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.

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.