Adiabatic Shearing Behavior Of TA2 Titanium Alloy And Its Influence Of Stress State, Grain Size

Adiabatic shear localization is an important and often dominating deformation and failure mechanism of materials under explosive and impact loading,and plays a particularly significant role in a variety of engineering fields,such as ballistic impact,penetration/perforation,high-speed machining and cutting,etc.In this paper,the adiabatic shearing behavior of TA2 Titanium alloy is investigated by the dynamic shear test carried out on the platform of Split Hopkinson Pressure Bar using flat-hat-shaped specimens.The initiation and evolution processes of the adiabatic shear bands?ASBs?are analyzed through the combination of macroscopic mechanical analysis and microscopic metallographic observation,then the effects of strain rate,stress state and alloy gain size on the initiation and evolution of adiabatic shear bands are also analyzed.The main conclusions are drawn as follows:The stress states of shear zone for hat-shaped specimens loaded by split Hopkinson pressure bars are studied by theoretical analysis and numerical simulation.It is found that the stress state can be considered as the state of pure shear as long as the Lode parameters?_?meet the condition of-0.46<?_?<0.46 and its experimental error is less than 3.4%.The equivalent stress-strain curve of TA2 titanium alloy obtained by the hat shear test is consistent with that of compression test.The experimental results show that the equivalent stress-strain curve obtained by shear test can directly reflect ASBs initiation and evolution processes in the shear region of the material,and the stress-strain curve degradation once ASBs formation in the hat-shaped specimens and material tend to soften.However,the equivalent stress-strain curve obtained by the dynamic compression test can not directly reveal the softening characteristics of the adiabatic shear band initiation and crack propagation processes,even if the symmetrical bi-conical ASBs and local crack distribution have appeared in the cylindrical specimen.The shear tests for the closed flat hat-shaped specimens of the TA2 titanium alloy under different loading rates are carried out.The results show that:1)There are two dynamic failure mechanisms of TA2 titanium alloy,which are related to the loading strain rate.Under lower strain rate(less than 2500 s^-1),the damage of material is caused by shear failure,while under the higher strain rate(greater than 2800^-1),the adiabatic shear localization occurs,and the crack propagates along the adiabatic shear band.2)The time corresponding to the peak of stress-strain curve reflects the initiation of adiabatic shear,and the"stress collapse"in the stress-strain curve characterizes the moment when the adiabatic shear bands propagate through the shear zone.3)From the perspective of macroscopic continuum mechanics,the thermal softening of materials caused by adiabatic temperature rise is not the critical condition for the initiation of adiabatic shear bands.On the contrary,the formation and development of adiabatic shear bands cause the temperature to rise sharply.Thus,thermal softening may be not the critical condition for the initiation of adiabatic shear bands.4)The critical strain of specimen for ASBs initiation decreases with the increase of strain rate,and its corresponding plastic work is almost constant.Therefore there is a critical plastic work W_i for the initiation of adiabatic shear band,and Rittel's"cold work"criterion may be reasonable.In order to investigate the effect of stress state on adiabatic shear characteristics,the dynamic shear tests of hat-shaped specimens under different compression-shear ratios are carried out.The experimental results show that the initial strain of TA2 titanium alloy adiabatic shear band increases with the compression-shear ratio,and the formation of ASBs is delayed.In other words,the increase of hydrostatic pressure is not conducive to the initiation and development of adiabatic shear bands.The adiabatic shear behavior of TA2 titanium alloys with different grain sizes is studied experimentally.The results show that the grain size has a great influence on the initial evolution of adiabatic shear bands,and the grain size reduction is beneficial to the initial evolution of adiabatic shear bands.On the other hand,once the ASBs initiated,the effect of the grain size on the evolution process of ASBs is not large.