chapter 5 fabrication procedures of the proposed post in...

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


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


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.


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


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


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


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.


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


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


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]


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.


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.


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.


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.


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.


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].


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.


Figure 5.11: Fabrication process of post structure in composite laminates

Preparation

Cutting

Lay-up

Vacuum Bag

Curing

Mechanisation

Bonding

Post-curing

chapter 5 fabrication procedures of the proposed post in...

Documents