High strain rate studies of armor materials

This thesis focuses on the high strain rate behavior of Maraging steel 300, High Hardness Armor (HHA) and Aluminum 5083 -- H131 Alloy. These materials are used by the Department of National Defense (DND) of Canada as armor plate materials in military applications. The aim of the research is to investigate the dynamic shear-strain response of these armor materials at high strain rate loading to study the occurrence of Adiabatic Shear Bands and the subsequent failure. The effects of impact momentum and strain rates on the dynamic stress-strain curve and on the adiabatic shear failure of these armor materials under impact and torsion loading need to be investigated to evaluate their capability to withstand military conditions.;The impact loading experiments involved striking cylindrical specimens at very high impact momentum using a projectile. The strain rates achieved on impact of a specimen is a function of impact momentum. The influence of strain rate and impact momentum on the occurrence of ASBs in the materials of interest was investigated. Microstructural evaluation of these armor materials showed that the formation of ASB requires a threshold level of impact momentum. ASBs in the form of white-etching bands and deformed bands at its tail end were observed in Maraging steel 300 subjected to an impact momentum of 44.54 kg/mls. On the other hand, deformed bands were observed in HHA specimens subjected to an impact momentum between 32.91 kg/mls and 45.49 kg/mls. Deformed bands were also visible in Aluminum 5083 -- H131 Alloy specimens subjected to impact loading. The deformed bands in aluminum were governed by plastic flow due to thermal softening.;Thin-walled tube specimens of Aluminum 5083 - H131 Alloy and Maraging steel 300 were subjected to 4°, 6° and 8° angles of twist using the TSHB. Deformed ASBs accompanied by cracks were found in aluminum subjected to 8° angles of twist. On the other hand, ASBs were not visible in Maraging steel 300 and the specimens failed at high angles of twist.;The mechanisms for the occurrence of ASB in steels and aluminum are presented m this thesis. Optical, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) were used. The utilization of AFM to study the characteristic of the white-etching band in Maraging steel 300 provides information on the atomistic evolution of ASBs and it is also discussed in this thesis. The dynamic stress-strain response from impact and torsion loading experiments supplemented with microstructural investigation form a basis for evaluation of the resistance of these materials to high strain rate loading conditions.;Specimens of these materials were subjected to direct impact and torsion loading using the Split Hopkinson Pressure Bar (SHPB) -- Direct Impact and Torsion Split Hopkinson Bar (TSHB) respectively. The dynamic stress-strain curves were generated to study the plastic deformation behavior during impact and torsion loading. High strain rate loading affects the mechanical properties in a wide range of metals and alloys by thermo-mechanical instability. A phenomenon called Adiabatic Shear Bands (ASBs) occurs due to extreme strain localization as a result of thermo-mechanical instability in the microstructure at high strain rate loading conditions. The presence of ASBs acts as a precursor to failure as they are harder and more brittle than the bulk material.