1. Fracture Behavior of Interfaces
Cleavage Fracture
Microscopic aspects
Ductile Fracture
Ductile-to-Brittle Transition

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”