Adiabatic shear banding

The subject of adiabatic shear banding is quite classical and the main assumption is that it consists of  structural/mechanical instability in which the thermal softening effects play a dominant role in the generation of the band (Zener-Hollomon).

I would like to focus on the phase that precedes global failure, namely up to the peak stress or strain) at which the material starts to exhibit negative strain hardening.

The point is that the anticipated temperature rise which leads to softening is seldom measured and more often calculated by converting all of the mechanical work into heat, assuming a beta factor of Taylor to be equal to 1. This is fine except that many materials do not have such a high beta which is most often a function of the strain and the strain rate as well! Moreover, mny materials, e.g. titanium alloys, magnesium alloys, maraging steel, tungsten heavy alloys and others fail at overall low stress levels, which results in a very modest temperature rise, even if the mechanical properties are high (e.g. maraging steel). This casts a doubt on the role of homogeneous heating during the hardening phase.

In a couple of recent works, we proposed to consider the dynamic strain energy as a  potential factor, and showed it remains quite constant at failure, irrespective of the prior thermomechanical static history of the material. The advantage of this parameter is that it ties naturally to the dynamically stored energy of cold work which is the complementary part of the energy balance, once thermal effects are neglected.

More recently, we could identify dynamic recrystallization in a Ti6Al4V alloy that was only deformed to roughly 50% of its dynamic failure strain, at a stage where thermal effects are still surely negligible while the shear band has not developped at all yet. But most of all, this suggests that the stored energy can be iterpreted as the driving force for dynamic recrystallization, whose multiplication will eventually form the shear band. Note that dynamic recrystallization is universally (or almost) observed post-mortem in shear bands, and considered to result from the high temperature in the band. The recrystallized grains are soft and dislocation free.

 To summarize: It seems like adiabatic shear banding is the result of aphase-transformation like process, occurring during the hardening phase, without marked thermal coupling effects, whose ultimate result is a shear band.

The destabilizing factor here is a microstructural transformation resulting in local strain softening.

These results certainly call for a second look on the whole subject and I will be happy to receive comemnts and suggestions from the community.

Sincerely,

Dany 

D. Rittel, Z. G. Wang and M. Merzer, "Adiabatic shear failure and dynamic stored
energy of cold work", (2006), Physical Review Letters, 96(7),
075502:1-4 A. 

D. Rittel, P.
Landau and A. Venkert, (2008), “Dynamic recrystallization as a potential cause
for adiabatic shear failure”, Physical Review Letters, 101, 165501.