Issue 28

P. Valentino et alii, Frattura ed Integrità Strutturale, 28 (2014) 1-11; DOI: 10.3221/IGF-ESIS.28.01 2 I NTRODUCTION oven fabric (WF) composite materials are of particular interest in the scientific community due to their mechanical performances compared to the unidirectional laminates. In particular, textile composites offer better dimensional stability over a large range of temperatures, better impact resistance and tolerance; subtle conformability anddeepdrawmoldability, compared to the common unidirectional laminated composites. The variety ofmanufacturingmethods havemade the textile composites cost-competitive, therefore they are used inmany application fields and they are being considered for intra and interlaminar strength and damage resistance. Among the various textile forms, woven fabrics are the most widely used in composites, they provide more balanced properties in the fabric plane and the interlacing of yarns provides higher out of plane strength, which can take up the secondary loads due to loadpath eccentricities, local buckling, etc. Simplified theoretical approaches to predict themechanical behaviour of such a kind of compositematerial are quite little, in factmost of themodels are limited to the unidirectional reinforced layers and they use the homogenization technique. Several studies, on 2-D and 3-D geometries, have been carried out tomodel and analyse themechanical properties of the reinforced composite materials. In particular, Barbero et al [1], developed an accurate model of a plain wave fabric in order to evaluate its mechanical properties assuming a sinusoidal shape of the tow fibres. Stress and strain averaging procedure has been studied by Yiwie Jiang et al. [2], for local/global analysis of plain-weave fabric composites, where, within a representative volume cell, using uniform stress and uniform strain assumptions, the constitutive equations are averaged along the thickness direction. Chou and Ito [3] analysed the strength and failure behaviour of plain weave composites. In particular, the geometrical characteristics of yarn shape, laminate stacking configuration, fibre volume fraction, and yarn packing fraction were investigated using three-dimensional geometrical models. Based on the geometrical characteristics, iso-strain approach was developed to predict elastic properties, stress distributions, and strengths under tensile loading. N. K. Naik et al. [4] developed a two-dimensional closed-form analytical method for the thermo-elastic analysis of two- dimensional orthogonal plain weave fabric laminas, considering the volume fraction of fibres and the possible gap between the two adjacent strands. Finally, analytical models of the plain weave laminated composites to evaluate the elastic properties of woven fabric composites are reported in [5]. However, most of these works are related to plainwave fabrics, while not many efforts to study the twill wave type have been carried out. The aim of this work is to evaluate the stiffness of a fabric reinforced composite inwarp and fill direction. In particular a numerical FE model, assuming elliptical sections and sinusoidal shape of the yarns, has been implemented and experimental tests have been carriedout inorder to validate the proposedmodel. Results in terms of stress-strain response and stiffness, for the different analysed structures are reported and discussed. Finally, the strength and the failure modes of the composite material, for each analysed structure and textile orientation, have been experimentally investigated. M ATERIALSANDTESTPROCEDURES oven fabric composites made in continuous basalt fibres and polymeric matrix has been investigated. Test panels have been produced by placing hand all the constituent layers, as shown inFig. 1, and a curing treatment at room temperature in vacuum bag has been carriedout, inorder to get the final product. All test panels have been produced using the same epoxy matrix system [6] and two different orientations of the fibres have been considered, as reported in Tab. 2. According to the German standardDINEN 2747 [7], each panel has been processedwith the aim to get a thickness of approx. 2mm. Two different fabric types have been used in this investigation, i.e. twill 2/2 and twill 1/3, details are reported in [5]. The corresponding layups and technical data, according to the provided data sheet [12] are listed inTab. 2. Tensile test specimens, with plane dimensions of 250 x 25mm [7], have been obtained bywater jet cutting from the panel produced in autoclave as shown in Fig. 2. In order to avoid the failure of the specimens near to the gripping zone, due to the local stress concentration, and with the aim to prevent eccentricity of load and limit bending phenomena during the setup, tabs have been bonded to both the ends of each sample, Fig. 3. According to [7], they were made of glass fiber reinforcedplasticswith a±45°-layup andwith dimensions of 25 x 45 x 1.5mm. W W