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. (6 + 4 + 2 = 12 marks)
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.
equation to calculate the endurance limit at a mean stress of 0 MPa for this
Hint: the following equations can be derived: