fixed
to the mould by high temperature resistant tape
preventing it from displacing during the cure cycle.
Consideration is given to positioning of the
teflon/laminate on the mould so that the vacuum
valves and sealant tape fit appropriately on the
shared surface.
2. High temperature sealant tape is adhered onto the
mould surface. In order to form a rectangular sealant
as shown in the image, four lengths of tape are cut
with the protective layer kept in place. One end of
the tape’s length is stuck to the mould and pulled so
that it is taut. The second end is then stuck. The
remaining length of tape is adhered by applying
slight pressure with one finger over the protective
layer starting at the centre and moving outwards to
the ends. The tape is overlapped at the corners.
Chapter 5. Fabrication procedures for proposed composite post 151
3. Cork tape is adhered directly against the laminate so
as to prevent resin escaping during curing. Attention
is required at the area where two cork tapes meet as
any slight gap between the two will be exposed by
the applied pressure during the cycle and resin will
escape. Depending on the thickness of the laminate,
the cork can be stacked on top of each other.
Because the rate of compaction of the cork is greater
than that of the prepreg during curing, the cork
should be thicker than the laminate so as to factor
this difference between the two components.
4. Thermocouples are fixed by high temperature tape
onto the assembly. One is fixed onto the edge of the
laminate and the second is fixed onto the mould
surface. The protective layer on the sealant is broken
at the point where the thermocouple exits the
assembly. An additional piece of sealant is fixed
beyond the original length and the wires of the
thermocouple are firmly embedded into the sealant.
Strips of sealant are fixed above the area where the
wires exit the assembly also. The broken protective
layer is reinstated onto the sealant.
5. Teflon film is placed over the prepreg and cork tape.
It is secured by high temperature tape. This prevents
the resin sticking to the breather membrane. Fibre
glass fabric tape is placed under the area in which
the valves are to be positioned. This helps absorb
stresses around the areas of the valves and
distributes the vacuum evenly.
6. The breather membrane is cut to size and placed
over the assembly but maintained within the
boundaries defined by the sealant tape. The
breather is a fabric that can be detrimental to the
bags performance if loose fabric strands come in
contact with the sealant and form a bridge across it.
This is one of the main reasons the protective layer is
kept on the sealant. The bottom parts of the vacuum
valves are placed upon extra breather or fibre glass
cushions.
Chapter 5. Fabrication procedures for proposed composite post 152
7. At this stage the plastic bagging film is ready to be
fixed onto the assembly. The first points of contact of
the bagging film and sealant is at two proximate
corners. The protective layer is removed from these
corners, film fixed at the first corner then drawn
tightly and fixed at the other end. This is repeated
for the remaining two corners. Between the corners
the bagging is lifted, the protective layer removed
and then fixed to the sealant beginning at the centre
and moving to the corners applying slight pressure
with one finger.
8. The vacuum line is connected to one of the valves to
induce vacuum (560 mmHg) into the bag. A teflon
spatula of smooth edges is ran over the areas of
sealant covered by the plastic so as to fasten the bag
completely. A vacuum gauge is connected to the
other valve to determine if the bag fully airtight. If
more than 0.17 bar of vacuum is lost after five
minutes, the bag must be repeated. All loose objects
are kept away from the vacuum bag so as not to risk
puncture of the bagging film.
Alterations to this process are made for the vacuum bag of the composite beam component
due to its pronounced 3-D profile. The bagging film must incorporate folds so as to eliminate
unwanted stresses in the bagging film that may cause rupture during the curing cycle. The area
of film is cut larger than the planar area mapped out by the sealant tape. The size of area cut
depends on the thickness of the piece/mould within, and the complexity of the profile, i.e. if it
contains apertures or sharp edges. The folds are sealed at the edges of the assembly by
transversal lengths of sealant connected to the sealant fixed onto the flat mould surface.
5.5.4 Curing
When the vacuum bag is determined to be completely sealed, it can then be introduced into
the autoclave. As part of the preparation, the sufficient pressure needs to be accumulated by
the compressor and stored in the device’s pressure vessel. The pressure amount required in the
process is related to the pressure applied in the cycle. The compressor is left switched on during
the cycle to ensure a sufficient amount of pressure is available at all times. The mould, piece
and vacuum assembly are introduced into the autoclave by a train-type structure shown in
figure 5.8. The vacuum and gauge lines are connected to the valves, and the thermocouples are
connected to the autoclave. If more than one piece is to be cured, care is required in that the
appropriate pairs of vacuum lines and thermocouples are connected to their proper locations.
Chapter 5. Fabrication procedures for proposed composite post 153
The program cycle is entered into the device’s computer system. The functionality of the
autoclave should be tested before the cycle is carried out. This involves verifying that all
vacuum lines are working and that the thermocouples read the correct temperature (room
temperature). If all systems prove functional, the autoclave is closed, secured and the cycle is
activated.
Figure 5.8: Train assembly entering autoclave
5.5.5 Mechanisation
In the case of Model 2, before all the components are unified to complete the structure, the
two plate components are mechanised to incorporate holes throughout its length but with
exception to the area around the fixed support. The individual plate components facilitate
mechanisation more easily than carrying out the same operation on the complete structure.
Cutting is one of the unique mechanisation methods applicable to carbon fibre composite
structures and the composite structure presented in this projected has been designed in such a
manner so as to incorporate its mechanisation limitation. The cutting method felt most suitable
is with a waterjet cutting device. Using CAD software, the waterjet cuts the desired areas to be
removed from the laminate. The main advantage of this type of mechanisation device is that it
does not cause burn marks, cracking, excess burr or thermal distortion. Figure 5.9 shows the
plate component before and after mechanisation.
Chapter 5. Fabrication procedures for proposed composite post 154
Figure 5.9: Before and after mechanization of plate component
5.5.6 Bonding and Post-Curing
Component areas that are to be joined together through adhesive bonding are given a special
finish after the lay-up process. Peel-ply is added to those specific surfaces that are to form parts
of union. These surfaces include the outside surfaces of the flanges of the UPN beams and on
one side of both plates at a thickness of 100 mm from the length’s edge. It must be noted that
the decision of which side of the plate components are picked is irrelevant as their ply
configurations are symmetric about the laminates’ middle surface. The peel-ply is removed
after curing. It leaves a surface that is clean, highly rough and chemically active ready for
bonding. With a temperature resistance of 180oC, the wet peel-ply EA9895 is considered
suitable for surface preparation. The adhesive is applied immediately after removing the peel-
ply so as to minimise contamination of the surface. FM300M film adhesive, with its
aeronautical applications, is deemed appropriate for bonding the post’s composite material
subcomponents. The adhesive is an epoxy-based or thermoset material that has the almost
same curing temperature (175oC) as that of the prepreg. The entire structure is returned to
autoclave for post-curing cycle to create an entirely unified structure as shown in figure 5.10.
The curing specifications in terms of maximum pressure applied is approximately 7 bars, with a
cycle of 30 to 60 minutes of a heating ramp-up reaching 175oC maintained for one hour
followed by a cooling ramp of another 30 to 60 minutes. Storage temperature of -18oC is
required, as any exposure above this will advance the cure state of the adhesive. The material
has a shelf life of 6 months from date of fabrication and 10 days at 32oC. Its service
temperature ranges form -55oC to 150
oC [32].
Chapter 5. Fabrication procedures for proposed composite post 155
Figure 5.10: Complete post structure made in carbon fibre/epoxy composite material
Figure 5.11 shows schematically the steps involved in the fabrication process of the post in
composite materials.
Chapter 5. Fabrication procedures for proposed composite post 156
Figure 5.11: Fabrication process of post structure in composite laminates
Preparation
Cutting
Lay-up
Vacuum Bag
Curing
Mechanisation
Bonding
Post-curing