Mechanical modelling of solid woven fabric composites 81 Table 3.4.Classification of the geometric parameters Know group M Number of fibres in the yarn d Diameter of the fibre p Yarn spacing Measure group f Aspect ratio of the yarns hi Crimp parameter for the filling yarn D Thickness of the composite K Fibre packing density Calculate group hoghw Crimp parameter for the warp yarns t Thickness of the yarn cross-section w Width of the yarn cross-section Orientation of the yarn 4 Fibre volume fraction Certainly,the most important output of the calculation procedure is the poo fibre volume fraction,the orientation of the yarns and the fractional volume of each cell.These data are the basis for a modelling of mechanical prop- L:00 erties.Moreover,the geometric model as such is most useful in determin- ing some textile properties as fabric thickness,but also in determining the allowable microstructural states of fabrics.A custom Microsoft Excel application,called TexComp,has been developed to perform all geometric calculations [34]. 3.3.3 Multilevel decomposition scheme ashierarchical system that can be decomposed.Two The geometric model treats a woven fabric composite unit cell,shown in 8 vations are here formulated.First,the calculation and bookkeeping of geometric data should become a simple task.It is easy to calculate the geometric parameters that fully describe the yarn architecture only based on the presented 'know'and 'measure'group.Second,a logic and simple geometric meshing of the unit cell is essential for the computation of the mechanical properties.Basically,the composite unit cell level(1)is split up into block cells or macro-cells (2),micro-cells (3),matrix and yarn layers (4)and matrix and fibres (5).This five-level decomposition scheme could be considered as an 'intelligent mesh generator'for 2-D woven fabric composites.A logic extension towards 3-D woven preforms and to braids is currently being carried out. The block partition of the unit cell consists of discretizing the unit cell in a number of rectangular block cells.At each crossover zone of a warp yarn and a weft yarn,one 'building block'is defined.The size of each block can easily be computed as a function of yarn spacings p,yarn widths w and com-Certainly, the most important output of the calculation procedure is the fibre volume fraction, the orientation of the yarns and the fractional volume of each cell. These data are the basis for a modelling of mechanical prop￾erties. Moreover, the geometric model as such is most useful in determin￾ing some textile properties as fabric thickness, but also in determining the allowable microstructural states of fabrics. A custom Microsoft Excel® application, called TexComp, has been developed to perform all geometric calculations [34]. 3.3.3 Multilevel decomposition scheme The geometric model treats a woven fabric composite unit cell, shown in Fig. 3.5, as a hierarchical system that can be decomposed. Two major moti￾vations are here formulated. First, the calculation and bookkeeping of geometric data should become a simple task. It is easy to calculate the geometric parameters that fully describe the yarn architecture only based on the presented ‘know’ and ‘measure’ group. Second, a logic and simple geometric meshing of the unit cell is essential for the computation of the mechanical properties. Basically, the composite unit cell level (1) is split up into block cells or macro-cells (2), micro-cells (3), matrix and yarn layers (4) and matrix and fibres (5). This five-level decomposition scheme could be considered as an ‘intelligent mesh generator’ for 2-D woven fabric composites. A logic extension towards 3-D woven preforms and to braids is currently being carried out. The block partition of the unit cell consists of discretizing the unit cell in a number of rectangular block cells. At each crossover zone of a warp yarn and a weft yarn, one ‘building block’ is defined. The size of each block can easily be computed as a function of yarn spacings p, yarn widths w and com￾Mechanical modelling of solid woven fabric composites 81 Table 3.4. Classification of the geometric parameters Know group Nf Number of fibres in the yarn d Diameter of the fibre p Yarn spacing Measure group f Aspect ratio of the yarns hf Crimp parameter for the filling yarn D Thickness of the composite K Fibre packing density Calculate group hw,hw* Crimp parameter for the warp yarns t Thickness of the yarn cross-section w Width of the yarn cross-section b Orientation of the yarn Vf Fibre volume fraction RIC3 7/10/99 7:37 PM Page 81 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:11 AM IP Address: 158.132.122.9