Full-scale vibration fatigue testing of 316L butt-welded small-bore piping in nuclear power plants and equivalent structural stress S-N curve

: To establish direct experimental evidence for vibration fatigue assessment of 316L stainless-steel small-bore piping butt-welded joints in nuclear power plants, full-scale TIG/GTAW butt-weld specimens (Φ16 mm × 3 mm) were tested using an electrodynamic shaker resonance excitation system. A total of 39 specimens were subjected to uniaxial constant-amplitude vibration fatigue tests at six nominal stress levels (130–210 MPa) with approximately fully reversed loading (R ≈ −1) and first-mode resonance frequencies of 55–83 Hz. Digital image correlation (DIC) was employed to measure the weld-toe strain field, X-ray diffraction (XRD) was used to determine residual stresses, scanning electron microscopy (SEM) was performed for fracture-surface analysis, and equivalent structural stress (ESS) was calculated by extracting nodal forces from ANSYS. The influence mechanism of weld reinforcement height on local mechanical states and fatigue life was systematically investigated. Specimens were classified into three groups according to weld reinforcement height: high reinforcement (h ≥ 1.0 mm), medium reinforcement (0.5 mm < h < 1.0 mm), and low reinforcement (h ≤ 0.5 mm). SEM observations on one failed specimen tested at 210 MPa indicated that crack propagation accounted for only ∼0.8% of the total life, supporting the approximation that leak-detection life was initiation-dominated. DIC measurements yielded weld-toe stress concentration factors (SCFs) of 1.26–1.47, which increased monotonically with reinforcement height. XRD results showed that reducing reinforcement height generally corresponds to a larger magnitude of compressive residual stress on the outer surface. Finite-element calculations gave bending ratios r ranging from 0.453 to 0.508 (mean 0.475); the mean r values for the high-, medium-, and low-reinforcement groups were 0.498, 0.472, and 0.458, respectively, indicating that reinforcement height affects fatigue performance through three coupled pathways: SCF, the load mode at the control section, and residual stress. Based on ESS S–N regression using 30 failed specimens, the fitted parameters were m = 7.88, A = 25.91, S logN = 0.331, and R 2 = 0.618; the P97.7% engineering design-line intercept was A d = 25.25. A supplementary right-censored maximum-likelihood analysis showed that, at N f = 10 6 , the mean-line ΔS s increased by only 4.2% compared with the baseline result, indicating local stability of the fitted curve within the observed life range. Compared with the ASME VIII Div. 2 Annex 3.F master S–N curve, the two mean lines intersect at ΔS s ≈ 417.4 MPa and N f ≈ 1.80×10 5 . At N f = 10 6 and 10 7 , the present mean line is higher than the ASME mean line by 39.2% and 116.9%, respectively, suggesting that the ASME master curve is conservative in the medium-to-high-cycle vibration fatigue regime covered herein. The scenario-specific ESS S–N design line developed in this work provides direct experimental support for vibration fatigue evaluation of 316L small-bore piping butt-welded joints under comparable material compositions, pipe dimensions, and resonance-bending loading conditions.