Issue 29

M. Romano et alii, Frattura ed Integrità Strutturale, 29 (2014) 385-398; DOI: 10.3221/IGF-ESIS.29.34 391 m f i i i 1 m A (Q n )     (3) where m is the mass, A is the area of the test panel, Q is the specific weight of the fabric, n is the number of the single layers of each fabric and the subscript f indicates the property of all fabrics in the layup. Considering the summation i is the counter for the different kinds of reinforcement fibres used and m is the upper limit of the counter, i. e. the number of the different kinds of reinforcement fibres used in the hybrid stacked material. The mass of the composite c m is obtained by weighing the single specimens on a precision balance. The fibre mass content in the specimen  can then be calculated by evaluating Eq. (1). For an analytical evaluation of the fibre volume content f  the density of the fibres f  is necessary. Because more than one kind of reinforcement fibre is used in the hybrid stacked materials the fibre density is not a uniform value for the material. Yet it is possible to analytically evaluate a mean value of the density of the fibre reinforcement in the composite material f  . Therefore the volume of each kind of dry fabrics is calculated by i i V A t n    (4) where A is the area of the specimen, n is the number of the single layers of each fabric, i is the counter for the different kinds of reinforcement fibres used and t is the thickness of each single fabric reinforced layers. Thereby the values of the thickness of the single fabric reinforced layers have been taken from the respective technical data sheets [12], [13] and [18]. The mean value of the density of the fibre reinforcement in the composite material is analytically calculated by f , f , 1 f 3 f , 1 ( ) m i i i i i V V         (5) where V is the volume of each kind of dry fabrics,  is the nominal density of each kind of reinforcement fibre and the subscript f indicates the property of the fabric reinforcement. Considering the summation i is the counter for the different kinds of reinforcement fibres used and m is the upper limit of the counter, i.e. the number of the different kinds of reinforcement fibres used in the hybrid stacked material. Type of test panel Mean value f  (%) Standard deviation f  (%) Carbon 63.60 0.46 Glass 58.65 0.94 Basalt 61.16 0.66 C+G+B 63.07 0.72 C+G+Hex+B 57.15 1.36 Table 3 : Fibre volume contents of the test panels. The calculation of the fibre volume content according to Eq. (2) can be carried out by using the mean value of the density of the fibre reinforcement in the composite material f  instead of the density of a single kind if reinforcement fibre  . Though, it is sufficient to average the density of the used fibre reinforcement and carry out the evaluation according to Eq. (5).