Fabric Drape
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4. Fabric drape
4.1 Deformation modes
The fibre configuration in a textile fabric affects the processing properties porosity andpermeability and also the mechanical properties of the finished composites part. Thus the
characterisation of the distribution of fibre angles, caused by local deformation effects, is an
important task in order to describe the manufacture and the performance of the part. Drape is
the deformation of two-dimensional textiles and fabrics caused by gravity or other external
forces for adaptation of the textiles on doubly-curved surfaces. Body forces such as gravity
act directly on the fibres, while contact forces are transferred by friction. The following
deformation modes can occur for textile materials [1].
In a single fabric layer:
Shearing Straightening Wrinkling Stretching SlippingIn a lay-up of several layers:
Slipping of fibre layers, especially at edges of the surface
Figure 4-1. Illustration of the drape modes fibre shear (a), straightening (b), wrinkling (c), stretching (d), slip (e)
and inter-layer slip (f).
a
b
c
d
e
f
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Fabric shear, as discussed e.g. by Skelton [2,3] and Behre [4], occurs, if the directions of
applied tensile forces do not coincide with the orientations of the fibre tow axes. The fibre tow
orientations change, until the fibre axes coincide with the directions of the applied forces, or
until a fabric specific maximum shear angle (locking angle) is achieved. Actual locking
angles as observed experimentally for various glass fibre woven fabrics are in the rangebetween 15 and 35 (Paper C), depending on the fabric architecture. In case of fibre locking,
the fabric starts to wrinkle due to local shear stresses. Aspects of fibre bending and fabric
wrinkling have been discussed by Grosberg and Swani [5] and Dahlberg [6]. Complex
buckling of fabrics has been described and mechanically analysed by Amirbayat and Hearle
[7]. Fabric shear has been found to be in general the most important fabric deformation effect
[8,9]. Fibre straightening with changes in the curvature of fibres under tensile load in general
is the deformation mode to occur first. This effect is significant for knitted fabrics, while for
most low-crimp textile architectures, the effect is low. Elastic fibre stretching is generally of
minor significance, since the reinforcement textiles are normally processed from fibres withhigh elastic tensile modulus. Slipping of the fibre tows can occur at sharp edges or corners of
the surface and is subject to friction in the crossing points. In addition to the effects mentioned
above, secondary effects such as bending and torsion caused by friction between the fibre
tows and compression caused by forces normal to the axes of the fibre tows can occur on a
microscopic scale in the fibre tows [10].
Parametric studies, carried out by Chen and Govindaraj [11], have shown that the draping
behaviour is mainly determined by
elastic tensile modulus in material principal directions shear modulus material thicknessTensile modulus and shear modulus depend on the type of the fibres and on the fabric
architecture, as has been shown by Long et al. [12] for various non-crimp fabrics. The
Poisson-ratio is of minor importance for the draping behaviour. Collier et al. [13] and Kang
and Yu [14] report experimental determination of the mechanical properties for
characterisation of textiles using the Kawabata evaluation system [15,16]. While Hu andZhang [17] discuss the applicability of the Kawabata system for determination of the fabric
shear modulus, Culpin [18] suggests an alternative approach for quantitative experimental
determination of the shear properties. Hu and Chan [19] investigated the relation between the
fabric drape coefficient, characterising the formability, and the mechanical properties.
Experimental results for the draping behaviour of various commercial fabrics have been
published by Lindberg et al. [20]. Mohammed et al. [21] studied the shear deformation of
woven fabrics of various architectures with respect to mechanical and microstructural aspects.
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4.2 Influence of fabric architecture
Figure 4-2. Qualitative classification of various reinforcement textile architectures with respect to drapability and
shear stiffness [22].
In the diagram shown in Figure 4-2, various reinforcement textiles are classified with respect
to drapability and shear stiffness. For most fabric architectures, there is a relation between
shear stiffness and drapability, i.e. the lower the shear stiffness, the higher the drapability.
