GRANULAR FLOW OF AN ADVANCED CERAMIC UNDER EXTREME MULTI-AXIAL LOADING
Johns Hopkins University
Boron carbide has been widely used in a variety of engineering fields, especially in extreme conditions, because of its low density, high compressive strength, high corrosion resistance and excellent thermal stability. Granular flow is a major deformation mechanism of boron carbide under high strain rates and large pressures. The dynamic rheology of a complex granular material system strongly depends on the imposed stress state. Granular flow can also be influenced by strain rate, and by the evolution of particle size and shape. However, the effects of these factors on granular flow under extreme loading conditions, with the associated deformation mechanisms, have not been well characterized. Developing a clear physics-based picture of dynamic granular flow is challenging due to the heterogeneous nature of granular materials. Here, we load three commercially available boron carbide powders with average particle sizes of ∼ 0.7 μm (fine), ∼ 10 μm (intermediate) and ∼ 70 μm (coarse), under uni-axial strain conditions using quasi-static MTS tests and dynamic Kolsky bar experiments. Quasi-static powder compaction is also conducted on a rounded powder (from heat-treatment of the fine powder) to investigate the shape effect. Pressure-shear plate impact (PSPI) is utilized to load the three granular boron carbide powders in a multi- axial fashion, with higher strain rates of 105 s−1 and larger pressures ranging from 1 to 3 GPa. Normal plate impact experiments are also conducted in comparison with the pressure-shear plate impact. Comparison between the shear stresses and the superimposed normal stresses shows a strong pressure dependency in the granular constitutive response. The mea- sured effective friction coefficient is around 0.16, which tends to be particle size in- dependent. Both the normal and shear stress-strain curves are obtained, with the corresponding shear strength and porosity evolution. Granular boron carbide shows a highly compressible behavior with significant amount of volume compaction. The estimated granular wave speed increases with granular density. Particle size, shape and strain rate effects are discussed accordingly. Microstructural characterizations of the deformed particles show that fracture and amorphization are active deformation mechanisms in addition to grain-grain frictional interactions and particle rearrangement. Large numbers of damaged particles are observed, indicating that particle fracture is a key deformation mechanism. Dynamic loading introduces more particle fracture and fragmentation than the quasi-static loading, which may contribute to the rate effect on granular flow. We also conduct morphological characterization of particle shapes showing that the deformation from powder compaction increases the overall regularities (Eq. 4.4.3) of the three powders. Comparing the shape regularity from the dynamic loading with that from the quasi- static loading, we can see that increasing the strain rate has the effect of increasing the particle regularity. This study will contribute to the development of integrative models and meso-scale simulations for granular flow of ceramics at ultra-high strain rates and large confinement pressures.
ceramic, granular flow, plate impact