Review on Composite Defects in Aircraft Part Manufacturing

D. Chin2, A.R. Othman†1 and S. Kamaruddin1

1School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia

2Spirit AeroSystems (M) Sdn Bhd, Malaysia International Aerospace Center (MIAC), Lapangan Terbang Sultan Abdul Aziz Shah, 47200 Subang, Selangor, Malaysia

ABSTRACT

For the past twenty years, efforts have been putting in to make the aircraft structures lighter

using composite materials; this effort has since been continuing to grow, primary structures are

increasingly made from advanced composite materials. As production rates for composite parts

are expanding very markedly, cost effectiveness for advanced composite parts manufacturing

must focus on both the manufacturing processes and at the same time target to achieve zero

defects, zero rework, zero repair, and zero scrap. Thus, this paper reviews the typical defects

identified in composite materials and the cured composite parts. In addition, the methods as

practiced by the aircraft composite part manufacturers to minimize these defects during the

composite parts manufacturing activities have also been introduced. Among all defects, the most

important manufacturing defect that is likely to occur in practice is porosity; the presence of

multiple small voids. Voids management methodologies for both autoclave, and the new VBO

(Vacuum-Bag-Only) oven cure processes - the advanced composite parts manufacturing

processes, were critically presented in this paper.

Keywords: Composite defects; Porosity; Voids, Composite processing; Permeability.

INTRODUCTION

† Corresponding author. Tel.: +6(0) 4 599 8320

Email add.: merahim@eng.usm.my [A.R. Othman]

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The fabrication of composite material in the aircraft industry, where the resin is combined

with the fibre materials, is via three methods [1]; there are wet resin layup, resin infusion and

pre-impregnating processes [2]. Due to the lower material cost, wet resin layup may still be used

in the fabrication for composite components for the general aviation industry, however this

process has reduced in its application over the recent years. The advanced composite material

used for commercial and military aircraft industries focuses on the later two, especially on pre-

impregnating process.

The material selection criteria depend on the finished composite component quality and

the structural requirements, and partly depend upon the size of the composite component

requirement as well. For example, in the general aviation industry, composite component that has

lower structural load carrying requirement such as interior panels, these components may be

manufactured by wet layup or resin infusion in order to reduce the manufacturing cost. The resin

may then cure at ambient temperature or elevated temperature. To achieve high quality laminates

using the wet layup technique can be extremely difficult, therefore the method is used when the

lower strength finished components can be tolerated and allowed for in design.

Composite part manufacturing processes involve a critical phase as the curing of composite

is completed. This occurs when the cross-linking phase of polymers takes place; the full load

carrying capability of the component is established during this phase. However the mechanical

property, i.e. the load carrying capability, could be reduced due to defects introduced during the

material and component manufacturing processes.

The following sections discuss the common defects related to composite components

which are introduced during the manufacturing processes. The current highly practiced

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manufacturing processes related to aircraft composite component fabrication are also discussed,

with their associated challenges are identified.

MANUFACTURING DEFECTS IN COMPOSITE

Higher quality composite components as in the typical load carrying aircraft components

are usually fabricated using processes involving positive pressure; techniques such as hot

pressing, RTM or autoclaving are typically used. These processes ensure the best control of fibre

to resin ratio which are the predominant factor for controlling the predicted load carrying

properties within the cured composite structure and therefore achieving the desirable mechanical

properties. Typically, other than the materials used for the RTM process, which are dry fibres or

woven fabric and wet resin, material in the form of prepreg [3], is usually used for hot press and

autoclaving processes, where all of these processes involve positive pressure during curing. The

quality of the last three mentioned processes; i.e. hot press, RTM and autoclaving, depends

strongly on the compaction pressure applied at the correct moment during the heating process.

The details of the technique adopted will depend upon the resin system used.

During all these manufacturing processes defects can be introduced into the composite

material, where the size and frequency of occurrences of each type of defects depends upon the

particular process cycle. The following defines the typical defects introduced in the sequence of

a manufacturing process cycle:

1. Fibre defects [4-6]. The presence of defects in the fibres themselves is one of the ultimate

limiting factors in determining strength of a cured laminate, and sometimes faulty fibres

can be identified as the locations from which damage growth has been initiated. These

defects can be present in fibres as supplied or introduced during downstream processes.