Exceptions are UD tapes, which have low shear stiffness and low drapability due to the lack
of fixation of parallel fibres, and random fibre mats, which have high isotropic shear stiffness
due to entanglement of the fibres but due to the lack of internal structure still conform easily
to arbitrary surfaces [23]. In general, the reinforcing effect of the textiles increases from the
upper right corner of the diagram to the lower left corner, although it is hardly possible to
compare all architectures due to the various fields of application (e.g. UD tapes and 3D
woven). Knitted fabrics conform well to arbitrary surfaces, since the mobility of the fibres is
high and there is a significant effect of fibre straightening of the highly curved fibres. The
reinforcing effect in terms of an increase in stiffness of the composite for knitted fabrics is
low. For three-dimensional woven fabrics, on the other hand, the mobility of the fibres is
highly constricted and the reinforcing effect is high. The drapability is low.
4.3 Description of drape
Attempts for description of fabric drape and for numerical drape simulation have not only
been made in the field of composites engineering, aiming at the characterisation of processing
parameters and mechanical properties of the finished parts, but also
in textile industry, for description of the material behaviour regarding visual and sensualimpressions in clothing and fashion [24]
in computer graphics, for realistic representation of textile deformation, e.g. in animatedmovie sequences [25].
shear stiffness
high low
drapability
low
high
3D braided,
wovenUD tape
fibre
mat
2D braided
2D woven
2D knitted
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While in composites engineering a quantitative description of the final state of the draped
textile is relevant, for textile industry ("virtual catwalk") and computer graphics qualitative
real time display of dynamic drape effects is of interest.
The fibre orientations of bi-directional fabrics draped on doubly curved surfaces can be
determined by applying drape simulation techniques. Approaches to describe the draping
behaviour of fabrics have been discussed e.g. by de Jong and Postle [26], van der Ween [27],
Breen et al. [28] and Shanahan et al. [29]. Kinematic models, mapping a geometrical pattern
representing the textile structure on a surface taking into account defined geometrical
constraints [27], and elastic models, describing the fabrics as anisotropic continuous structures
[30], that are discretised to calculate the deformation and determine the fibre orientations
using finite-element-methods (FEM) [13,14], will be discussed in the following sections. In
particle models [28], each point of intersection of fibres is represented by a discrete particle,to which the physical properties of the fabric are attributed. For each particle the energy,
given by interaction with the adjacent particles, is determined. Minimisation of the particle
energy gives the most probable configuration. This approach is similar to the kinematic
model, but does not require a surface for fabric deposition. Since fibre bending is also taken
into consideration, free forming of fabrics under the influence of gravity can be simulated.
Chen et al. [31,32] also suggest application of the finite-volume-method for simulation of
complex deformations of woven fabrics under self-weight or external loads.
4.4 Theory of kinematic / geometrical drape simulation
The fitting of woven fabrics to surfaces has first been discussed by Mack and Taylor [33].
While their method is restricted to fitting of fabrics to analytical surfaces, Heisey and Haller
[34] describe fitting to non-analytical surfaces using numerical analysis techniques. Robertson
et al. determined criteria for fabric wrinkling, applying a kinematic algorithm for calculation
of the fibre arrangement of woven cloth draped on a hemisphere [35] and on a cone [36].
Examples for kinematic simulation of fabric drape and its applications have been presentede.g. by van West et al. [37], Bergsma [38] (for fibre reinforced thermoplastics), Long and
Rudd [39], Trochu et al. [40] and Wang et al. [41]. Potluri et al. [42] extended the capabilities
of kinematic drape algorithms to draping of closed preforms. The basics of kinematic drape
simulation are discussed by van der Ween [27] and Tucker [43]. Each point x on a doubly
curved surface can be described parametrically in surface co-ordinates ui:
),( 21 uuxx= .
The elementary length ds of a surface segment between two points is given by the firstfundamental form of the surface
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jiji ududGsd =2 (4-1)
with the coefficients
ji
jiuu
G
=
xx. (4-2)
The textile is described by co-ordinates vi oriented along the fibre directions. The elementary
length ds of a section of the deformed textile is given by
jijiji vdvdEsd )2(2
+= d . (4-3)
Eij is the Green-Lagrange-tensor with
011 =E , 022 =E (4-4)
(inextensible fibres) and
acos2 21 =E , (4-5)
i.e. the textile deformation is assumed to be given by pure shear with a fibre angle a.
