Chen, Y., Lee, J.D., Eskandarian, A., and Bedewi, N.E. (2000) “Adibatic Shear Localization in Impact problems”, International Conference of Crashworthiness, London, September 2000.

Abstract: Highly localized deformation, generally referred to as adiabatic shear band, can occur in variety of metals when deformed at high strain rates. Adiabatic shear bands are areas of intensive shear deformation in which plastic work can cause temperature to rise significantly and thus soften the material and allow for greater deformation and further heating. This kind of deformation is usually referred to as adiabatic, because there is no sufficient time for the heat to be conducted away before thermal softening occurs. Once localization has taken place, the strain and the temperature are locally very large without contributing much to the overall deformation of the object, and the highly localized strains can precipitate a shear fracture. This phenomenon has been known to be crucial in situations such as high-speed impact, crash, penetration, metal forming, machining and manufacturing.

In this paper, we study the initiation and development of shear bands in high-speed impact/contact problem. The deformable material used in this study is modeled to be strain hardening, strain rate hardening, and thermal softening with the flow stress specified by the Johnson-Cook relation and the Gruneisen state equation. The deformation is assumed to be adiabatic and thus the effect of heat conduction is ignored.

The numerical simulation of a cylindrical rod impacting with a flat rigid surface at normal incidence (Talor anvil or reverse ballistic impact test) is performed by using the large-scale crash analysis program, LS-DYNA. For comparison purpose, the simulations of the plane strain compression and the plane strain impact problems are also studied. The formation of shear bands has been observed in both plane strain and axisymmetric impact problems for steel. At higher impact speed, the deformations were found to be more localized along a curve path extending from the transition zone, between the mushroom region and the relatively underformed region, towards the impact surface, as was observed in Talor’s and Dick’s tests.

We have also investigated the effects of thermal softening, strain rate, plastic strain, and friction (between the impact/contact objects) on the deformation localization. It is found that the impact speed, the geometry constrains, and the thermal softening characteristics have significant effects on shear localization. Among those, the thermal softening characteristics is the dominant factor on the formation of shear band.