Adiabatic Plastic Deformation

Adiabatic plastic deformation may play a role in such widely diverse areas as ballistic impact, explosive fragmentation, cryogenic behavior of materials, high velocity shaping and forming, machining, grinding 0), surface frictional effects, erosion (2), and seismic faulting. All of the adiabatic shearing phenomena are based on two facts: Approximately 90% of the work of plastic deformation is converted to heat, and the flow stress of most metals is quite sensitive to temperature, decreasin g as the temperature increases. That localized temperature increases and strain concentration play a major part in high speed deformation of metals was recognized by Zener & Hollomon (3) in 1944. The phenomenon is most clearly identified in most steels, in which heating above the transforma­ tion temperature causes the transformation of ferrite to austenite. On rapid cooling the austenite retransforms to a product that etches with difficulty and appears as a white band against the dark background of the remainder of the etched steel. These materials thus retain evidence for adiabaticity of the deformation, while in most other metals the evidence is significantly less definite. The use of the term "adiabatic deformation" is obviously an over­ simplification in the sense that some heat always transfers out of any deforming region. Moreover, to categorize one situation as "adiabatic" and another not is in many instances equivalent to labeling shades of gray as black or white. It