Study of shear localization using a novel test specimen loaded an a split-Hopkinson compression bar

Large plastic shearing of materials at various loading rates has been known to affect the material microstructure and behavior in a significant manner. These effects include: (1) localization of shear strain into bands of several microns to several tens of microns thick called shear bands, (2) strain and temperature induced phase transformations, (3) grain refinement, and (4) profuse twinning.

In this research, a split-Hopkinson compression bar was used to study the effects of high strain rate loading and large plastic in three novel "shear" specimens. Each specimen had a unique geometry producing various degrees of shear deformation when dynamically loaded. The region of large shear deformation was known prior to testing, allowing research to be focused on that specific region of the test specimen. The specimens were tested using projectile velocities ranging from 11 m/s to 22 m/s. The objectives of this research were to investigate the effects of large shear deformation at high loading rates on various metals and furthermore to modal some of the experimentally observed phenomena using finite element analysis.

The amount and nature of shear deformation varied depending on the specimen geometry, materials, and metallurgical condition. Adiabatic shear bands were generated within AISI Ti-6A1-4V titanium and AISI 4142 steel. AISI 4142 steel also underwent a significant refinement in grain structure. Profuse twinning and strain induced phase transformations were observed within AISI 304 stainless steel. The microstructure of AISI D-2 tool steel displayed rotation and fracture of carbide particles. Microstructural changes were analyzed by means of optical and scanning electron microscopy without having to cut, section or destroy the test specimens. The geometry of the test specimens allowed the microstructure to be analyzed before and after loading.

Loading of the three specimen geometries were simulated for the titanium and steel alloys using the finite element analysis (FEA) software ABAQUS version 6.3. Simulations consisted of a 2-dimensional, plane strain full-scale model that included the effects of adiabatic heating and employed the Johnson-Cook equation in modeling plastic behavior. The FEA predicted adiabatic shear band formation to occur where shear banding was observed experimentally.