86 F. D. Thomas et al. 14.2 Experimental Setup An experimental technique is developed by modifying the 0.25 inch diameter Kolsky bar to conduct single fiber transverse impact, as shown in Fig. 14.1. The indenter is attached to the incident bar end through a sleeve, and a U frame is used to secure the load cells (Kistler 9712B5) attached to the single fiber mounted on a window frame. A Photron Fastcam SA-5 high speed camera is used to record the deformation. Projectile-fiber contact induces multiaxial loading and strain concentration in the fibers. The setup allows measuring load at failure, strain at failure, failure angle θ, and impact velocity. A series of experiments are performed to characterize fiber failure under dynamic multiaxial loading conditions and to better understand the influence of different projectile geometry (blunt to sharp to razor) on fiber failure. Diameter measurements are performed on each sample using a confocal optical microscope in a region of the fiber which is proximal to the intended impact location and averaged for use in appropriate stress calculations based on load measurements. As a result, failure at the impact site is a necessary criterion for an experiment to be considered valid in this study. The fibers are impacted at velocities in the range of 10–20 m/s. Measurements made with camera recordings utilize the overall diameter of the projectile, which is 1 mm, for scale. In order to record at a higher framerate (100,000 fps), resolution is reduced to 320×192. The location of the impactor tip is measured in the initial impact frame and in the frame before failure, and the distance between the two points is divided by the elapsed time to yield the average velocity. A pressure vessel is used to control the velocity, which is filled to specific high and low pressures to vary velocity. Outlier velocities for a given pressure setting are classified as irregular test conditions and are therefore excluded from this study. Analytical calculations are used to determine strains and strain rates from observed geometric changes. The maximum angle of deflection before failure (θ) provides a useful means of approximating far-field strains (ε) according to Eq. 14.1. Strain rates under the indenter are difficult to evaluate based on the available data, so a hybrid finite element approach must be used to quantify the relationship between strain rate at the failure location and impact velocity. However, for discussion purposes in this chapter, the approximate strain rate (SR) is related to velocity v according to Eq. 14.2, where gage length L0 is 41.5 mm. The piezoelectric load cells to which the fiber ends are mounted measure axial loads in the fiber at a high frequency and can be used to estimate average stress in the fiber. ε = 1 cosθ − 1 (14.1) SR= v 1 2L0 (14.2) In order to examine the effects of high-rate multiaxial loading on single UHMWPE fibers, several indenters with varying loading geometries have been designed and manufactured through wire EDM. Loading geometry in all cases is circular and altered by changing the radius in orders of magnitude relative to the fiber diameter. The average fiber diameter of Dyneema® SK76 is measured to be 17.7 μm. The sharp indenter has a radius on the same order of magnitude as the fiber diameter Fig. 14.1 Schematic of experimental setup
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