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2. Wavy fibres [4, 5]. These are produced by in-plane kinking of the fibres in a ply and can

seriously affect laminate strength. This defect may be introduced during the material

manufacturing process at material supplier site for prepreg material, or during composite

parts manufacturing period such as wet layup and resin infusion processes, where fibres

in-plane kinking occurred.

3. Moisture entrapment [7-12]. The epoxy resin and many other resin systems absorb

moisture from the environment which turns to condensation as the composites laminate is

heat-cured. This causes micro-bubbles which may infuse into larger voids. Voids will

degrade the quality and the appearance of the structure produced. The typical practice

guideline is that materials should not be exposed to the environment unless the

environment is within the required humidity and temperature limits. In the case of

prepreg where cold storage is required to preserve the shelf life, during the removal of the

prepreg material from the freezer, the material should be allowed for thawing to reach

room temperature before the seal on the storage bag is broken and the material is

removed. Thaw time will vary depending on the amount of material to be thawed; the

more the material, the more time is required for thawing.

4. Ply/Fibre misalignment [5]. This is produced as a result of wrong fibre orientation

introduced during the kitting process or during the lay-up of the component plies. This

alters the overall stiffness and strength and may introduce warping and deformation of

cured laminate, this may also cause local changes in volume fraction by preventing ideal

packing of fibres.

5. Foreign bodies inclusions [4]. For example, the prepreg backing paper or the poly film,

blade, and etc were unintentionally left entrapped between the laminates during the layup

process. These foreign objects typically cause delamination within the cured laminates or

become stress concentration of the composite structure.

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6. Bridging of laminate [13] is due to layers of composite plies are not placed down

properly in place, especially over complex curvatures, concave surfaces and tight corners

causing poor draping of the composite plies, thus introducing high resin concentration [4,

13, 14] and the same time ply delamination [13, 15] with high voids and porosity content.

7. Incorrect fibre volume fraction [16] is due to excess or insufficient resin. Local variations

in volume fraction will always occur but large departures from specifications may be

caused by inappropriate process conditions, such a bridging of the laminates,

voids/porosity within the resin, and vacuum leakages during the curing process.

8. Bonding defects [17, 18]. This typically applied to composite structure that contains core

materials or processes require co-curing of multiple laminates to form higher level of

assembled cure composite structure. During the manufacturing process, where these

components are bonded together, it is possible for defects to occur in the bond line which

may due to incorrect cure conditions for the laminate or contaminated bonding surface on

the adhesive or surfaces to be bonded.

9. Incompletely cured matrix [19] is due to incorrect curing cycle or faulty material.

10. Ply cracking [20, 21]. Thermally induced cracks occur with certain ply lay-ups due to

differential contraction of the plies after cure.

11. Delaminations, disbonds [14, 19, 15]. These are planar defects usually at ply boundaries

and are fairly rare during the manufacture of the basic material but may be produced by

contamination during layup, or by bridging as mentioned previously or by machining

process after the curing process.

12. Voids (porosity) [7, 14, 8, 4, 19] are due to volatile formation [11] within resin, or

chemical shrinkage within the resin during the curing process, or air and moisture

entrapment as mentioned earlier during the handling of material. These voids (porosity)

are not efficiently removed prior to the gelation of the resin during the curing. Voids

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contain within cured resin have major negative impact on the structural integrity in term

of reducing the physical and mechanical properties of the composite component, as high

voids content will result in [22, 23]; (i) weaker interfacial strength due to inadequate

adhesion and therefore causing disbonds and delaminations between laminates, (ii)

mutual abrasion of fiber as the cushioning resin that separating fibers is replaced by the

voids, (iii) crack initiation and growth causes by voids, and (iv) increased moisture

intake. Based on investigation performed by Zhu Hong-Yan [24] and Ling Liu [25],

voids affect the mechanical properties of cured composite laminates; both the strength

and modulus decrease with increasing porosity. Inter-lamina shear strength (ILSS),

flexural strength and flexural modulus of cured composite laminates are the properties

with higher void sensitivity, and similarly, tensile strength and tensile modulus decrease

with the increase in the void content [26]. As stated by Ray [19], typically with the

reduction in void content from 40% to 10% will allow the flexural strength to increase by

nearly three-fold, and almost doubles the modulus. The interaction between voids and

moisture has a severe impact on the transient hygro-elastic stresses at both microscopic

(fiber and matrix) and macroscopic (ply) scales [27].

