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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 1 Examinations 2010/
Module Title: Mechanical Materials (3D)
Module Code: MECH
School: School of Mechanical & Process Engineering
Programme Title: Bachelor of Engineering (Honours) in Mechanical Engineering
Programme Code: EMECH_8_Y
External Examiner(s): Prof. R. Clarke, Mr. J.J. Hayes Internal Examiner(s): Mr. S.F. O Leary
Instructions: Answer THREE Questions. All Questions carry equal marks.
Duration: 2 Hours
Sitting: Winter-Spring 2010/
Requirements for this examination:
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination. If in doubt please contact an Invigilator.
Q3. (a) Explain what is meant by the term “autofrettage” as applied to thick cylinder design. What benefits are obtained from autofrettage and what precautions should be taken in its application? (10 marks)
(b) A thick cylinder, inside radius 62.5mm and outside radius 190mm, forms the pressure vessel of an isostatic compacting press used in the manufacture of sparking plug components. Calculate, using the maximum shear stress theory of elastic failure, the safety factor on initial yield of the cylinder when an internal working pressure of 240 MN/m^2 is applied. (9 marks)
(c) The same cylinder from 3(b) is now subjected to an autofrettage pressure of 580 MN/m^2. Calculate the residual stresses produced at the bore of the cylinder when the autofrettage pressure is removed and hence determine the new value of the safety factor at the bore when the working pressure is applied. Axial stresses may be ignored. The yield stress of the cylinder material is 850 MN/m^2. (15 marks)
Q4. A shaking mechanism of a machine, as shown in Figure 1 Q4, has a crank of diameter 15mm at the critical fatigue section. The crank is made of a ductile steel with the following properties:- Static Tensile Ultimate Stress = = 430 M Fatigue Strength Reduction Factor = 0. A load P is applied to the crank pin, normal to the plane of the crank. This load varies cyclically between zero and a maximum load of 500N tensile. Calculate the fatigue life of the crank utilising the Von Mises criterion of elastic failure together with:- (a) The Society of American Engineers Fatigue Criterion (b) The Gerber Fatigue Criterion Comment on the reliability of the fatigue lives predicted by the two criteria.
Given: ( ) ⁄⁄
And Table 1 Q4. (34 marks)
Fatigue Criterion
Ductile Steels (
Brittle (hard) steels ( Nf Nf Ne Modified Goodman 0.9 103 1/2K σu 106 0.9 σu 103 1/3 K σu 108 Soderberg 0.9 103 1/2K σu 106 0.9 σu 103 1/3 K σu 108 Gerber 0.9 103 1/2K σu 106 0.9 σu 103 1/3 K σu 108 SAE 6 1 1/2K σu 106 σu 1 1/3 K σu 108
Table 1 Q4. End Points for S-N Diagram
