English: Supplementary Movie 8 MD simulation of crack propagation in nanotwinned aragonite (with twin boundary spacing of 10nm). Under increasing loading, an edge crack starts to propagate toward an array of TBs. When the crack reaches a TB, it is trapped by the TB over a time period. The crack tip becomes blunted due to sliding on the TB. Plastic deformation via structural transformation ahead of the crack tip is confined within the nanoscale twins. As the applied loading continues, the crack eventually penetrates through the TB.