2009 Volume 49 Issue 11 Pages 1806-1813
The effects of microscopic surface stress concentrations on the fatigue endurance of thin-walled high strength steel were systematically estimated by numerical analysis and plane-bending fatigue tests with Schenck-type specimens, using the commercially available static implicit method FEA software I-DEAS ver. 11 for the stress distribution calculations. The microscopic stress concentration factor αi, from notch depth t=50 μm and notch root radius ρ=6 μm microscopic surface ridges, monotonously increased with increases in the roughness ridge direction, θ, from 1 to 7 in the bending mode. A fitted curve was developed for deriving the calculated stress concentration, αθ, from the superposition of the principal stresses. In the twisting mode, αi varied from about 4 to 7. The θ dependency of αi was smaller than that in bending mode. The empirical rule that the specimen collection direction has a lesser effect in twisting mode fatigue was supported by the αi value. It is reported that the fatigue notch factor β increased linearly with increases in the macroscopic stress concentration factor αa of up to 3. On the other hand, β slowly increased with increases in αi until it exceeded about 2. This marked difference might be due to differences between their respective stress gradients, which was well described by Nisitani and Endo by using a parameter ρ. A plane-bending fatigue test was performed with an artificial surface micro-groove of θ=0, 90° using 590 MPa class strength circumferentially flattened electric resistance welded tube. The θ=0° micro-groove had little effect on the fatigue endurance in bending. On the other hand, the fatigue cracks of all the θ=90° specimens initiated at the basilar part of the micro-groove without any nonpropagating cracks. The fatigue notch factor β seems to be determined by only αi independent of ρ in the microscopic stress concentration field.