ON THE STRAIN AND STRAIN-RATE DEPENDENCE OF THE FRACTION OF PLASTIC WORK CONVERTED TO HEAT - AN EXPERIMENTAL-STUDY USING HIGH-SPEED INFRARED DETECTORS AND THE KOLSKY BAR

The conversion of plastic work to heat at high strain rates gives rise to a significant temperature increase which contributes to thermal softening in the constitutive response of many materials. This investigation systematically examines the rate of conversion of plastic work to heat in metals using a Kolsky (split Hopkinson) pressure bar and a high-speed infrared detector array. Several experiments are performed, and the work rate to heat rate conversion fraction, the relative rate at which plastic work is converted to heat, is reported for 4340 steel, 2024 aluminum and Ti-6Al-4V titanium alloys undergoing high strain and high strain rate deformation. The functional dependence of this quantity upon strain and strain rate is also reported for these metals. This quantity represents the strength of the coupling term between temperature and mechanical fields in thermomechanical problems involving plastic flow. The experimental measurement of this constitutive function is important since it is an integral part of the formulation of coupled thermomechanical field equations, and it plays an important role in failure mode selection - such as the formation of adiabatic shear bands - in metals deforming at high strain rates.