Fracture

Fracture is the final stage of material failure, occurring when a material can no longer withstand applied stress and breaks apart. The nature of fracture depends on the material’s ductility and loading conditions, and it is broadly classified into ductile fracture and brittle fracture.

Ductile Fracture

Ductile fracture occurs after significant plastic deformation, often involving necking before failure. Ductile materials, such as mild steel and aluminum, exhibit high energy absorption before breaking, making them more resistant to sudden failure.

  • Appearance on the Stress-Strain Diagram: Ductile fracture happens after the ultimate tensile strength (UTS) and follows a period of localized necking.
  • Failure Mechanism: Microvoids form within the material due to stress concentrations. These voids coalesce, leading to internal cracks that eventually grow and cause fracture.
  • Cup-and-Cone Fracture: A characteristic feature of ductile fracture in tensile loading is the cup-and-cone fracture, where the broken surfaces show a rough, fibrous interior (due to void coalescence) and an outer region that fails in shear, forming a slanted “cone” shape on one side and a “cup” shape on the other.
Ductile fracture exhibiting necking and cup-and-cone fracture (idealized).
Actual specimen exhibiting cup and cone fracture characteristics

Brittle Fracture

Brittle fracture occurs with little or no plastic deformation and happens suddenly, often at much lower strain levels compared to ductile fracture. Materials such as ceramics, glass, and some hardened steels exhibit brittle fracture.

  • Appearance on the Stress-Strain Diagram: Brittle fracture typically occurs before or just after yielding, meaning the material does not undergo significant plastic deformation.
  • Failure Mechanism: Instead of microvoid coalescence, brittle fracture is characterized by rapid crack propagation along grain boundaries or internal defects, often following a cleavage plane in crystalline materials.
  • Fracture Surface: The fracture surface appears smooth and flat, sometimes showing chevron or radial patterns that point toward the fracture origin.
Brittle Fracture (idealized)
Brittle Fracture Specimen