Microstructural simulation of adiabatic shear band formation in AISI 4340 steel using Voronoi Tessellation

Using the stress strain response data obtained from an experimental investigation carried out by the first author (Polyzois, 2014), a finite element model was developed in Matlab and explicit FEA software ANSYS and LSDYNA to simulate Adiabatic Shear Band (ASB) formation in AISI 4340 steel on the microstructural level under high strain rate impact. This FEA model uses the Voronoi Tessellation to generate geometry simulating the microstructure of heat treated AISI 4340 steel in a 2D axi-symmetric cross-section based on the prior austenite grain size and 2D geometric orientation of martensitic lath blocks. It also incorporates the microstructural material inhomogeneity between the grains and the grain boundaries. The model was discretized using the meshless Smooth Particle Hydrodynamics (SPH) method and utilizes the Johnson-Cook plasticity model parameters for various heat treatments of the steel as well as a rupture failure criterion. The model provides a good representation of the kinematics of ASB formation based on grain refinement showing a progressive reorientation and elongation of the grains in the direction of shear in 2D. Severe strain localization and ASB formation were shown to nucleate at the grain boundaries of the elongated grains, creating micro-voids, which grew and propagated as micro-cracks through the grains, separating them into smaller sizes. Under continued deformation, the grains continued to elongate and refine. Final refined grain size within the band is represented by the size of the smallest cluster of intact SPH particles.