Fracture Process Zone 1

1
Fracture Process Zone -1

• Crack is the material separation with separation
  distance substantially smaller than the
  separation length, the crack length.
• Separation distance is comparable to some
  microstructural length dimension, ex. Distance
  between inhomogeneities or inclusions
• This distance can be as low as a few atomic
  distance while the crack length is considerable
  larger.
• The length scales involved are typically smaller
  than the characteristic length scale of the
  material, e.g. grain sizes
• The cracks at these length scales are called
  micro-cracks, and material separates at micro
  level and indicates micro separation.

Refernce Cracks and Fracture by K. B. Broberg
2
Fracture Process Zone -2

• The fracture process always takes place in a
  small region near the edge of the crack and is
  the process zone where the material separation
  occurs.
• While continuum theories are valid in other
  region, the theory is not valid in the process
  zone.
• Micromechanistic processes occurring in the
  process zone vary from material to material even
  for the same material they vary under different
  temperatures and rates of loading
• Micro separation is the decohesion on a
  microstructural level.
• They can be classified as
• Micro cracks (metals at low temperatures,
  ceramics)
• Voids (metals at room temperatures)
• Cavities at grain boundaries (metals at high
  temperature)
• Rupture/disentaglement (polymers)
• Rupture at atomic scale (ceramics like glass)

3
FPZ-3 Dominant kernels of micro-separation

• Note that there are two populations of particles,
  large and small. Cracks nucleate in each case at
  a nucleating kernel.
• Nucleating kernels can be the large or small
  particles. Typically it starts with the large and
  connects with the ones originating from the
  smaller ones.

4
FPZ 4- Intrinsic Material Length Parameter
5
FPZ-5 Micro separation in metals

• Metals are polycrystalline in nature and
  micro-separation occur as a rule after severe
  plastic flow.
• Micro cracks are generated by high stresses
  (usually at lower temperatures).
• Cleavage is a micro crack occuring along specific
  planes, and is promoted by lower plastic flow.
  Since high triaxial state suppress plasticity,
  cleavage is promoted by triaxial state.
• Voids originate by large strains with a stong
  shear component.(typically above room
  temperature)
• Typically microcracks and voids grow inside the
  grain leading to transgranular fracture.
• Cavities occur at higher temperatures typically
  at grain boundaries and triple points, and lead
  to intergranular fracture.

6
FPZ 6- Micro cracks in metals

• Various types of mechanisms are active in the
  formation of micro cracks within a grain.

7
FPZ 7- Cleavage fracture in metals

• Micro cracks occur in different grains with
  different orientations. Once the ligaments are
  sheared off, the fractured surfaces contain
  glittering facets with fibrous borders

8
FPZ-8 Micro separation in ceramics

• Crack is the material separation with separation
  distance substantially smaller than the
  separation length, the crack length.

9
FPZ-9 Micro separation in polymers

• Bond between macromolecules are very weak they
  stretch/slide/disentangle under load.
• Crazing at the edge of a crack in a polymer.
  Fibrils link the top and bottom of the crack
  surface. They break at the left.

10
FPZ-10 Cell Model of Materials

• Cell Model permits the modeling of separation
  within the context of continuum mechanics.

11
FPZ-11 Cohesion-Decohesion curve

• Crack is the material separation with separation
  distance substantially smaller than the
  separation length, the crack length.

12
FPZ-12 Model of forward and wake regions

• Ahead of the crack tip the process zone has a
  forward region where the material undergoes
  separation.
• Behind the crack tip we have elastic unloading.

13
FPZ-13 Model of a process zone

• The top figure indicates the cell model while the
  bottom shows the traction-separation profile.

14
FPZ-12 The sequence of the fracture process

• Crack is the material separation with separation
  distance substantially smaller than the
  separation length, the crack length.

15
FPZ-12 Model of forward and wake regions

• Crack is the material separation with separation
  distance substantially smaller than the
  separation length, the crack length.

16
FPZ-12 Model of forward and wake regions

• Crack is the material separation with separation
  distance substantially smaller than the
  separation length, the crack length.