Computation of Strength and Failure of Textile Composites in a Multiscale Simulation

Textile composites describe a broad range of polymer composite materials with textile reinforcements, from woven and non-crimp commodity fabrics to three dimensional textiles. In a general manner textile composites are based on textile preforms manufactured by some textile processing technique and on some resin infiltration and consolidation technique. Due to the complex three-dimensional structure of textile composites, experimental determination of strength parameters is not an easy procedure. Especially the through-thickness parameters are hardly to obtain. Therefore, in addition to real material testings, virtual material testings are performed by use of an information-passing multiscale approach. The multiscale approach consists of three scales and is based on computation of representative volume elements (RVE’s) on micro-, mesoand macroscale. The micromechanical RVE enables to determine stiffness and strength parameters of unidirectional fiber bundle material. Further, statistical distribution of fibers is investigated. The homogenized material parameters at microscale are used as input data for the next scale, the mesoscale. In the mesomechanical RVE, fiber architecture, in particular fiber undulations and the influence of through-thickness reinforcements, are studied. The obtained stiffnesses and strengths are used as input for the macroscale. On macroscale, structural components are calculated. On each scale, numerical results are compared with experimental test data for validating the numerical models. 1. Textile Composites Textile composites are characterized by the manufacturing process which involves machines usually used for production of textiles. With these machines, the dry rovings are laid and connected, e.g. knitted, woven or braided, in a preform. Figure 1 shows different preforms used in textile composites. The lay-up of these dry preforms is easier than with less flexible pre-impregnated layers and allows for more draping and easier connection of the layers via pinning or stitching etc. The resin infiltration of the fibers after the lay-up is followed immediately by the consolidation. During the infiltration process the fibers are held in place by the textile structure of the preform. Because the consolidation process, beginning with the infiltration, does not have to be stopped, textile composites are cheaper than prepreg material, which generate storage cost. Compared to prepreg-composites, the structure of textile composites is much more heterogeneous. In textile composites, the fibers are only equally dispersed throughout the fiber bundles, but not over the whole layer. Between the fiber bundles there are epoxy resin pockets and fiber undulations due to reinforcements in thickness direction, which have an influence on the mechanical properties. Compared to prepregs, textile composites have advantages in through-thickness strength, delamination sensitivity and crashworthiness. Figure 1: Different preforms used in textile composites 2. Multiscale Analysis Figure 3 shows the scheme of the multiscale algorithm. At microscale, fiber and matrix are modeled. As input serve experimentally obtained stress-strain curves for epoxy resin under different loading states and the elasticity parameters of the fiber material. The exakt determination of matrix and fiber properties is essential for the applicability of the information passing multiscale simulation. With the micromechanical unit cell stiffnesses and strengths of unidirectional fiber bundle material can be determined. For the elastic properties representative volume elements with a stochastic fiber distribution are investigated, see Figure 3a. Figure 3b shows the unit cell of a weft-knitted fabric. By comparison of test data and results of numerical analysis the numerical models are validated. The strengths computed on the mesoscale serve as input parameters for the failure criterion of Juhasz [1] that is used on macroscale for the evaluation of failure. (a) Micromechanical RVE with stochastic fiber distribution (b) Mesomechanical unit cell of weft-knitted fabric Figure 2: Representative Volume Elements (RVE) Figure 3: Multiscale algorithm