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Question 1
One of the important and practical aspects of materials
selection in the design,development and production of new
components (or parts/ structures) is the possibility
of failure of the component (or parts/ structures) under normal
operation.
a) In term of materials selection and design, define the failure
of materials or
components and give five (5) types of that failure.[30
marks]
Answer
Failure is defined as inability of materials or components
to
i) perform the intended function (5)
ii) Meet performance criteria although it may still be
operational (5)
iii) Perform safely(5) and reliably even after deterioration.
(5)
Examplesi) Fracture (2)
ii) Corrosion (2)
iii) Wear(2)
iv) Yielding (2)
v) Buckling (elastic instability) (2)
b) Explain why analysis of a failed component is important and
what are steps
taken in the failure analysis process[70 marks]
Answer
The important of failure analysis are as stated below
i) Identify corrective actions based on a determination of the
root cause of thefailure. (4)
ii) Minimize future failures of the same type. (4)iii) Tool that
can be used to
a) reduce maintenance costs, (4)
b) increase system availability, (4)
c) reduce the potential losses involved in catastrophicfailures.
(4)
iv) Used to identify the party at fault. (4)
v) Financial losses resulting from a failure can often be
recovered whendeficiencies in design, workmanship or materials can
be identified as the rootcause of the failure. (4)
Steps should be taken in investigating the failure of the
component
1. Collecting background data(3)
2. Visual Examination (3)
3. NDT evaluation (3)
4. Fractographic examination bySEM (3)
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5. Chemical composition analysis(3)
6. Mechanical Properties (3)
7. Macroscopic Examination (3)
8. Metallographic Examination(3)
9. Microhardness evaluation (3)
10. Summarize findings (3)
11. Root cause determination (3)
12. Failure mechanism (3)
13. Recommendation to overcomefuture failure (3)
14. Report writing (3)
Question 2
Generally, two types of fracture failure (ductile and brittle
fracture) are based on the
ability of a material to experience plastic deformation.
a) In term of plastic deformation, explain the characteristics
ductile andbrittle fracture.
[20 marks]
Answer
A ductile fracture surface has a dull (4), fibrous appearance
(4) and oftenresembles a cup-and cone(4) configuration.
A brittle fracture surface typically appears shiny(4) with flat
facets (4) which arecreated during cleavage fracture.
b) Describe the process of fracture of metals and differentiate
between
ductile and brittle fracture[80 marks]
Answer
The process of fracture:
The specimen elongates (4), forming a necked region (4) in which
cavitiesform.
The cavities coalesce (4) in the neck center, forming a crack
(4) whichpropagates toward the specimen surface in a direction
perpendicular to theapplied stress.
As the crack approaches the surface, its growth direction shifts
to 45 (4)with respect to the tension axis. This redirection allows
for the formation of thecup-and-cone (4) configuration and
facilitates fracture.
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Ductile fracture Brittle fracture
Plastic deformation (3) Small/ no plastic deformation (3)
High energy absorption beforefracture (3)
Low energy absorption beforefracture (3)
Characterized by slow crackpropagation (3)
Characterized by rapid crackpropagation (3)
Detectable failure (3) Unexpected failure (3)Stable crack(3)
Unstable crack(3)Eg: Metals, polymers (3) Eg: Ceramics, polymers
(3)
