(1) The coffin manson exponent (m) is typically quoted as ~ 6-8 for brittle materials, ~ 3-5 for hard solders and ~ 1-3 for soft solders (or something close to these values)
(2) We also know from plotting S-N curves, that the slope (actually -1/slope = m) is dependent upon whether we are in the "plastic" or "elastic" regime. The "elastic" regime (i.e., higher cycles to failure) results in a larger value of m.
I would like to know why harder materials have higher values for m.
Are the m values quoted in the literture assuming that the S-N experiments are done so that the N (number of cycles to failure) is held constant? In other words, are the S values chosen in an experiment higher for the harder materials (so as to keep the same nominal cycles to failure a constant, regardless of material type)?
Or is this simply a result that the S-N experiments are performed at the same nominal values of S (stress), in which case, harder materials will natural experience a greater fraction of strain in the form of elastic strain (not plastic) as compared to softer materials? In other words, the m value changing with material hardness is simply a result that we are on a different part of the S-N curve, and the S-N curve has a slope that is a function of N.
Thanks, Jay