Promising method for the shear testing of textile composites

Abstract

The development of the production of textile composites has created the need for the testing of these materials, particularly for their shear strength. The test methods used at present in practice, were developed for laminar and simple 2D woven materials [1, 2], which are characterized by the presence of planes having a low shear strength. For the various 2D and 3D woven, sewn, and knitted composites that have appeared during the last years, the criteria for the preparation and testing of specimens have not-yet been established [3]. These materials are characterized by a significantly higher shear strength than the laminar composites. The textile materials that differ essentially by their mechanical properties, differ from the laminar materials by the scale of the structural inhomogeneity: by a relatively large diameter of the braids, forming the unit cell, and by the space between them, filled with the binder. The unit cell of such materials is defined as the smallest unit which repeats the structure of the fibers and which can be considered as the structural unit of the material. The size of the unit cell, which depends on the size of the braid, the angles and the complexity of interweaving, can be significant and reach 10-12 mm and more. A significant inhomogeneity of the deformation field exists within such a cell. According to [3] the deformation gradient, for instance in a 2D triaxially woven textile composite, can reach 3. The testing of textile composites required new methods, which take into account their peculiar characteristics: a shear strength which is higher than that of the laminar composites, and structural inhomogeneity. The accurate determination of the mechanical properties of the material requires the correct selection of its representative volume and loading, if possible, by a uniform pure shear stress. One of the most promising methods for the testing of this class of materials is the asymmetrical bending of a beam with V-shaped notches [4]. It has been shown in [5] that the method is suited for the testing of three-dimensionally reinforced composites of the carbon-carbon type with orthogonal packing of the tibers. The present article assesses the possibility of expanding the field of applications of this method to the testing of textile composites. It is necessary to select a certain shape and dimensions of the specimen, the parameters of the neck, and the loading scheme. The influence of the parameters of the notch and of the width of the specimen on the shear strength was therefore investigated experimentally. The experimental data were compared with the strength, determined by other methods. The experimental data were obtained on specimens of textile glass fiber plastic; the spatial links in the specimens were formed by the distortion and interweaving of the warp with the rectilinear fibers of the weft, by using the traditional two-thread scheme [2]. The warp fibers linked only the adjacent weft fibers according to height. The configuration and dimensions of the specimen are given in Fig. 1. The specimens were cut out, so that the direction of the warp coincided with the longitudinal axis of the specimen. Variants of the notch geometry and the coefficient of orthotropy of the material are given in Table i. The loading of the samples was based on the asymmetrical four-point bending scheme (AFPB) of the beam (Fig. 2). The dimension of the inner arm b (see Fig. 2) was 10 mm, of the outer arm a 30 mm. The average shear stress ~'av was calculated from the equation