Figure 1 describes typical process flow of an autoclave composite part manufacturing process,

where the above mentioned defects are usually introduced at particular process steps.

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Figure 1: Flow diagram of autoclave-based manufacturing process & the occurrence of composite defects [28, 29].

Lay out roll

Cut kit

Core stabilization

Prepare tool

Lay down ply

Debulk

Apply vacuum

Apply pressure Cure

Load batch

Form vacuum bag

Unload batch

Remove vacuum bag Remove component

Automated kit cuttingLayup

Autoclave loading

Collect batch

Autoclave cure process Unloading + Debagging

To trim & drill

Thawing

Moisture entrapment

Automated kit cutting

Layup

Autoclave loading

Autoclave cure process

Unloading + Debagging

Trim & drill

Thawing

Manufacturer

Wavy fibres, Fibre defects, Fibre misalignment

Ply misalignment

Foreign bodies, Bridging of laminate, Void

Incorrect fibre volume fraction, Bonding defects, Incomplete cured matrix, Ply cracking, Delamination disbands, Void

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KEY PRACTISE FOR DEFECTS REDUCTION

Composite laminates which has been layup [30, 31] on top of the corresponding mould

surface may be placed inside a vacuum bagging assembly. As vacuum is being created within the

sealed vacuum system, the atmospheric pressure increases against the external surfaces of the

vacuum system, causing the pressure inside the bag decreases while the outside pressure remains

constant at one atmospheric pressure (14.7 PSI or approximately 30"Hg) [30, 31]. This creates

compacting force against the composite laminate. Pressurizing with force acting on a composite

lamination serves several functions. Firstly, it removes trapped air between layers during the air

extraction process. Secondly, as vacuum is created within an enclosed composite laminate, the

external force compacts against the fibre layers and fibre bundles, preventing the shifting of fibre

orientation during cure. Thirdly, the vacuum system creates a barrier that reduces moisture intake

of the composite laminate. Finally, the vacuum bagging technique optimizes the fibre-to-resin

ratio in the cured composite part. These advantages have for years enabled aerospace and other

composite industries to maximize the mechanical properties of cured composite parts.

Good practices are always the fundamental rule of thumb applicable during the

laminating/layup of the composite laminates before the curing process; they will further ensuring

the voids (porosity) formation is minimized. Typical good best practices as implemented by the

composite parts manufacturers during the laminating/layup activities are highlighted as follows.

Minimize the exposure of moisture to the composite material

Per the discussion previously, composite materials should not be exposed to the

environment unless the environment is within the required humidity and temperature limits. In

the case of prepreg where cold storage is required to preserve the shelf life, during the removal of

the prepreg material from the freezer, the material should be allowed for thawing to reach room

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temperature before the seal on the storage bag is broken and the material is removed. This will

avoid the condensation of moisture on the material surfaces, which will eventually migrate into

the material.

Assist in moving out entrapped air

In order to achieve voids (porosity) free laminate and ultimately the best quality cured

laminate, the removal of entrapped air during the layup is essential; this can be achieved by

mechanical mean through rubbing of every ply laid down during the layup activity. In the

composite part manufacturing industry, typically plastic spatula, rubbing stick or rollers are used

to help manually pushing the trapped air moving towards the edges of the laminate. In addition,

another mechanical mean of removing air can be further improved by using vacuum bag system

to perform debulk [28, 32, 33] activity. Debulk is an intermediate vacuum evacuation step

involving the utilization of atmospheric pressure and vacuum suction to remove trapped air,

where one atmospheric pressure is acting on the laminate stack to squeeze out the air towards the

edges of laminate and the vacuum system provides continuity of air removal via the available

breathing channel. Typically as practiced in the composite parts manufacturing industry, the first

ply of layup laminate should be vacuum-debulked to the mould surface. Vacuum-debulking of

subsequent plies may be necessary to ensure removal of air trapped during the lay-up process,

where the frequency of debulking depends on part size and complexity. Composites components

manufacturers may vary the debulking frequencies, periods and methodologies based on their

experience.

