Issue 29

M. Romano et alii, Frattura ed Integrità Strutturale, 29 (2014) 385-398; DOI: 10.3221/IGF-ESIS.29.34 390 Experimental determination of the fibre volume content Basically there are two methods for experimentally investigating the fibre volume content. Whereas for inorganic fibres like glass or basalt fibres it can be determined by thermal vaporization the polymeric matrix system according to DIN EN ISO 1172 [15], [23], for organic fibres like carbon fibres it can be determined by chemical extraction of the fibres according to DIN EN 2564 [16]. For a statistically ensured determination five specimens of every test panel with geometric dimensions of approx. 15 x 15 mm have been investigated. In both cases knowing the mass of the initial specimen and the remaining fibres as well as the densities of the single components, namely fibre and matrix, it is possible to determine the fibre volume content f  . In the following the two afore mentioned experimental methods are described. Thermal vaporization of the polymeric matrix system according to DIN EN ISO 1172 For the panels that contain only glass or basalt fibres as inorganic reinforcement fibres the fibre volume content f  can be determined experimentally using a muffle furnace and a precision balance following DIN EN ISO 1172 [15]. The furnace is of the type C ARBOLITE EML 11/6, required to reach the temperature of 620 °C. This temperature is necessary to evaporate the polymeric matrix without affecting the mass of the fibres [2], [22], [23], [24]. The precision balance is of the type M ETTLER T OLEDO 204-S being able to supply an accuracy of measurements in a range of 0.1 mg. With the precision balance it is possible to weigh the specimens before and the remaining fibres after the thermal treatment and so determining the fibre volume content f  . Chemical extraction of the fibres according to DIN EN 2564 The afore described process is not suitable for carbon fibres. These kinds of reinforcement fibres show a distinct loss of weight under thermal loads [1], [24] as they are of organic structure. Therefore the process of chemical extraction was used for test panels that only contain carbon fibres following DIN EN 2564 [16]. The fibres are extracted from the matrix by placing the specimens in concentrated sulphuric acid and heating up to 160 °C. After the solution turns brown, hydrogen peroxide solution is added until the solution turns clear again. This effect is an indicator that the fibres have been completely extracted from the matrix. Redrying wet fibres enables weighing them and thereby the calculation of the fibre volume content f  . Evaluation of the results and calculation of the fibre volume content As previously described, the specimens have been weighted before and the remaining fibres after the respective treatment. Knowing these weights the fibre mass content can be calculated by f c m m   (1) where m indicates the mass and the subscripts f and c indicate the properties of the fibres and of the composite, respectively. Knowing the fibre mass content  and the densities of the single components, namely reinforcement fibres and polymeric matrix system, out of the respective technical data sheets [12, 13, 17, 18] the fibre volume content can be calculated by f f m 1 1 1         (2) where  is the density,  is the fibre mass content and the subscripts f and m indicate the properties of the fibre and the matrix, respectively [6], [24]. Analytical approximation of fibre volume content for test panels with hybrid stacking sequences When the hybrid stacked layups are considered none of the two afore described experimental methods for the determination of the fibre volume content are suitable because both organic and inorganic kinds of reinforcement fibres are used in the respective layups, as listed in Tab. 3. Neither the thermal vaporization method according to DIN EN ISO 1172 [15] nor the chemical extraction method according to DIN EN 2564 [16] is applicable. Therefore an approximation method has been developed in order to being able to analytically evaluate the fibre volume content of the hybrid stacked layups. The approximation is based on the specific weights of the dry fabrics and the weight of the prepared specimens. The mass of the fabrics in the specimen can be calculated by