2.18
Composite Preforming
Techniques
I. VERPOEST
Katholieke Universiteit Leuven, Belgium
2.18.1 INTRODUCTION
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2.18.2 ONE-DIMENSIONAL PREFORMS
2
2.18.2.1 Description
2.18.2.2 Processibility
2.18.2.3 Properties
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2.18.3 TWO-DIMENSIONAL PREFORMS
7
2.18.3.1 UD Layers
2.18.3.1.1 Single layer UD prepregs
2.18.3.1.2 Multiple layer preforms
2.18.3.2 Woven Fabrics
2.18.3.2.1 Introduction
2.18.3.2.2 Description
2.18.3.2.3 Processibility
2.18.3.2.4 Properties
2.18.3.3 2-D Braided Fabrics
2.18.3.3.1 Description
2.18.3.3.2 Shaped 2-D braids
2.18.3.3.3 Processing
2.18.3.3.4 Properties
2.18.3.4 Knitted Fabrics
2.18.3.4.1 Description
2.18.3.4.2 Processibility
2.18.3.4.3 Properties
2.18.3.5 Random Fiber Preforms
2.18.3.5.1 Dry preforms
2.18.3.5.2 Impregnated preforms
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2.18.4 THREE-DIMENSIONAL PREFORMS
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2.18.4.1 Solid 3-D Textiles
2.18.4.1.1 3-D woven fabrics
2.18.4.1.2 3-D braids
2.18.4.1.3 3-D knits
2.18.4.1.4 Stitching
2.18.4.1.5 Processing
2.18.4.1.6 Properties
2.18.4.2 Preforms for Woven Sandwich Structures
2.18.4.2.1 Description
2.18.4.2.2 Properties
2.18.4.3 Preforms for Knitted Sandwich Structures
2.18.4.3.1 Description
2.18.4.3.2 Processibility
2.18.4.3.3 Properties
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1
2
Composite Preforming Techniques
2.18.5 MECHANICAL PROPERTIES AND PROCESSIBILITY
38
2.18.5.1 Analytical Models for Stiffness and Strength Predictions of Textile Composites
2.18.5.1.1 Introduction
2.18.5.1.2 Basic hypotheses and principles of the models
2.18.5.1.3 Comparison of the performance of the different models
2.18.5.2 Processibility
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2.18.6 CONCLUSION
45
2.18.7 REFERENCES
46
2.18.1
INTRODUCTION
Only since the beginning of the 1990s has the
word ªpreformº become a common term in the
composites literature. This reflects the growing
awareness of the importance of the fiber architecture in composites, not only for the thermomechanical properties, but also for the
processing characteristics of composites.
This chapter deals with all fiber architectures,
which bridge the gap between the two extremes:
discontinuous, random fiber mats on one side
and continuous unidirectional layers on the
other. The former has low but almost isotropic
mechanical properties, due to the low fiber volume fraction and the discontinuous nature of
the fibers; the latter has high mechanical properties in the fiber direction, but is very anisotropic.
Both preforms have, however, some characteristics in common: a limited formability and
drapability over complex shapes, and a rather
poor impact resistance. Textile based preforms
will close the gap between these two extremes
and moreover show a considerably improved
drapability and impact resistance.
Textiles have been around as long as mankind has existed. Already in the very early
stages of modern composites technology, glass
fibers were woven into fabrics. The lower
strain-to-failure of carbon fibers and the creation of a conductive carbon dust makes the
production of carbon fiber based textiles
much more difficult. Aramid and other polymeric fibers (natural or synthetic), however, are
easy to handle in a textile process. Most types of
preforms are nowadays available in all three of
the most used reinforcing fibers (glass, carbon,
and aramid). Hence, the chapter has not been
organized according to the different fiber types,
but following a dimensional hierarchy.
First, the one-dimensional preforms, namely
the yarns or linear fiber assemblies, are presented. Then, the two-dimensional preforms
are treated in depth, because they are most
widely used in the actual composite material
applications. For many different reasons,
which will be explained later, there is, however,
a strongly increasing interest in three-dimensional preforms, both as a solid reinforcement
and as a preform for integrated sandwich structures. The individual sections, dealing with the
different types of preforms, will all follow the
same structure: description of the textile as such
(yarn architecture, manufacture of the textile,
etc.), discussion of the processibility into a composite part, evaluation of the mechanical properties of the textile composite. The chapter ends
with two more general sections: one on models
for calculating the thermomechanical properties of textile based composites, another on their
two most important processing characteristics,
namely permeability and drapability.
This chapter cannot aim for completeness, as
the field of textile based composites has grown
too fast and in too many directions over the
past few years. An attempt has, however, been
made to present, in a well-structured way, the
most important and relevant preform types.
Certainly, many (still?) more exotic types of
preforms, like embroidery or some complex
three-dimensional preforms, have not been
treated. Even so, this chapter cannot give a
complete overview of all existing literature for
each of the preform types, and hence some
important papers will not have been referenced,
for which the author apologizes. Keeping the
number of references within decent limits often
forced the author to difficult and sometimes
almost arbitrary but inevitable choices.
2.18.2
2.18.2.1
ONE-DIMENSIONAL PREFORMS
Description
The one-dimensional preforms that will be
discussed here are yarns. Yarns are linear
assemblies of fibers characterized by a substantial length and relatively small cross-section.
Their inherent flexibility enables them to be
processed into subsequent textile structures
such as weaves, braids, and knits. The fundamental terms that characterize yarns are listed
in Table 1.
Textile preforms have established an important role in the fabrication of composite structures primarily due to their high strength to
One-dimensional Preforms
3
Table 1 Fundamental yarn terms.
Property
Description
linear density (tex = grams km71)
thickness/bulk (microns)
direction and no. of turns per meter
(polymer) composition
chemical or mechanical finish
number of individual strands
filaments run essentially the whole length
of the yarn/short filaments
mixture of two or more fiber types
heat set, fibrillated
Count
Diameter
Twist
Fiber type
Surface finish
Single (1) or ply (>1)
Continuous/ discontinuous
Blend/hybrid
Texture
Figure 1
Types of yarns.
light weight ratios. The orientation and the
composition of the reinforcement materials,
namely fibers, determine the properties of all
textile preforms. Therefore, the properties of
textile preforms are directly influenced by the
properties of the fibers.
In composites, either organic or inorganic
fibers are used. The microstructure of organic
or polymeric fibers is similar to that of yarns
because they are also composed of linear structures (polymeric chains) with high length to
width ratios. The most effective microstructures
are achieved when the polymeric chains are
packed in crystallites, and oriented parallel to
the yarn axis, and when the amount of amorphous regions is reduced to a minimum. The
alignment of the crystallites may occur naturally in fibers such as wool, however, for extruded fibers such as polyethylene, they have to
be drawn (i.e., stretched).
Among the inorganic fibers, carbon or graphitic fibers yield the highest stiffness (up to
600 GPa) and strength (up to 4000 MPa), because they are composed of highly oriented,
nearly perfect graphitic plates. Glass fibers are
amorphous, and hence reach a much lower
stiffness (70±75 GPa). Ceramic fibers are
mostly crystalline, but not oriented, and have
properties in between those of glass and carbon
fibers. The different types of yarns are illustrated in Figure 1. Figure 2 illustrates examples of four different types of yarns.
The more the fibers are aligned parallel to the
yarn axis, the higher the reinforcement efficiency (along the fiber direction) of the composite. Continuous filaments are by nature
straight fibers, and the yarns are only slightly
twisted. Staple fibers need to be processed
through carding or combing (analogous to
combing of hair where fibers are disentangled
and aligned) in order to reach a high degree of
orientation. However, the waviness of the staple fibers also alters the effective fiber orientation along the yarn axis. In order for the
individual, discontinuous staple fibers to behave as a unit in the yarn structure, there
needs to be a minimum cohesion between the
fibers. This can be achieved by twisting the yarn
and hence introducing compressive and frictional forces in between the yarns. There is an
optimum twist level for yarns that will impart
the maximum tensile strength. Figure 3 shows
the typical strength (or tenacity) vs. twist behavior of spun and continuous-filament yarn.
4
Figure 2
Composite Preforming Techniques
Schematic of different yarns. (a) Spun yarn (z-twist); (b) spun yarn (s-twist); (c) twisted
continuous-filament yarn; (d) untwisted continuous-filament yarn (roving).
Figure 3 Tenacity vs. twist behavior of yarns.
This rule need not to apply to preforms which
will be eventually processed into a composite,
because the matrix provides adequate binding
between the individual fibers. Hence, a minimum twist will be preferred, which results in a
yarn strength which is sufficient to guarantee a
good handling during the subsequent textile
processing (weaving, knitting, etc.).
Surface treatments on the yarns will also
affect the properties of the composite. The adhesion between the resin and preform or any
surface treatment that may be present on the
preform must be adequate to prevent weak
regions in the composite. Surface chemical
finishes such as sizes are usually applied to
yarns to improve the interfacial mechanical
properties in the subsequent composite. Similarly, texturing techniques such as fibrillation
and crimp may be used to improve the interfacial mechanical interlocking between the resin
and the yarn. However, since the composite
tensile properties are proportional to the linearity of the fibers, straight fiber orientations
along the yarn axis are preferred for high tensile
stress applications.
The properties of the yarn can also be manipulated by combining two or more fiber types
to form a hybrid. The resultant structure will
thus contain a combination of the properties of
its constituents. However, not all the components of the hybrid need constitute the reinforcement as will be discussed in Section 2.18.2.2.
2.18.2.2
Processibility
Yarns may be processed into composites via
many different routes. Often the yarn only contains the reinforcing fibers, and then goes
through a textile process (weaving, knitting,
etc.) before being impregnated with a matrix.
This route is preferable when thermoset resins
are used, because the handleability of the unimpregnated preform is superior over the impregnated one. Moreover, its storage time is
unlimited, and the same preform can be used
for many different resin systems.
For thermoplastic matrix systems on the contrary, the high viscosity of the polymer melt
One-dimensional Preforms
Figure 4
5
Types of co-mingled yarns (a) intermingled; (b) co-wrapped; (c) core-spun; (d) stretch-breaking.
often hinders a good impregnation. Hence,
yarns are impregnated with the thermoplastic
polymer before any further textile processing.
The polymer can be brought into the yarn in
basically two conditions: either as a solid or as a
liquid.
Solid impregnation can be categorized into
powder impregnation and co-mingling techniques, while pultrusion techniques are commonly used for liquid impregnation.
During powder impregnation, the fibers are
initially spread in order to enable access into the
yarn structure and the impregnating thermoplastic powder is deposited. Dry powder particles adhere electrostatiscally to the fibers.
Sometimes particle distribution and adhesion
is enhanced by dissolving in a liquid. The thermoplastic particles are then melted immediately
after impregnation to form a preconsolidated
composite yarn (Iyer and Drzal, 1990). Alternatively, a polymer sheath similar to the impregnating powder can be used to prevent the
powders from falling out of the tow.
The co-mingling process can be divided into
four different techniques: intermingling, cowrapping, core spinning, and stretch breaking.
During the intermingling process, the fibers
are spread and blended either using air jet or
water jet systems (Figure 4(a)) The advantage of
using air over water is that the fibers do not have
to be subsequently dried. Alternatively, electrostatic charges can also be used in the intermingling process (Jou et al., 1995, Lennox-Kerr,
1995).
The co-wrapping process involves wrapping
a binder yarn around a core of reinforcing
fibers (Figure 4(b)). The material is usually
heated, melting the binder. The disadvantage
with this technique is that the resultant composite lacks homogeneity. In order to overcome
this problem, elevated temperatures and pressures need to be used resulting in high processing costs (Braches, 1991; Svenson et al., 1998).
The core-spinning technique is similar to the
co-wrapping technique but instead of using
continuous filaments, short fibers are spun
around a core of continuous reinforcing fibers
(Figure 4(c)).
During the stretch-breaking technique continuous reinforcing and matrix fibers are
stretched and broken at predetermined lengths,
and reformed into a yarn via insertion of twist
(Figure 4(d)).
In general, the co-mingled yarns containing
spun yarns rather than continuous filaments
tend to be more intimately blended, thus resulting in composites with homogeneous
fiber±matrix distributions. For instance, the
intermingling and stretch-breaking techniques
will give more homogeneous composites than
the co-wrapping and core spinning techniques.
For liquid impregnation, the process of pultrusion, explained in greater detail elsewhere
(Bijsterbosch and Gaymans, 1992; Miller and
Gibson, 1996; Moyer, 1976) (see Chapter 2.24,
this volume), is most often used. In brief, pultrusion is a continuous process that uses unidirectional tows, passing them through the
resin and pulling them through a heated die
(Schwartz, 1984). The advantage with this process is the cost benefit of a continuous process,
while the disadvantages are impregnation difficulties and consequently inhomogeneous fiber
matrix ratios.
The viscosity of the thermoplastic can be
reduced by dissolving it with solvent (solvent
or solution impregnation) and/or by increasing
the temperature (melt impregnation). The latter
is of course preferable as many solvents are
harmful to the environment and to humans.
Even then the viscosity of thermoplastic polymers is often considerably higher than that of
thermoset polymers, liquid impregnation can
only happen successfully under hydrostatic
pressure or by applying a shear force. Solvent
impregnation is widely used for amorphous
thermoplastics, because most crystalline thermoplastics are not readily soluble in solvents.
After the impregnation, the solvent must be
6
Composite Preforming Techniques
Figure 5 Longitudinal modulus and strength, respectively (after Lauke et al., 1998).
extracted by evaporation, otherwise voids
might be formed in the final composites (Goodman and Loos, 1990).
2.18.2.3
Properties
The composite properties will depend primarily on the mechanical properties (i.e., tensile
and compressive) of the reinforcing fibers as
well as on the fiber volume fraction and
the homogeneity of the fiber distribution in
the matrix. Also the length and orientation
of the fibers will determine the mechanical
properties of the composite.