Avoiding locking/trapping of air path ways

In the study performed by Tavares et al. [34], it has mentioned that the air permeability of

semipregs or in a different terminology, EVaCs (Engineered Vacuum Channels) [35] are

designed to improve air path ways within the materials themselves. Depending on the

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manufacturer, the strategy of resin distribution varies, some promoting the increase of the in-

plane air permeability, others of the through thickness air permeability. However, localized air

escape path way on the surfaces on the laminates next to the vacuum bagging materials can be

locked/trapped if insufficient breathing media such as breather cloth, and release films are used;

typically this phenomena occurs between the mould tool surfaces that contact the composite

laminate. The use of peel ply or release film such PTFE [34] release film as separator between

the composite laminate and mould tool surface can be used to minimize localized surface lock

off which will allow the trapped air to move out via the breathing channel within the vacuum bag

system.

Promoting the breathability of the composite laminate

Further improvement of the breathability of the composite laminate is by introducing fibre glass

roving [36], connecting the laminate edges to the breather cloth; this will ensure the breathing

channel is kept opened. Alternative similar method is allow net trimming on composite laminate

edges, in which these edges are then connected to the breather cloth [33, 37] using rolled up fibre

glass cloth/fabric as shown in Figure 2.

Assist the external pressure to work more efficiently

The use of caul plate or intensifier [12, 38] together with the vacuum bag system is an

improvement option, which will improve the efficiency and evenness of pressure acting onto the

composite laminate, and therefore improving the voids removal process.

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To vacuum supply

Vacuum bag

Breather clothRelease film perforated

Sealant tape

Composite laminate

Vacuum port

PTFE release filmFiberglass cloth

Figure 2: Vacuum bag construction for VBO Vacuum Bag Only process with edge breathing features [33, 37].

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Mould tool

Ensuring the vacuum integrity of the vacuum system is at 100%

Prior to the loading the vacuum bagged composite laminate for curing, a vacuum hold at

full vacuum (minimum 28 inch mercury (in Hg) at sea level) is required. Full vacuum should be

within 2 in Hg of absolute vacuum for the given altitude. Vacuum hold times will depend on the

part size and complexity, but general recommendations are 4 hours minimum hold for any

uniform thickness parts smaller than 2 ft x 2 ft (0.6 m x 0.6 m) and 16 hours minimum hold for

larger or more complex parts [37]. Composite components manufacturers may vary the vacuum

holding time based on their experience. Similarly a vacuum leak check should be performed

prior to cure. The test should not show more than a 2 in Hg vacuum loss in 5 minutes. Good

vacuum integrity will promote air trapped within the laminates to be effectively removed during

curing, and therefore reduce the chance of porosity/voids formation within the curing laminates.

VOIDS MANAGEMENT FOR VBO OVEN CURE PREPREG MATERIAL

Though VBO prepreg materials are improved material with built in features that will ease

trapped moisture and air removal, the full potential of the cured composite component cannot be

developed without reliable and reproducible manufacturing processes for the composite

component fabrication [39]. Voids (porosity) affect the finishing appearance of the cured

laminate, but they have significant negative impact on the quality in term of lowering the load

carrying capability of the cured structures. Voids (porosity) management is a typical challenge

in any composites part manufacturing process; therefore robust composite part manufacturing

process for voids (porosity) management is of strong interest to the composite part

manufacturing industry.

Understanding of the trapped moistures, volatiles and air escape routes, which is related to

the air permeability through the composite laminates, is critical to ensure voids free cured

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composite laminates. As air tends to flow towards low pressure areas, mechanical mean via

effective vacuum evacuation of the trapped moisture and air is the key to successful voids

management, and therefore providing reliable and reproducible manufacturing processes for the

composite component.

The air permeability however depend on many factors, these include the physical and

chemical characteristics of the prepreg material and the related process system. The physical

properties relates to a few elements, these include the tackiness of the resin, the fibre architecture

in term of the weave patterns, ply orientation during the layup process, and how each ply

connected at ply edges such if ending plies are close edges joining or overlapping joining. Other

properties also include the thickness and plies quantity of the laminate, the laminate size and its

shape in term of complexity, the mould tool profiles (whether if it is a male, a female tools or

complex curvatures), the vacuum bag construction, if debulk process is introduced and its

frequency applied, and the integrity and quality of vacuum and the associated pressure during

curing. On the other hand, the chemical properties are related to the chemical reaction, the

morphology during cure which is driven by temperature change. The natural routes for air to

escape, i.e. the air permeability for any laminate, are via two channels, they are the permeability

through the ply thickness direction [34, 40], and the permeability through the plane of plies

direction, i.e. the in plane permeability.

