AbstractStarting with the basic fatigue life formulaΔε/ΔεT= (Nf/NT)b+ (Nf/NT)can inversion formula is derived in the formNf/NT= [(Δε/ΔεT)z/b+ (Δε/ΔεT)z/c]1/zwherezis a function of strain range and the ratioc/b.The inversion formula is valid over the entire life range of engineering interest for all materials examined. Conformity between the two equations is extremely close, suitable for all engineering problems. The approach used to invert the life relation is also suitable for the inversion of other formulas involving the sum of two powe