The S-N curve which represents the fatigue strength, which is measured as a stress amplitude, as a function of number of stress cycles, can be approximated by a straight line in the range of 103 cycles to 106 cycles when plotted as log(S) versus log(N). Beyond
The S-N curve which represents the fatigue strength, which is measured as a stress amplitude, as a function of number of stress cycles, can be approximated by a straight line in the
range of 103
cycles to 106
cycles when plotted as log(S) versus log(N). Beyond 106
cycles,
the fatigue strength remains constant. The fatigue strength of a certain alloy at 103
cycles
is given as 80 % of the ultimate tensile strength and its endurance limit is 50 % of the
ultimate tensile strength.
(a) The alloy part fails in 15 000 cycles. Calculate the percent change (increase or decrease)
of the maximum applied cyclic stress amplitude if the life of the part needs to be
extended by 3.5 times. Is this considered under a high or low cycle fatigue failure?
State any assumptions made. (12 marks)
(b) If the applied cyclic stress amplitude remains unchanged when the life is extended by
3.5 times, calculate the percent change (increase or decrease) of the ultimate tensile
strength of a new alloy required. (8 marks)
(c) If the new allow in question (b) is now used to sustain 1.5 × 106
cycles, what is the
ratio of the maximum applied cyclic stress amplitude to its ultimate tensile strength.
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