14 Influence of Dynamic Multiaxial Transverse Loading on Dyneema® SK76 Single Fiber Failure 91 Fig. 14.9 Broken fiber ends for each experimental case: (a) blunt, 1018.6 s−1; (b) blunt, 507.2 s−1; (c) sharp, 976.8 s−1; (d) sharp, 487.9 s−1; (e) razor, 975.4 s−1; (f) razor, 475.1 s−1 the blunt indenter at 1019 s−1, failure strain was reduced by 2.1% relative to the 913 s−1 tensile experiments at 5 mm gage length and 2.4% relative to the 1156 s−1 tensile experiments at 7 mm gage length. The consistency of the geometric effects when controlling for strain rate indicate the reliability of the results despite relatively large standard deviations. Therefore, the slight decrease in apparent strength in the sharp indenter experiments relative to the blunt indenter for both strain rates despite the increase in apparent strain further supports the conclusion that the indenter geometry is influencing the failure mode even though the differences in the strength and strain data are subtle. The microscope imaging is useful in elucidating failure modes of the fibers. Blunt tests clearly undergo fibrillation similar to tensile tests. For razor loading, fibrils are much more likely to shear, which appears to occur at significantly smaller levels of deformation. Sharp tests, in contrast to both, demonstrate a combination of fibrillation and fibril shear. This phenomenon could explain the apparent mismatch between failure strain and strength in the case of the sharp experiments compared to the blunt experiments, where failure strain was increased slightly, yet strength was reduced. Initially impacted fibrils may undergo shear failure while fibrils that are not on the impact side fail later due to fibrillation. Strain localization may not be significant enough to result in early macroscopic fiber failure but the reduced load-carrying capacity would result in lower apparent failure stress. Further modeling efforts may yield more information regarding the stress-strain states, strain concentration, and the level of multiaxial loading experienced by the fibers. Future investigation using finite element modeling of the experiments is necessary to develop a strain rate–dependent multiaxial failure criterion [7].
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