Dynamic Fracture Behavior and Enhancement of Ballistic Performance in High-Strength Armor Steel

Abstract

Selection of suitable armor materials for defense applications is crucial with respect to protective capability, maneuverability, and transportability for achieving the ensured survival of combat crew members. High-strength bainitic steels exhibit a great potential as a steel armor plate because of their excellent combination of strength, ductility, toughness and high ballistic mass efficiency. A carbide-free design concept that uses increased Si content has led to active utilization of retained austenite (RA), and has also increased the toughness of high-strength bainitic steel. Transformation-induced plasticity (TRIP) by RA has been a main key to improve mechanical properties. However, few studies have considered on the contribution of RA under high-strain-rate deformation, and an improvement in mechanical properties alone does not simply enhance ballistic performance because many parameters affect it. Ballistic impact tests are the most accurate test to evaluate the ballistic performance of armor steel, but they are time-consuming and costly, have limitation in microstructural analyses because of difficulties of collecting samples from fragmented or penetrated target specimens. First, in this study, microstructural evolution and fracture behavior under high-strain-rate deformation in high-strength armor steel were investigated using a laboratory-scale split Hopkinson pressure bar (SHPB); the results were then correlated with the steels’ actual responses to ballistic impact. Cracks initiated and propagated along adiabatic shear bands (ASBs) that formed during both dynamic compression and ballistic impact. These cracks often result in catastrophic failure under high-strain-rate deformation, so the fracture behavior was investigated in detail the context of formation behavior and mechanism of ASB. Results of interrupted dynamic compressive tests revealed that a deformed ASB (dASB) started to form right after a stress collapse, then developed into a transformed ASB (tASB). After a ballistic impact, wide tASBs formed mostly at the penetrated surface with narrow tASBs branching from the main tASB. Very fine equiaxed grains ~190 nm in size, developed in the tASB during dynamic compression; this trend indicates that dynamic recrystallization occurred within 86.5 μs after impact, and that the grains then grew to ~260 nm in 9.5 μs. A rotational dynamic recrystallization mechanism and grain-growth rate model were proposed in this study based on a calculation of temperature rise using the thermo-elasto-plastic finite element method. It provided a reasonable explanation of the formation and growth of the fine equiaxed grains during either dynamic compression or ballistic impact. These results showed that the formation mechanisms and kinetics of ASBs formed by ballistic impact could be entirely explained by the dynamic-compression results achieved in the laboratory. Second, high-strength bainitic steels that had various volume fraction or stability of RA while maintaining high hardness were designed, and the role of RA on ballistic performance was investigated in relation to the ASB formation. SHPB test was conducted to quantify ASB formation, and V50 ballistic impact tests were conducted to evaluate ballistic performance. The results were correlated and analyzed in terms of the microstructural evolution and resistance to ASB formation. The results showed that an active TRIP mechanism achieved by a large quantity of metastable RA can significantly enhance the ballistic performance of high-strength bainitic steel by increasing its resistance to ASB formation. This study thus suggests that dynamic compression testing using SHPB is a reliable method to interpret ASB formation and fracture mechanisms from ballistic impacts, and that the utilization of active TRIP by RA greatly contributes to enhancing the ballistic performance of high-strength bainitic steel.