Experimental Study on Low Cycle Fatigue Life of Carbon Steel Pipe Fittings by a Uniaxial Shaking Table

Abstract When the cyclic application of dynamic loads induces considerable alternating plastic deformation in metallic components, low-cycle fatigue (LCF) poses a risk to their structural stability. LCF is a potential failure mode in the design of nuclear piping systems. During fatigue life evaluation, allowable cycle counts for dynamic load reversals are derived from fatigue design curves, which are based on strain-controlled fatigue testing of small-scale specimens. Maintaining a sufficient safety margin is crucial because uncertainties stemming from material variability and data dispersion are typically unavoidable. To assess the conservatism inherent in fatigue evaluation procedures outlined in existing design codes, the authors performed a series of shaking table experiments. The shaking table generated uniaxial sinusoidal acceleration to induce resonance in three distinct types of pipe fittings: elbows, equal tees, and reducing tees. The specimens were fabricated from JIS G3456 STPT370 seamless carbon steel pipe, equivalent to ASTM A106 Grade A. Accelerometers and strain gauges were used to capture the dynamic behavior of the specimens. Fatigue cracks were initiated and propagated by continuous sinusoidal excitation, and ultimately led to penetration. Using the applied stress and the number of cycles to crack penetration, the authors evaluated the margin between the fatigue design curve and the actual number of cycles to failure.