Experimental investigation and characterization of the vibration fatigue S-N curve for small-diameter 316L stainless steel butt-welded pipes

Abstract

Vibration fatigue in small-diameter pipes (SDP) represents a key failure mode in industrial pipeline systems, with butt-welded joints constituting the critical weak links. This study addresses the lack of specific component-level vibration fatigue data for 316L stainless steel butt-welded SDP and the insufficient accuracy of traditional evaluation methods (based on base material data with generic Fatigue Strength Reduction Factors), aiming to experimentally obtain and characterize the vibration fatigue S-N curves for these components.The experimental methodology employed an electromagnetic vibration table to conduct component-level resonance bending fatigue tests on 316L butt-welded pipe specimens with an outer diameter (OD) of 16 mm and a wall thickness of 3 mm. Constant amplitude loading was applied at the first-order bending resonance frequency of the specimens (approximately 58 Hz), with stress amplitude monitored and controlled via strain gauges. Failure detection was implemented through gas leakage monitoring. Residual stresses after welding and dynamic stress concentration factors (SCF) were measured using X-ray diffraction (XRD) and digital image correlation (DIC) techniques, respectively. The results successfully established the component-level S-N curve for the joint, determining the median fatigue strength at 10⁷ cycles (162.5 MPa) and the lower limit of design fatigue strength (148.7 MPa). All failures initiated at the weld toe, where SCF values ranged from 1.26 to 1.47. Significant residual compressive stresses were identified on the outer surface of the weld, exhibiting a negative correlation with weld reinforcement height. Under medium and high stress conditions, fatigue life decreased with increasing reinforcement height. Compared to ASME standard reference data, the component-level S-N curves developed in this study exhibited superior fatigue resistance in the medium–high cycle and fatigue limit regions. The component-level S-N curves obtained in this research incorporate the influences of actual weld geometry, residual stress distribution, and microstructural characteristics, providing a more accurate and reliable fatigue assessment basis for 316L butt-welded small-diameter pipes. These findings support optimized design approaches and provide a reference for evaluating the conservatism of standard assessment methods, which has significant implications for improving the safety and reliability of pipeline systems.