For deposition of the textile on the surface the elementary length of a surface segment and a
textile section are equal
jijijijiji vdvdEududG )2( += d . (4-6)
With the Einstein convention and substitution of Equations (4-4) and (4-5), Equation (4-6)
gives
2
221
2
1
2
2222121
2
111 cos22 vdvdvdvdudGududGudG ++=++ a . (4-7)
Drape of the fabric on the surface is described by mapping
),( 21 vvuu ii = ,
for which various approaches can be chosen [27]. Inserting of
2
2
11
1
11 vd
v
uvd
v
uud
+
= and 2
2
21
1
22 vd
v
uvd
v
uud
+
= (4-8)
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in Equation (4-7) gives
2
221
2
1
21
2
2
1
222
1
2
2
1
2
2
1
121
2
1
1
111
2
2
2
2
222
2
2
2
121
2
2
111
2
1
2
1
2
221
2
1
1
21
2
1
1
11
cos2
2
2
2
vdvdvdvd
vdvdv
u
v
uG
v
u
v
u
v
u
v
uG
v
u
v
uG
vdv
uG
v
u
v
uG
v
uG
vdv
uG
v
u
v
uG
v
uG
++=
+
+
+
+
+
+
+
+
+
a
(4-9)
Comparison of the left hand side and right hand side of Equation (4-9) gives the set ofequations
12
2
1
222
1
2
1
121
2
1
111 =
+
+
v
uG
v
u
v
uG
v
uG , (4-10)
12
2
2
222
2
2
2
121
2
2
111 =
+
+
v
uG
v
u
v
uG
v
uG , (4-11)
acos2
2
1
222
1
2
2
1
2
2
1
121
2
1
1
111 =
+
+
+
v
u
v
uG
v
u
v
u
v
u
v
uG
v
u
v
uG . (4-12)
Boundary conditions are defined e.g. by the requirement that for
01 =v and 02 =v (4-13)
the fibres are put on the surface along geodesic lines.
For numerical solution of the non-linear set of Equations (4-10) to (4-12), the fabric is
discretised in a grid with edge length d, such that the grid point (i,j) is characterised by the
fibre co-ordinates
div =1 and djv =2 (4-14)
as illustrated in Figure 4-3. Transformation of the position of each grid point to surface co-
ordinates or spatial co-ordinates allows to determine the angle a between the fibre
orientations.
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(0,0)v1v2
(i,j)
(i - 1,j - 1)
d
a
d
Figure 4-3. Kinematic simulation of drape on a spherical surface; illustration of discretised fabric.
The drape simulation is based on an algorithm with the following steps:
1. choose starting point for deposition of the fabric on the surface2. define initial orientation for fibres with v2 = 03. put fibre with v2 = 0 along geodesic line on surface4. repeat steps 2 and 3 for fibres with v1 = 05. loop overi andj and determine the positions of the grid point (i,j), such that for each cell
with grid points (i,j) and (i-1, j-1) as diagonally opposite corners the conditions imposed
by Equations (4-10) to (4-12) are satisfied
Inversion of the mapping
),( 21 uuvv ii =
gives the set of equations
11
2
1
2
1
2
1
1
2
1
1 cos2 Gu
v
u
v
u
v
u
v=
+
+
a (4-15)
22
2
2
2
2
2
2
1
2
2
1 cos2 Gu
v
u
v
u
v
u
v=
+
+
a (4-16)
21
2
2
1
2
1
2
2
1
2
2
1
1
2
1
1
1 cos G
u
v
u
v
u
v
u
v
u
v
u
v
u
v
u
v=
+
+
+
a (4-17)
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The results of the drape simulation, depending on the starting point for draping of the textile
and on the initial orientation of the fibres [41], are angles for the two directions of fibre
orientation with respect to the local co-ordinate axes of the corresponding finite element and
the thickness of the draped material for each element. For laminates with different propertiesof the layers, fabric drape is simulated for each single layer. The results are saved in text
format in a data file and can be read for manipulation. Since the drape simulation is performed
on a mesh representing the final geometry of the composite part, the mesh can be simulated
such that it can be used for the finite element injection simulation.
