S-N Curve Shape

[Ian H]

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 from S). Can anyone help with a reference please.

S(N) = S(inf) * (1 + (A-1) * (1+(N/B))^(-C))
(A is ratio between start and end values of curve, B is horizontal shift factor and C is steepness factor)

 

[rb1957]

looks like it's for steel (with an infinite life asymptote).

surprised it doesn't use log(N) ... S/N tend to be log/linear.

another day in paradise, or is paradise one day closer ?

 

[Ian H]

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.

 

[Ian H]