James Q. Zheng

Mechanical behavior of A265 single fibers

The mechanical behavior of A265 high-performance fibers was experimentally investigated at both low and high strain rates. Axial, transverse, and torsional experiments were performed to measure the five material constants on a single fiber assumed as a linear, transversely isotropic material. In order to determine the tensile response of the fiber at high rates, a modified Kolsky tensile bar, also known as a split Hopkinson tension bar (SHTB) for single-fiber tests, was used. The diameter of each fiber was measured individually using a high-resolution scanning electron microscope for accurate stress calculation. A pulse shaper technique was adopted to generate a smooth and constant-amplitude incident pulse to produce deformation in the fiber specimen at a nearly constant strain rate. The tensile strength of the fiber exhibits both rate and gage-length effects.

Influence of stress state and strain rate on structural amorphization in boron carbide

The reduced performance of B4C armor plate for impact against tungsten carbide penetrators beyond a critical velocity has been attributed in the literature to localized amorphization. However, it is unclear if this reduction in strength is a consequence of high pressure or high velocity. Despite numerous fundamental studies of B4C under indentation and impact, the roles of strain rate and pressure on amorphization have not been fully established. Toward this end, rate dependent uniaxial compressive strength and rate dependent indentation hardness, along with Raman spectroscopy, have been employed to show that high strain rate deformation alone (without concurrent high pressure) cannot trigger localized amorphization in B4C. Based on our analysis, it is also suggested that rate dependent indentation hardness can be used to reveal if a given B4C ceramic exhibits amorphization under high pressure and high strain rate loading. It is argued that when amorphization does occur in B4C, its dynamic inelastic prope...

Raman spectroscopic characterization of the core-rim structure in reaction bonded boron carbide ceramics

Raman spectroscopy was used to characterize the microstructure of reaction bonded boron carbide ceramics. Compositional and structural gradation in the silicon-doped boron carbide phase (rim), which develops around the parent boron carbide region (core) due to the reaction between silicon and boron carbide, was evaluated using changes in Raman peak position and intensity. Peak shifting and intensity variation from the core to the rim region was attributed to changes in the boron carbide crystal structure based on experimental Raman observations and ab initio calculations reported in literature. The results were consistent with compositional analysis determined by energy dispersive spectroscopy. The Raman analysis revealed the substitution of silicon atoms first into the linear 3-atom chain, and then into icosahedral units of the boron carbide structure. Thus, micro-Raman spectroscopy provided a non-destructive means of identifying the preferential positions of Si atoms in the boron carbide lattice.