4.6 Theory of finite element drape simulation
In finite element drape simulation, textiles are treated as continuous structures, discretised for
numerical analysis. For modelling of the material in finite element analysis two approaches
exist. One approach considers the microscopic structure of the material with micromechanical
interactions between the fibres. The continuous fibres are discretised in beam elements,
representing segments of the fibres between two intersection points [6]. Mass and elastic
modulus of the fibres are attributed to the beams. If, in addition to fibre shear, also the effect
of fibre slip is considered, contacts are simulated in the nodes of the beam elements. In a
macroscopic approach, textiles are treated as two-dimensional orthotropic continuous
structures [45,46]. Averaged homogeneous properties are attributed to each finite element.Micromechanical effects are implicitly taken into consideration by specification of effective
values for elastic tensile modulus, shear modulus and Poisson-number of the textiles. This
statistical treatment is justified, if the elements include a minimum number of textile unit
cells. In two dimensions, fabrics are mostly modelled as shells [14,47,48], while there are also
approaches to simulate sheet forming processes implementing membrane [49] or plate [13]
elements. Due to the large deformations occurring in fabric drape, the forming behaviour of
textiles is to be considered geometrically non-linear [29,50,51]. Similar considerations are
applied for forming processes of fabrics pre-impregnated with thermoset or thermoplastic
matrix material [52,53].
The description of forming processes is based on continuum mechanics. The momentum of a
continuous system with density distribution r, velocity distribution v and volume Vis
=V
dVvP r . (4-18)
The system is subject to body forces
=V
V VdaF r (4-19)
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with acceleration a. Contact forces
=A
C dAnF (4-20)
with the stress tensors and the normal vectorn of dA are exerted on the surface A of thesystem. With Gau's theorem this formulation can be replaced by
=V
C VdF . (4-21)
In each inertial system the balance of momentum equation, stating that the change of
momentum corresponds to the sum of body and contact forces
CVtd
dFF
P+= , (4-22)
must be satisfied. The change of momentum is in Lagrange formulation expressed as
Vdtd
d
td
d
V
vPr= . (4-23)
Substitution of body and contact forces:
+=VVV
VdVdVdtd
da
vrr . (4-24)
The weak formulation of Equation (4-24) is
+=
VVVVduVduVdu
ax rr&&
(4-25)
with the weight function u. With Green's theorem
+-=V AV
AduVduVdu n (4-26)
Equation (4-25) gives
+-=V AVV
AduVduVduVdu nax rr && . (4-27)
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Equation (4-27) is discretised in finite elements, on which the acceleration is described by
jjW Xx&&&& = (4-28)
with the vectors of nodal accelerations jX&& and the shape functions Wj. Choice of the weight
functions according to Galerkin
iWu = (4-29)
gives
+-= V Aii
V
ij
V
ji AdWVdWVdWVdWW naX rr&&
. (4-30)
Equation (4-30) corresponds to a set of equations
uKFuM +=&& (4-31)
with mass matrix
VdWW jV
i r=M (4-32)
and acceleration vector
jXu&&&& = . (4-33)
The term
AdWVdWA
i
V
i naF += r (4-34)
corresponds to external forces Fext, the term
VdWV
i uK -= (4-35)
is identified with internal forces Fint. Transformation of Equation (4-31) gives a system of
equations for the vector of nodal acceleration
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)(1 intext FFMu -=-
&& (4-36)
with given right hand side. If the mass of the system is "lumped" in the nodes, i.e. the
continuous mass distribution is approximated by discrete masses located in the nodes, the
inversion of the resulting diagonal matrix is trivial and Equation (4-36) gives a system ofdecoupled equations that can be solved for the nodal accelerations.
Equation (4-36) is discretised in time such that
)(1 nintnextn FFMu -=-
&& (4-37)
with the notation
)( nn tuu &&&& = .
The nodal velocities and displacements are calculated from the nodal accelerations according
to the explicit central differences scheme
nnnn tD+= -+ uuu &&&& 2/12/1 , 2/12/1 -+ -=D nnn ttt (4-38)
and
2/12/11 +++ D+= nnnn tuuu & , nnn ttt -=D ++ 12/1 (4-39)
with the initial conditions
0)0( uu && = and 0)0( uu = .
The values with index n+1/2 are interpolated in the middle of the interval from tn to tn+1.
Forming of thin sheets is simulated using shell elements with 6 degrees of freedom per node,
i.e. 3 degrees of freedom for translation and 3 for rotation. Equation (4-37) then corresponds
to a set of 6Nequations forNnodes. In practice, four-noded quadrilateral elements with bi-
linear shape functions are often used. Procedures like this [48] are extensively discussed in the
standard literature, e.g. by Zienkiewicz and Taylor [54] and Bathe [55].
