Adiabatic shear banding under pure shear loading Part II: fractographic and metallographic observations

Abstract A metallographic and fractographic investigation was performed of dynamically loaded torsion specimens of VAR 4340 steel (HRC 40). Macroscopically, it was found that the shear bands propagated on several distinct planes around the circumference of the specimen. Microscopically, scanning electron microscope observations of the fracture surfaces reveal that microvoid nucleation and growth is the mechanism leading to shear band fracture. Patches of highly smeared and knobbly material, produced by sliding of the fracture surfaces against each other, were also found at several locations and suggest very high local temperatures. Metallographic cross sections revealed white-etching material, characteristic of transformed bands, only at limited locations, corresponding to regions of the fracture surface exhibiting a highly smeared or knobbly texture. These results indicate that microvoid nucleation and growth play a significant role in adiabatic shear banding and that one of the mechanisms for the formation of transformed bands is the extensive sliding and rubbing of the shear fracture surfaces against each other. Thus transformed bands may be, at least in some cases, a consequence rather than a cause of the final shear band fracture.

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