widgeo said, "I know the S-61 was designed before safe life calculation was used ."
Widg, I don't know what that means, since the S61 and the S76 and the S92 were all designed to have no life limit on the fuselage. This is perfomed by designing the fuselage so that the stresses are low enough to assure no fatigue failures, and then the monitoring and updating in areas where field experience proves this not to be true. I believe at least one S76 has also reached 40,000 hours.