Brittle and Ductile Fracture Flashcards
What is the plastic brittle transition temperature?
Plastic deformation occurs by thermally activation movement of dislocations leading to ductile failure. If these mechanisms can’t operate then there is brittle fracture.
When does brittle fracture occur>
When there is insufficient time, too low a temperature, too complex a stress state, insufficient operative slip systems
Which metals undergo a ductile to brittle transtition?
bcc and hcp metals
Why do fcc metals no have a ductile to brittle transition temperature?
The stress required to move cracks in fcc structures are not temperature dependent. They are ductile at low temperatures.
Three mechanisms which increase the probability of brittle fracture rather than ductile fracture.
- increasing strain rate
- decreasing temperature
- increasing stress raisers e.g. notches, corners, sharp change in cross-section, concentration stress (should be radiussed)
Other possibilities in changing ductile/brittle behaviour
- composition
- processing/cold working
Example of brittle failure
Liberty Ship
combination of cracks in weld and the low temperature at which it was being used. The steel was being used at temperatures below the brittle-to-ductile temperature. Rivets are now installed so cracks propagate along the rivet rather than the seam.
Polymer Helmets
What is ductile fracture?
Ductile fracture is fracture accompanied by deformation processes e.g. nucleation of dislocation at crack tip, or flow processes in polymers
Ductile fracture surfaces
Dull and there are slip lines because there is plastic deformation occuring
Properties of brittle fracture
e.g. elastic deformation occurring, plastic deformation, energy absorption?
Minimal energy absorption, little plastic deformation, mostly elastic deformation, it occurs rapidly.
What is intergranular fracture?
The fracture propagates along the grain boundaries rather than through grains. This is because oxygen usually segregates to the grain boundaries and this makes the grain boundaries weaker than the grain.
What is the homologous temperature?
It is the temperature that the material is being used at/ melting temperature of the material.
What happens when the homologous temperature ~0.7?
Then flow processes can occur as well as plastic deformation. This is why ice can be considered to be working at “high” temperatures.
