CHARACTERIZATION OF FAILURE OF COMPOSITE STRIPS AND SINGLE FIBERS UNDER EXTREME TRANSVERSE LOADING
When a composite laminate is transversely impacted by a projectile at the ballistic limit, its failure mode transits from global conical deformation to localized perforation. This Ph.D. dissertation aims to reveal the fundamental material failure mechanism at the ballistic limit to control perforation. First, transverse impact experiments were designed on composite strips to isolate the interaction between plies and tows. Three failure modes were identified, divided by no, partial, and complete failure before the transverse wave deformed the entire composite strip. The failure phenomenon and critical velocity region can differ with the fiber type and projectile nose geometry and dimension. In most impact events, the composite strips all failed in tension in the front of the projectiles, although they failed at different positions as the projectile nose geometry and fiber type changed. A special failure phenomenon was uncovered when the composite strips were impacted onto razor blades above the upper limit of the critical velocity region: the composite strips seemed to be cut through completely by the razor blades. To further investigate the failure by razor blade, a microscopic method was developed to cut a single fiber extracted from the composite strip and simultaneously image the failure process inside a Scanning Electron Microscope (SEM). The experiments revealed that the razor blade cannot cut through the inorganic S-2 glass fibers while can partially incision the aramid Kevlar® KM2 Plus fibers and completely shear through the ultra-high-molecular-weight polyethylene (UHMWPE) Dyneema® SK76 fibers. Further investigations on the fiber’s failure under dynamic cut revealed that there was no variation in the failure mode when the cut speed was increased from 1.67 μm/s to ~5.34 m/s. To record the local dynamic failure inside the composite strips and single fibers at high-velocity impact, an advanced imaging technique, high-speed synchrotron X-ray phase-contrast imaging, was introduced, which allows to capture the composite’s internal failure with a resolution of up to 1.6 μm/pixel and at a time interval 0.1 μs. Integrated with a reverse impact technique, such an advanced imaging technique is believed to be capable of revealing the mechanism involved in the impact-induced cut in single fibers, yarns, and composite strips. The relevant studies will be the extended work of this Ph.D. dissertation and published in the future.