The longitudinal and transverse tensile properties of yarn reinforced composites are not
only influenced by fiber strength, volume fraction and orientation, but also whether it contains continuous or discontinuous fibers. For
instance, Figure 5 illustrates the tensile properties of the various co-mingled yarn types. SBS
(side by side) are continuous filaments with the
matrix and reinforcement aligned together.
KEM (Kemafil) is a type of co-wrapped yarn,
COM (air textured) is a type of intermingled
yarn, FS (friction spinning) is a type of corespun yarn, and SCH (Schappe technology) is a
type of stretch broken yarn.
The tensile strength is higher in the continuous filament (SBS, KEM, FS) yarns compared
to the strength of the spun yarns which contain
discontinuous fibers (COM, SCH). In fact, if
we also take into consideration the differences
in the fiber volume fraction, it becomes even
more apparent that the continuous nature of
the fibers together with their orientation along
the yarn axis improves the longitudinal tensile
strength. However, the trend for the longitudinal modulus will be influenced more by the
fiber volume fraction. For instance, when the
modulus is normalized with the modulus of
SBS for an identical fiber volume fraction of
45%, the modulus ratios are: for FS:1.15,
SBS:1, KEM: 0.96, COM: 0.95, and SCH:
0.96 (see Table 2).
The transverse modulus and strength are
illustrated in Figure 6 (top) while the interlaminar and intralaminar shear are illustrated in
Figure 6 (bottom). The yarns with discontinuous fibers, namely COM and SCH, have higher
transverse and shear properties compared with
the continuous filament yarns.
Two-dimensional Preforms
Table 2
Hybrid yarn
Fiber volume fraction
Normalized moduli over
SBS @ Vf = 45%
7
Fiber volume fractions.
SBS
KEM
COM
FS
SCH
45
1
53
0.96
48
0.95
59
1.15
56
0.96
Figure 6 Transverse and shear properties, respectively.
2.18.3
TWO-DIMENSIONAL PREFORMS
2.18.3.1
2.18.3.1.1
(i)
UD Layers
Single layer UD prepregs
Description
UD-tapes are the most simple type of 2-D
preforms: rovings (untwisted filament yarns)
are laid parallel to each other and impregnated
with matrix material (see Chapter 2.17, this
volume). The impregnation procedures are
slightly different for thermoset and thermoplastic resins. For thermosets (mostly epoxies), impregnation can basically happen in two ways:
(i) by immersing the fiber into liquid resin.
This can be carried out either in a bath or
through contact with a dipping roll. The process requires a low viscosity resin, which can
often only be realized by adding solvents to the
resin.
(ii) by spreading out a thin resin film on one
or two backing sheets, in between which the
fibers are squeezed, so that the resin penetrates
into the rovings. As a higher viscosity is allowed
in this process, just heating the resin can already
be sufficient.
After both impregnation processes, the prepreg is further heated in order to remove the
solvents and/or to start a partial gelation and
cross-linking called ªB-staging.º
8
Composite Preforming Techniques
Table 3 Typical properties for UD laminates.
Thickness (mm)
Resin content (vol.%)
Dry fiber areal weight
(g m72)
Width (cm)
Range
Typical values
0.08±0.25
28±45
30±300
0.125
40
120
2.5±150
30
For thermoplastics, the viscosity of the molten polymer remains high, even after adding
solvents. Real liquid impregnation will hence
be more difficult but not impossible. Alternative impregnation techniques have therefore
been developed, however, this information is
often inaccessible due to the proprietary restrictions. Possible techniques include:
(i) continuous film stacking: one or more
thermoplastic films are squeezed into the rovings held between two backing sheets, similar to
the resin film process for thermosets. High
temperatures and high pressures are required.
(ii) by using powder impregnated or comingled yarns (see Section 2.18.2.2), it is possible to manufacture a thermoplastic prepreg by
continuously heating and compacting the
yarns, which are fed into the machine parallel
to each other.
(ii)
Processibility
UD-prepreg tapes are mostly used in highly
loaded structures because 100% strength and
stiffness levels can be achieved. This is realized
by carefully manipulating the UD-tape, avoiding distortion of the prepreg and misorientation
of the fibers. Four properties are crucial in this
respect:
(i) tack: is the measure of adhesion of the
tape to the tool surfaces and to other prepregs.
An optimum tack makes lay-up easier, as the
consecutive layers are immediately but do not
excessively stick. Moreover, it indicates that
thermoset prepregs are not overtime (surpassed
the B-stage). If they are, they loose their tack, so
that air is entrapped more easily during lay up.
Heavy tack can be made more manageable by
reducing temperature and overly dry prepregs
can be slightly heated up to improve their tack.
As thermoplastic prepregs do not show any
tack (apart from special exceptions), laying up
thermoplastic prepregs creates additional problems: the layers do not stay in their original
position and slide very easily over each other. A
way to overcome this problem is by spot welding them together.
(ii) flow: is the amount of resin squeezed out
from a specimen as it is heated up and pressur-
ized. During curing of thermosets, some flow is
desirable to allow volatiles and reaction gases
to bleed out and for perfect fusing of successive
plies. Excessive flow, however, will result in an
inhomogeneous and/or an unpredictable fiber
volume fraction and in fiber migration. Newer
thermoset prepregs are no-bleed systems, with
flow characteristics well controlled by thickening or thixotropic additives. During consolidation of thermoplastics, flow is essential to
further wet out the filament yarns. While processing, the flow is mainly temperature controlled but also depends on the chemistry of
the thermoplastic.
(iii) gel time: for thermoset prepregs, the gel
time is an additional important characteristic.
It is the time needed to reach gelation (sudden
viscosity increase) for a given temperature. This
may happen prior to lay up when it is stored for
too long at ambient temperature and the prepreg becomes stiff and cannot be manipulated
any more. The gel time can be increased by
storing at a lower temperature (±20 8C). Gel
time at higher temperatures is relevant for optimization of the cure process as gelation and
cross-linking have to be perfectly matched to
each other.
(iv) drape of the prepreg will mainly depend
on the actual viscosity of the polymer. For
thermosets, this means that the prepregs should
still be in the B-stage. If it has surpassed the Bstage, the prepreg becomes too stiff and will
behave more like a thermoplastic prepreg (i.e., a
thin consolidated UD laminate). In order not to
cause fiber damage, thermoplastic prepregs
have to be handled with extreme care. They
could be softened by warming up, but such a
process is not very practical.
Second, the drape also depends on the transverse strength of the UD prepreg. Therefore, if
the prepreg is too weak, it will easily tear,
especially when curved surfaces have to be
laid up. This leads to uncontrolled fiber orientations.
(iii)
Properties
UD tapes typically have the characteristics
shown in Table 3.
UD tapes show the optimum reinforcement
efficiency when loaded in the longitudinal direction. This is clearly shown in Table 4 where
UD tape stiffness and strength are compared to
the values for woven fabric prepregs.
Staple fiber UD prepregs are a special case of
UD prepregs, which have seen growth recently
due to:
(i) their low material cost since recycled prepreg material can be utilized, and
(ii) their potential for improved drapability.
Two-dimensional Preforms
Figure 7
9
Multiaxial weft inserted warp knit (MWK).
Table 4 Typical values for the tensile strength and stiffness for UD tape and balanced fabric (vf % 60%).
Tensile strength (MPa)
Tensile modulus (GPa)
2800
1800
1500
175
75
45
1000
600
500
90
40
30
Unidirectional tape
Intermediate modulus graphite
Aramid
S-glass
Balanced fabric
Intermediate modulus graphite
Aramid
S-glass
The expected loss in longitudinal strength
(720%) and stiffness (710%) are still acceptable for some applications.
2.18.3.1.2
(i)
Multiple layer preforms
Description
The main disadvantage of woven fabrics is
the crimp factor due to the fact that the yarns
are never perfectly straight. Consequently, even
cross-plied UD tapes can potentially support
higher loads than woven reinforcements. On
the other hand, the assembly of UD tapes can
be very time consuming and hence expensive.
This could be overcome by using several types
of multilayered preforms such as:
(i) Multidirectional tape prepreg: this material consists of different layers of UD prepregs
stacked on top of each other in predefined
directions. The properties of the final composite can be controlled by varying the orientation
and the number of plies. This material will
mainly be used when repetitive fiber orientations are required and the drapability is not so
important.
(ii) Multiaxial warp knit (MWK): the disadvantages of UD tapes can also be overcome
by using knitted UD tapes. Figure 7 illustrates
the MWK process with possible stacking
sequences. The process consists of laying one
ply on various oriented layers of UD plies. The
entire stack of layers is either stitched or knitted
together by a very fine yarn.
(iii) Stitched UD preforms (Pattyn et al.,
1999a). Here, heavy tows (12±80 k; the number
of fibers in a yarn: 1 k = 1000) are spread into a
tape, laid down, and oriented layer by layer. A
belt transports the lay-up to the stitching unit,
which assembles the lay-up into the final preform. One of the advantages of this system is
that the orientations and stacking sequence
choice are completely flexible.
(ii)
Processibility
The main advantages of using multiple
layer preforms for composites include an absence of crimp and an enhanced drapability if
plies in different orientations are present. The
transverse strength of the tape can be improved by knitting or stitching the yarns together. However, the number of plies should
not exceed a maximum (i.e., eight for MWK),
so that sufficient conformability and permeability for impregnating resins can be maintained. Finally, the lay up time can be
drastically reduced.
The close packing of yarns in stitched UD
preforms provides a densely consolidated
material. Consequently, the through-the-thickness-permeability is also lower. On the other
10
Composite Preforming Techniques
Figure 8 Comparison of a Liba multiaxial weft inserted warp knit MWK (left) with a Hexcel stitched
multiaxial preform (right) (after Pattyn et al., 1999b).
2.18.3.2
2.18.3.2.1
Figure 9 Weaving (after AÊstroÈm, 1997).
hand, very fine plies can be obtained starting
from heavy tows and resulting in more homogenous properties (Pattyn et al., 1999b).
For the weft inserted warp knits using nonspreaded yarns (Liba type), the permeability
will be higher because the gaps between the
yarns of the material act as channels for the
resin to flow through (see Figure 8). This material will have more resin-rich zones because of a
coarser fiber distribution that will reduce the
maximum mechanical properties obtainable.
These multiaxial materials can be impregnated via RTM or by stacking resin films between the different preforms and autoclaving,
similar to the processing of high quality thin
laminates.
(iii)
Properties
The mechanical properties of the composite
will be very close to those of laminates built up
of UD layers. Moreover, the knitting yarn provides reinforcement through the thickness,
hence improving the delamination properties
of the composite. On the other hand, the stitch
yarn also keeps the other yarns in the right
place.
Woven Fabrics
Introduction
Weaving, see Figure 9, is perhaps the oldest
fabric-forming technology. Hand weaving has
been done for millennia, mechanical weaving
for centuries, automated weaving of complex
fabrics for decennia. Much technological development in terms of machinery and fabric structures has been carried out. Modern weaving is
one of those technological areas where hightech, very sophisticated machines are used for
the manufacturing of low-tech, mass-production goods.
However, during the whole history, woven
fabrics have not only uniquely been used for
clothing, but also for load-carrying applications; a typical example is a sail for ships, for
which centuries ago very durable fabrics were
developed.
The family of ªtechnical textilesº has gained
increasing importance over the past decennia,
and within this family textiles for composites
play a special role as they have to fulfill special
requirements: they have to perform as a reinforcement, not as much as a textile. Woven
fabrics are by far the most commonly used
textiles for composite applications. Weaves
offer a low-cost method of fabricating large
areas of material, and provide preforms with
only a small sacrifice in the properties that
would have been obtained with laminates of
unidirectional tape. Woven broad goods may
be found either as a dry preform or preimpregnated with a B-staged thermoset matrix or preimpregnated with a thermoplastic matrix. In
most applications, multiple layers of 2-D
weaves are laminated together.
Compared with unidirectional laminates,
woven fabrics provide more balanced properties as in a single layer fabric they consist of a
bidirectional reinforcement. The low fabrication cost, ease of handling, and rise of impact
Two-dimensional Preforms
11
Figure 10 Schematic illustration of a simple weaving loom (after Bogdanovich and Pastore, 1996).
resistance have made fabrics attractive for
structural applications. Disadvantages of
woven fabrics are the limited conformability
of the fabric, reduced yarn-to-fabric tensile
properties, translation efficiency due to yarn
crimp, and poor in-plane shear resistance of
the fabric composite.
2.18.3.2.2
Description
The woven fabric is realized through a loom
(Figure 10) which fundamentally consists of
five components: a yarn supply, harnesses, a
filling insertion mechanism, a combing
mechanism, and a take-up mechanism (Bogdanovich and Pastore, 1996). It is made by interlacing two or more orthogonal sets of yarns,
called warp and weft yarns. The lengthwise
yarns aligned with the direction of the fabric
leaving the loom are called warps. A warp yarn
may also be called an end. The weft yarns run
perpendicular to the warp direction (widthways yarns), and are sometimes called fill
yarns or pick yarns. The interlacing of the
yarns causes yarn undulation or yarn crimp.
Details of the weaving process can be found in
Lord and Mohamed (1982).
A variety of weave patterns can be used to
interlace the warp yarns (in the weaving direction) and the weft (or filling) yarns to form a
stable fabric.
(i)
Biaxial weaves
Weaving terminology is, for an outsider, not
very transparent; different words are used to
indicate the same type of fabric, and moreover
different methods of graphical presentations of
these weaving patterns can be used. Figure 11
illustrates some typical weave patterns used in
composites.
The plain weave (Figure 11(a)) is the simplest
pattern in which one warp yarn interlaces over
and under one weft yarn to give a checkerboard
effect. Although very simple to manufacture,
this fabric type has disadvantages due to its
high yarn crimp.
