View
28
Download
3
Category
Tags:
Preview:
DESCRIPTION
1233456
Citation preview
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]
1
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
2
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.
3
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.
4
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
5
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.
6
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
7
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
8
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
9
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.
10
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].
11
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
12
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
13
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].
14
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].
15
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.
16
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.
References
1. S. C. Joshi, Pragmatism in Semi-Steady Modular Finite-Grid Simulation Methodology for Aerospace Composites Manufacturing, Simulation Modelling Practice and Theory, 17, 839–849, 2009
2. K. F. Karlsson and B. T. Astrom, February, Manufacturing and Applications of Structural Sandwich Components, Composites Part A, 28, 97-111, 1997
3. A. Lystrup, and T. L. Andersen, Autoclave Consolidation of Fibre Composites with a High Temperature Thermoplastic Matrix, Journal of Materials Processing Technology , 77, 80–85, 1998
4. Z. S. Guo, L. Liu, B. M. Zhang and S. Du, Critical Void Content for Thermoset Composite Laminates, Journal of Composite Materials, 43: 1775, 2009
5. H. Huang, R. Talreja, Effects of void geometry on elastic properties of unidirectional fiber reinforced composites, Composites Science and Technology, 65, 1964–1981, 2005
6. P. Feraboli, E. Peitso, T. Cleveland, P. B. Stickler, and J. C. Halpin, Notched Behavior of Prepreg-Based Discontinuous Carbon Fiber Epoxy Systems, Composites: Part A, 40, 289–299, 2009
7. P. Hurbert and A. Poursartip, Aspects of the Compaction of Composite Angle Laminates An Experimental Investigation, J. of Composite Materials 35: 2, 2001
8. Y. Ledru, G. Bernhart, R. Piquet, F. Schmidt, L. Michel, Coupled Visco-Mechanical and Diffusion Void Growth Modelling During Composite Curing, Composites Science and Technology, 70, 2139–2145, 2010
17
9. F.U. Buehler, J.C. Seferis, Effect of Reinforcement and Solvent Content on Moisture Absorption in Epoxy Composite Materials, Composites: Part A, 31, 741–748, 2000
10. D. Gueribiz, M. Rahmani, F. Jacquemin, S. Fréour, R. Guillen and K. Loucif, Homogenization of Moisture Diffusing Behavior of Composite Materials with Impermeable or Permeable Fibers Application to Porous Composite Materials, Journal of Composite Materials 43: 1391,2009
11. L. A. Khan, A. Nesbitt, R. J. Day, Hygrothermal Degradation of 977-2A Carbon Epoxy Composite Laminates Cured and Quickstep, Composites: Part A, 41, 942–953,2010
12. M. L. Costa, M. C. Rezende, and S.F.M. De Almeida, Strength of Hygrothermally Conditioned Polymer Composites with Voids, Journal of Composite Materials, 39: 1943, 2005
13. X. Wang, F. Xie, M. Li and Z. Zhang, Influence of Tool Assembly Schemes and Integral Molding Technologies on Compaction of T-stiffened Skins in Autoclave Process, Journal of Reinforced Plastics and Composites, 29: 1311,2010
14. X. Wang, Z. Zhang, F. Xie, M. Li, D.I. Dai and F. Wang, Correlated Rules between Complex Structure of Composite Components and Manufacturing Defects in Autoclave Molding Technology, Journal of Reinforced Plastics and Composites, 28: 2791, 2009
15. B. Khan, K. Potter and M. R. Wisnom, Suppression of Delamination at Ply Drops in Tapered Composites by Ply Chamfering, Journal of Composite Materials, 40: 157, 2006
16. C.P. Jock, Quantitative Optical Microscopy Fiber Volume Methods for Composites, Journal of Reinforced Plastics and Composites, 5: 110, 1986
17. S.K. Panigrahi, B. Pradhan, Onset and Growth of Adhesion Failure and Delamination Induced Damages in Double Lap Joint of Laminated FRP Composites, Composite Structures, 85, 326–336, 2008
18. T.S. Gates, X. Su, F. Abdi, G. M. Odegard and H. M. Herring, Facesheet Delamination of Composite Sandwich Materials at Cryogenic Temperatures, Composites Science and Technology, 66, 2423–2435, 2006
