Fatigue Strength Assessment of S235JR Steel

Assessing S235JR fatigue strength isn't about a single number. Unlike yield strength (235 MPa), fatigue performance varies widely with loading conditions. For most structural applications, the endurance limit sits around 100–120 MPa at 10 million cycles under fully reversed bending.

Three factors dominate fatigue life. First, stress concentration – sharp corners, weld toes, or drilled holes can reduce fatigue strength by 40% or more. Smooth transitions and grinding weld toes help dramatically. Second, surface finish – rolled or machined surfaces outperform as-rolled or flame-cut edges. Third, mean stress – tensile mean stresses lower fatigue life; compressive ones help.

For welded joints, fatigue design follows standards like Eurocode 3 with detail categories. S235JR welds typically fall into FAT 80–100 range, meaning much lower allowable stresses.

Best practice? Keep working stresses below 60–80 MPa for long-life welded structures. Use finite element analysis (FEA) to identify hot spots. And always factor in real load spectra – don't rely on static safety margins alone.