The yarn crimp is particularly important in
composite applications. Crimp levels influence
fiber volume fraction, thickness of the fabric,
and mechanical performance of the fabric
based composite. The greater the number of
points of interlacing per unit area, the higher
the crimp, which results in reduced drapability
or conformability, due to the higher resistance
to shearing. Higher crimp also means less
straight yarns, which translates into lower
strength translation efficiency. The benefit of
higher crimp is an improved degree of stability/
integrity with respect to yarn slippage and fabric distortion. For composite applications, the
fabric stability is important in order to reduce
and maintain the prescribed yarn orientations
during the lay-up and consolidation processes.
The basket weave (Figure 11(b)) is a variation
of plain weave, with better drapability properties. It has warp and weft yarns that are paired:
two or more up and two or more down. A twill
weave (Figure 11(c) and (d)), with one or more
or warp yarns interlaced under two or more weft
yarns in a regular pattern, offers even greater
drapability. However, basket and twill weaves
tend to be less stable than plain weaves.
12
Composite Preforming Techniques
Figure 11
Fabric construction forms and their fabric repeat (after Dominguez, 1989).
Most fabrics used for composites belong to
the satin family (Figure 11(e) and (f)). The satin
weave has a good drapability, a smooth surface,
and a minimum thickness. Here, one warp yarn
interlaces over at least three weft yarns and
under a single one in an irregular pattern. The
number of variants is enormous, as two parameters can be changed:
(i) The harness number (typically 4 (which is
equivalent to twill 3/1), 5, and 8 used in the
selection of composite preforms) is equal to the
number of warp yarns in the fabric repeat.
(ii) The base number is the number of yarns
in between two closest interlacing points. Both
parameters will control both the drapability
and the strength translation efficiency.
(ii)
Triaxial and multiaxial weaves
Using a special weaving technique, in which
two sets of warp yarns and one set of weft
yarns are interlaced at 608 angles, the three
sets of yarns can form a multitude of triangles.
The forms, called triaxial woven fabrics
(Figure 12), provide higher isotropy and inplane shear rigidity than orthogonal wovens
(Chou, 1992) but they are very expensive and
now face competition from multiaxial knitted
UD layers, which show a better strength translation efficiency. The potential properties of
these structures has led to intense activities
and resulted in patents and research papers
with regard to the structures, manufacturing
techniques, and properties (Dow, 1975; Fujita
et al., 1993).
The benefit of having fibers at angles other
than the primary weaving axis is clear as it
results in composites with nearly isotropic mechanical properties. In the field of woven textile
composites, some very interesting developments have been achieved for producing quadriaxial woven structures. Figure 13(a))
illustrates quadriaxial fabric made by a modified lappet weaving technology to introduce
458 fibers into the basic weave structure
(Ruzand and Guenot, 1994; Farley, 1993).
The samples made using this mechanism are
very open. Another quadriaxial type structure
comprises bias yarns sandwiched between weft
yarns with structural integrity provided by the
warp binding (Figure 13(b)). This is made via a
bias yarn forming assembly, used in conjunction with a jacquard shedding mechanism
(Addis, 1996). The latest development as can
be seen in Figure 13(c) is the quadriaxial fabric
with a sheet of highly concentrated bias yarns
which are integrally woven into the surface of
conventional woven structures (Durie et al.,
1999).
Fabrics are not only characterized by the
weaving pattern, but also by the weaving density, which is the number of weft or warp yarns
per length unit (i.e., picks per cm, tows per cm,
or ends per cm). It is clear that fabric weight,
thickness, and breaking strength are influenced
by the weaving density and also by the types of
yarns used to weave fabrics.
All the above mentioned fabrics are considered to be balanced, as the same yarn type and
dimension and also the same weaving density
Two-dimensional Preforms
13
Figure 12 Triaxial woven fabrics (after Yang and Chou, 1989).
Figure 13
Quartaxial woven fabrics.
are used in the warp and the weft directions.
However, when the properties of weft and warp
are different, it will result in an unbalanced
structure. Unbalanced fabrics are sometimes
used when anisotropic properties are required,
though the need for fabric stability can also be
the reason. This may be achieved either by
using an unequal ratio of warp to weft weaving
density or by using different yarn dimensions in
the warp and weft directions.
The leno weave (Figure 14(a)) has two or
more parallel warp yarns that are twisted
around consecutive weft, effectively locking
each weft in place. It tends to minimize sleaziness (easily damage) in ªopenº fabrics that have
a low cover factor. An extreme form of unbalanced fabrics are the quasi-UD fabrics
(Figure 14(b)), in which the weft yarns are
much thinner, and woven with a much lower
density than the warp yarns. Quasi-UD fabrics
are no longer that popular due to competition
from knitted UD layers (see Section 2.18.3.1).
The other type of unbalanced fabrics include
hybrid woven fabrics which refer to fabrics that
have more than one type of fiber material.
There are two major reasons why these preforms are attractive for structural materials
(Vandeurzen, 1998):
(i) These fabrics allow the designer considerable flexibility. They offer the potential of
improved composite mechanical properties,
weight saving, or excellent impact resistance.
(ii) A more cost-effective use of expensive
fibers can be obtained by replacing them partially with less expensive fibers.
Glass, aramid, carbon, boron, ceramics, and
natural fibers are examples of fiber types that
can constitute a hybrid fabric.
14
Composite Preforming Techniques
Figure 14 Examples of unbalanced fabric forms (after Cripps, 1998).
Figure 15 Shear deformation of plain weave (after
Kawabata, 1989).
2.18.3.2.3
Processibility
The important parameters to consider when
dealing with woven fabrics are the fiber type,
yarn type, weave style, crimp, weaving density,
and areal weight (Dominguez, 1989). Also of
great importance to the weaving operation is
the size or yarn finish, which ideally should
eliminate any fiber damage as it helps lubricate
and protect the yarn.
Single curved surfaces can be easily draped
by woven fabrics. The interlacing of warp and
weft yarns prevents splitting of the preform
along one yarn, as can happen in UD preforms.
Surfaces with double curvature are, however,
much more difficult to realize with woven fabrics, as they require a shear deformation of the
initial rectangular fabric structure (Figure 15).
The tightness of the weave will determine its
resistance to draping over such geometries (Bailie, 1989). This is clear from Figure 16, where a
low-density fabric has been draped over a halfsphere. The shear deformability of a fabric is
limited by:
(i) the number of interlacings per unit area,
which can be represented by the weaving den-
sity and the weave pattern. In general, satin
weaves can take more shear as the number of
interlacings per unit area decreases or the number of crossed/float yarns increase (eight-harness is better than four-harness, which is better
than a plain weave). If the weave pattern is too
tight, the fabric will not conform to various
contours and will not accept resin, resulting in
a weak composite.
(ii) the yarn density, related to the yarn
dimensions: a low density, thin yarn fabric
shears to a larger extent than a fabric with
thicker, high-density yarns.
When a fabric is deformed beyond its shear
limit, it will start wrinkling. Conversely, when
the weave pattern is too open, the composite
will not contain sufficient fiber to attain its
maximum possible strength. The open fabric
will be easily distorted, precluding alignment
of the fibers with preferred strength axes.
The potential of triaxial woven fabrics for
composite materials has been recognized with
respect to not only their in-plane shear rigidity
(Chou, 1992; Yang and Chou, 1989), but also
their conformability to curved surfaces (Trost,
1984). The conformability to the double curved
surface of triaxial woven fabrics is better than
to biaxially woven fabrics. The use of this triaxial woven fabric in complex structure parts will
minimize the biased fiber orientation problem
(Yang and Chou, 1989).
Table 5 gives a comparative assessment of
various biaxial weaves and Table 6 gives an
overview of the different types of bi- and triaxial weave structures.
The production of curved components from
flat woven fabrics with constant fiber orientation around the structure is difficult. However,
some recent methods, such as shape1 weaving
and circular disk weaving (Lee et al., 1997) have
shown developments of woven fabrics for composite applications in shell-type structures. For
the same purpose, triaxial interlaced oblique
Two-dimensional Preforms
2.18.3.2.4
Figure 16
Shear deformation over a half-sphere.
fabrics have been developed to produce a 3-D
shell fabric, which can be easily formed into
domed shapes (Fukuta et al., 1991).
Finally, the type of fabric prepreg preforms
can also influence the drapability of the preforms (Dominguez, 1989). The use of solvent
coating processing results in fabric prepregs
with better drape and higher tack than prepregs
generated by hot-melt processing. The use of
hot-melt processing incorporates higher resin
viscosity, which in turn is due to the lack of
residual solvent in the prepreg.
15
Properties
At first sight, the stiffness of woven fabric
composites could be derived from the stress±
strain curves of the fabric itself. This assumption is however erroneous, as the yarns do not
move freely anymore: they are restrained by the
surrounding solid polymer matrix.
It is, however, evident from experimental
data that both the weave pattern and the yarn
dimensions play an important role in the stiffness and strength of woven fabric composites.
Figures 17 and 18 show that:
(i) stiffness and strength increase with
decreasing yarn dimensions, because the yarn
crimp becomes smaller.
(ii) stiffness and strength increase with
decreasing number of crossovers, which is controlled both by the weave pattern (eight-harness
compared to crowfoot or four-harness satin)
and by the yarn density along the weft (and
warp) direction.
An optimum strength and stiffness translation
efficiency is hence obtained by using thin yarns
in a satin weave configuration with high crossover length and not too high yarn densities. The
relationship between the various composite
properties is also influenced by the yarn type.
For instance, in Table 7, it is shown that aramid
yarns achieve a higher stiffness and a much
Table 5 Properties of biaxial weaves.
Property
Plain
Twill
Satin
Basket
Leno
Good stability
Good drape
Low porosity
Smoothness
Balance
Symmetrical
(in plane)
Low crimp
****
**
***
**
****
*****
***
****
****
***
****
***
**
*****
*****
*****
***
**
**
***
**
**
****
***
*****
*
*
*
**
*
**
***
*****
**
**/*****
Source: Cripps, 1998. ***** = excellent, **** = good, *** = acceptable, ** = poor, * = very poor.
Table 6 Overview of biaxial and triaxial weaves.
Fabric
construction
Directional
stability
Directional
conformability
Woven
warp
(MD)
weft
(CD)
bias
(BD)
warp
(MD)
Biaxial
Triaxial
x
x
x
x
x
weft
(CD)
Substantial in-plane
shear resistance
bias
(BD)
x
Source: Scardino, 1989. MD = machine direction; CD = crosswise direction; BD = bias direction.
warp
(MD)
weft
(CD)
bias
(BD)
x
x
x
x
16
Composite Preforming Techniques
Figure 17 Influence of fabric construction on graphite composite flexural properties (after Dominguez,
1989).
Figure 18 Comparison of in-plane shear strengths
of various T300/934 weaves (after Cox and Flanagan, 1997).
higher strength translation efficiency than carbon yarns. This efficiency is expressed here
numerically by comparing the fabric properties
with those of a cross-ply laminate (which has
the same fiber volume fraction).
As mentioned before, the properties of triaxial woven fabrics yield more isotropic responses
than orthogonal wovens as the load-bearing
yarns are arranged in three instead of two
directions. Their dimensional stability, good
shear resistance, and superior interlaminar
shear properties are highly desirable for aerospace structures (Yang and Chou, 1989).
Figure 19 shows the isotropic characteristic of
two types of triaxial woven composites compared to orthotropic plain weave composites.
The lower isotropy in tensile strength, however,
can happen due to variation in fracture
mechanisms.
Table 7 Properties of graphite, aramid, and hybrid fabric composites compared to 08/908 laminates made
from unidirectional layers (data normalized to 65 vol.% fiber).
Ratio of
aramid to
graphite
fiber
100/0
50/50
25/75
0/100
Tensile modulus
(GPa)
08/908
fabric
36.5
55.1
69.6
72.3
35.8
48.2
57.2
59.9
Source: Dominguez, 1989.
Fabric
efficiency
(%)
98
87
82
83
Tensile strength
(MPa)
08/908
fabric
579
572
661
730
544
400
434
434
Fabric
efficiency
(%)
94
70
66
59
Compressive strength
(MPa)
08/908
fabric
165
407
641
965
152
227
317
558
Fabric
efficiency
(%)
92
46
49
58
Two-dimensional Preforms
Figure 19
17
Comparison of tensile properties between triaxial and orthotropic woven composites (after Fujita
et al., 1993).
Figure 20 Typical unit cells for 2-D braids: (a) 1 6 1 bias braid, (b) 2 6 2 bias braid.
2.18.3.3
2-D Braided Fabrics
Braiding is a technique that is most probably
the oldest textile technology known to humans.
It is known for its simplicity and versatility and
has been used for a variety of applications in
many different areas.
2.18.3.3.1
Description
In the traditional form of this process two or
more yarn systems are interlaced by following
intersecting circular paths in opposite directions. The resulting crossover sequence has a
tubular shape (called a bias braid). Flat braids
also exist; the production method is similar to
the one for tubular braids, the difference being
that the yarn carriers follow a modified path
compared to circular braiding machines. Manipulation of the yarn intersection sequence
results in changes to the structure of the textile.
Braids are almost identical to weaves. The main
difference between the two structures is that the
angle between the yarn systems in a braid can
be less than 90 8. Braid geometry is character-
ized by the braiding angle, which is defined as
half the value of the angle between the two
interlacing yarn systems. The braiding angle
can range from around 58 to almost 858. Therefore, even if in both structures the yarns are
orthogonal, the unit cell of the braid will be
oriented at 458 to the production direction
while the one for a weave would be perpendicular to it (see Figure 20).
Let us consider a braided and a woven fabric
having identical constructions and dimensions.
One can see that the unit cells of both materials
are highly deformable in shear. In addition, the
tubular braid is highly deformable in the radial
and axial directions; similarly, the weave is
highly deformable under tension in the bias
direction. It should be pointed out that the
ability of braids to deform radially allows
them to conform to complex tubular shapes
with varying diameters.
