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.

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