Baris Sabuncuoglu

Micro-scale finite element analysis of stress concentrations in steel fiber composites under transverse loading:

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.

Analysis of creep behavior of polypropylene fibers

Time and stress-dependent viscoelastic properties of polypropylene fibers are determined by means of a series of creep tests and data analysis technique. Relaxation tests are carried out to verify and update the suggested model. The tests are performed for single polypropylene fibers that form a thermally bonded low-density nonwoven. The obtained properties are implemented into the finite element analysis software MSC. Marc - Mentat. Tensile tests and simulations are performed in order to demonstrate suitability of the developed creep model. The obtained results are discussed and further recommendations are given.

On the high stress concentrations in steel fiber composites under transverse loading

Transverse stress concentrations are mainly governed by the polygonal cross section of fibers generating stress concentrations near the corners and the high stiffness mismatch of steel fibers and epoxy matrix. Micro-scale images show that there is a slight radius at the polygon corners instead of sharp corners. First, the effects of these radii on the stress concentrations are investigated. Then, in order to decrease the stress concentrations, the idea of introducing a transition region with nano-reinforcements between fiber and matrix is investigated. Finite element models are generated in micro-scale to investigate these cases in which fibers and matrix are generated separately. The results show that the polygonal cross sections significantly decrease stress concentrations, whereas the variation introduced to the fiber–matrix interface reduces only the normal component of stresses and if the fiber/volume fraction is low.

Analysis of rate-dependent tensile properties of polypropylene fibres used in thermally bonded nonwovens

The stress–strain behaviour of polypropylene fibres is evaluated for various tensile strain rates. Fibre samples are extracted from a thermally bonded nonwoven and fixed in a low-load tensile test machine. A methodology is introduced to implement a constant true strain rate at high strain tests for conventional tensile test machines. The obtained results indicate that polypropylene fibres show a highly viscous behaviour, especially during the initial stage of load application. No significant difference in a tensile behaviour of fibres was observed for loading regimes with a constant true strain rate and a constant engineering strain rate.