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How many rotor revolutions can be applied before fatigue failure is expected?
For convenience and better understanding, we will work with Von Mises stresses in this problem. We will follow the following 11 steps:
1. calculate the mean and alternating torsional stresses
2. calculate the corresponding Von Mises stresses
3. Using Goodman's finite life relation, calculate the effective fatigue stress
4. Using the effective fatigue stress, determine the fatigue life (number of cycles) using S-
N relation for given a and b values
5. Calculate the fatigue stress for infinite life ( 0.1 million cycles), also known as endurance limit
6. As we are already working with Von Mises stresses, a factor of safety would be the ratio of quantities calculated in step 5 and step 3
7. 200000 cycles lie in the infinite life domain. So, calculate the alternating and mean values of Von Mises stresses that would be developed with fatigue stress equal to endurance limit. Here we will not consider any factor of safety
8. With these Von Mises stresses, calculate the respective torsional stresses
9. Using the given alternating and mean torsional loads, calculate the safe diameter of the shaft
10. Consider a Factor of safety equal to 1.5 and repeat part c