Stress-State and Strain-Rate Dependent Multiscale Characterization of ARMOX 500T

A strain-rate and stress-state dependent experimental characterization is conducted for the parameterization of a triaxiality and lode angle parameter (LAP) dependent Generalized Incremental Stress-State Dependent Damage Model (GISSMO) for ARMOX 500T (AX500) armour steel. 100+ mechanical tests have been conducted which differentiate the effects of triaxiality, LAP, and strain-rate on instability and fracture strains. Quasistatic characterization tests have been conducted at 18 different stress-states abiding by previous GISSMO literature and ASTM standards. LaVision’s Digital Image Correlation (DIC) system is employed in 2D & stereo 3D configurations to acquire high resolution full-field strain measurements. The stain-paths are quantified in the fracture regions of all specimens, from which in-situ equivalent plastic strains are derived.

A novel and low-cost Tensile Hopkinson bar has been designed and constructed for dynamic characterization of ductile metals at intermediate to high strain rates (500-1500 /s). High strain rate mechanical tests coupled with high-speed 2D-DIC have been conducted to provide a strain-rate dependent GISSMO extension to the model. Two Hopkinson bars (direct compression, split-tension) have been used to provide lode angle dependent strain-rate hardening data on stress-states of axisymmetric compression and tension covering the lode angle parameter values of -1 and 1, respectively. In addition, two cylindrical inclined compression-shear specimens with varied angles have been impacted at high strain rate to quantify the effect of stress-state on the formation and evolution of Adiabatic Shear Bands (ASBs) and their consequential effect on ductility. This innovative dynamic characterization procedure is conducted to stipulate diligent test matrices and enable improved multiscale terminal ballistics simulations on novel combat vehicle development, with the purpose to increase the predictability of shear plugging.

High strain rate axisymmetric compression, compression-shear and tension specimens have been investigated using a combination of optical (OM) and electron microscopy (SEM/TEM) to elucidate their microstructural evolution. Ductile fracture is observed under all stress-states, with changes from mode I to mode II crack formation from positive to negative lode angles. Under axisymmetric dynamic tension, enhanced damage tolerance in comparison to quasistatic loading is found attributed to increased dislocation pileups (work hardening) and subsequent ductile void growth responsible for enhanced plastic flow during necking. Axisymmetric dynamic compression reveals a severe loss of global ductility and strengthening not observed under quasistatic loading, with continuous work hardening until premature fracture and localized hardening in the ASB regions. Compression-shear specimens reveal higher susceptibility to ASB initiation with increasing angle of inclination (shear stress) and corresponding ductility loss due to increased strain localization along the plane of maximum shear. Lastly, ASB multisite microcrack initiation and coalescence, multi-directional cracking, secondary ASBs and bifurcation, nanosized grain refinement, nanoscale twinning, and dislocation cell networks are found within triaxial ASB regions revealing that AX500 has various energy absorbing mechanisms to delay crack propagation and fracture after the onset of ASB initiation.