ESDU Fat A.00.01 introduces an R-M diagram which relates the alternating and mean stresses for some endurance. A number of such curves are plotted, each characterised by a value of an exponent m and the appropriate curve for a plain test piece is readily selected if the alternating stress at zero mean stress is known for the endurance. If not, a value of unity is recommended for m. Any straight line joining the axes represents constant maximum stress in the cycle, that is the sum of the alternating and mean stresses. Its intersection with the R-M diagram divides the region in which fatigue failure is the result of substantially elastic conditions from the region where it is preceded by marked plastic yield.