Shaft A, shown below, is made from AISI 1020 hot-rolled steel, is welded to a fixed support and is subjected to loading by equal and opposite forces F via shaft B. A theoretical stress-concentration factor Kts of 1.6 is induced by the 1/8′′ fillet. The length of shaft A from the fixed support to the connection at shaft B is 2 ft. The load F cycles from 150 to 500 lbf. (a) For shaft A, the factor of safety for infinite life using the Modified Goodman fatigue failure criterion is