Materials Engineering (Week 6)

Question 1

Failure is almost always undesirable.
However sometimes failure is built into the design, can you give some examples?

Answer 1

Ring pull on a can
Fuse
Egg
Crumple zone of a car

Question 2

Mechanical Properties
Elastic region

Answer 2

deformation is reversible (work or
energy required for deformation low)

Question 3

Mechanical Properties
Plastic region

Answer 3

-deformation is permanent (work of
deformation high) due to movement of dislocations
(defects or holes in the atomic structure)

Question 4

Different classes of materials have
different failure characteristics

Ductile fracture:

Answer 4

– Occurs with plastic deformation

Question 5

Young’s Modulus

Answer 5

Slope of line in elastic
region

Question 6

• Yield Stress

Answer 6

Point at which elastic
behaviour ceases

Question 7

Failure Stress

Answer 7

Break point

Question 8

Different classes of materials have
different failure characteristics

Brittle fracture

Answer 8

Little or no plastic deformation
– Catastrophic

Question 9

Different classes of materials have
different failure characteristics- Why?

Answer 9

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).

Question 10

Ductile fracture is usually desirable! why?

Answer 10

Ductile: warning before fracture
Brittle: no warning

You can look at % Area Reduction or % Elongation Length, when and before material fractures.

Question 11

Ductile vs Brittle Failure:
Fracture involves two steps:

Answer 11

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

Question 12

Example: Failure of a Pipe
for a ductile pipe and brittle one:

Answer 12

Ductile failure:
–one piece
–large deformation

Brittle failure:
–many pieces
–small deformation

Question 13

Moderately Ductile Failure
Evolution to failure: (5 steps)

Answer 13

-Necking
-Cavity formation
-Coalescence of cavities to form crack
-Crack propagation
-Shear fracture at a 45 degree angle relative to tensile direction

Question 14

Why do materials fail differently?

The shape of the potential energy curve describes the difficulty in separating different atoms
* A ‘sharper’ potential well indicates …

Answer 14

more difficulty in separating the two atoms

Question 15

Mechanical Properties
* Pure metals

Answer 15

– 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)

Question 16

Mechanical Properties
Both ionic and covalently bonded ceramics
tend to be very hard.

The rigid atomic structure of most ceramics means that…

Answer 16

they are intrinsically hard
– Dislocation movement difficult in covalent structures (requires breaking and reforming of bonds as the dislocation moves through the structure) and can only occur in limited directions (slip planes)

Question 17

Mechanical Properties
However, most materials are polycrystalline

Answer 17

-Dislocation movement along limited slip planes is hindered by grain boundaries through variation in grain-grain orientation

-Deformation of one grain constrained by neighbours

Question 18

Mechanical Properties:
Strength of metals and alloys can be
increased if slip motion is made more difficult

Slip motion in polycrystalline materials (such
as metals and alloys) hindered by

Answer 18

Reducing grain size and Solid solutions
(dislocation movement impaired)

– Cold working (Dislocations already piled up at grain boundaries

Question 19

Mechanical Properties:

However the presence of grain boundaries in a
polycrystalline ceramic does not allow for much…

Answer 19

plastic deformation (adsorption of stress)

Question 20

What is Cleavage

Answer 20

Successive and repeated breaking of atomic
bonds along specific crystallographic planes

Question 21

Transgranular Fracture

Answer 21

is characterised by cracks growing
through the grains

Question 22

Intergranular Fracture

Answer 22

Inter-granular
fracture is characterised growing along the grain boundaries

Question 23

What are microcracks caused by?

Answer 23

Thermal or mechanical stress
microcracks are undesirable

Question 24

Fracture Pathways

Answer 24

Will always fracture on the path of least
resistance (Lowest energy expenditure), if pores are present they will go through them

Question 25

What are pores left by?

Answer 25

Processing
Pores are undesirable

Question 26

Fracture Pathways
* How much energy needed related to: (2)

Answer 26

– Surface area created

– intrinsic properties of the atomic structure(s) of the phases present

Question 27

Ideal vs Real Materials:

DaVinci (500 yrs ago!) observed
The longer the wire, the smaller
the load for failure, Why?

Answer 27

Larger samples contain more flaws!
Flaws cause premature failure

TS(perfect materials)&raquo_space; TS(engineering materials)

I.e On paper materials have a much high tensile strength, then in reality due to potential flaws.