What is the stress amplitude of a cyclic stress with a maximum of +300 MPa and a minimum of −100 MPa?

Answer: C. 200 MPa

The stress amplitude is σa = (σmax − σmin)/2 = (300 − (−100))/2 = 200 MPa. The mean stress is σm = (σmax + σmin)/2 = (300 + (−100))/2 = 100 MPa, and the stress ratio is R = σmin/σmax = −100/300 ≈ −0.33. 100 MPa is the mean stress, and 400 MPa is the stress range Δσ, the difference between maximum and minimum. Stress amplitude and stress range differ by a factor of two, so always check which one a data sheet or standard is quoting. Fully reversed loading with zero mean stress (R = −1) is the basic condition for fatigue testing.