This work presents a combined theoretical-experimental study of strain rate behavior in metals. The method is to experimentally calibrate and validate an Internal State Variable (ISV) constitutive model with a wide range of strain rate sensitivity. Therefore a practical apparatus and methodology for performing highly sought-after intermediate strain rate experimentation was created. For the first time in reported literature, the structure-property relations of Rolled Homogeneous Armor is quantified at the microscale and modeled with varying strain rates, temperatures, and stress states to capture plasticity and damage with a single set of constants that includes intermediate strain rates. A rolled homogeneous armor (RHA) was used as a material system to prove the methodology. In doing so, a newly implemented strain rate dependent nucleation parameter for RHA was implemented to transition the dominant damage mechanism from void growth to void nucleation as strain rate increased. The ISVs were utilized in finite element analysis for robust predictability of mechanical performance as well as predictability of microstructural evolution with regards to void size and number distribution. For intermediate strain rate experiments, robust load acquisition was achieved using a novel serpentine transmittal bar that allowed for long stress waves to traverse a short bar system; this system eliminated load- ringing that plagues servo-hydraulic systems. A direct hydraulic loading apparatus was developed to provide uniform strain rates throughout intermediate rate tests to improve on the current limitations of the state-of-the-art. Key recommendations on the advancement of predictive modeling of dynamic materials, as well as performing advanced dynamic experimentation, are elucidated.

An investigation of the microstructure-mechanical property relationships for gray cast iron and a vintage ASTM A7 steel are presented herein. Gray cast iron was shown to have a moderate sensitivity to strain rate and a large disparity in behavior between compression, tension, and torsion. ASTM A7 steel was shown to behave in a more complex manor with the strain rate sensitivity having a negative relationship in tension and positive relationship in compression and torsion, the tensile stress-state producing the highest stress response, and the material producing a higher stress response when exposed to elevated temperatures. The counterintuitive behavior observed in A7 steel was attributed to dynamic strain aging. The Mississippi State University Internal State Variable Plasticity-Damage model was updated to accurately capture negative strain rate sensitivity and DSA embrittlement by developing kinematic, thermodynamic, and kinetic constitutive relationships for dynamic strain aging. A parametric study was performed to elucidate the behavior of the new internal state variable for dynamic strain aging. Gray cast iron was successfully calibrated to a pre-DSA version of the plasticity-damage model and A7 steel was successfully calibrated to the updated plasticity-damage model.