In addition to bias braids, triaxial braids exist
in which the material is reinforced in three inplane orientations. Triaxial braids consist of
adding axially oriented yarns into the braid
structure, through guide tubes mounted on
the undulating yarn carriers. As a result they
18
Composite Preforming Techniques
Figure 21 Example of tubular braids with inlays: (a) is the preform and (b) the processed part.
are ªtrappedº between the bias yarns and are
not interwoven but locked in the center of the
two interlaced yarn sets. Axial yarns are of
particular importance for applications of tubular braids where bending moments are more
important relative to hoop stresses and torque
loads, and in the case of compressive structural
members (see Figure 21). Also in the case of flat
braids they provide for more quasi-isotropic
behavior.
2.18.3.3.2
Shaped 2-D braids
The important characteristic of braids is their
ability to conform to a variety of shapes.
Braided fabrics can be produced over mandrels
or cores, which are passed through the fabric as
it is formed. In this case the final configuration
of the textile is set by the geometry of the core
while the orientation of the bias yarns depends
on the ratio of the rotational speed of the yarn
carriers to the transverse speed through the
machine. The mandrel can have a varying
cross-section. Provided no re-entrant geometry
exists, it is possible, in most cases, to achieve
conformal fitting to the shape of the mandrel
without any postprocessing.
Braiding machines can produce a fabric in a
single direction of motion. Unfortunately, in
the case of producing thick or complex-shaped
composite preforms, it is often called for to
traverse the work part through the braiding
point in different directions so that a series of
layers of the textile are laid (see Figure 22). This
results in the yarns being continuous throughout the part and no major discontinuities exist
around turnaround points.
During the braiding process cut-outs, fasteners, pins, and other fittings can be incorporated
with relative ease. In contrast to other types of
preforms (e.g., weaves, knits), such types of
special features are not detrimental to the mechanical properties of the structure. An example would be the incorporation of a connector
transversely to a mandrel. The bias yarns will
just accommodate themselves around the connector and even though their paths are distorted there are no cuts and therefore they
maintain their strength.
The past few years have also seen the modification of traditional 2-D braiding techniques
to produce near-net-shape parts. Murata Machinery of Japan has developed the ªintegrated
braidingº concept to produce complex parts.
Integrally braided I-beams have been demonstrated using this technique. Daimler Chrysler
has also demonstrated the manufacture of complex knot elements primarily for automotive
structures (Figure 22). Foster Miller, in the
US, has also recently demonstrated flat braiding machines consisting of series of straight
braiders. These are utilized to produce thinwalled structures in various shapes.
In general it can be stated that the design
potential of braided structures is significant.
Design limitations are primarily set by the machinery used.
2.18.3.3.3
Processing
Braided preforms for composites are processed in a variety of ways with the addition
of a suitable impregnating matrix material at
some point during the production process. A
common way of manufacturing braided composite parts is to first produce a dry preform
and then impregnate it using resin transfer
molding (RTM) (see Figure 23). This method
consists of braiding the preform on some mandrel or core. These core materials should be
capable of thermal expansion (required if the
core is placed in a solid female mold) which
would result in compaction of the composite
during the curing cycle. Once the preform is
complete it is removed from the braiding
machine, introduced into a mold, closed, and
sealed. A vacuum is formed into the mold
followed by resin injection. Once the resin fills
Two-dimensional Preforms
Figure 22 T-knot elements produced using braiding. The lower part shows the preform over the
mandrel.
the mold, it is cured. The main advantage of
RTM processing of braided composites is that
the braiding and impregnation processes are
separated. This results in reduced braiding
machine idle times. The RTM process produces
parts with very good dimensional tolerances
and surface quality in addition to having very
low void content.
A modified RTM method can be used to
impregnate preforms on line. In this process
the braiding machine is fitted with an impregnation ring. This ring is placed before the fabric
formation point in such a way so that yarns
come in contact with it. Resin is pumped
through holes on the ring onto the yarns,
which pick up the resin as they come in contact
with the ring moving towards the braiding point
19
resulting in a wet laid composite preform. This
can be consolidated by a series of pressure rolls
placed after the raiding point. The final production step consists of curing the part using autoclaving. A modification of this method exists
which looks like the traditional pultrusion
method for UD reinforced materials.
Another way to produce braid reinforced
composites is to use prepreg yarns. Since these
yarns already contain the matrix they are a
practical alternative to producing a fully impregnated part at the end of the braiding process. The part can then be cured by autoclaving.
A major problem with using prepreg yarns is
that the partly cured resin affects the stiffness
and tackiness of the yarns. These pose problems
in the handling of yarns by braiding machines.
Users of prepreg yarns have to adjust their
machine settings as well as control yarn tackiness to be able to use prepreg yarns effectively.
Finally, co-mingled yarns (see Section
2.18.2.2) have been used successfully in braiding to produce thermoplastic composite parts.
2.18.3.3.4
Properties
Braided structures provide users with high
stiffness and strength, excellent damage tolerance, as well as with high structural integrity.
They have been used, among others, by companies in the aerospace, military, medical,
recreational, and transport fields. Braiding is
also of interest in overcoming limitations presented by some preforms produced using other
textile processes (weaving, knitting, and stitching). These shortcomings include poor shear
Figure 23 An automotive A-pillar realized using 2-D braiding and processed using autoclaving. Part (a) is
the wax mandrel over which the part is braided. The braided preform is shown in (b). This is a glass/carbon
hybrid. (c) The finished part after testing.
20
Composite Preforming Techniques
Table 8 Mechanical properties for fiberglass braids having different braiding
angles.
Tensile strength
Braid angle
(Degrees)
Compressive strength
Hoop
(MPa)
Longitudinal
(MPa)
Hoop
(MPa)
Longitudinal
(MPa)
In-plane shear
(MPa)
1320
1250
1030
730
21
83
...
...
700
380
330
275
220
100
...
...
55
75
...
...
89
86.75
82.5
78
Source: Chou and Ko, 1989.
Table 9
Properties of triaxial braids.
Braid angle
(Degrees)
nf
(%)
ELT
(GPa)
ELC
(GPa)
EHT
(GPa)
nLHT
nLHC
nHLT
45
63
80
33.8
29.3
56.3
61.4
49.0
...
62.7
49.6
52.4
6.8
15.2
43.6
0.56
0.43
...
0.64
0.45
0.13
0.044
0.088
0.110
Source: Chou and Ko, 1989.
resistance and limited strength in primary loading directions. The mechanical properties of
braids can be changed by manipulating a number of factors such as the orientations of the
bias yarns, the inclusion, or not, of axial yarns,
by using more than one yarn type, and by the
choice of the yarns themselves. Tables 8 and 9
give typical properties for two-dimensional
braided composites. Table 8 refers to braided
S-2 fiberglass-epoxy composites with a fiber
volume fraction of 75%.
Table 9 presents the results of a study on
triaxial braids (08 + y). The results show that
in the axial direction the modulus is not significantly affected by braiding angle variations as a
result of the axial yarns. This is not the case for
the hoop moduli, which are clearly affected by
variations in the braiding angle.
2.18.3.4
2.18.3.4.1
Knitted Fabrics
Description
Knitting is by definition where loops of yarns
are meshed to form a fabric. Furthermore,
knitting is divided into the two categories of
warp and weft knitting, classified by the
method of production of the fabric. Warp
knitted fabrics are produced by forming loops
from each warp thread, along the length of the
fabric. Weft knitted fabrics are produced by
forming loops from one or more weft threads,
across the width of the fabric (refer to
Figure 24). Similarly, the wales refer to columns
of knitted stitches (i.e., along the length of the
fabric) and courses refer to rows of knitted
stitches (i.e., across the fabric width).
Knitting technology is explained in greater
detail elsewhere (Spencer, 1997; Raz, 1987).
However, the basic principles of both warp
and weft knitting will be explained in the following.
Warp knitting requires appropriate vertical
and horizontal movements of the needle and
sinker bars, respectively, resulting in the yarns
wrapping around the needles. The horizontal
movements of the guide bars provide the knitting pattern and are controlled by either cam or
chain link profiles. The distance between the
needles (machine gauge), the knitting pattern,
and the yarn properties will determine the properties of the preform. Examples of some typical
warp-knitted fabrics are given in Figure 25.
Weft knits are realized by a combination of
the vertical movements of the needles together
with the horizontal traversing of the yarn. All
weft-knit structures are essentially combinations of knit, tuck, and float stitches. According
to the official textile definition (Tubbs and
Daniels, 1991), a knit stitch (Figure 26(a)) is
defined as the basic unit of weft-knitted fabrics
consisting of a loop of yarn meshed with a
previously formed loop at its base. A float stitch
(Figure 26(b)), otherwise referred to as a miss
stitch, is the length of yarn not received by a
needle and connecting two loops of the same
course that are not in adjacent wales (Tubbs
Two-dimensional Preforms
Figure 24 Weft and warp knitted fabrics, respectively (after Spencer, 1997).
Figure 25
Examples of warp-knitted preforms (after Spencer, 1997).
Figure 26 (a) Knit stitch; (b) float stitch; (c) tuck stitches (after Spencer, 1997).
21
22
Composite Preforming Techniques
Figure 27 (a) Plain single jersey; (b) 1x1 rib; (c) full cardigan; (d) full milano (after Spencer, 1997).
Table 10
Term
Course
Abbreviation
Definition
c
A row of loops either across the width of a flat
fabric or around the circumference of a circular fabric
A column of loops along the length of a fabric
The length of yarn to form one complete knitted stitch
Total length of yarn to form a repeat (or unit cell)
divided by the number of stitches in the repeat structure
The number of visible loops per unit length measured
along a wale
The number of visible loops per unit length measured
along a course
Mass of fabric in grams per area in square meters
HTex per average loop length (it also indicates the extend
to which the area of the fabric is covered by yarn);
SI units = tex‰ mm71
Ratio of loop lengths in a structural repeat, where n is the
number of different courses
Linear density of yarn in g km71
Wale
Loop length
Average loop
length
Course density
w
L (mm)
Lav (mm)
Wale density
w cm71
Areal density
Tightness factor
Run-in ratio
Tex
Fundamental terms for knitted structures.
c cm71
g m72
K
L 1:L 2: . . . .Ln
Td
and Daniels, 1991). A tuck stitch (Figure 26(c))
consists of a held loop and a tuck loop, both of
which are intermeshed in the same course
(Tubbs and Daniels, 1991). Cam profiles determine the type of stitch formed and the use of
jacquard systems enable the production of
numerous knitting designs. Examples of some
typical weft-knitted fabrics are shown in
Figure 27.
The fundamental terms used to characterize
knitted structures are given in Table 10. Loop
length is especially important since it dictates,
together with bending and compression properties of the yarns, and the types of loops present,
the final geometry of the fabric. Loop length is
also of particular interest in composite processing since it remains constant even after deformation of the preform.
Knitted fabrics can also be inserted with
inlay yarns. In warp knitting, it is possible to
insert yarns in 08, 908, and any angle in between, provided it lies in the plane of the fabric
(refer to Section 2.18.3.1.2). However, in weft
knitting the only possible combinations include
08 and 908 directions (Marvin, 1970). The inclusion of straight fibers alters the properties in
the principal direction of insertion by reducing
extensibility and enhancing the tensile properties. Figure 28(a) illustrates a uniaxially inserted
weft-knitted fabric, Figure 28(b) illustrates a
combined woven-knitted fabric where the
inlay yarns are interlaced, and Figure 28(c)
shows an orthogonal biaxial inlay. Hence, the
density and orientation of inlay yarns used in
the knitted preform can tailor the stiffness and
strength of the composite.
Finally, integral knits or near net shapes may
also be produced. The diversity of the types of
shapes with the use of warp knitting is limited
to tubes and bifurcated tubes. However, the
Two-dimensional Preforms
23
Figure 28 (a) Uniaxial inlay; (b) biaxial inlay; (c) orthogonal biaxial inlay (after Marvin, 1970; Spencer,
1997).
Figure 29 Examples of integrally weft-knitted preforms (after Williams, 1987).
achievable complexity of the shapes is substantially greater with weft knitting. The main advantages of integrally knitted structures include
the cost saving due to lack of material waste
and the replacement of a traditionally laborintensive process with full automation. The resultant preform is not only free of any joints but
it can also be made with inlay yarns. Therefore,
knitted shapes can be used for high tensile
applications since fibers can be reinforced in
the desired direction. Figure 29 illustrates some
integrally knitted preforms.
2.18.3.4.2
Processibility
Knitted preforms are known for their ability
to conform to complex 3-D shapes due to a
built-in reserve of yarn in their 3-D path. The
mechanical behavior of knits is explained in
greater detail in Section 2.18.5, but basically,
knitted fabrics deform though the stretching of
the needle and sinker loops (i.e., curved segments) and the straightening of the legs (i.e.,
yarns connecting the curved portions). At high
tensile stress levels, the yarns will also slip over
one another until locking at the points of inter-
lacing occurs. Therefore, knitted fabrics are
commonly either stretched in the plane of the
fabric or deformed from a flat panel into a 3-D
shape and fabricated into a composite.
Knitted fabrics can be processed into composite materials either through liquid or solid
impregnation. In common solid impregnation
techniques for thermoplastic composites, either
co-mingled or powder impregnated yarns are
used. Alternatively, reinforcing and matrix
forming yarns can be co-knitted. Finally, film
stacking is also used. For thermoset resins, the
usual liquid impregnation techniques are used.
The architecture of the preform will greatly
affect the quality of the composite. For instance, lower fiber volume fraction preforms
are more easily infiltrated than preforms with
higher fiber volume fractions. Generally speaking, woven preforms are preferred for straight
contoured shapes while braided and knitted
preforms are selected for more complex shapes
but often confined to nonload bearing applications due to the low fiber volume fractions
achievable. However, vacuum assisted RTM
enables fiber volume fractions with weft knitting in excess of 60% (Drews and Kaldenhoff,
1994).
24
Composite Preforming Techniques
Figure 30 Comparison of woven, random mat, and warp-knitted composite stiffness and strength (after
Huysmans et al., 1996).
