Analyses of fabric tensile behaviour: determination of the biaxial tension-strain surfaces and their use in forming simulations

The forming of fibre fabric reinforcements without a matrix is possible because of their very specific mechanical behaviour. The lack of some rigidities is due to possible motions between the fibres. For the fabrics used as reinforcement in the R.T.M. process and composed of warp and weft yarns made with untwisted fibres, the tension stiffness is very preponderant compared to the others. The tensile behaviour of such a fabric is biaxial, i.e. the tension-deformation states in warp or weft directions depend on the other direction because of the interweaving. It is given by the knowledge of two surfaces relating the warp and weft tensions to the two strains in these directions (or that of a single surface if the fabric is balanced). In the present paper, three complementary methods are investigated in order to determine these surfaces. A biaxial tensile device on a cross-shaped specimen is first used. 3D finite element simulations of the unit woven cell are then presented. This mesoscopic study permits to understand some phenomena at the elementary woven cell level. Finally a simplified model, which is consistent with the geometry of the plain weave woven mesh is presented. The agreement of the two last methods with experimental results is shown. From these tensile behaviour surfaces, a dynamic explicit approach for the simulations of a fabric sheet forming process is presented. The interests of the method are both its good numerical efficiency, particularly due to the direct use of the biaxial tension surfaces, and its proximity with fabric physics. (C) 2001 Elsevier Science Ltd. All rights reserved.