When examining fracture surfaces, three typical zones are identified, corresponding to the different stages of crack initiation, propagation, and final failure.

Fracture Origin Zone

  • Definition:​ The area where the crack first initiates and begins to propagate. It is the "starting point" of the fracture.
  • Location:​ Typically located at the edge of the fracture surface or at internal defects (e.g., stress concentration points, inclusions, surface scratches).
  • Macroscopic Features:​ A relatively small and flat area, often appearing as the convergence point of radial lines or "chevron patterns."
  • Microscopic Features:​ May show cleavage steps, fatigue striations (in fatigue fractures), or the initiation point of dimples (in ductile fractures).
  • Analytical Significance:​ Identifying the origin zone is crucial for determining the cause of failure, as it allows tracing back to the initial defect or stress condition that led to fracture.

Rapid Fracture Zone (Instability Zone)

  • Definition:​ The area where the crack undergoes rapid, unstable propagation until complete failure occurs.
  • Location:​ Represents the final stage of the fracture process, typically on the opposite side of the fracture surface from the origin zone.
  • Macroscopic Features:​ The surface is rough and may exhibit radial marks (in brittle materials) or inclined shear lips (in ductile materials).
  • Microscopic Features:​ The morphology is complex, potentially showing cleavage facets, tear ridges, or rapidly expanded dimple structures.
  • Analytical Significance:​ Reflects the material's resistance to fracture and the fracture mode (brittle or ductile) at the moment of final failure.

Ductile Fracture Zone

  • Definition:​ The area formed by significant plastic deformation before final fracture.
  • Location:​ May be distributed across the entire fracture surface (in purely ductile fractures) or concentrated at the edges of the rapid fracture zone, forming "shear lips."
  • Macroscopic Features:​ The surface appears dark gray and fibrous, with 45° shear lips at the edges.
  • Microscopic Features:​ Characterized by a "dimpled" structure—craters left by the nucleation, growth, and coalescence of microvoids.
  • Analytical Significance:​ Direct evidence of good material toughness and energy absorption capacity. Its size and distribution reflect the material's plasticity.

On a mixed-mode fracture surface, these three zones are generally arranged in the following sequence:

Fracture Origin Zone → Propagation Zone (which may include a fatigue zone, etc.) → Rapid Fracture Zone (whose edges or interior may contain the Ductile Fracture Zone/Shear Lips).

Alternatively, the fracture surface might exhibit this pattern: Fracture Origin Zone → Large-area Rapid Fracture Zone → A small amount of Ductile Zone in the center.

Diagnostic Indicators:

  • If the vast majority of the fracture surface is rough and radial, with the origin zone directly connected to a large rough area → this indicates instantaneous overload fracture.
  • If the vast majority of the fracture surface is relatively flat and smooth, with only a small rough area on one side → this strongly suggests fatigue fracture.

Illustration of Features:

  • A large-area Rapid Fracture Zone​ indicates that once the crack initiated, it propagated unstably at a very high speed, leaving the material insufficient time for extensive plastic deformation. This is a typical characteristic of impact or high-stress rapid loading.
  • A Ductile Zone that is compressed and very small in the center​ represents only the area of final rapid shear separation that occurred the instant the remaining cross-section could no longer sustain the load. Its small size indicates that the overall plastic deformation capability was very limited.
  • This pattern is highly indicative of a rapid brittle fracture caused by a single, significant overload force.