ME4255
Feedback of ME4255 – Materials Failure, AY2014/15 SEM2 Examination and CA
QUESTION 1 Explaining the significant of the data for unirradiated and irradiated A533B-1 steel expressed in Figure 1. (15 marks)
Figure 1
This question is 15 marks, the average mark for this question in the class is 8.3 and mean is 10. The key to this question is that, for radiated specimens, at the same temperature, the fracture toughness is lower than the unirradiated one. Some of you get confused by the figure, and answered opposite to the fact.
ME4255
QUESTION 2 (i) (ii) (iii) (iv) (v)
What is generalized plane stress state? Explain the possible metal oxidation mechanisms. How a safety factor against failure is defined? What are the adhesive wear and abrasive wear? What is electrochemical cell and what are the components in such cell?
(25 marks) This question is 25 marks. The average marks for this question in the class is 19.6, and the mean value is 20. Most of you do not have problems, but some of you did not complete the answer. Some of you have problem on the first one, the question ask “generalized plane stress state” which is different from the “plane stress state”.
ME4255
QUESTION 3 What are the possible failure modes in the four pictures shown in Figure 2?
Figure 2 (20 marks)
This question is 20 marks. The average mark for this question in class is 8.6 with mean of 7. Some of you did not complete the answer, for example, some of you cannot identify the fracture surface correctly, such as ductile failure (dimp) (a), fatigue failure (b), intergranular failure (c), and corrosion failure (d).
ME4255
QUESTION 4 An engineering component made of alloy S-590 will be subjected in actual service to temperature of 800oC and a stress of 100 MPa. (i) (ii)
(iii)
What life to creep rupture is expected for the given service conditions? It has been suggested that the components of this type should be replaced after they have reached to half of the expected life time. What safety factor on stress would be provided by this replacement? Do you agree with this suggestion of replacement at half of the expected life? Why or why not?
(For S-590 alloy, C=17 log(hours), b0=38,405, b1=-8,206, b2=b3=0). (20 marks)
This question is 20 marks, the average for this question is 15.5 with mean of 20. Most of you do not have problems with this question, except some of the answers are not completed, especially the discussion on part (iii).
ME4255
QUESTION 5 (i) For the notch members shown in Figure 3, estimate the repeatedly applied zero-tomaximum nominal stress Smax that corresponds to a life of Nf = 105 cycles: (a) using Goodman equation for ductile materials; and (b) using a different equation: 1 R . S fat Smax Sa Smax 2 (ii) One of the same notched members is subjected to constant amplitude cycling of Pa=32kN and a mean force of Pm=25kN. What fatigue life in cycle is expected? Note: 1. The nominal stress is calculated from its net area of the notch position. 2. You need first to determine the S-N relationship from the curve.
Figure 3 (20 Marks) This question is 20 marks, the average mark for this question in class is 7.9 with mean of 7. The main problems are the calculations using the Goodman relationship, as well as to computer the stress from the load. The average for the final exam in the class is 66 with the mean at 62. The average mark for quiz is 48. The relative low average is because of one student did not attend the exam and CA, therefore is zero (0) for his grade, this pulls-down the average marks of the whole class. The term papers do not have major issues, except some of you did not address what you have learnt from the term paper.
