Adiabatic Shear: Pre- and Post-Critical Dynamic Plasticity Modelling and Study of Impact Penetration. Heat Generation in this Context

Adiabatic Shear Banding (ASB) is recognized as a phenomenon of notable importance, being a failure precursor in the context of dynamic deformation for a large class of metals and alloys (in particular high-strength steels and alloys) and non-metals (polymers). Stemming from the pioneering work of Zener & Hollomon (1944), Recht (1964), extensive investigation – metallurgical and mechanical, experimental and theoretical –, and relevant literature have been devoted to the matter, see for instance numerous references given in the books by Bai & Dodd (1992), Wright (2002). These authors have attempted complementary syntheses of the field ranging from materials science oriented research to non linear mechanics issues. The special issue ‘Shear Instabilities and Viscoplasticity Theories’ of Mechanics of Materials published in 1994, including notably the papers by Mason et al. (1994), Nemat-Nasser et al. (1994), keeps also its topical importance. The seminal contribution by Marchand & Duffy (1988) should be cited as a major experimental work. The emergence of ASB is attributed predominantly to the opposite influence of strain and strain rate hardening and thermal softening effects, respectively. Thermal softening is assumed to lead to a stage when the material can no longer harden and, in this way, looses its stability, making possible the formation of a localized discontinuity/failure mode. This is why many studies of instability inception are concerned, in this context, with perturbation analysis of the mechanical and thermal fields, see for instance Molinari & Clifton (1987). Very recent results regarding the ASB phenomenon bring out some finer points to the picture mentioned above. They tend to clarify the role of microstructural evolutions and point out a particular phase transition, namely dynamic recrystallization as a possible factor in the ASB generation (Rittel et al., 2008). Adiabatic shear mode requires that thermal conductivity effects be attenuated by a small deformation time, i.e. high strain rate involved. In such a way this mode is considered sometimes as ‘a characteristics’ of impact loading (Woodward, 1990). Depending on the thermomechanical properties of the target material and on the intensity of loading, the penetration of a flat end projectile into, say, a hard steel plate can be accompanied by the formation of a ring shape intense (localized) shear zone inside the target. Intense shearing can lead to the development of adiabatic shear bands which are