Dynamics of three-dimensional stepped cracks, bistability, and their transition to simple cracks

Slow cracks may be simple, with no internal structure. The leading edge of a simple crack, the crack front, forms a single fracture plane in its wake. Slow cracks may also develop segmented crack fronts, each segment propagating along a separate fracture plane. These planes merge at locations that form steps along fracture surfaces. Steps are not stationary, but instead propagate within a crack front. Real-time measurements of crack front structure and energy flux reveal that step dynamics significantly increase energy dissipation and drastically alter crack dynamics. Simple and stepped cracks are each stable. By extending the use of energy balance to include 3D crack front structure, we find that, while energy balance is obeyed, it is insufficient to select the energetically favorable crack growth mode. Transitions from stepped cracks to simple cracks occur only when their in-plane front lengths become equal and a perturbation momentarily changes step topology. Such 3D crack dynamics challenge our traditional understanding of fracture.

Automated summary

Slow cracks can be simple with a single fracture plane or segmented with multiple fracture planes. The merging of these planes forms steps along the fracture surfaces, which dynamically propagate within a crack front. Real-time measurements show that step dynamics significantly increase energy dissipation and change crack dynamics. The transition from stepped cracks to simple cracks occurs when their in-plane front lengths become equal and a perturbation momentarily changes step topology. These 3D crack dynamics challenge traditional understanding of fracture.