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”