The influence of various types of small defects on the fatigue limit of precipitation-hardened 17-4PH stainless steel

- The influence of artificial defects on the fatigue limit is investigated.
- The fatigue limit can be estimated using the √area parameter model.
- For defects larger than √area = 80 μm, the fatigue limit is determined by ΔKth,lc.
- The stress ratio dependency is determined.
- Drilled holes with diameters >50 μm cannot be compared with corrosion pits.

Fatigue tests were performed on 17-4PH stainless steel specimens containing small artificial defects, with area ranging from 30 to 900 μm at stress ratios, R, of −1, 0.05 and 0.4. An investigation was thus conducted to examine the influence of various types of small artificial defects on fatigue strength, including circumferential notches, corrosion pits, drilled holes and pre-cracked holes. The fatigue limit was determined by the threshold condition for the propagation of a crack emanating from the defects. The threshold stress intensity factor range, ΔKth, exhibited a defect size dependency for area ≤ 80 µm , and it became a constant value for area > 80 µm independent of R. Based on the area parameter model and a material constant of ΔKth for long cracks, the fatigue limit could be predicted as a function of R, with the exception of drilled holes with relatively large diameters of 100 and 300 μm, for which the fatigue limit was determined by the critical condition for crack initiation. When artificial defects were absent or non-detrimental, intrinsic defects, such as non-metallic inclusions, were found to control the fatigue strength and, in addition, were responsible for the scatter in the fatigue limit. The proposed method enables the quantitative evaluation of the lower bound of the scatter as a function of the number of test specimens, or the overall control volume of fatigue-loaded components.