If this is a student question, please refer to the following text book for information;
Mechanical Metallurgy, 2nd Edition, by G. Dieter. Under Fatigue of Metals, page 437.
If this is for a specific engineering application, please post some additional information so we can provide an answer. For starters, you will need to know the following information;
1) material properties,
2) an S/N curve for the material to select an endurance limit and
3) stress range (stress max-stress min) in service to calculate cycles to failure.
The data from items 1-3 are used to plot a Goodman diagram, and using the equation for the Goodman line and the S/N curve you should be able to calculate the cycles to failure.
We are making 20mm studs out of MP35N, which has a uts 260ksi min. The mean stress is 142ksi, with an alternating stress of 16.4ksi. I do not have an S/N curve for MP35N.
Ok. From what I remember you need to have the S/N curve for MP35N to establish an endurance limit. The Goodman diagram is constructed as a plot of stress amplitude versus mean stress. The Goodman line is drawn using two points with the following equation;
The S/N fatigue curve is required. Once you plug in the endurance limit and mean stress, the value obtained for stress amplitude is used to pick off the reduced cycles to failure IF you are above the Goodman line. Obviously, if your stress amplitude and mean stress data point is BELOW the Goodman line, this indicates that failure will not occur because you have not exceeded the endurance limit for the material.
The web site below contains information for MP35N that could be of help regarding fatigue data;
The link that metengr provided also has some information on reversed bending fatigue. For MP35N that has been cold drawn and aged to UTS = 265 ksi (1825 MPa), the fatigue strength at 108 cycles is 97 ksi (668 MPa).
