Dynamic deformation of silicon carbide

The objective of this investigation is to develop a micromechanical understanding of ceramics subjected to impact loading and penetration. Silicon carbide was chosen because it is a primary candidate material for armor applications. The investigation had two components: (a) small strains {dollar}(varepsilonapprox{lcub}-{rcub}0.01),{dollar} and (b) large strains {dollar}(varepsilonapprox{lcub}-{rcub}0.22).{dollar}; Two types of small strain experiments were conducted: (1) uni-axial compression, using split Hopkinson bar {dollar}(dotvarepsilonapprox 6times 10sp2{dollar} s{dollar}sp{lcub}-1{rcub}){dollar} to study the microstructural damage, and (2) high velocity rod impact.; Considerable plastic deformation, as dislocations and stacking faults, was observed during the uni-axial dynamic compression. A polytype transformation was identified through a significant increase in the 6H-SiC phase at compressive stresses higher than 4.5 GPa. Two mechanisms are proposed for the initiation of fracture: dilatant cracks and Zener-Stroh cracks. Finite element calculations indicated that elastic anisotropy between adjacent grains can generate tensile stresses. Dilatant cracks are nucleated by the induced tensile stresses, leading to transgranular fracture. Zener-Stroh cracks are nucleated by piled-up dislocations along grain boundaries, resulting in intergranular fracture.; In the rod-impacted SiC specimens, there was a frontal layer, separating the comminution (Mescall) zone and impact interface. Profuse dislocation activity was observed in the frontal layer. The impact pressure was higher than the Hugoniot elastic limit, but the frontal layer did not exhibit comminution because of the lateral confinement, imposed by the surrounding material.; The large strain, high-strain-rate deformation was studied using the thick-walled cylinder method {dollar}(dotvarepsilonapprox 3times 10sp4{dollar} s{dollar}sp{lcub}-1{rcub}).{dollar} Both solid and granular SiC were subjected to large strain, high-strain-rate deformation by radial symmetric collapse of a thick-walled cylinder. The global strain was accommodated by both homogeneous deformation and shear localization.; For the fragmented SiC, the difference in microstructure affected the microcrack propagation, but did not affect the shear localization. Shear bands are formed through: (a) shear crack formation, (b) comminution by friction of crack surfaces, (c) incorporation of adjacent fragments into the shear band, and (d) erosion of large fragments inside the shear band.; For the granular SiC, shear bands are formed through particle comminution and rearrangement. A model was derived, and demonstrated the effect of particle size on comminution. The shear localization at sufficiently high displacement leads to the formation of a heat-affected zone. Calculations showed that the temperature can reach {dollar}2300spcirc{dollar}C. The generated heat and the imposed pressure create a thin layer of well-bonded material.