ML1: Structural performance intro and ductile/brittle recap Flashcards
(6 cards)
What are five different modes of failure?
- Fracture (ducitle or brittle): behaviour in the presence of a stress concentration
- Fatigue (cyclic loading): initiation and growth of a defect
- Corrosion (dry or wet): an oxidation process
- Wear (surface degredation): local properties important (use of coatings)
- Creep: time dependent deformation (temp dependent, usually @ high temps).
Note: many of these processes can occur at once.
What is the characterisitics of ductile failure modes?
How it occurs:
1. Initial necking, causing stress concentrations there
2. Small voids form around the hard secondary particles
3. Small voids join (coalesce) to give internal crack
4. Remaining ligaments shear and fail under maximum tensile stress
5. final failure.
Features of a ductile failure:
* shiny
* cup-cone shape
What is the characterisitics of Brittle failure modes?
How it occurs:
1. integranular or transgranular cracks will occur through the micro structure.
2. Catastrophic sudden failure
3. Little to no prior deformation
Feature of a brittle failure:
* dull looking
* riverlines which shows where the first initiation point occured.
What is meant by Toughness?
- Toughness is the ability of a material to resist fracture in the presence of a notch/defect.
- Often measured by energy it absorbs
How can toughness be measured?
Charpy impact test:
What is the Ductile to Brittle (DBT) transition point?
- Shows how differing micro structures (such as FCC, HCP and BCC) will be affected by temperature.
- i.e FCC is close packed in both planes and directions so plastic deformations from dislocations will occur always before any other type of failure. Thus it always has ductile failure.
- BCC and HCP can be closely packed in directions, but not planes, therefore diffusion processes will also play a role in failure. Thats why if temps are higher it tends to fail in a ductile mode, and lower temps it fails brittly.