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4.7 Drape simulation with PAM-FORM
The commercial software code PAM-FORM for forming simulation of metallic or non-
metallic sheet materials [53] has originally been developed for the simulation of structural
responses to transient dynamic loading, especially in crash situations. The Lagrangian non-linear dynamic numerical analysis is based on an explicit finite element method with central
differences time integration scheme as discussed above [56]. Forming by contact forces is
simulated, taking into consideration the material properties of the sheet materials.
The sheet material is described as an orthotropic continuum without modelling of the
individual fibres and their interactions. Forming of thin sheets is simulated using shell
elements. For the shell elements macroscopic material models
)(=
are implemented. The composite material is treated as a three-phase material [56]:
The first phase represents a uni-directional or bi-directional fibre material with linear-elastic properties. Two initial fibre orientations can be defined by vectors. The fibres do
not only contribute to membrane stresses, but also to bending moments calculated by
classical beam theory.
The second phase represents the effect of the textile architecture on the material propertiesdue to interaction of the fibre tows. This isotropic linear-elastic phase has no real
equivalent. It is characterised by a shear modulus G and a Poisson-numbern. The various
drape effects that can occur are expressed in the effective shear stiffness and are not
formulated separately. The shear modulus changes for a given value of the fibre angle
(locking angle alock). For values of the fibre angle smaller than the locking angle, the shear
stiffness changes abruptly and the fabric starts to buckle or to wrinkle (Figure 4-5).
1GG = , lockaa
2GG= , lockaa (4-40)
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Figure 4-5. Shear modulus as function of fibre angle a for a fabric with an initial fibre angle of 90.
A third optional phase represents a thermo-visco-elastic matrix, characterised by aMaxwell model with a spring and a dashpot in series. Introduction of this phase is useful
for description of the forming behaviour of fibre reinforced thermoplastic materials.
The mechanical behaviour of the fabric is given by superposition of the three phases in
parallel. When all phases are in parallel, the strains are identical
tmf == , (4-41)
while the stresses in the three phases sum up as
tmf ++= . (4-42)
The fibre orientations in the fabric are calculated from the shear strains of the deformed shell
elements. Forming of multi-layer structures such as preforms for laminates is simulated by
layering of shell elements, each layer representing one material ply.
Figure 4-6. Example for finite element drape simulation: Forming of a hemisphere. Surfaces of punch, die,
blankholder and fabric sheet are modelled.
a
G
alock
G1
G2
90
punch
die
blankholder
fabric sheet
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In forming processes, forces are transferred from the tools to the fabric by contacts. For the
simulation the surfaces of the sheet, the tools (punch and die) and blankholders are modelled
(Figure 4-6). The surfaces of the tools and blankholders are modelled using "null"-elements
with no calculation of nodal displacements, that are used for contact definition only. Since no
deformations are supposed to occur in the tools, they are modelled as rigid bodies, i.e.translational and rotational degrees of freedom are fixed relative to the centre of mass. A
contact between two objects is detected, if the distance between the surfaces of the objects
vanishes locally. Contacts are characterised by restrictions on the relative movement of the
objects normal to the contact interface. The relative movement of the objects tangential to the
contact interface is subject to frictional forces. For simulation of contacts an asymmetric
formulation is applied [56]. The surfaces of the tools are defined as "Master", the surface of
the sheet as "Slave". In given time intervals the distance between master-surface and slave-
nodes is controlled. If the distance between any segment of the master-surface and any slave-
node is smaller than the contact thickness, penetration is detected. The contact thickness isgiven by the thickness of the sheet, represented by the thickness of the shell elements. In case
of penetration, restoring forces in form of step functions are activated, avoiding further
penetration of the surfaces. The restoring forces are proportional to the depth of penetration
and the contact stiffness (penalty contact). The contact stiffness is again dependent on the
stiffness and density values of the objects being in contact with each other.
