In this study, we perform molecular dynamics simulations to investigate plastic deformation modes of a magnesium (Mg) nanotwinned structure. Periodic units including (1011) twin boundaries (TBs) are analyzed using an embedded atom method potential. Equal spaces between the TBs are assumed at the initial state and the space is parametrized in the range between about 5 nm and 30 nm. It is found that plastic deformation is triggered by the slip along a (1011) twinning plane near a TB, and that this event induces two different deformation modes depending on the space, i.e., the migration of the TBs and the evolution of double twinning, which leads to void nucleation and polycrystallization. The plastic deformation provided by the two different modes is quantitatively verified from geometric analysis. As the space decreases, the migration of the TBs is superior to the evolution of double twinning.
The present paper proposes a shape optimization of the adhesive interface to improve the strength of adhesive structure under a multiaxial stress state. Its strength was evaluated by the failure function based on the first invariant of stress tensor I1 and the second invariant of deviatoric stress tensor J2, which has been discussed in the related previous paper. We defined a sum of squares of the failure function as an objective function in the optimization. Two types of the adhesive material properties, which put the major weights on only I1 and J2, respectively, were numerically examined. As a multi-material structure model bonded by the adhesive, a thin-walled butt-jointed cylinder was employed to avoid stress concentration at free edges in the adhesive layer. Three kinds of loadings were applied: only tension, only torsion, and a tension-torsion combined load. The shape of optimal adhesive layer according to each condition was in good agreement with the simplified theoretical solution. The kinked part of the optimal adhesive interface necessary for the pipe configuration was also reasonable to diminish the stress concentration around the corner. The obtained optimal shapes for the two typical adhesive material properties suggest that the present optimization method would be applicable dependent to the adhesive material parameters under multiaxial stress states.