ME4254/ME4254E

NATIONAL UNIVERSITY OF SINGAPORE

ME4254/ME4254E MATERIALS IN ENGINEERING DESIGN

(Semester I : AY2010/2011)

Time Allowed : 2 Hours

INSTRUCTIONS TO CANDIDATES: 1. This examination paper contains FOUR (4) questions and comprises FOUR (4)

printed pages. 2. Answer ALL FOUR (4) questions. 3. Questions do not carry equal marks. 4. This is a CLOSED-BOOK EXAMINATION with authorized materials: Students

are allowed to bring one (1) A4 size sheet of notes/formulae written on both sides.

PAGE 2 ME4254/ME4254E

QUESTION 1 (a) Despite developments in the predictive capability of fatigue life of materials, fatigue

failures are very common in the industry. What important measures would you take at the design stage to ensure that the designed machine has sufficiently high fatigue life? Where applicable, use examples from aircraft and railway industry in your answer.

(15 marks)

(b) Weight saving measure in aircrafts has forced a design engineer to change a structural component from solid type to tubular type with possible change of the material. It is important to maximize the stiffness and at least maintain the same strength of the beam while saving on the weight. Assume that a beam is rigidly fixed at one end and free at the other with a concentrated load applied at the free end, and the outer radius of the tubular beam is much greater than the thickness of the tube. Derive an appropriate index to select the lightest material from the list given in Table 1 for high stiffness. The material of the component must be able to withstand a static stress of 105 MPa with a factor of safety of 2. Finally, discuss the cost effectiveness of your choice.

Table 1

Materials Elastic modulus Tensile strength Density Price

(GPa) MPa g/cm3 $/kg

Al-alloy 7075 71 572 2.80 9

Ti-alloy Ti6Al4V 114 1103 4.43 80

Mg-alloy AZ31B 45 200 1.81 8.8

Stainless steel 204 1310 7.65 9

17-7PH

CFRP 220 700 1.7 200

(15 marks)

PAGE 3 ME4254/ME4254E

QUESTION 2 (a) Small and light cylindrical pressure vessels have to be designed for pneumatic energy

storage. Find a suitable index to compare several candidate materials. The outer radius of the vessel is 50 cm with a wall thickness of 7 mm, and the vessel must be able to withstand an internal pressure of 6 MPa with a factor of safety of 2. A list of materials and their mechanical properties is provided in Table 2 for the selection purpose.

Table 2 Materials Fracture Yield/Fracture Density

Toughness (MPa m1/2) Strength (MPa) (g/cm3)

Oak 10 14 0.5

Low alloy steel 170 650 7.8

Ti-alloy 100 1100 4.4

Al-alloy 35 450 2.8

GFRP 60 1020 2.1 (15 marks)

(b) During the quality control inspection of a component in the fuselage of an airplane a

crack of depth 2 mm was discovered. Calculate the total number of flights this airplane can be allowed to take if we assume each pressurization and depressurization of the plane as one cycle of stress. For safety reason, it is recommended to use only half of the total calculated life. The stress in the component is 200 MPa with a factor of safety of 2. The metal alloy of the component has a fracture toughness of 45 MPa m1/2. In laboratory tests, the metal alloy was found to follow Paris Law of the following form:

da/dN = 3.4 x 10-14 (MN m-2)-4 m-1 (K)4

(15 marks) QUESTION 3 (a) What materials and testing would you recommend to find a suitable wear resistant

bearing material for application in extremely high contact stress conditions? Suggest a possible material or system of materials such as composite coatings, solid/liquid lubricants etc., that can be used for this purpose. What would be the important characteristics required of such a material or system?

(15 marks) (b) Explain some environmental factors that can increase the rate of corrosion of a metal

in an aqueous electrolyte. What can we do at the design stage to make stainless steel welded joint safe from possible corrosion in industrially corrosive environment?

PAGE 4 ME4254/ME4254E

(15 marks) QUESTION 4 Write a short essay on materials selection issues for orthopaedic applications where high stresses are to be carried by the prostheses. What problems would be encountered if a Ti6Al4V alloy femoral head is allowed to mate with an UHMWPE acetabular cup for total joint replacement?

(10 marks)

- END OF PAPER -

NATIONAL UNIVERSITY OF SINGAPORE