| This study describes a new version of the Split Hopkinson Pressure Bar (SHPB) apparatus capable of subjecting specimens to dynamic shear at rates of strain between 3.6 E04 and 1.2 E05 /s. Results are given for two heat treatments (Type A as received alloy, and Type B was heat treated alloy) of the titanium alloy Ti6Al4V. The flow curve for each type was obtained from the recorded signals of the instrumented bars of the SHPB for this range of strain rate. Also, for each specimen tested, a metallurgical examination of its micro-structure was done to trace and mark the onset of adiabatic shear formation as a function of the applied strain rate.; Both types of the Ti6Ala4V alloy showed a strain rate sensitivity, but the degree was less for type B alloy. Yield strengths for some of the specimens tested were observed up to 8.46 E04 /s. No evidence was obtained of adiabatic shear band formation or thermal softening up to a strain rate of 7.87 Eo4 /s for type A treatment, and up to 8.83 E04 /s for the other type. Above these values of strain rate failure of specimens by adiabatic shear banding was evident, and at higher values of strain rates voids and micro cracks were observed along these shear bands.; A new experimental finding was found to be useful to mark the onset of such plastic instability at high strain rate deformation. This criterion was based on the global amount of specific energy absorbed by the plastically deformed zone. The function relating specific energy and strain rate has a discontinuity which divide the behavior under dynamic loading into two zones. Before the discontinuity there is a steady increase in the specific energy with the strain rate, and there was no evidence of formation of adiabatic shear bands. At the discontinuity there is a large increase in specific strain energy. After the discontinuity there was always the formation of shear bands. Such a criterion was found to be suitable to mark the point of plastic instability by adiabatic shear banding. |
