A+ Answers

i Explain what an S-N curve is, including a sketch. How is such a curve typically determined in the laboratory?
ii. Explain what is meant by the terms endurance limit and fatigue limit.
iii. Fatigue testing of metals is often conducted at a constant R ratio. Define the R ratio for a fatigue test.
b. Commercially pure titanium alloy with an ultimate tensile strength of 650 MPa is tested at an R ratio of 0.1. The endurance limit at 107 cycles is found to be 180 MPa.
i. Calculate the mean stress in a fatigue test with a stress amplitude of 180 MPa and an R ratio of 0.1.
ii. Use the Goodman equation to calculate the endurance limit at a mean stress of 0 MPa for this alloy.
Hint: the following equations can be derived:
δm=(1+R)δmax/2
and δm=(1-R)δmax/2
1)         The mean stress is given by:

2.         The stress amplitude is given by:

3.         Solving the last equation for gives:

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