19. B. C. Ray, Evaluation of Defects in FRP Composites by NDT Techniques, Journal of Reinforced Plastics and Composites, Vol. 26, 12, 2007
20. T. Yokozekia, T. Aoki, and T. Ishikawa, Consecutive Matrix Cracking in Contiguous Plies of Composite Laminates, Int. J. of Solids and Structures, 42, 2785–2802, 2005
21. J. Andersons and R. Joffe, Statistical Model of the Transverse Ply Cracking in Cross-Ply Laminates by Strength and Fracture Toughness Based Failure Criteria, Engineering Fracture Mechanics, 75, 2651–2665, 2008
18
22. F.Y.C. Boey and S.W. Lye, Effects of Vacuum and Pressure in Autoclave Curing Process for Thermosetting Fibre Reinforced Composite, Journal of Materials Processing Technology, 23, 121-131, 1990
23. F.Y.C. Boey and S.W. Lye, Effects of Vacuum and Pressure in an Autoclave Curing Process for a Thermosetting Fibre Reinforced Composite, Journal of Materials Processing Technology, 23, 121-131, 1990
24. H. Zhu, D. Li, D. Zhang, B. Wu, Y. Chen, Influence of Voids on Interlaminar Shear Strength of Carbon Epoxy Fabric Laminates, Trans Nonferrous, Met Soc. China, 19, 470-475, 2009
25. L. Liu, B. M. Zhang, D. F. Wang, Z. J. Wu, Effects of Cure Cycles on Void Content and Mechanical Properties of Composite Laminates, Composite Structures, 73, 303–309, 2006
26. H. Zhu, B. Wu, D. Li, D. Zhang and Y. Chen, Influence of Voids on the Tensile Performance of Carbon epoxy Fabric Laminates, J. Mater. Sci. Technol., 27(1), 69-73, 2011
27. Z. Youssef, F. Jacquemin, D. Gloaguen and R. Guillén, Hygro-Elastic Internal Stresses in Porous Composite Materials A Multi-scale Analysis, Journal of Reinforced Plastics and Composites 27: 1417, 2008
28. D.A. Crump, J.M. Dulieu-Barton and J. Savage, The Manufacturing Procedure for Aerospace Secondary Sandwich Structure Panels, Journal of Sandwich Structures and Materials, 12: 421, 2010
29. F. C. Campbell, Manufacturing Processes for Advanced Composites, pg. 1-37, Elsevier, 2003, ISBN: 978-1-85617-415-2,
30. Vacuum Bagging Techniques On Sandwich Constructions, publication of DIVINYCELL
31. Vacuum Bagging Techniques publication of WEST SYSTEM® Epoxy, 7th Edition—April, 2010
32. M. I. Naji and S. V. Hoa, Curing of Thick Angle-Bend Thermoset Composite Part Curing Cycle Effect on Thickness Variation and Fiber Volume Fraction, Journal of Reinforced Plastics and Composites 18: 702, 1999
33. DURATOOL® 450 BMI TOOLING PREPREG, Cytec Published document AECM-00004 REV: 0 1 OCTOBER 2009
34. S. S. Tavares, V. Michaud, J.-A.E. Månson, Assessment of semi-impregnated fabrics in honeycomb sandwich structures, Composites: Part A 41, 8–15,2010
35. M. Wysocki, R. Larsson, S. Toll, Modeling The Consolidation of Partially Impregnated Prepregs, Swerea SICOMP, Sweden & Chalmers University of Technology, Göteborg, Sweden
19
36. Product information ACG MTM44-1, document number PDS1189/02.11/7
37. Product information Cytec 5320, Revision 1.3 – 03.18.09
38. J. H. Oh and D. G. Lee, Cure Cycle for Thick Glass Epoxy Composite Laminates, Journal of Composite Materials 36: 19, 2002
39. T. H. Hou, J. M. Baughman, T. J. Zimmerman, J. K. Sutter, and J. M. Gardner, Evaluation of Sandwich Structure Bonding In Out-of-Autoclave processing, 42nd Int. SAMPE Technical Conf. , Salt Lake City, UT, 2010
40. S. S. Tavares, Y. Roulin, V. Michaud, J.-A.E. Månson, Hybrid Processing of Thick Skins for Honeycomb Sandwich Structures, Composites Science and Technology, 71 183–189, 2011
41. C. Xin, M. Li, Y. Gu, Y. Li and Z. Zhang, Measurement and Analysis on in-Plane and Through-Thickness air Permeation of Fiber Resin Prepreg, Journal of Reinforced Plastics and Composites, 1–13, 2011
42. A.R.A. Arafath, G. Fernlund, A. Poursartip, Gas Trasport in prepreg Models & Permesbility Experiments, 17th Int. Conference on Composite Materials, UK, 2009
43. L.W. Davies, R.J. Day, D. Bond, A. Nesbitt, J. Ellis, E. Gardon, Effect of cure cycle heat transfer rates on the physical and mechanical properties of an epoxy matrix composite, Composites Science and Technology, 67 , 1892–1899,2007
44. S. Hernández , F. Sket , J.M. Molina-Aldareguı´a, C. González, J. Llorca, Effect of Curing Cycle on Void Distribution and Interlaminar Shear Strength in Polymer-Matrix Composites, Composites Science and Technology, 71, 1331–1341,2011
20
Recommended