Bend a paperclip back and forth and it snaps long before its rated strength — that is fatigue, the silent killer of axles, wings, and bridges. Materials survive far fewer stress cycles than their single-pull strength suggests, so engineers derate the tensile strength with a fatigue limit factor to get the safe cyclic stress: fatigue strength equals the factor times the ultimate tensile strength. In symbols: S_e = factor × S_ut. For polished steel the factor hovers near 0.5; for rough, notched, or corroded parts it drops much lower.

Airplane fuselages live this math every flight — each pressurization is one more cycle on the aluminum skin, which is why jets retire after a rated number of flights, not years. Truck axles, crankshafts, and even the wire in a repeatedly flexed phone charger fail the same way: not from one big load, but from thousands of small ones.

This calculator applies the derating. Type the material's ultimate tensile strength in megapascals in the Tensile Strength (S_ut, MPa) box and your combined fatigue factor — accounting for surface finish, size, and stress concentrations — in the Fatigue limit factor box. The Fatigue Limit / Strength (MPa) box reports the safe cyclic stress. Design below it, and the part theoretically lasts forever; design above it, and you are counting cycles until the crack wins.

With the pre-filled defaults — Tensile Strength (S_ut, MPa) 500, Fatigue limit factor 0.5 — the Fatigue Strength Calculator returns Fatigue Limit / Strength (MPa) 250.00. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.