Naval Structural Steels
Shear banding is a deformation phenomenon that involves intense localized shear deformation in a material. The intense shear strains signify localization of energy dissipation as deformation progresses. The eventual outcome of localized deformation is ductile rupture and material separation. For this reason, the combination of shear strain localization and eventual rupture is called shear failure. This phenomenon occurs and plays an important role in many applications. For example, shear bands are observed in ballistic impact, explosive fragmentation, high speed machining, metal forming, grinding, interfacial friction, powder compaction, granular flow, and seismic events. The formation of shear bands is a self-feeding process. Plastic dissipation generates heat which causes the material to soften as temperature increases. The thermal softening causes the material to deform at higher strain rates, resulting in further dissipation, temperature increase and thermal softening. This catastrophic cycle does not occur uniformly in the material. The result is the formation of narrow and distinct bands of high rates of shear deformation and high temperature. An important outcome of this process is the decrease of the material’s stress-carrying capability, leading to precipitous drop in stress in later stages of the process. If deformation is allowed to continue, the eventual result is the ductile rupture of the material and total separation through the center of the shear band.
This research is part of an effort to quantify the shear failure resistance of materials. The objective of this research is to identify, on macro- and microscopic scales, the factors that determine the resistance of materials to dynamic shear failure in the form of shear band formation and eventual rupture and provide an assessment of the relative resistance to this form of failure. The focus is on both the evolution of the load-carrying capacity of these materials during shear band development and associated microscopic changes. The materials studied are structural metals HY-80, HY-100, HSLA-80, 4340 VAR and Ti-6Al-4V. These are the materials for many structures and shear banding is the major mode of failure under certain dynamic loading conditions. The experiments used in this analysis provide a range of loading rates and superimposed hydrostatic pressures. Deformations can be controlled to occur to various stages of shear localization and failure, allowing shear bands to be “frozen” at different levels of straining and analyzed using optical and electron microscopy. The experiments also allow the evolution of the load-carrying capacity of the materials to be obtained and evaluated. Since a range of materials with different macroscopic properties and microstructures are analyzed under similar conditions, the results of this research can contribute to the quantification of the “shear band toughness” of materials.