The study of adiabatic shear band instability in a pearlitic 4340 steel using a dynamic punch test
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INTRODUCTION
AT low strain rates and moderate levels of strain, slip and twinning are the most common deformation mechanisms in metals and alloys. Both mechanisms are highly correlated with the crystallography of the material. At higher strain rates and levels of strain, deformation instabilities, such as adiabatic shear bands (ASB), may develop. These bands are planar in nature, and their formation is related more to the specimen geometry, deformation process, and mechanical properties of a material than to its local crystallography, t~l Adiabatic shear bands can form during compressive shear loading and are frequently observed to lead to the initiation and propagation of cracks/voids, followed by the fracture and fragmentation of the components. The first reported observation of adiabatic shear bands was made by Zener and Hollomon in 1944. t21 They inferred that shear localization was due to the deformation-induced temperature rise at rapid rates of loading. Thermal softening thus over-rode strengthening effects due to strain and strain-rate increases. In the case of steels, there is often a "signature" left by the adiabatic shear band in the form of the "white etching band" .[3] (Similar etching phenomena were also found in other materials, such as aluminum and uranium.) The white band is caused by the optical microscope resolution limit to resolve the nanometer substructure that is enhanced by etching with Nital. It is believed u'31 that these white etching bands are areas of extremely fine martensite that formed during the deformation by locally heating the steel to a temperature above that required for austenitization and followed by a rapid quench by the surrounding metal. Definitive proof for this transformation remains to be found in the literature. In addition to the white etching signature, these bands are also found to be much harder than the surrounding matrix and are believed to fracture in a brittle manner in steels in the direction normal to the local tensile stress. 131 Since Zener and Hollomon's article, there have been numerous works published that have presented experimental observations and models on the subject, t4-~SJ At present, it is generally accepted that adiabatic shear bands occur most readily in materials with ANNA K. ZUREK, Staff Member, is with Los Alamos National Laboratory, Los Alamos, NM 87544. Manuscript submitted February 8, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
a low-strain-hardening coefficient, a low-strain-rate sensitivity, a low thermal conductivity, and a high thermalsoftening rate. In addition, shear bands form more readily in high-strength materials, where the plastic work due to deformation is greater for a given plastic strain increment, t41 In recent years, several investigations have concentrated on the microstructural study of adiabatic shear bands, ts-l~ Glenn and Leslie t51 studied shear bands produced in ballistic experiments with low carbon martensitic steels. Using a diffraction method, they identified the structure of the white etching bands a
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