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 properties degrade more severely than its static properties. Finally, it is suggested that dynamic hardness, in conjunction with static hardness, can be used as a measurable mechanical property to reveal the incidence of amorphization in B4C without the need for postmortem TEM or Raman spectroscopy analyses.
We exposed a headform instrumented with 10 pressure sensors mounted flush with the surface to a shock wave with three nominal intensities: 70, 140 and 210 kPa. The headform was mounted on a Hybrid III neck, in a rigid configuration to eliminate motion and associated pressure variations. We evaluated the effect of the test location by placing the headform inside, at the end and outside of the shock tube. The shock wave intensity gradually decreases the further it travels in the shock tube and the end effect degrades shock wave characteristics, which makes comparison of the results obtained at three locations a difficult task. To resolve these issues, we developed a simple strategy of data reduction: the respective pressure parameters recorded by headform sensors were divided by their equivalents associated with the incident shock wave. As a result, we obtained a comprehensive set of non-dimensional parameters. These non-dimensional parameters (or amplification factors) allow for direct comparison of pressure waveform characteristic parameters generated by a range of incident shock waves differing in intensity and for the headform located in different locations. Using this approach, we found a correlation function which allows prediction of the peak pressure on the headform that depends only on the peak pressure of the incident shock wave (for specific sensor location on the headform), and itis independent on the headform location. We also found a similar relationship for the rise time. However, for the duration and impulse, comparable correlation functions do not exist. These findings using a headform with simplified geometry are baseline values and address a need for the development of standardized parameters for the evaluation of personal protective equipment (PPE) under shock wave loading.
A methodology is described for characterizing the spatial distribution of thermal mismatch stresses at grain level in B 4 C-SiC-Si ceramic composites using Raman spectroscopy. Unlike traditional methods to detect residual stress (e.g., X-ray diffraction) which provide average values over the entire specimen surface, Raman peak-shift analysis provides residual stress distributions within the microstructure at high spatial resolution. While the classical formulation predicts uniform compressive stress within a Si-phase surrounded by the ceramic matrix, the Raman measurements revealed non-uniform residual stress distributions in Si when the particle size was larger than 5 microns. For large irregular shaped particles, the two methods coincide only along the interface between the particle and matrix, but vary drastically both in magnitude and nature in the interior of the particle where large tensile stresses have been measured. The average residual stress within the microstructure was found to correlate well with the volume fraction of the constituents and material properties. The presence of anomalous tensile stress in the interior of the minor Si-phase results in defect generation and structural disorder which has been confirmed by a subsequent TEM analysis.
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