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Languerand Lagies, Dulcie

Mechanical Engineering

2007

MS

Kevlar and Spectra fibers are increasingly present in many products where low density and high strength are sought-after. Tensile behavior and fracture mechanisms were examined in Kevlar and Spectra fiber bundles at quasi-static and high strain rates. The mechanical properties of Spectra fibers were also explored by nanoindentation using atomic force microscopy. Dynamic strain rate tests were conducted using a tension Kolsky bar. No significant strain-rate dependence was found in the strength and fracture of Kevlar fiber bundles; this was confirmed through fracture surface analysis using scanning electron microscopy.
The underlying mechanisms including fibrillation and pointed break are also shown to have limited contribution on the strain energy dissipation process during deformation. Furthermore, Spectra fibers were found to exhibit significant strain-rate dependence on tensile strength and fracture. As opposed to Kevlar, this behavior can be directly correlated with a change of fracture, mechanism from the different strain rate regimes. Fracture surface microscopy shows a predominance of crazing at high strain rates and increase of plate formation under quasi-static condition.
This trend is more substantial in Spectra fibers with lower crystallinity, which also exhibit less pronounced strain-rate dependency. This finding is interpreted in terms of the deformation behavior in the crystalline regions of the fibers. Better knowledge of the deformation and fracture behavior of Spectra and Kevlar is needed in order to enhance and optimize these materials for present and future uses.