Fracture mechanisms Flashcards
What can the fracture surface give us information about?
- Fracture mechanism that had operated during failure of the part
- nucleation site
- growth direction
- amount of plasticity involved
- “crystallinty”
What are the steps during failure mechanisms in ductile alloys ?
- Nucleation of voids by fracture of second-phase particles or by decohesion at their interface with matrix
- Growth of microvoids (large energy “consumption”)
- Coalescence of microvoids and failure by ligament instability
How does ductile fracture for pure metal single-crystals (theoretically) happen?
By extensive shearing along slip planes.
What are the ways a nucleation of a void can begin?
- onset of voids by achievement of a stress limit
- onset of voids by achievement of a strain limit
- onset of voids by achievement of critical absorbed energy
Why does growth of micro voids happen?
Due to extensive plastic deformation
What is the picture of and what is happening here?
Typical microstructural features of ductile fracture surfaces found in metals.
Nucleation and coalescence of microvoids starting from globular inclusions.
What causes the cleavage mechanism in a brittle fracture to activate? And what happens after?
When a critical stress value is exceeded at the tip of a pre-existing defect, where peak stresses cannot be redistributed by plasticity. Very high local stress causes decohesion of atomic bonds. Unstable growth then follows along specific crystallographic planes
Where does the cleavage fracture develop?
Along flat surfaces corresponding to crystalline planes of grains
What type of fracture is this?
Transgranular fracture (cleavage fracture)
What type of fracture is this?
Cleavage fracture
When does intergranular fracture occur?
When cracks find a “preferential path” along grain boundaries that are “weakened” by some metallurgical embrittling phenomena
What type of fracture is this?
Intergranular fracture
How can thoughness be improved in ductile materials?
Inclusions and second-phase particles should be limited to a minimum and reduced in size, so as to prevent or delay as much as possible void nucleation and growth phenomena.
How can thoughness be improved in brittle materials?
Cracks should have few sites where they can potentially nucleate and should be forced to propagate along tortuous paths in order to increase the overall energy absorbed for fracture.
What are some metallurgical features that can affect toughness?
-Intrinsic mechanisms (use of suitable tough constituents in alloy structure)
-Extrinsic mechanisms (promotion of a tortuous crack path to improve absorbed energy)
The figure shows the complex fibering path found in a forged steel part. Which loading direction (arrows) should be avoided and which is preferred?
Red is negative and dangerous, blue is positive.
The main stress direction should conveniently lie parallel to fibres (blue arrow). Condition of loading normal to fibers (red arrow) should be avoided when possible.
