Brittle and Ductile Fracture

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 there is insufficient time, too low a temperature, too complex a stress state, insufficient operative slip systems

bcc and hcp metals

The stress required to move cracks in fcc structures are not temperature dependent. They are ductile at low temperatures.

• increasing strain rate • decreasing temperature • increasing stress raisers e.g. notches, corners, sharp change in cross-section, concentration stress (should be radiussed)

• composition - processing/cold working

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

Ductile fracture is fracture accompanied by deformation processes e.g. nucleation of dislocation at crack tip, or flow processes in polymers

Dull and there are slip lines because there is plastic deformation occuring

Minimal energy absorption, little plastic deformation, mostly elastic deformation, it occurs rapidly.

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.

It is the temperature that the material is being used at/ melting temperature of the material.

Then flow processes can occur as well as plastic deformation. This is why ice can be considered to be working at “high” temperatures.