Fatigue-resistant structural optimization and vibration testing of a spherical tube fitting in liquid rocket engines

Vibration-induced fatigue at stress-concentrated regions is a critical concern for high-pressure pipeline fittings in aerospace propulsion systems. This study focuses on a spherical tube fitting widely used in liquid rocket engines, which experiences high bending stresses at the transition fillet due to assembly preload and dynamic vibration. To enhance its fatigue resistance, we propose a rapid structural optimization framework integrating Latin hypercube sampling, nonparametric regression, and multi-objective genetic algorithms. Two optimized designs were derived based on stiffness compensation and pressure redistribution strategies, reducing peak stress by 19.6% and 7.6%, respectively. A resonance-based vibration fatigue testing system was developed for experimental validation, and strain–life (ε–N) curves were constructed under fully reversed loading. The results confirm substantial improvements in fatigue life, with strain-based fatigue limits increased by 63.7% and 18.3%. The proposed design–test framework is broadly applicable for improving the reliability of vibration-sensitive components in rocket engines and other aerospace systems.