These findings can thereby significantly influence the rules for structural engineering design. While fracture toughness in the low-temperature brittle fracture regime is dominated by the availability and activity of sources for dislocations, the transition to the ductile response at higher temperatures does not depend on the availability of sources but is controlled by the mobility of the dislocations.
Some materials are brittle and shatter like glass, while others are ductile and deformable.
The use of these materials in structural applications is often restricted by this transition from ductile response to brittle fracture with decreasing temperature. Irreversible plastic deformation is mainly carried by dislocations which are line-defects of the crystal lattice. Crack-tip plasticity comprises two distinct processes, the nucleation of dislocations at or near the crack tip and their propagation away from the crack. Two observations support this conclusion. Firstly, deformation prior to fracture toughness testing, which introduces dislocations and dislocation sources into the material, is shown to increase toughness at low temperatures.