In the study conducted by Xin [41] on air permeability, the results demonstrated that both

air permeation in term of in-plane and through-thickness can be quantitatively measured, where

the compacting pressure and temperature have important effects on the air permeability of the

prepreg stack. In the case of compacting pressure, as the applied compacting pressure increases;

the in plane and through-thickness air permeabilities for the prepreg reduce, and then gradually

reach a constant value. This is explained as under pressure condition, the dimension of the air

paths will greatly reduce; and therefore decrease in the gas permeabilities. On the other hand, the

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influence of temperature on gas permeability is mainly contributed by the effect of the viscosity

of the resin. As the temperature increases, the viscosity of the resin will decrease, and as a result,

the resin flows with low viscosity can easily seal off the gas paths; this is more significant under

higher compacting pressure condition. Beyond the threshold of temperature, the air paths in the

prepreg stack for gas flow are entirely sealed off, and it directly results in the cease of gas flow.

In the same study for the same prepreg system, the in-plane air permeability is two orders of

magnitudes higher than the through-thickness one. The schematic of gas flows through prepreg

stacks is shown in Figure 3.

Figure 3: Schematic of gas flows through prepreg stacks, (a) in-plane gas flow, (b) through-thickness gas flow [41].

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As proposed by Arafath [42], void management can be achieved in two ways, firstly by

controlling void source, and secondly by improving the void sinks. Void source is related to the

mechanisms that generate voids, which include, the condensation of moistures onto the prepreg

materials due to insufficiently thawed of the frozen material, trapped air introduced during layup

activity, volatiles released from the resin, and vacuum bag or mould leakage. On the other hand,

void sinks are the mechanisms that remove or mitigate voids, these include mechanism that

avoiding physical lock off of the air path ways of the vacuum bag system, and efficient vacuum

system construction to promote good air evacuation of the laminate; a well established void sinks

will therefore promote the air permeability within the laminate. Therefore an effective void

management should be strategized to minimize void sources and maximize void sinks, with all

gas evacuation path ways should be kept opened, and vacuum application must be continued to

the maximum extent until trapped air and volatiles are evacuated.

There are several theories in the literatures about phenomena that promotes the removal of

voids; however the complex characteristics of these phenomena and the inter relationship are not

yet fully understood. However a successful processing of the OOA oven cure prepreg material

will require the solutions of a variety of problems. Those are:

1. Stable and robust manufacturing processes to maintain low voids (porosity) content. For

the aircraft load carrying structures, the allowable voids (porosity) content within the

composite monolithic laminates is below 1% [43, 25] for the primary load-carrying

structures.

2. Manufacturing solutions that prevent the occurrence of voids (porosity) on the composite

core face sheets which attached to the honeycomb core surfaces or porosity within the

bond line that binds the composite core face sheets and the honeycomb core [ 34, 40].

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3. Manufacturing solutions that ensure good quality cured composite laminates which have

complex curvatures [14] applicable to both monolithic (solid composite laminate) and

core sandwich structures, including thick laminates.

Conclusion

Vast majority of advance composite structures in production today are still cured using

autoclaves [44]. With autoclave curing method, the autoclave pressure compresses the trapped

air, moisture and volatiles within the resin of the composite material into smaller size at micro

level and further diluted into the resin before the cross-linking phase of resin. With proper

process management, autoclave curing process is able to produce cured laminate with very low

voids (porosity) content.

In the drive for lower manufacturing cost and the solution for large component size

constraint, a considerable amount of effort has been put into the areas focusing on the capability

of composite parts manufacturing moving away from the high-cost and size constrained

autoclave process. With the potential cost savings associated with OOA manufacturing, there is a

growing interest in the use of this process for the aerospace industry. There are some great

amount of research works done in the area of void growth and dissolution in composites

processing and there are fairly well established theories regarding the resin pressure required to

keep volatiles in solution. However, these are typically applicable to autoclave process where

pressure is available. Although void formation, flow and compaction theories and models

developed for autoclave-cured laminates will still be applied for VBO process, the extent to

which the reduction in compaction pressure affects part quality and whether current vacuum

bagging methods are appropriate for the specially designed prepregs is still poorly understood.

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The removal of air, moistures and volatiles through vacuum evacuation has received less

attention and some of these techniques may still be kept secret in most companies.

Acknowledgements

The authors would like to thank the Ministry of Higher Education, Malaysia, and Universiti

Sains Malaysia for the financial support for this research work.

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