Adiabatic shear bands

C.G. SteUy, C.C. Dormeval, Adiabatic shear band phenomena, In L.E. Murr, M.A. Meyers, Editors., Metallurgical applications of shock-wave and high-rate phenomena, (1998), Marcel Dekker, New York-Basel. pp. 607-632. 

Comparison of the peak temperature and strain-rate estimates with the pertinent hot-deformation processing maps (Ref 19, 20, 23, 24) suggests that FSW of Ti-6A1-4V involves adiabatic shear banding (P instability) above the P-transus temperature. The instability in this temperature/strain-rate regime is also manifested by the broad oscillations observed on the stress-strain curves at these conditions reported by several researchers for this type of alloy (Ref... 

Observed dependencies of the critical hVc combinations for the onset of adiabatic shear banding on the material being machined and the tool geometry may be considered in terms of Fig. 3b. Observed transitions to localized shear and serrated chip formation with increasing h and Vc are rmly likely to be due to increasingly adiabatic conditions if hv tantp/a is in the range 10-100. Certainly any transition at hv tantp/a... 

Once shear localization is initiated, flow of heat from the shear band is predominantly normal to its surface. From the theory of heat diffusion and given that a shear band is active (Fig. 1) for a time (h/Vc), the minimum width over which conditions may be considered adiabatic (a[h/Vc])° This is expected to be the minimum width 5 of an adiabatic shear band, or (S/h)mm (a/[hVc])° . For typical machining conditions, 5mm is of the order of 10 pm. Experiments generally do show 8 to be of this order and reducing with reducing h/Vc but not always to the power of 0.5. 

Taking into account adiabatic shear bands as discussed above only leads to a slight segmentation on comparison with real chips so that some authors recommend a combination with failure simulations (Sievert et al. 2005). By contrast, good results without additional failure models were achieved by others. It is possible to reinforce segmentation by manipulating the flow curves used, but at the same time, material behavior that is not validated by experiments may be anticipated. 

Fig. 8.50 a Voids formation and b coalescence in an adiabatic shear band in titanium alloy... 

Strain softening, shear localization, and shear banding are all associated with adiabatic shearing, but they are not synonyms as they can also occur, for other reasrais, in isothermal conditions. 

Shear localization or banding due to thermal softening does not require truly adiabatic (i.e., no heat flow) conditions. All that is required is a conditiOTi in which enough heating occurs. The term catastrophic thermal shear covers this. 

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