Fracture Behavior of Interfaces

1
Fracture Behavior of Interfaces

• Cleavage Fracture
• Microscopic aspects
• Ductile Fracture
• Microscopic aspects
• Ductile-to-Brittle Transition
• Microscopic aspects

2
Microscopic Aspects of Cleavage Fracture

• Cleavage fracture occurs by separation along
  crystallographic planes of a material
• When fracture travels through grains, it must
  change direction with respect to the normal
  fracture direction
• Features of cleavage fracture include
• Ledges or steps
• Cleavage tongues and river patterns

3
Microscopic Aspects of Cleavage Fracture

• Figure 8.13 pg 234 of Nonlinear Fracture
  Mechanics for Engineers

4
Microscopic Aspects of Cleavage Fracture

• Figure 8.14a and b
• with descriptions

5
Microscopic Aspects of Cleavage Fracture

• River patterns are caused by the merging of
  several cleavage steps in the vicinity of grain
  boundaries
• The cleavage steps give the appearance of a river
  pattern from which the crack direction may be
  seen
• Tongues form when the cleavage crack meets a site
  of deformation twinning
• The cleavage crack continues to form on the
  interface of the twin boundary, eventually
  sticking out like a tongue

6
Microscopic Aspects of Cleavage Fracture

• When a polycrystalline sample is deformed, the
  deformed grains are constrained by the
  surrounding grains
• This is necessary to maintain continuity of the
  material
• If the grains are not constrained and allowed to
  deform as single crystals, gaps form and grains
  overlap
• This occurs due to the fact that the slip planes
  in each of the grains is oriented differently
  form one another and react differently to the
  applied strain

7
Microscopic Aspects of Cleavage Fracture

• Figure 8.16 pg 238 in Nonlinear Fracture
  Mechanics for Engineers

8
Microscopic Aspects of Cleavage Fracture

• Cleavage fracture occurs due to a lack of
  available slip systems
• Each grain requires a minimum of five slip
  systems to accommodate a state of strain
• When these are not available, stresses occur,
  especially on grain boundaries, giving rise to
  cleavage fracture
• FCC materials, containing 12 slip systems, do not
  exhibit cleavage fracture

9
Microscopic Aspects of Ductile Fracture

• The classic example of ductile fracture
• FCC pure metals in tensile testing experience
  severe necking
• Avoid cleavage fracture due to their 12 slip
  system
• Figure 8.22 pg 247 Nonlinear Fracture Mechanics
  for Engineers

10
Microscopic Aspects of Ductile Fracture

• Engineering alloys contain particles which alter
  ductile fracture behavior
• Some of these are added deliberately to
  strengthen the material
• These particles create voids in the material
  which eventually grow large enough for failure to
  occur
• By contrast, in cleavage fracture, these
  particles become sites where microcracks form and
  eventually grow large enough to cause failure

11
Microscopic Aspects of Ductile Fracture

• Figure 8.23 pg 248 Nonlinear Fracture Mechanics
  for Engineers

12
Ductile to Brittle Transition Microscopic Aspects

• Occurs as a competition between ductile tearing
  and cleavage fracture
• Ductile crack growth occurs via void growth
• Cleavage fracture by a stress controlled process
• The method of fracture depends upon the size and
  geometry of the specimen
• Either may occur at a fixed temperature as seen
  in the figure on the next page
• Cleavage fracture usually occurs at a higher
  constraint than ductile fracture

13
Ductile to Brittle Transition Microscopic Aspects

• Figure 8.30 pg 258 Nonlinear Fracture Mechanics
  for Engineers