Anisotropy in Woven Fabric Stress and Elongation at Break

Abstract

Anisotropy is a characteristic of most fabrics, especially woven; the impact of the direction of loading on tensile properties can be enormous and is frequently examined, for example in (Dai & Zhang, 2003; Hu, 2004; Kilby, 1963; Kovar & Dolatabadi, 2009; Kovar, 2003; Lo & Hu, 2002; Pan & Yoon, 1996; Postle et al., 1988 etc.). Anisotropy of properties comes out of anisotropy of the structure, based on longitudinal fibers. For woven fabric there are two principal directions – warp and weft (fill), in which yarns and majority of fibres are oriented. Load in principal directions results in minimum breaking elongation and maximum initial modulus. For arbitrary load direction the values of tensile properties change and fabric deformation becomes more complex, often incorporating fabric shear and bend deformation. Although weave anisotropy is well known, tensile properties are usually theoretically and experimentally investigated namely for principal directions; the main reason is probably complexity of deformation and stress distribution when the load is put at non-principal direction. In this section we shall try to make a step to describe and perhaps to overcome some of these problems. In practical use, the fabrics are often imposed load in arbitrary direction, bi-axial load or complex load composed of elongation, bend, shear and lateral compression. To predict tensile properties becomes more and more important with development of technical textiles. Now only main difficulties, connected with the topic of this section, will be outlined: a. At diagonal load great lateral contraction occurs. It causes complex distribution of stresses. It results in stress concentration at jaws when experiment in accordance with EN ISO 13934-1 is used. b. There are yarns cut ends in the sample where tensile stress starts from zero. c. Shear deformation causes jamming of yarns, what can change yarn properties. Strength of the yarn in the fabric can be higher than the strength of free yarn. There are not available many publications, based on real fabric structure and solving the problem of woven fabric tensile properties in different directions. The reason is mentioned long range of problems and difficulties. Monographs (Hearle et al., 1969 and Postle et al., 1988) are involved in problems of bias fabric load only marginally. (Hu, 2004) is oriented on influence of direction on properties such as tensile work, tensile extension, tensile linearity etc. and uses another approach. Fabric shear at bias extension is investigated in (Du & Yu, 2008). Model of all stress-strain curve of fabric, imposed bias load, is introduced for example in (King, M. J. et al., 2005) with the respect to boundary conditions (stress concentration at