Elastic response and strength of perfect crystals is calculated for triaxial loading conditions from first principles. The triaxial stress state is constituted by uniaxial tensile stress and superimposed transverse biaxial stresses. The maximum uniaxial tensile stress is evaluated as a function of the transverse stresses. Results for eight crystals of cubic metals and two orientations (⟨110⟩ and ⟨111⟩) of the primary loading axis are presented and compared with data for ⟨100⟩ direction of loading. Obtained results show that, within a studied range of biaxial stresses, the maximum tensile stress monotonically increases with increasing biaxial tensile stress for most of the studied metals. Within a certain range, the dependence can be mostly approximated by a linear function.

Influence of biaxial stresses applied perpendicularly to the [100] loading axis on the theoretical tensile strength is studied from first principles. Ten crystals of cubic metals and three crystals of diamond ceramics were selected as particular case studies. Obtained results show that, within a limited range of biaxial stresses, the tensile strength monotonously increases with increasing biaxial tensile stress for most of the studied metals. Within the range, the dependence can be approximated by a linear function. Beyond the range, the dependence shows a maximum that usually appears in the tensile range of biaxial stresses. On the other hand, some of the materials (Si, Ni, Cu, and Ge) exhibit a maximum tensile strength at nearly uniaxial stress state, and the superposition of both tensile and compressive biaxial stresses reduces the tensile strength. Unlike the other crystals, diamond revealed a maximum under compressive biaxial stress.