ON THE MEASUREMENT OF LOCAL STRAIN AND TEMPERATURE DURING THE FORMATION OF ADIABATIC SHEAR BANDS

A series of experiments was performed to study the process of adiabatic shear band initiation and formation in steels. The steels include a low carbon cold-rolled steel and three martensitic steels (HY-100 and two tempers of AISI 4340 VAR steel of varying hardness). In each case the specimens are machined as thin-walled tubes that are deformed dynamically in a torsional Kolsky bar (torsional split Hopkinson bar). Shear band initiation and formation are observed by ultrahigh-speed photography of a fine grid pattern deposited on the specimen's surface. It is shown that the critical strain for shear band initiation depends on the magnitude of a preexisting defect, in accordance with the predictions of Molinari and Clifton, J Appl. Mech., 54 (1991) 806-812. Ultrahigh-speed photographs of the grid pattern show that local strains of 100-1000% may be attained and that the local strain rates reach 10(5) s-1. In addition, the local temperature in the shear band is measured by employing an array of small high-speed infrared detectors that provide a plot of temperature as a function of time and position. Within the shear band region, temperatures of 600-degrees-C have been measured.