lomapaseo,
You are confusing 'steady' stress such as that due to centrifugal forces on fan blades and disc from rotation, with 'varying' stress (ie stress which alternates) such as might arise in non rotating parts like casings etc. A steady stress which is below the safe stress for the material is quite different to an alternating stress which will cause failure if the cycles reach the fatigue limit for the material. I think we may need to ask people to state their engineering qualifications before entering this discussion. pm me and I'll tell you mine if you are interested.