Lecture 5

Question 1

Static loads

Answer 1

Loads over a constant period of time 
Failure by
-Elastic deformation
-Elastic Instability 
-Brittle/ ductile fracture 
-Plastic collapse
-Creep at elevated temperatures

Question 2

Erection loads

Answer 2

Loads unexpected in the original design but arise when the structure is assembled
Failure by
-Plastic collapse
-Ductile/ brittle fracture

Question 3

Live loads

Answer 3

Dynamic loading

Failure by fatigue

Question 4

Residual Stresses

Answer 4

Stresses which remain in the material after processing
Failure by 
-Fatigue 
-Brittle fracture
Can be combatted by annealing

Question 5

Thermal stresses

Answer 5

Stresses induced by thermal expansion in service
Failure by creep or fatigue (in boilers)
Failure by fatigue (liquefied gas storage)
Failure by brittle fracture or elastic instability (bridges)
Failure by elastic instability (railway lines)

Question 6

Stress Concentrations

Answer 6

Local stresses around holes, defects in the material, abrupt changes in the sections
Failure by brittle fracture and fatigue failure

Question 7

Elastic deformation (stiffness, modulus)

Answer 7

Too much deflection renders design inoperative

Question 8

Creep Extension (stiffness, modulus)

Answer 8

Continuous plastic deformation under constant load, which could render design inoperative. No fracture occurs but can still cause component failure

Question 9

Elastic Instability (stiffness, modulus)

Answer 9

Buckling of structures in compression. Occurs in slender structures. Stiffness gives resistance to buckling.

Question 10

Plastic Collapse (Yield, strength)

Answer 10

General yielding of the material somewhere in the structure leading to collapse. General yielding is often made use of for high energy absorption

Question 11

Brittle fracture

Answer 11

In ductile materials fracture occurs below the design or yield load. Results from local stresses concentrations not allowed by the designer

Question 12

Fatigue

Answer 12

Development and propagation of cracks under cyclic load. When the crack is big enough brittle fracture occurs

Question 13

Creep rupture

Answer 13

The increase in extension under static load leads to damage accumulation in the material.
Failure by
-Brittle fracture
-Ductile fracture

Question 14

Pumps

Answer 14

Centrifugal pumps
Vacuum cleaner
Turbocharger
Jet engine

Question 15

Types of loading

Answer 15

Tensile load –> dead load

Live load –> vibration, variations in properties of medium being pumped

Question 16

Failure modes

Answer 16

Elastic deformation
Creep
Ductile / Brittle fracture
Fatigue

Question 17

Microstructure depends on

Answer 17

Thermal history and Mechanical history, so the processing technique is important

Question 18

Alloy Steel 4140

Answer 18

Elastic limit
Annealed: 417 MPa
Normalised: 655 Mpa
Q&T: 1570 MPa
Percent elongation
Annealed: 25.7%
Normalised: 17.7%
Q&T: 11.5%

Question 19

Pure copper

Answer 19

Hot rolled 
Elastic limit: 69 MPa
Percent elongation: 50%
Cold rolled 
Elastic limit: 310 MPs
Percent elongation: 12%