Damage at negative triaxiality

The damage process under compressive hydrostatic stress in ductile metals has been observed in many experiments. However, no damage theory explains the physical mechanism of damage due to compressive hydrostatic stress in ductile metals. Recently, one possible physical mechanism of shear damage at low and negative stress triaxiality (The stress triaxiality is defined as ( ( 1 / τ eqv ) ( τ k k / 3 ) ) where τ eqv = ( 2 / 3 ) τ ′ : τ ′ , τ is the Kirchhoff stress tensor. Triaxiality in this paper refers to the stress triaxiality defined above.) was shown to be the development of tensile hydrostatic stress due to grain-to-grain interaction (Kweon, S., Beaudoin, A.J., McDonald, R.J., 2010. Experimental characterization of damage processes in aluminum AA2024-O. Journal of Engineering Materials and Technology 132.) Kweon (2009. Edge cracking in rolling of an aluminum alloy AA2024-O. Mechanical Science and Engineering, University of Illinois, Urbana.) proposed a mesoscale theoretical framework that can be used to quantitatively investigate the amount of shear damage at all triaxiality levels, incorporating the physical mechanism of shear damage. This theoretical framework is based on crystal plasticity and the theory of void growth due to hydrostatic stress. The damage process at negative triaxiality is particularly important since many industrial processes for metals involve a compressive hydrostatic stress state such as rolling. Using the mesoscale theoretical framework, damage at negative triaxiality is theoretically quantified. It is shown that damage does exist at small negative triaxiality, and that the shear deformation component drives damage in the small negative triaxiality regime.