Estimated tensile strength of bcc crystals from shear instabilities.

M. Cerny, P. Sestak, J. Pokluda

This work presents a simple but efficient way how to estimate the theoretical tensile strength from calculated values of the theoretical shear strength taking its dependence on a superimposed normal stress into account. The presented procedure enables us to avoid complicated and time-consuming analyses of elastic stability of crystals under tensile loading. The atomistic simulations of coupled shear and tensile deformations in cubic crystals are performed using first principles computational code based on pseudo-potentials and plane wave basis set. Selected bcc crystals are subjected to shear deformations in two convenient shear systems (<111>{110} and <111>{112}) and a special relaxation procedure controls the stress tensor. Computed ideal shear strength seems to be monotonically decreasing function of the normal tensile stress for most of the investigated crystals. Taking these results into account, the uniaxial tensile strength values in  three crystallographic directions were evaluated by assuming a collapse of the weakest shear system.