The curve is for steel, The equation gives a curve which is flat at the left end(low cycle) and right end(infinite life). Dont know hoew to include a picture here.
I am working on analysis of fatigue specimens and was given the following equation for S-N curves many years ago which I have used ever since but can find no reference for it. It has the advantage of giving the correct "S" shape for the curves and is also reversible (Can calculate S for N or N...
