AN EXPERINffiNTAL STUDY OF THE FORMATION PROCESS OF ADIABATIC SHEAR BANDS IN A STRUCTURAL STEEL

A series of experiments is described in which the local temperature and local strain are measured during the formation of an adiabatic shear band in a low alloy structural steel (HY-100). The specimen employed consists of a short thin-walled tube and the required rapid deformation rates are imposed by loading the specimen in a torsional Kolsky bar (split-Hopkinson bar). The local temperature is determined by measuring the infrared radiation emanating at twelve neighboring points on the specimen's surface, including the shear band area. Indium-antimode elements are employed for this purpose to give the temperature history during deformation. In addition, high speed photographs are made of a grid pattern deposited on the specimen's surface, thus providing a measure of the strain distribution at various stages during shear band formation. By testing a number of specimens it is possible to form a picture of the developing strain localization process, of the temperature history within the forming shear band, and of the consequent loss in the load capacity of the steel. It appears that plastic deformation follows a three stage process which begins with a homogeneous strain state followed by a generally inhomogeneous strain distribution, and finally by a narrowing of the localization into a fine shear band. It is estimated that the shear band propagates at a speed of about 510 m/s in the material tested. Results also include data on the stress-strain behavior of HY-100 steel over the temperature range -190C to 250C and at quasi-static as well as dynamic strain rates.