Analysis of the phenomenon of adiabatic flow instability during the torsional straining of CP titanium has been made and the instability shear strain predicted. The constitutive equation for isothermal deformation of CP titanium was integrated over the total shear strain received by the material. Thermal softening caused by adiabatic heating was calculated and used to derive the adiabatic shear stress-shear strain curves. The instability shear strain was determined as the value that corresponds to zero slope of these curves. The effect of strain rate, temperature, and specific heat on the instability shear strain was also discussed. Comparison of the experimental results with predictions showed that flow instability in the torsion test occurred at strains approximately equal to the theoretical thermo-plastic instability strains for a temperature of 150°C but at lower strains at higher temperatures. The difference between the theoretical and experimental instability strains increases with an increase in temperature. A discussion of this difference is also presented.

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