ME4254/ME4254E

NATIONAL UNIVERSITY OF SINGAPORE

ME4254/ME4254E MATERIALS IN ENGINEERING DESIGN

(Semester I : AY2009/2010)

Time Allowed : 2 Hours

INSTRUCTIONS TO CANDIDATES: 1. This examination paper contains FOUR (4) questions and comprises THREE (3)

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 1 A4 size sheet of notes/formulae written on both sides.

Page 2 ME4254/ME4254E

Question 1 (a) Despite being very ductile in nature, why do metals suffer from the fatigue failure

problem even if the maximum applied tensile stress may be much lower than the 0.2% offset yield strength of the material? Answer in the context of fatigue mechanism. Describe a fatigue design concept for a highly weight-sensitive application.

(15 marks) (b) What wear problem(s) in human body joints was highlighted in this module? Describe

the common materials used and comment on their effectiveness in solving the specific wear issue(s).

(10 marks) Question 2 (a) When building the worlds longest bridge, Akashi-Kaikyo Bridge (Kobe, Japan), there

was one problem faced in the beginning; i.e., the weight of the steel to be used to make the suspension wires was too high for its strength. What material solution(s) would you recommend to make such kind of very long structures?

(10 marks) (b) What is meant by metallurgical effects in wet corrosion? Provide your answer with

examples. Is this relevant to a welded structure made of grade 304 stainless steel? Provide explanation.

(15 marks) Question 3 (a) What are possible regimes of liquid lubrication in the Stribeck curve? How is the

surface of a modern bearing designed to ensure minimum wear during its service life? Provide sketches and examples where necessary.

(10 marks) (b) A horizontal beam to support the roof of a large lecture theater is to be built.

Rectangular cross-section of the beam is selected. To avoid other design changes, it is important that the maximum deflection in the beam should not exceed 4 mm. If the width is a fixed parameter by design, compute an appropriate index for the selection of the right material and provide your recommended material from the list given in Table 1. The beam may be considered a case of uniform loading of 500 kg over a length of 25 m which is simply supported at two ends. Take a factor of safety = 3, width of the beam = 30 cm and maximum allowed thickness = 40 cm. Mechanical properties and density of the given materials are provided in Table 1.

TABLE 1 Material Elastic Modulus Tensile strength Density (GPa) (MPa) (x 103 kg/m3) Low alloy steel 207 745 7.85 Ductile Iron 160 420 7.00 Red Oak 13 90 0.66 GFRP 48 1030 2.1 (20 marks)

Page 3 ME4254/ME4254E

Question 4 The tensile stress element of a boom of a crane experiences a constant static stress of 80 MPa. An additional tensile stress of 240 MPa is added every time the crane is loaded to perform a work. If the fracture toughness of the steel used is 60 MN/m3/2 and it is assumed that the steel may contain an internal crack defect of 2 mm length, what will be the maximum loading cycle of this crane? What will be your recommended inspection interval if the crane performs on average 200 loadings each day? Assume that the material follows the Paris Law with the constant value A = 2.43x10-14 (MNm-2)-4m-1 and the exponent m = 4. (20 marks)

- End of Paper -

NATIONAL UNIVERSITY OF SINGAPORE