Stress Concentration: As the crack advances, the stress ahead of the crack tip intensifies. This stress concentration is the driving force for crack propagation.
Bond Breaking: At the crack tip, atomic bonds between atoms or molecules in the material directly ahead of the crack are severed. This breaking of interatomic bonds requires energy, which can come from various sources such as applied loads, residual stresses, or temperature gradients.
Plastic Deformation: In some materials, plastic deformation may occur near the crack tip. This involves localized, irreversible deformation of the material to relieve the high stresses. Plastic deformation can contribute to crack growth by providing a path of least resistance for the crack to advance.
Microcracking: In brittle materials, the stress concentration ahead of the crack can cause the formation of microcracks or voids. These microcracks may coalesce with the main crack or lead to additional branching of the crack.
Crack Propagation: The combination of bond breaking and plastic deformation leads to the propagation of the crack. As new bonds break and material separates at the crack tip, the crack advances, extending the damage zone within the material.
The behaviour at the moving edge of a crack depends on the material properties, the applied stress conditions, and the material's microstructure. In some cases, the crack may propagate in a straight or brittle manner, while in other cases, it may exhibit deviations, branching, or other complex patterns of crack growth.