2.18.3.4.3
Properties
Figure 30 gives a comparative analysis
between various warp-knitted composite properties with typical woven and random mat composite properties. The composites all have a
40% glass fiber volume fraction and the resin
is epoxy. Concerning stiffness, knitted fabric
composites show comparable properties with
woven fabric composites and are remarkably
superior to random mat composites. It can also
be noted from the polar plots in Figure 30 that
knit reinforced composites enable the manipulation of isotropy/anisotropy. Therefore, the
mechanical properties can be controlled in the
various fabric orientations by simply using different knit structures in the composite. The
strength of knitted fabric composites is inferior
to woven fabric composites in the course and
wale directions. However, knitted fabric composites do not show the pronounced strength
loss in the bias direction which is characteristic
of woven fabric composites.
An advantage of knitted fabric composites is
the intermeshing of the loop structure in between the different fabric layers. Consequently,
the out-of-plane properties that have traditionally been a weak point of UD laminates and to
a lesser extent of woven fabric composites are
superior in knitted composites. Improved transverse properties are also reflected in increased
damage and impact resistance.
Figure 31 shows typical mode I crack
resistance curves for two warp-knitted fabric
Two-dimensional Preforms
25
Figure 31 Mode I fracture toughness of glass/epoxy composites tested in the wale direction.
composites (BDD/450 and BDF/555) for a
crack running in the course direction. In addition, typical toughness values for E-glass/epoxy
woven fabric composites (*1200 J m72) and Eglass/epoxy UD-composites (500±700 J m72)
are indicated. It is clear from this figure that
knitted fabric composites have excellent fracture toughness properties which far exceed
those of woven fabric and UD-based composites. The loop structure of the fabric forces a
continuous microscopic branching of the crack
path, thus improving the crack propagation
resistance. It is suggested that smaller loops
increase the number of crack branches and
lead to higher toughness values (BDD/450).
Generally speaking, the ultimate failure of
knit reinforced composites occurs at a lower
stress level than for weaves due to the level of
damage that develops around the yarns prior to
fracture of the yarn. An increase in the fiber
volume fractions and the insertion of inlay
yarns will result in increases in composite tensile
strengths with reference to the direction of fiber
insertion. The use of float stitches, which can be
considered as inlay yarns restricted to short
specific lengths, will result in improvements in
the composite stiffness. The use of inlay yarns
also enables higher energy absorption, improved impact resistance, and tensile strength
(Chou et al., 1992). An increase in the stitch
density and the number of plies used in the
composite will also improve the impact damage
resistance of weft knit reinforced composites
(Ramakrishna and Hamada, 1995). Knitted
preforms also enable the formation of holes
without machining, hence the mechanical properties around these areas are improved due to
higher local fiber volume fractions (Ruffieux
et al., 1992). The effect of stretching weft knitted
preforms will increase the composite tensile
strength and stiffening (Mayer et al., 1992; Verpoest and Dendauw, 1992; Ha et al., 1993).
2.18.3.5
Random Fiber Preforms
Random fiber preforms (or mats) are characterized by an almost random distribution of
the fiber orientations in the textile. This random fiber orientation results in low mechanical
properties of mat reinforced composites as the
strength and stiffness transfer efficiency is poor.
Moreover, the random orientation also limits
the maximum fiber content of the final material. Nevertheless, mats are the most used preform for composites. This is due to their
handleability, isotropy, processability, and
especially low cost (to achieve it, mats are
mainly produced with glass fibers).
Random fiber preforms can be categorized in
different ways:
(i) The length of the fibers: finite length
(chopped) or continuous,
(ii) The bonding method: mechanical or chemical,
(iii) The areal weight: fleeces or mats, and
(iv) The presence of matrix: dry or impregnated preforms.
2.18.3.5.1
(i)
Dry preforms
Chopped strand mats
Flat two-dimensional random fiber preforms
are mainly produced by fiber manufacturers.
Figure 32 schematically shows the production
of glass fiber products. Traditionally, twodimensional chopped strand mats are produced
26
Composite Preforming Techniques
Figure 32 Manufacturing of different types of random mats, based on glass fibers (own figure after
Vetrotex brochure).
by chopping roving and spreading them out
over a moving belt. Recently, however,
chopped strand mats are produced directly
after the bushing, without going through the
intermediate step of roving production. Fiber
lengths up to 50 mm are commonly used. The
deposition of the fibers is done by gravity, or
assisted by air stream or water dispersion.
After deposition of the fibers, some bonding
is required to maintain the integrity of the preform. The bonding can be achieved chemically
(using a binder) or mechanically.
A chemical bond is obtained by spraying a
low amount of binder (usually 2±10 % by mass)
on the sheet during the preform production.
This binder can be a thermoset or a thermoplastic in the form of a powder, an emulsion, or
a solution. The important requirements for a
binder are the uniformity of the distribution,
and the compatibility and solubility with respect to the matrix system.
As mentioned before, the binder stabilizes
the dry preform to improve the handleability.
If preform strength is required in the dry state
only, a soluble binder is preferably used: a
polyester or polystyrene binder will dissolve in
a polyester or vinylester resin upon impregnation, resulting in a fast wet-out of the preform.
Preforms with soluble binders do not have any
strength in the wet state, resulting in tearing of
the preform or fiber washing during resin injection. In those cases, medium- or low-solubility
binders (for instance, silanated acrylic resin)
have to be used, which have a negative effect
on the mechanical properties. The presence of
the binder inevitably limits the deformability of
the preform and thus the complexity of the
parts that can be produced. This problem can
be overcome by directly producing complex
shaped preforms (see Section 2.18.3.5.1(iv)).
Alternatively, a mechanical bonding can be
obtained by entanglement, stitching, or knitting. Entanglement can be achieved by needle
punching, or by air or water jet punching. At
the punching points, the discontinuous fibers
are three-dimensionally entangled. Entangled
preforms still have deformability up to 25%
in both directions, making them more deformable than chemically bonded preforms, mentioned above. Moreover, the absence of a
binder and the presence of fibers oriented in
the thickness direction results in faster wet-out
of the preform during composite production.
In the case of stitching or knitting, additional
fibers are added to keep the preform together.
This method can be used to form complexes,
Two-dimensional Preforms
27
Figure 33 Shaped preforms based on chopped roving are produced by spraying the chopped roving, together
with a binder, on a perforated preform mold (own figure based on book by Chris Rudd on mold technologies).
where several preform layers (e.g., mats, noncrimp fabrics, and weaves) are combined.
The commonly used areal densities for
chopped mats are 250±1000 g m72. Composites
based on chopped strand mats generally have a
fiber volume fraction between 25 and 40%,
which is much lower than for woven fabric
composites. The random orientation of the fibers results in almost isotropic strength and
stiffness characteristics. While it is in many
cases advantageous for a material to exhibit
the same mechanical response whatever the
direction, this benefit is only obtained by accepting very low levels of overall strength. The
inferior strength and stiffness is partly due to
the short fiber length, but mostly because of the
random arrangement of the fibers and the already mentioned lower fiber volume fraction.
(ii)
Continuous filament random mats
Continuous filament random mats are produced by swirling continuous strands onto a
moving belt. The binder, similar to chopped
strand mats, is then applied. A typical continuous random mat consists of multiple layers.
Although continuous filament random mats
look similar to chopped strand mats, they have
several advantages. The use of continuous fibers results in better mechanical properties,
improved resistance to fiber washing, and preform tearing during processing. Therefore, a
lower amount of binder can be used, improving
the wet-out during composite processing. Using
the right kind of binder, for instance, a water
emulsion of thermoplastic polyester or a two
stage UV-curing binder, makes continuous filament random mats very suitable for thermoforming (cf. Section 2.18.3.5.1(v)).
The most commonly used areal densities are
450 and 600 g m72, although other areal densities in the same range as chopped strand mats
are available. The fiber volume fractions that
can be obtained are somewhat lower than for
chopped strand mats (25±35%), because it is
more difficult to compact the fabric. On the
other hand, this characteristic results in a
higher permeability of the preform.
Although most continuous filament mats
have a random fiber distribution, it is possible
to introduce some directionality in the fiber
orientation by varying the belt speed. In the
most extreme case, directionalized glass mats
are obtained: the glass fibers are almost unidirectional.
(iii)
Surface veils
Special kinds of nonwovens are surface mats
or veils, with a very low areal density, usually
30±100 g m72, and a fiber volume fraction of
10±25 %. The chopped or continuous fibers are
held together with a polyester or PVA binder.
They are used as surface layers in open mold
technology, pultrusion, and filament winding.
They provide a smooth surface by eliminating
fiber strike-through (i.e., groups of fibers that
are clearly visible at the part surface) and a
resin-rich layer and reinforce the gel coat (in
some cases, the surface veil may eliminate the
use of a gel coat). Veils with special ªARº- or
ªCº-glass fibers or synthetic fibers provide improved chemical resistance.
(iv)
Shaped preforms
Shaped chopped strand random preforms are
produced as shown in Figure 33. Roving is
chopped and sprayed, together with a thermoset emulsion binder onto a perforated mold. By
means of airflow through the perforated mold,
the fibers are kept in place. After the spray-up,
the preform is heated to cure the binder. The
advantage of this production lies in the low-cost
materials and the complexity of the preforms
that can be made in this way. As for manual
28
Composite Preforming Techniques
Figure 34 In the P4-technology, the chopper gun is mounted on to a robot for perfect control of the fiber
distribution (Website USCAR).
spray-up, the quality of the manually produced
preforms (i.e., a homogenous fiber distribution)
strongly depends on the skills of the operator.
To obtain a uniform fiber distribution and
reproducible preforms, automated equipment
has been developed. The most advanced technology is the P4-Technology, where the spray
gun is mounted onto a robot arm (Figure 34).
In this case, a thermoplastic powder binder is
used. The equipment can also be used for continuous fiber placement (random or aligned).
(v)
Thermoforming
Complex shaped preforms are mostly
produced, starting from two-dimensional
continuous filament random mats, with a thermoplastic or two-stage curing binder. Thermoplastic binder based random mats are preheated
to melt the binder and subsequently transferred
to a preforming press where the preform is
pressed into a complex shape. A two-stage curing binder is a thermoset binder that is partially
cured during the mat production to provide
stability for the 2-D preform. The second curing stage is done in the preforming press.
As the preforming of complex parts can result in large deformation variations in the preform, high strains can result in significant
thinning and even tearing of the continuous
mat during preforming.
2.18.3.5.2
Impregnated preforms
For completeness, it should be mentioned
that impregnated preforms are also available.
The approach to intensely mix fibers and
matrix on the preform level is caused by the
fact that the infiltration of the matrix into the
preform is often a limiting factor in the reduction of production times, especially if thermoplastic resins are used. The impregnation of
thermoplastics is therefore often separated
from the actual part production, as is the case
for glass mat thermoplastic (GMT) preforms.
Thermoplastic composite processing is discussed in Chapters 2.25±2.33 in this volume.
The low viscosity of most thermoset resins
allow short impregnation times, but the use of
liquid resins creates extra cost for the part
manufacturer in terms of storage, cleaning,
and safety precautions in dealing with reactive
chemicals. The use of preimpregnated preforms
is therefore also a solution for thermosets. The
two main thermoset preforms are sheet molding
compounds (SMC) and bulk molding compounds (BMC). These preforms are discussed
in detail in Chapters 2.25±2.33 in this volume.
2.18.4
THREE-DIMENSIONAL
PREFORMS
2.18.4.1
2.18.4.1.1
Solid 3-D Textiles
3-D woven fabrics
A variety of processes exist which can produce 3-D woven textile preforms. Three-dimensional weaves have not only three-dimensional
shapes but have yarns in at least three directions. They have been primarily developed for
military and aerospace applications, e.g., missile cones and radar covers.
Three-dimensional Preforms
Figure 35 The yarn structure of a generic 3-D
weave.
(i)
Traditional 3-D woven structures
3-D woven structures can be divided into two
generic categories (see Figure 35). In the first
category textiles are created on multiwarp
looms. Fabrics consist of several distinct layers
of warp and weft yarns. These can be woven in
thick dense structures or with spacing between
them forming spacer or core materials. The
process is similar to the one used for 2-D
weaves but in this case separate harnesses lift
groups of warp yarns to different heights which
results in some of them being formed into layers
while others weave the layers together. The
shedding mechanism, which controls the height
to which harnesses are lifted, sets the number of
layers in a fabric. Chou and Ko (1989) reported
that weaves with up to 17 layers have been
produced. Contrary to the case of triaxial
weaves, it is not possible to introduce yarns in
the bias direction. Some beat up of the yarns is
required for better consolidation of the structure. This has the side effect that some yarns
may be slightly damaged.
A variety of fibers can be used including
glass, carbon, ceramic, and metal fibers. Various complex architectures have been produced
using the multiwarp approach: panels with variable cross-sectional thickness, T and I beams,
trusses, and others. Some recent developments
in this area allow for weaving with linkage
structures, e.g., with a bushing. As a result no
major degradation of properties due to machining exhibits itself. These new methods have the
major disadvantage that they have very slow
preform production rates which makes the final
product very expensive.
(ii)
Orthogonal nonwoven structures
The second generic category includes what
are called orthogonal nonwoven fabrics (the
reader is cautioned not to confuse the term
29
ªnonwovensº as used here with traditional
mats). Processes in this category were originally
developed for the needs of the aerospace industry with work focusing on carbon±carbon composites. Original work was carried out by
General Electric, AVCO, and FMI. More recent methods have been developed by Aerospatiale, Brochier and in Japan, by Fukuta of the
Research Institute for Polymers and Textiles
(Chou and Ko, 1989).
The techniques developed by AVCO, Brochier (Autoweave), and FMI (Ultra Loom) are
limited to the production of bodies of revolution. In these processes yarns are inserted radially into a mandrel. Following this,
circumferential yarns are wound and axial
yarns are laid simultaneously to form layers in
the space between radial rods. Therefore, the
final product has three sets of fibers. One set
runs vertically, the second set radially, and the
last set circumferentially. The integrity of preforms is generally low and in most cases impregnation has to be carried out before
removing the mandrel.
