This is in reference to John C Luf's interesting article 'Sustained Stresses' in the COADE Mechanical Engineering News dated January 2001.
Under heading Sustained Stress Mulipliers (Indices) John refers to an equation for SIF:

i=a/S.(N^b) where a = 245000 PSI and b=0.2
Factors a, and b are material specific, values shown are for carbon steel.

As I am doing some personal research in to SIF vs SCF what I would like to find out from John (or any body else) is where did he obtain the values of a and b from and furthermore where did the above SIF equation originate from.

I look forward to your comments.

Regards
Aaron.