MM3202 : Mechanical Properties of Materials
Course Teacher: Dr. Snehanshu Pal
Assignment 6: Creep, Fatigue and Fracture
1. Data for the steady-state creep of a carbon steel are plotted in Figure below;
Creep data for a carbon steel. Data from P. N. Randall, Proc. ASTM, v. 57 (1957).
3. For steels, the endurance limit is approximately half of the tensile strength, and the fatigue
strength at 103 cycles is approximately 90% of the tensile strength. The S-N curves can be
approximated by a straight line between 103 and 106 cycles when plotted as log(S) versus
log(N). Beyond 106 cycles, the curves are horizontal.
A. Write a mathematical expression for S as a function of N for the sloping part of the S-N
curve, evaluating the constants in terms of the previous approximations.
B. A steel part fails in 12,000 cycles. Use the previous expression to find what percent decrease
of applied (cyclic) stress would be necessary to increase in the life of the part by a factor of 2.5
(to 30,000 cycles).
C. Alternatively, what percent increase in tensile strength would achieve the same increase in
life without decreasing the stress.
4.
5. For creep in Ni of grain size, d = 200 μm, determine the boundary between Coble and
Nabarro-Herring creep on a deformation mechanism map using the data in Figure below. The
activation energy for lattice diffusion and grain boundary diffusion in Ni is QL = 286 kJ/mole
and Qgb = 115 kJ/mole.
6. A. Tungsten has a melting point of 3,400◦C. Why is it not considered for use in jet engines?
B. What advantages do aluminum alloys have over more refractory materials at operating
temperatures of 400◦F?
7.
8. There are different Modes of failure :
Also there are different yield / failure criteria. Now discuss on correlation between different Failure
criteria and Modes of Failure.
11. Derive an expression for the local uniform strain across the neck of a round bar being loaded
in tension. Then, determine its magnitude if the original diameter is reduced 80%.
13. What does mean by plane stress and plane strain conditions?
14. What is effect of Grain size on the creep behaviour of materials?
15. Write a note on “Discontinuous Dynamic Recrystallization”.