The other techniques produce 3-D block
structures. These have to be machined to the
desired shape after consolidation. In general
preform manufacture consists of inserting
weft yarns (Y-direction) between layers of
warp yarns (X-direction). This is followed by
the insertion of binder yarns (Z-direction) between the rows of warp yarns perpendicular to
the X- and Y-directions. Finally the yarns are
packed together using a beat-up mechanism. By
controlling how the axial yarns are arranged,
different structures can be achieved. These include densely packed highly stiff structures,
open lattice ones, and flexible structures,
which can conform to a variety of geometries.
As for the traditional 3-D weaves the manufacturing processes for nonwovens are slow and
expensive.
2.18.4.1.2
3-D braids
Three-dimensional braiding is an extension of
traditional two-dimensional braiding. The preform is the result of interlacing a number of yarn
systems to form an integral structure. Two
major categories of three-dimensional braids
exist: Cartesian 3-D braids and circumferential
3-D braids (see Figure 36). Circumferential 3-D
braids are similar to their 2-D tubular counterparts with the difference that many layers are
laid on top of each other forming a preform
with thick walls. The basic difference between
3-D Cartesian braids and 2-D braiding is in the
way the yarn carriers are displaced to create the
30
Composite Preforming Techniques
Figure 36 Examples of 3-D braided structures.
braid geometry. In 2-D braiding the carriers
move in a maypole fashion while in 3-D methods they move in a discrete, sequential manner.
Three-dimensional braiding techniques can be
characterized as two-step, four-step, and multistep processes depending on the number of
movements required for the yarn carriers to
return to their original positions. Contrary to
2-D braiding methods, 3-D methods use a beater to help with compaction after yarn interlacing. This is an important parameter, which
affects the final fabric geometry.
Near-net-shape parts having multidirectional reinforcement can be achieved. Thin
and thick highly damage-resistant structures
having complex shapes can be produced. Traditionally, the market for these reinforcement
architectures has been limited to aerospace
applications (e.g., rocket motor components)
but now attempts are being made to expand
into other application areas (sea and ground
transport). Factors affecting the widespread
adaptation of braided parts by industries
other than aerospace ones are the slow production rates and the difficulty in setting up computer programs to automate design analysis
and production due to the complex nature of
their yarn architecture. Additionally part dimensions are limited by the currently available
machines.
2.18.4.1.3
3-D knits
Weft-inserted multiaxial warp knits can be
considered as three-dimensional, as many
layers of UD material are knitted together
into one preform. These preforms are covered
more in detail in Section 2.18.3.1.
Another three-dimensional knitted material
is the MULTIMAT1 (Luo and Verpoest, 1999)
which is made up of knits and mats. MULTIMAT1 utilizes a layer of deformable knitted
fabric as the core and mats as skins. The knit
and mats are stitched together to form an integrated structure. The presence of the knitted
fabric results in two distinct advantages for
composites using MULTIMAT1 textiles. The
deformability of knitted fabrics is present in the
MULTIMAT1, which allows ease of forming
during manufacturing processes. Additionally,
due to the high permeability of knitted fabrics,
the knitted layer serves as a fast flowing channel
during the injection phase of RTM processes
with a significant reduction in injection time.
Composites produced using MULTIMAT1
materials have a sandwich structure with the
mats serving as skins and the knit as the core
(see Figure 37). Fiber volume fractions
achieved are in the range 22±45%. Good flexural and impact properties (due to stitching)
have been reported. These materials are used in
a wide range of applications including ground
transport (civilian and military vehicles, trains),
sea transport, and head protection.
2.18.4.1.4
Stitching
Stitching has been in use for many years to
provide through-the-thickness reinforcement
for composite preforms (see Figure 38). The
major advantages of current stitching technologies are that they enhance damage tolerance
and aid fabrication. In the past stitching could
only be carried out with prepregs but the introduction of resin transfer manufacturing processes for composites saw the use of stitching
for dry preforms. As a result significant gains
have been realized in stitching speed. In addition stitching can now be carried out through
thicker materials and the damage to in-plane
yarns has been reduced.
In terms of fabrication, stitching allows the
mechanical joining of preforms into a complete
structure before processing (see Figure 39). In
doing so the final preform can be processed
with limited or no damage.
Three-dimensional Preforms
31
Figure 37 (a) Schematic cross-section of a MULTIMAT material. (b) The cross-section of a MULTIMAT1 material (processed sample) obtained using a SEM. The mat and knitted layers can be clearly
distinguished.
Figure 38
Multiaxial warp knitted fabrics stitched together to form rib structures. Part (a) is an
unimpregnated preform while parts (b) and (c) are partly impregnated preforms.
Figure 39 (a), (b), and (c) were produced by folding and stitching MWK textiles.
2.18.4.1.5
Processing
Processing wise, most 3-D textiles can be
transformed into composites using methods
typical for 2-D materials like RTM and autoclaving. In some cases though, e.g., for orthogonal nonwovens, impregnation and even
curing have to be done while the preform is
still on the mandrel.
2.18.4.1.6
Properties
A major characteristic of all three-dimensional textile reinforced composites is that the
through-the-thickness reinforcement tackles
quite successfully delamination problems. On
the other hand, fiber volume fractions are usually not as high as those for their 2-D equivalents (Table 11). This is due to a reduction in the
32
Composite Preforming Techniques
Table 11 Comparison of elastic moduli for different Multimat reinforced composites and a 2-D woven
fabric composite.
Fabric
MULTIMAT (glass mat + knit)
2D-Woven (glass)
Vf (%)
E (GPa)
E (GPa)
E (GPa)
G12 (GPa)
12
25.0
34.9
36.3
08
7.3
13.3
16.1
18.2
908
6.9
9.8
14.4
17.9
458
6.7
11.2
14.4
11.1
2.5
4.0
5.4
3.6
Table 12 Properties of three-dimensional braided glass/polyester I-beams.
I-beam 1
I-beam 2
I-beam 3
Braid
Glass
50
18.34
21.10
150.5
145.1
Braid/inlay
Glass/glass
60
30.54
30.54
237.9
176.4
Braid/inlay
Glass/carbon
65
44.82
68.26
292.0
175.9
Geometry
Fiber
Vf (%)
Tensile modulus (GPa)
Compressive modulus (GPa)
Flexural strength (MPa)
Compressive strength (MPa)
Source: Engineered Materials Handbook, 1993.
Figure 40 Schematic of velvet weaving.
preform manufacturing, the size of the yarn
bundles, and the geometric construction. Considering the aforementioned parameters and the
quite extensive range of 3-D textiles and preforms that can be manufactured, it is almost
impossible to identify a superior system. The
choice of a textile and final mechanical properties depend on the end user's requirements. As
reported by Chou and Ko (1989), the data
in the literature focus primarily on tensile and
flexural characteristics. A few results are also
available on impact behavior and damage
(Chou and Ko, 1989).
2.18.4.2
2.18.4.2.1
Figure 41 Sandwich-fabric preform.
yarn packing density because of the introduction of additional yarn systems. For 3-D braids
(Table 12), the Vf can be as high as 68% (Chou
and Ko, 1989), while for orthogonal nonwovens that value is around 60% (Chou and Ko,
1989). Additionally, the structural integrity, the
fiber volume fraction, and yarn orientation in
each 3-D fabric depend on the method used for
Preforms for Woven Sandwich
Structures
Description
The distance woven (or sandwich) fabric preform is woven with the classical velvet weaving
process (Figure 40). If the last step of cutting
the vertical pile fiber in the core is skipped an
integral sandwich-fabric preform can be
obtained (see Figure 41). In this way, the two
skins of the future sandwich structure are connected by the pile yarns.
Many variations in fabric layout can be
made. Some examples of different configurations that can be obtained are shown in
Figure 42. Up to now mostly glass and some
carbon fabrics have been produced.
Three-dimensional Preforms
33
Figure 42 Examples of configurations made by velvet weaving.
The pile yarns are warp yarns that link two
skins together; they also contribute to the
woven structure of skins. Some important parameters that can be varied are pile yarn type, pile
length, pile density (piles per cm), pile spacing,
and the introduction of piles inclined to the
skins (Van Vuure et al., 1995a, 1995b). All
these parameters will influence the processability and the properties of the sandwich structure
and the composite made out of it.
The skins can be varied almost as much as in
any 2-D woven structure (yarn type, yarn density, and weave pattern). However, up to now,
only orthotropic weave patterns are used. It is
important that the pile yarns in the core adequately fix in the skins to guarantee a sufficient
resiliency of the fabric in the thickness direction.
The manufacture of sandwich structures
from woven distance fabrics has been done by
impregnation with a thermoset resin (i.e.,
polyester, epoxy, and phenolic resin). Several
techniques can be used although hand lay-up
and hot melt impregnation are the common
techniques that have been applied (Van Vuure
et al., 1995a, 1995b; Judawisastra et al., 1999).
Resin films, used in the latter, make the control
of resin content and distribution easy. Impregnation can be made in a continuous way and the
various kinds of additional skins can be bonded
in situ to the base panels. The core of the
structure can also be filled up with expandable
foams to improve the mechanical properties of
the composites: compression, shear, and bending resistance.
When the fabric is being impregnated, the
piles can stretch up themselves due to the resiliency effect and capillary action of the pile fiber
bundles. This effect decreases for finer yarns,
longer pile lengths, lower pile densities, and
distribution. It is possible to achieve high pile
lengths (>16 mm) or to have fully stretch conditions by the adhesive foil stretching process.
The impregnated fabric is placed in between
two foils, which are attached by vacuum to
the plates of a specially designed hot press.
When the skins are partially cured, and hence
the piles are still soft and flexible, the distance
between the plates is gradually increased until
the desired panel thickness is reached. Then, the
curing continues until the piles are stiff enough.
By this unique processing method maximum
sandwich thickness can be obtained (Van
Vuure et al., 1995a, 1995b).
The sandwich structures produced by woven
distance fabrics will inherently have advantages
over conventional sandwich structures (with
honeycomb, foam, or balsa cores) due to the
integral core structure. The advantages include
a more cost-effective alternative and a high
delamination resistance (Van Vuure et al.,
1995a, 1995b; Judawisastra et al., 1999; Ivens
et al., 1992).
2.18.4.2.2
Properties
The basic mechanical properties of the sandwich-fabric have been presented in references
(Van Vuure et al., 1995a, 1995b; Judawisastra
et al., 1999; Ivens et al., 1992). The properties of
this sandwich material strongly depend on the
microstructure of the core. Extensive research
has been carried out to find the most relevant
parameters. These parameters are related to the
fabric (type of fiber, pile density, pile length,
woven pile angles), the resin (type, resin content), the foam (type, density), and the production technique (degree of stretching of pile
fibers, deviation of the piles).
Figure 43 shows skin-core delamination
resistance for various sandwich fabric panels
compared with other sandwich panels. The
most important advantage of 3-D-woven sandwich panels is a superior skin-core delamination resistance, especially if it is considered that
the other sandwich panels are all of 20 mm
thickness.
The two most critical sandwich properties,
flatwise compression and shear strength, are
compared with those of alternative sandwich
structures in Figures 44 and 45. Although the
honeycomb exhibits better mechanical properties for the same density of woven sandwich
structure, woven sandwich-fabrics offer an
appealing alternative due to their lower cost
and higher delamination resistance which
leads to higher damage tolerance.
34
Composite Preforming Techniques
Figure 43 Skin peel strength: results and test set-up (LD = low pile density, HD = high pile density,
F = foamed panels, other sandwich panels have 20 mm core thickness).
Figure 44 Comparison of compressive strengths (Brandt et al., 1996).
Figure 45 Comparison of shear strengths (Brandt et al., 1996).
Three-dimensional Preforms
35
Table 13 Comparison of 3-D woven with 2-D knitted composites. The disadvantages of the 3-D woven
structures are the advantages of the knitted materials.
Composite
Advantages
Disadvantages
3-D Woven
Good specific properties
The textiles are not drapable
on double curved surfaces
Closed skins
Good delamination and impact
properties
Efficient processing
Textiles are extremely drapable
Open structures are possible
Controllable (an)isotropy
2-D Knitted
The sandwich core properties can be further
improved by injecting the into sandwich-fabric
structure rigid foam. However, this results in an
increased weight of the panels. The compression strength of the sandwich is inversely related to the pile length and rises dramatically if
the core is foamed.
Good and a higher shear resistance can be
obtained by either the use of 45 8 pile yarns in
the core, or by using a higher pile density with
an even pile distribution and a lower degree of
stretching, or by using a higher resin content
and also by using foam in the core.
2.18.4.3
2.18.4.3.1
Preforms for Knitted Sandwich
Structures
Description
In the previous sections, 3-D woven sandwich (Section 2.18.4.2) and 2-D knitted (Section
2.18.3.4) composites have been discussed. The
main characteristics of 3-D woven composites
are the integral sandwich structure, which
makes these materials quite damage tolerant.
Moreover, these textile preforms also make the
processing easier. On the other hand, the main
disadvantage of woven preforms is that no
complex double-curved surfaces can be produced.
For composites based on 2-D knits, the most
important advantage is the extreme drapability
of the knitted textile. Another asset is the possibility to control the (an)isotropy of the knit
and hence the mechanical properties of the final
composite. Unfortunately, the mechanical
properties such as stiffness are somewhat
lower because the knitting yarns are oriented
in all directions and are not straight.
All this has been summarized in Table 13,
which compares the advantages and disadvantages of 3-D woven and 2-D knitted composites. It can be noticed that the disadvantages
Fixed anisotropy
Acceptable properties
of 3-D woven materials are exactly the same as
the advantages of the 2-D knitted materials.
Hence, if it were possible to knit a 3-D sandwich textile and to make a composite with it, a
new class of materials with an interesting combination of properties would emerge (Philips,
1999).
