Steel fibers, with their high stiffness and high ductility, have a potential to provide a new range of properties in polymer composites, in comparison with carbon and glass fiber composites. However, the high stiffness contrast between the steel fiber and the polymer matrix plus the fiber’s non-circular cross-section are likely to generate high stress concentrations in a composite under transverse loading. In the present study, these stress concentrations are analyzed using finite element modeling and compared with the case of carbon and glass fiber composites. The study is performed for an isolated fiber and multiple fibers in hexagonal and random packings with 40% and 60% of fiber volume fractions. According to the results, in spite of a high contrast between the stiffness values of steel and glass fibers, no significant difference between the transverse stress concentrations was observed for steel and glass fibers in the hexagonal packing due to the difference in material properties. Differences in stress concentrations were noted for the case of randomly packed fibers. The polygonal cross-section of steel fibers was found to introduce extreme stress concentrations.
In this study, a new method for the finite element analysis of low-density thermally bonded nonwoven materials is proposed and compared with tensile tests. By using advantages of parametric modelling, the model with a large number of fibres is developed. It has also the advantage to implement easily any changes in its parameters such as dimensions and material properties easily. In the suggested model, bond points in a nonwoven are connected by fibres according to criteria in the input file to prevent cross-ing over bond points and unrealistic long-distance connections. Orientation distribution of fibres is deter-mined by an image analysis technique based on the Hough transform and implemented into the model with the variation of crosssectional areas of fibres. Creep tests are performed with single fibres to deter-mine a time-dependent response under constant load due to their visco-elastic behaviour. A case of uni-form tension is simulated with various elastic, elasto-plastic and creep material properties. The results for various formulations are compared with each other as well as with the data from tensile tests. The obtained results are discussed and suggestions for further development of the model are presented.
Due to random orientation of fibres and presence of voids in their microstructure, lowdensity thermally bonded polymer-based nonwovens demonstrate complex processes of deformation and damage initiation and evolution. This paper aims to introduce a micro-scale discontinuous finite element model to simulate an onset of damage in low-density nonwovens. In the model, structural randomness of a nonwoven fabric was introduced in terms of orientation distribution function (ODF) obtained by an algorithm based on the Hough Transform. Fibres were represented in the model with truss elements with orientations defined according to the computed ODF. Another structural element of nonwovens -bond points-were modelled with shell elements having isotropic mechanical properties. The numerical scheme employed direct modelling of fibres at micro level, naturally introducing the presence of voids into the model and thus making it suitable for simulations of low-density nonwovens. The obtained results of FE simulations were compared with our data of tensile tests performed in principal directions until the onset of damage in the specimens.
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