Materials Engineering (Week 6) Flashcards
Failure is almost always undesirable.
However sometimes failure is built into the design, can you give some examples?
Ring pull on a can
Fuse
Egg
Crumple zone of a car
Mechanical Properties
Elastic region
deformation is reversible (work or
energy required for deformation low)
Mechanical Properties
Plastic region
-deformation is permanent (work of
deformation high) due to movement of dislocations
(defects or holes in the atomic structure)
Different classes of materials have
different failure characteristics
Ductile fracture:
– Occurs with plastic deformation
Young’s Modulus
Slope of line in elastic
region
- Yield Stress
Point at which elastic
behaviour ceases
Failure Stress
Break point
Different classes of materials have
different failure characteristics
Brittle fracture
Little or no plastic deformation
– Catastrophic
Different classes of materials have
different failure characteristics- Why?
Ductility is a function of intrinsic (e.g atomic structure) and
extrinsic materials properties (e.g microstructure) and
‘test’ or ‘service’ conditions (the material is being loaded under) (e.g temperature, strain rate, stress state).
Ductile fracture is usually desirable! why?
Ductile: warning before fracture
Brittle: no warning
You can look at % Area Reduction or % Elongation Length, when and before material fractures.
Ductile vs Brittle Failure:
Fracture involves two steps:
Crack formation and then Crack Propagation (where crack gets larger to the point where the material fails / breaks).
Ductile: plastic deformation in vicinity of advancing
crack progresses slowly stable cracks
Brittle: cracks can spread very rapidly unstable crack
propagates spontaneously without an increase in
applied stress
Ductile materials are generally tougher more strain
energy required to induce ductile fracture
Examples of very ductile materials: pure gold or lead
Example: Failure of a Pipe
for a ductile pipe and brittle one:
Ductile failure:
–one piece
–large deformation
Brittle failure:
–many pieces
–small deformation
Moderately Ductile Failure
Evolution to failure: (5 steps)
-Necking
-Cavity formation
-Coalescence of cavities to form crack
-Crack propagation
-Shear fracture at a 45 degree angle relative to tensile direction
Why do materials fail differently?
The shape of the potential energy curve describes the difficulty in separating different atoms
* A ‘sharper’ potential well indicates …
more difficulty in separating the two atoms
Mechanical Properties
* Pure metals
– Dislocation movement facilitated along
Slip planes
– Can be several slip planes in a metal lattice
* eg 12 options in FCC and 12-24 in BCC packed structures!
– Ductile
* Only 3-6 in HCP
– Brittle (relatively)