The production of 3-D knittings is very similar to that of flat knittings. In principle, two
knitting machines are put back to back in order
to produce the top and the bottom skins of the
sandwich fabric simultaneously. In practice, a
double-bed Raschel knitting machine is used
for this purpose. This machine has two needle
beds that can be controlled more or less independently. By changing the distance between
the needle beds it is possible to vary the fabric
height (Figure 46).
Figure 47 shows the two basic types of 3-D
knitted sandwich fabrics: knits with open or
with closed skins. The left sample is a closedskin 3-D knitted distance fabric, which is very
similar to a 3-D sandwich weave. The main
difference is that the yarns have a loop shape
in the knitted skin while they are more or less
straight in the woven skin. Hence, the deformability of closed 3-D knits will already be better
than that of 3-D weaves. A second difference is
that 3-D knits need thick thermoplastic monofilaments to stabilize the core, which is not
required for the 3-D weaves.
The deformability of 3-D sandwich knits can
be improved by introducing pores (cell structure) in the skins. This can easily be done by not
connecting all the loops in the skins with each
other. In this way, an expandable, cellular 3-D
knit can be produced as shown in Figure 47 on
the right. While this fabric is still on the knitting
machine, it looks like a fully closed 3-D knit.
However, once it comes off the machine, it is
expanded in the width direction so that the
holes become visible. For this very open and
loose fabric it is absolutely necessary to have
monofilaments in the pile fibers to stabilize the
structure.
36
Composite Preforming Techniques
Figure 46 Schematic drawing of the 3-D knitting production process. Top and bottom skin of the textile
are simultaneously knitted on the two beds of the Rashell knitting machine while the pile fibers are knitted
in both skins so that they become connected.
Figure 47 Comparison of a closed-skin 3-D knit with an open 3-D knit. The closed-skin knit is very similar
to a 3-D weave.
Figure 48 Complex shape demonstrating the extreme drapability of open 3-D knits.
2.18.4.3.2
(i)
Processibility
Drapability of the knitted preform
Because of the knitted structure, this textile is
more deformable than woven preforms. Especially open 3-D knits have a very good drapability because of the cellular structure, which
allows many complex types of deformations
(Figure 48).
(ii)
Thermofixing of the knitted preform
Since many residual stresses are present in
the thermoplastic yarns due to the knitting
process, it is better to remove them so that a
more thermally stable preform is obtained. For
this reason, the textile is put in an oven at a
temperature where the residual stresses in the
thermoplastic monofilaments can be relaxed.
After cooling the fabric, the fabric is stress-
Three-dimensional Preforms
37
stresses. When cooling down, the material is
frozen in the cellular geometry.
It is interesting to note that this technique
can be applied to thermoforming of complex
parts in the desired shape to facilitate further
processing into a composite structure.
(iii)
Figure 49 Bad and inhomogenous impregnation of
pure PET monofilament 3-D knits for composites.
Figure 50 Combined yarn (polyester multifilament
and PET monofilament) used for the second generation of 3-D knits.
Figure 51 Good and homogenous impregnation of
combined pile fibers.
free so that the geometrical stability during
further processing is secured.
For the open cell 3-D knits, there is also
another reason for thermofixing: when the fabric has been removed from the knitting machine, it has to be expanded in the width
(weft) direction to open the cellular structure.
Then the fabric is heated to release the internal
Impregnation and curing
For 3-D knitted composites, the core of the
knit will (almost) always contain thermoplastic
monofilament pile fibers because their high
bending rigidity is needed to keep the skins
apart during impregnation.
For closed 3-D knitted composites, it is the
aim to impregnate the skins and most of the
time also the core. If only the skins have to be
impregnated, resin films can be placed onto the
skins, the stack is put between heated plates, the
resin melts and it is absorbed by the skins. The
core will hardly be impregnated in this case.
However, if, i.e., glass fiber bundles are used in
the core, a small amount of resin will also be
present in the core because of capillary effects.
For open 3-D knitted composites, resin films
cannot be applied anymore. Here, liquid resin
impregnation will have to be used. The resin is
diluted with solvent and the textile is soaked
into this mixture. The excess resin is squeezed
out, the solvent is evaporated, and the sample is
cured in an oven. The main point here is that
the resin has to be homogeneously distributed
all over the sample. For the open materials,
especially the core, they will have to be impregnated well and this can only be achieved if the
pile fibers have also been impregnated. Pure
monofilament piles are quite difficult to ªimpregnateº (coat) as demonstrated in Figure 49.
Combined pile fibers consist of a monofilament
core covered with multifilaments that absorb
resin (Figure 50). Here, the resin is absorbed
more evenly (Figure 51). It can be expected that
the resin distribution will have a profound
influence on the final composite properties.
2.18.4.3.3
Properties
Figure 52 gives an indication of the bending
properties that can be achieved with open 3-D
knitted composite materials. The apparent
E-modulus (the sandwich structure behaves
like a solid material and is treated accordingly)
calculated from bending tests is plotted for
three types of material. The first group of
samples on the left is based on the original
PET-monofilament knits. The poor impregnation (Figure 50) explains why the properties
are very low, even at high resin contents. The
38
Composite Preforming Techniques
Figure 52 Comparison of the apparent E-modulus for different 3-D knitted epoxy composites tested in
three-point bending with a span length of 100 mm.
second group of materials has ramie-viscose
fibers inside the core so that resin will be
absorbed well and will be distributed homogeneously. The third group of materials additionally has glass fibers in the skins that lead to a
further improvement of the properties. Within
this last group, the apparent E-modulus is
extremely dependent on the direction in which
the sample is tested. Warp and diagonal direction have more or less the same modulus while
the weft direction has very low values.
2.18.5
MECHANICAL PROPERTIES AND
PROCESSIBILITY
2.18.5.1
2.18.5.1.1
Analytical Models for Stiffness and
Strength Predictions of Textile
Composites
Introduction
Developing models for thermomechanical
properties of composites can have two goals.
First, one can aim at analyzing the properties
and understanding the specific influence of material and geometrical parameters. A second
goal can be to predict the composite properties,
starting from the properties of the constituent
materials, and from an accurate description of
their geometry. Ultimately, one would like to
predict the composite properties even without
having to produce the actual material.
Both goals are equally important in the case
of modeling the thermomechanical properties
of composites based on textile preforms. Understanding is important, because the sometimes intricate yarn architecture can create
very complex local stress states, the influence
of which on the average stiffness and on the
damage development and final failure is not at
all straightforward. Prediction is important,
because the variety of textile preforms is so
immensely large that producing all of them,
let alone making composites out of them, is
inconceivable.
Predictions of properties, based only on the
geometry and the thermomechanical properties
of the constituent materials, are hence very
necessary. Moreover, during processing, the
textile preforms can be subjected to large extensional and shear deformations, creating in fact
a ªnewº preform. Predictive models should include the effect of processing-induced deformations on the eventual properties of the textile
composite.
Thermomechanical models should be able to
analyze and predict the following. First, the
three-dimensional stiffness matrix of the representative volume element (RVE) of the textile
composite should be predicted, and hence not
only the in-plane elastic properties, as many
earlier models do. Second, the local stresses
and strains in the RVE should be calculated,
under any arbitrary, externally imposed threedimensional stresses or strains. Third, by applying appropriate failure criteria locally, not only
the initial cracking but also damage development and final failure should be predicted
accurately. Fourth, the three-dimensional thermal expansion coefficients should be predicted
as well, and local thermal stresses, originating
from processing, should be taken into account.
Moreover, the model should be applicable to all
types of textile preforms (weaves, braids, knits,
stitched and embroided fabrics, etc.), both single layered (two-dimensional) and multilayered
or truly three-dimensional.
Figure 53 shows schematically how the models used for calculating the mechanical properties of textile based composites differ in
accuracy and calculation time or effort.
Mechanical Properties and Processibility
39
Figure 53 Comparison of different families of micromechanical models for textile based composites.
It is obvious that the early models, like the
Krenchel model (Krenchel, 1964) or the different types of FGMs (fabric geometry models) or
Series/Parallel models are easy to apply but at
the same time rather inaccurate. The FGM
models are based on oversimplifying isostrain
(Voigt) or isostress (Reuss) assumptions, and
the geometrical information used is too limited.
In this chapter, focus will be on three classes of
more sophisticated and more performant models: the cell-based models, the inclusion type
models (both being analytical), and the numerical models, like the finite element or the
boundary element models. (It should, however,
be mentioned that the analytical models are
highly ªnumericalº because they use intricate
iterative procedures.)
Three criteria can be used to compare the
different models, two of which can be expressed
quantitatively: the accuracy of the predicted
properties, and the calculation time to achieve
these predictions (Figure 53). The third group
of criteria are more qualitative, but contribute
very much to the user's satisfaction: the ease of
carrying out parametric studies, the sensitivity
to small errors or variations in the RVE geometry, the possibility of linking the model to
geometrical preprocessors (e.g., generating the
yarn architecture in the RVE from the textile
machine settings and the yarn properties), or
being able to link the predicted properties to
structural FE packages, where the model can
itself serve as a material data generating preprocessor. The latter criteria not only depend
on the model concept itself, but also on the way
the model has been implemented in a specific
software package; they will hence not be discussed in detail.
2.18.5.1.2
Basic hypotheses and principles of
the models
The general structure of thermomechanical
models for textile composites is represented in
Figure 54. It consists of three basic parts: geometrical modeling, local stress/strain calculation, and stiffness and strength prediction.
The modeling of the textile geometry as such
will not be discussed in depth. It is, however,
important to state that all three presented micromechanical model types start from an accurate description of the yarn architecture within
a representative volume element (RVE). By a
three-dimensional translation operation, the
complete textile can then be reconstructed. In
the following models, it is hence assumed that
an accurate geometrical description of the yarn
architecture is possible. Basically, two routes
can be followed to achieve this goal (Figure 54).
First, ab initio geometrical models, also
called ªprocess models,º in which only two
sets of input data are needed. On the one
hand, the mechanical properties of the fibers
and yarns (mainly bending and compression).
On the other, some basic geometrical data on
the textile structure (e.g., the weave type, the
yarn density in warp and weft, etc.). Lomov
et al. (2000) has developed a generic model
based on the minimization of energy (mainly
due to bending) of the yarns in the textile RVE,
and has shown convincing examples of 2- and
3-D weaves.
Second, experimental data can be, manually
or automatically, translated into a full geometrical description of the yarn architecture in the
RVE. For most 2-D textiles, microscopic
images of the unimpregnated textile already
40
Composite Preforming Techniques
Figure 54 Schematic representation of the different steps in modeling thermo-mechanical properties of
textile composites.
Figure 55
Schematic representation of the hierarchical top-bottom decomposition and bottom-up
homogenization procedure (after Vandeurzen et al., 1997).
offer a lot of information, although the out-ofplane positions and shapes of the yarns are
more difficult to determine. Alternatively, the
textiles are impregnated with a thermoset resin
and cured, and the through-the-thickness sections are investigated under the microscope.
X-ray tomography has been used to directly
view the three-dimensional yarn geometry inside a textile or a textile composite (Lomov and
Verpoest, 2000).
As a compromise, mixed methods have been
developed (Huysmans, 2000). They make use of
a limited number of experimental observations,
and use some basic characteristics of the textile
to reconstruct the yarn heartline spline function
and the cross-sectional shape and orientation
along the yarns.
(i)
Cell based models
The basic principle of the cell-based models,
as they have been developed for textile composites by Vandeurzen et al. (1996, 1997), are
schematically presented in Figure 55. It consists
of two hierarchically structured operations,
which will be explained here for woven fabrics,
although the same principle holds for braided
fabrics. It is clear that knits cannot be easily
handled by this hierarchical approach.
The hierarchical approach, as developed by
Vandeurzen, is not absolutely necessary in cell
models. More recently, Huysmans (2000) suggested applying the cell based models in one
step, from the macro- directly to the microlevel.
From a mathematical point of view, this
Mechanical Properties and Processibility
approach seems more complex, but it can be
computationally more effective. Vandeurzen's
hierarchical approach offers a good compromise. However, one should be aware that the
periodic boundary conditions, used throughout
the analysis, are, strictly speaking, not present
at all levels.
First, the unit cell or RVE is sequentially split
up into increasingly smaller material entities,
up to the level of fibers and matrix, of which the
thermomechanical properties are supposed to
be known. The division of the RVE into block
cells is dictated by the way the warp and weft
yarns cross each other. The further division
along the yarn axis into microcells is governed
by the out-of-plane curvature of the yarns,
because the curved yarn will be segmented
into straight yarn segments. Then a division in
the direction perpendicular to the yarn axis
takes into account the lenticular shape of the
yarn.
The smallest unit, namely the microcell, now
consists of an impregnated yarn with a certain
shape and orientation, embedded in matrix; it is
further simplified into a bimaterial, where only
the relative volume of impregnated yarn and
matrix is retained. Finally, the impregnated
yarn bundle can be split up into fibers and
matrix.
Second, opposed to this top-down decomposition scheme, a bottom-up homogenization
procedure has been developed to come up
with the local stresses and strains. As a byproduct, the three-dimensional stiffness matrix
can be easily derived. Existing micromechanical
models have been used to calculate (at level 5),
from the fiber and matrix properties, the elastic
properties of the impregnated yarn. At level 4,
for the first time the complementary variational
principle is applied to derive the stiffness of the
bimaterial. In contradiction to well-established
fabric geometry models, the method does not
assume either an isotrain or an isostress condition. It is stated that from all admissible stress
fields, the true field is that which minimizes the
total complementary energy of the cell. This
energy minimization problem can be solved,
with the additional constraints of stress
continuity across the internal cell subdivision
borders and with the stress averaging laws.
Basically, this procedure also provides one
with so-called stress concentration tensors,
which relate the average stresses acting in the
cell with the stresses acting in the cell subdivisions. These stress concentration tensors will
later on be used to calculate, given an external
stress on the RVE, hierarchically top-down the
local stresses, up to the fiber and matrix level.
