Estimating the effects of microscopic stress concentrations on the fatigue endurance of thin-walled high strength steel

The effects of microscopic surface stress concentrations on the fatigue endurance of thin-walled high strength steel were systematicatly estimated by numerical analysis and plane-bending fatigue tests with Schenck-type specimens, using the commercially available static implicit method FEA software l-DEAS ver. 11 for the stress distribution calculations. The microscopic stress concentration factor α _μ from notch depth t=50 μm and notch root radius p=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, α _θ, from the superposition of the principal stresses. In the twisting mode, α _i, varied from about 4 to 7. The θ dependency of α, 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 bγ the α _i, value. It is reported that the fatigue notch factor β increased linearlγ with increases in the macroscopic stress concentration factor α _a of up to 3. On the other hand, β slowly increased with increases in α _i,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 p. 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 α _i independent of p in the microscopic stress concentration field.