4.8 Comparison of kinematic and finite element drape simulation
Figure 4-7 shows the results of the kinematic drape simulation for fabric drape on a
hemisphere. The simulation gives the pattern of the fibres placed on the surface of the final
geometry. The results of the FE simulation for draping of the same fabric on the same surface
geometry are illustrated in Figure 4-8, showing the vectors for the fibre axes on the deformed
sheet. Comparison of Figures 4-7 and 4-8 shows qualitative correspondence of the fibre
orientation patterns achieved by both simulation methods. Quantitative differences can be
observed, since Figure 4-8 does not show the final state of the simulation. In the finite
element simulation, local deviations of the vectors from the general fibre orientations arecaused by numerical effects such as hourglassing of the shell elements. This effect can occur
for shell elements with reduced numerical integration schemes. Discussion of the theory of
shell elements is beyond the scope of this work and can be found e.g. in the work published
by Belytschko et al. [57,58].
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Figure 4-7. Drape of a 0/90 fabric on a hemisphere; kinematic simulation using PATRAN Laminate Modeler.
Figure 4-8. Drape of a 0/90 fabric on a hemisphere; Lagrangian non-linear dynamic finite element simulation
using PAM-FORM.
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Figure 4-9. Drape of a 0/90 twill weave 2x2 glass fibre fabric on a hemisphere.
Figure 4-10. Drape of a 0/90 twill weave 2x2 carbon/aramid hybrid fabric on a hemisphere.
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Figures 4-9 and 4-10 show typical results of drape experiments of 0/90 fabrics on a
hemisphere. Especially from Figure 4-9 it can be seen, that under experimental conditions, it
is difficult to achieve a perfectly symmetrical fibre pattern. Nonetheless, both experimental
pictures show the same characteristics: Under 0 and 90, there is no change in fibre angle
and no fabric extension. Under 45, the fibre angle is reduced and the fabric is extended dueto fibre shear. These observations are in agreement with the quantitative results of draping
experiments of various fabrics on a hemisphere, presented by Mohammed et al. [59].
Comparison of the simulated and experimentally determined fibre patterns shows qualitative
correspondence of the results. Quantitative correspondence depends on correct information on
the locking angle (for the kinematic simulation) and of the effective stiffness values of the
shell elements (for the finite element simulation).
Draping experiments on a hemisphere for validation of results for both kinematic and finiteelement drape simulation have frequently been documented in the literature. In general, good
agreement between experiment and simulation has been found: Van West et al. [37] and Long
et al. [12], for example, report good agreement between predicted and measured fibre patterns
for kinematic drape simulation methods. For unbalanced fabrics, however, Long et al. [10]
found the results of standard kinematic drape simulation less satisfactory and developed a
strain energy based iterative mapping scheme. For finite element methods, Dong et al. [48]
studied the influence of numerical parameters on the simulation results for explicit dynamic
analysis implementing shell elements, applying the ABAQUS commercial code. The
presented results are similar to the results shown in Figure 4-8. Simon et al. [52] simulated thehot drape forming of carbon fibre/epoxy prepregs using PAM-STAMP and found good
coincidence with experiments. For a finite element method implementing membrane
elements, Boisse et al. [49] report good agreement with drape experiments.
The kinematic simulation does not consider the actual material behaviour and the mechanics
of the draping process and is based on geometrical information only. Main problems of the
FEM approach, on the other hand, are related to the experimental determination of the
effective material stiffness [45] and to the numerical solution of the complex mathematical
problem (non-linear effects in material behaviour, large deformations, contacts, friction). Forthe example shown in Figures 4-7 and 4-8, the CPU time for kinematic drape simulation is in
the order of magnitude of 1 s on a PC (Pentium II, 400 MHz), for finite element drape
simulation it is in the order of magnitude of 20 h on a workstation (SUN Blade 1000). Finite
element modelling is complex since a high number of null elements is required to mesh the
spherical surfaces of the tools with reasonable accuracy.
The mesh for kinematic drape simulation can be generated with respect to the needs of the
LCM resin flow simulation formulated in Euler co-ordinates. In case of FE drape simulation
formulated in Lagrange co-ordinates, the information on fibre orientations needs to bemapped from the deformed mesh representing the draped fabric onto a second mesh, meeting
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the requirements of the flow simulation. Kinematic drape simulation is thus to be preferred to
finite element drape simulation for combination with LCM-simulation.
References
[1] Long, A.C., Rudd, C.D., Blagdon, M., Kendall, K.N., Demeri, M.Y.: "Simulation andMeasurement of Reinforcement Deformation During Preform Manufacture", Polymers
& Polymer Composites 4(5), 1996
[2] Skelton, J.: "Fundamentals of Fabric Shear", Textile Research Journal46(1), 1976
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