On level 3, the stiffness matrix of the microcell is calculated by transforming the stiffness
41
matrix of the combi-cell, calculated on level 4 in
the on-axis or local coordinate system, to the
global coordinate system; in this way the yarn
orientation in the microcell is accounted for
correctly. Finally, the same complementary
variational principle is applied on levels 2 and
1 to finally come up with the stiffness (or compliance) matrix of the RVE, and hence of the
total textile composite. Full details of this calculation procedure are given in Vandeurzen
et al. (1996), where it is also explained that a
similar procedure can be followed to calculate
the thermal expansion coefficients and consequently the thermal stresses.
(ii)
Inclusion models
Inclusion models are widely used for micromechanical modeling of polycrystalline materials or short fiber reinforced composites. They
are based on Eshelby's equivalent inclusion
principle, and have been further developed
and adapted, first to knitted and later to
woven and braided fabric composites, by Huysmans et al. (1998) and Gommers et al. (1996).
As in the cell models, the impregnated yarns
are split up into short segments, which are
considered to be straight. They are characterized by an orientation (coinciding with the fiber
orientation in the yarn), a cross-section, and a
segment length. The split-up procedure can
differ for weaves, braids, and knits, but Huysmans et al. (1998) proposed a spline function
representation of the curved yarns, which is
equally applicable to all textile types.
Each yarn segment is considered as a (transversally isotropic) heterogeneity, embedded in
the matrix, and causing a perturbation of the
macroscopic strain field. Each heterogeneity
can be replaced, according to the general
Eshelby approach, by an equivalent inclusion
with the same properties as the matrix, but with
a fictitious ªeigenstrain.º It can be shown that,
if a relationship between perturbation and eigenstrains is known, and if the equilibrium
conditions for the whole RVE are applied, the
stresses and strains at any position in the RVE
can be solved.
In this approach, two major difficulties have
to be overcome. First, the relationship between
the eigenstrain and the perturbation strain in
the same inclusion (given by the so-called
Eshelby tensor); a solution is known for single,
ellipsoid-shaped inclusions, and hence yarn
segments have to be mapped as ellipsoids.
Second, the relationship between the eigenstrain and the perturbation strain in different
inclusions; this requires some ªaveraging
schemes.º Two routes have been explored to
accomplish this:
42
Composite Preforming Techniques
(i) In the Mori±Tanaka scheme, it is
assumed that each inclusion feels its neighboring inclusions indirectly via the total strain in
the matrix, and that this influence is homogeneously distributed over the different inclusions. Although this allows for an explicit
solution, Huysmans et al. (1998) indicated
that this assumption might be invalid for the
case of yarn segments, which are anisotropic
inclusions with nontrivial orientation distributions.
(ii) Alternatively, the singly embedded selfconsistent scheme assumes that each inclusion
feels the influence of the others directly through
the composite as a whole. Hence, the solution
becomes implicit, because the inclusions are
considered to embedded in a ªmatrixº with
yet unknown composite properties. This seriously increases the computational effort, especially because Eshelby's tensor has to be
computed numerically for each yarn segment
during every iteration.
(iii)
Numerical methods
The major problem for the application of
numerical models, like finite element or boundary element models, to textile composites, lies in
the intricate yarn architecture inside the RVE.
This makes the mesh generation very complex,
and the required number of elements to describe the RVE with sufficient accuracy extremely large. A full FEM analysis, including
volume elements, hence becomes almost prohibitive, except for simple geometries (Paumelle,
1990). Moreover, small changes in the yarn
architecture within the RVE (during parametric
studies, or as a consequence of processing) require a complete remeshing. Automatic mesh
generating procedures could cure this problem.
Zako (see Huysmans, 2000) has solved this
problem for woven fabrics by constructing the
RVE as a set of deformed yarns. The initially
straight yarns with ellipsoidal or lenticular
cross-sections are rather easy to mesh. Then,
they are bent into the interlaced weave structure, a matrix is added, and the new mesh is
generated automatically. Calculation of the
local stresses and strains is still computationally
intensive, but results in accurate predictions.
The huge computational effort can be reduced by using simplified approaches. Hamada
et al. (1994) presented a finite element model for
plain weft knitted fabrics, in which the composite was treated as an assembly of matrix and
impregnated yarn beam elements. Although the
in-plane moduli were predicted satisfactorily,
the shear moduli were not treated and the
strength properties were predicted less accurately. Due to the beam representation of the
matrix, an extension to more complex knits is
not evident either.
A generalized FEM model has been developed for complex knitted fabric composites
(Huysmans, 2000) using the binary approach
originally developed by Cox et al. (1994). In this
approach, the composite is decomposed into
linear truss elements surrounded by three-dimensional volume elements. In-plane elastic
properties were predicted with high accuracy,
except for the shear modulus, which is largely
underestimated. Moreover, these models require large computation time and are less practical for strength analysis.
2.18.5.1.3
Comparison of the performance of
the different models
This comparison will be limited to the elastic
properties. All three model types will be compared, and the results will be compared with
experimental data.
As an example, the predicted elastic constants
of some woven fabric composites are calculated
using the Flexcomp program (Huysmans,
2000), which is based on the inclusion models
following either the Mori±Tanaka (MT) or the
self-consistent (SC) schemes. The results are
compared with those of the cell-based models
(complementary energy models, CEM, using
the Texcomp program), with fabric geometry
models (FGM), and with experimental data.
This is in addition to the parametric calculations
carried out previously using the data of Paumelle, and where the results were compared with
his FEM predictions (Vandeurzen et al., 1996).
Ten types of woven fabric composites were
evaluated: six E-glass-epoxy (four plain and two
basket weaves), a carbon-Kevlar and a glassKevlar hybrid weave, and two plain weaves
containing only monofilament steel wires.
Figure 56 shows a typical polar plot of extensional (E) and shear (G) moduli predictions for
glass-fiber fabrics. The quality of both the MT
and SC predictions is very good, with overall
errors less than 10%.
The accuracy of the carbon/Kevlar and glass/
Kevlar predictions were found to be similar (be
it slightly less). More interestingly, Figure 57
shows the results for one of the exotic steel
fabrics, for which the isostrain model is totally
inadequate. The SC model overestimates the
on-axis moduli (and hence the shear modulus
in the bias direction), but the on-axis shear was
better than the MT estimate.
To demonstrate the overall behavior of the
inclusion models applied to woven fabric
composites, performance maps were made.
Mechanical Properties and Processibility
43
Figure 56 Polar representation of the Young's
modulus (a) and shear modulus (b) predictions for
an E-glass/epoxy woven fabric composite (RE280L)
(CEM: cell model; FC.MT and FC.SC: inclusion
models; exp: experimental data).
Figure 57 Polar representation of the Young's
modulus (a) and shear modulus (b) predictions for
a steel fabric/epoxy matrix composite (for symbols
see Figure 56).
Figure 58 shows the relative errors for the
different material combinations based on the
overall in-plane behavior (i.e., an orientation
average over all in-plane orientations).
For the traditional material combinations,
the self-consistent inclusion model performs
slightly better than the Mori±Tanaka method.
The improvement of the CEM model with respect to the SC model is mainly attributed to
the E-moduli predictions. As mentioned above,
the error in the Poisson effect is comparable for
all methods. (This could also be caused by the
larger experimental error, often encountered in
Poisson's ratio measurements.)
From Figure 58 it might seem that the FGM
model might be the most attractive model from
an accuracy/performance viewpoint. First of
all, it does not work for isotropic yarns (like
the steel monofilament yarns). Second, isostrain models perform do not well when outof-plane properties are considered (e.g., no yarn
shape effects included). From comparison with
the FEM data of Paumelle, it was shown that
the isostrain models overestimate out-of-plane
properties. The inclusion models do not show
this shortcoming.
In a forthcoming publication (Huysmans and
Verpoest, 2001), a similar, extensive comparison will be presented for knitted fabric composites. As already indicated in Huysmans (2000),
the validity of the inclusion models for knits is
confirmed by comparison with a large number
of experimental data.
As a conclusion, one can present a ranking
for micromechanical models for woven fabric
composites:
(i) If the largest accuracy is desired (e.g.,
local stress/damage analysis), the cell-based
models (CEM method) should be used for
woven and braided fabric composites. For
knitted fabric composites, the application of
CEM remains doubtful.
(ii) If a quick estimate of elastic properties is
desired, an inclusion model using the Mori±
Tanaka scheme is clearly preferable over the
fabric geometry (FGM) models. MT is far more
accurate than FGM, if the average over all the
in-plane orientations is taken into account.
Elsewhere (Huysmans, 2000), it is shown that
the out-of-plane properties are also predicted
more accurately using MT or CEM. Over
CEM, using the Mori±Tanaka scheme has the
44
Composite Preforming Techniques
advantage of being very fast. The self-consistent inclusion model (SC model) is for most
applications a little more accurate than MT,
but it has comparable calculation times with
the CEM model. Hence, if one has the choice
(like for weaves or braids), it is better to switch
directly to the CEM model if accuracy is the
major issue.
2.18.5.2
Processibility
When one designs a composite structure, one
needs to consider not only the mechanical properties, but also the processability of the chosen
material. Drapability and permeability are two
of the most important aspects of the processability.
Drapability is the ability of a textile preform
to conform to complex-shaped surfaces. In
many of the composite manufacturing techniques, a flat piece of textile reinforcement needs
to be brought into contact with a tool surface.
For instance, the first step of the liquid composite molding technique involves placing and
draping the dry fabric in the mold cavity. In
thermoplastic composite processing with comingled yarns, the fabric is first deep drawn
into the mold, then heat and pressure are applied to consolidate the part.
Two types of faults can result from a draping
operation: fabric wrinkling, due to excessive
shear deformation; and fabric tearing, due to
large tensile deformation. Moreover, the deformation causes the change of local fiber orientation and volume fraction (Bickerton et al.,
1997) and thus has a significant effect on the
processing and mechanical properties of the
textile composites. It has been shown (Smith
et al., 1997) that the shear deformation of the
woven fabric can influence the stiffness and
permeability dramatically.
The ability to predict the fabric configuration
after the draping process enables us to predict
the potential flaws of the textile prior to the
investment of expensive equipment. Furthermore, it allows us to determine the fiber rearrangement during the draping operation, which
can be used as input for an integrated design
tool.
It was observed that the major deformation
modes for the woven fabrics are shear and
interfiber slip (Potter, 1979). The shear deformation is realized by the free rotation of warp
and weft yarns around the yarn crossovers, the
so-called trellis effect. Wrinkling occurs at a
critical shear angle (locking angle) when the
yarns are jammed together. The measured locking angle for several types of weaves varies
between 20 and 408.
For the knitted fabrics, the deformation is
contributed mainly by the fiber extension in
both wale and course directions. By doing
biaxial tensile tests with different displacement
Figure 58 Relative error in the prediction of the in-plane elastic constants for different woven material
combinations: Average over all in-plane orientations (for symbols see Figure 60, except: FGM-NX-P: fabric
geometry model)
Conclusion
45
Figure 59 Deformation limit curves for glass knitted fabrics. Typical absolute values of woven fabrics are
shown for comparison.
ratios, deformation limit curves can be constructed and used as the guideline for draping
(see Figure 59).
Another important material property influencing the processability is permeability. Permeability is an inherent property of porous
materials that can be characterized by their
resistance to fluid flow under the driving force
of a pressure gradient. During the liquid composite molding techniques, such as resin transfer molding, vacuum assisted resin infusion,
etc., the resin has to flow through a dry fiber
bed. Darcy's law (Darcy, 1856) describes the
fluid flow in a porous media
uˆ
Q
k P
ˆÿ
A
Z L
where Q is the volumetric flow rate, A is the
cross-section, u is the superficial velocity, k is
the permeability of the fiber bed, Z is the viscosity of the fluid, and DP/L is the pressure
gradient.
When making large complex parts, it is important to know how the resin is going to fill the
mold before the actual production. This can be
done with flow simulation software, which uses
the permeability of the fabrics as input. With
the knowledge of the permeability, the flow
simulation software can predict the resin flow
in the mold by using Darcy's law. This gives the
mold designers information to make decisions
on where to locate mold inlets and exits, as well
as an estimation of mold filling time. Besides,
the pressure distribution inside the mold can be
solved. This helps to define the clamping pressure, the pump power, and the required mold
stiffness.
The permeability of several types of glass
fabrics is shown in Figure 60. Generally speaking permeability depends on fiber volume fraction and fabric architecture. It decreases as the
fiber volume fraction increases. At the same
fiber volume fraction the permeability of different fabrics can be very different due to the
difference in fabric architecture.
2.18.6
CONCLUSION
In this chapter, an attempt has been made to
give a concise overview of the state of the art in
preform technologies for composites. The
length of this chapter in itself indicates that
preforms have become, over the past decade,
a major topic of research and development in
the composites world. Certainly not all existing
preform types have been presented, but it is
hoped that at least a useful framework has
been provided to better understand the processing and properties of composites as they are
related to the preform type and architecture.
ACKNOWLEDGMENTS
This chapter is the result of a collaborative
effort of a group of researchers within the
Composites and Ceramics group of the Department of Metallurgy and Materials Engineering
of the Katholieke Universiteit Leuven, Belgium. The enormous help and important
contributions of all the co-authors should be
acknowledged explicitly: Jan Ivens, Hermawan
Judawisastra, Yiwen Luo, Surya Darma
46
Composite Preforming Techniques
Figure 60
Permeability values of several types of knitted, woven fabrics and a random mat measured by
radial flow method.
Pandita, Dirk Philips, Andreas Prodromou,
and Sule Savci. Special thanks also to Stepan
Lomov for reading the final manuscript, and to
Katrien Lammertyn for merging all the individual contributions into one file, and for the
final editing of the text. The quality of the
product of this collaborative exercise has to be
judged by the reader; the process towards this
product was anyway a most interesting and
rewarding experience!
2.18.7
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Comprehensive Composite Materials
ISBN (set): 0-08 0429939
Volume 2; (ISBN: 0-080437206); pp. 623±669