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Abstract
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This study experimentally examines the effect of ratcheting behavior on the fatigue life of U-shaped SS316L stainless steel pipes subjected to constant internal pressure and cyclic bending. The experiments were carried out using a Zwick/Roell Amsler HB100 servohydraulic testing system. Each test included a pre-ratcheting phase consisting of 400 loading cycles, during which the internal pressure, bending force amplitude, and mean bending force were systematically varied. Force–displacement hysteresis loops were recorded and analyzed to charac terize the mechanical response of the material. The results show that, under constant internal pressure and mean force, increasing the force range leads to larger hysteresis loop areas and higher displacement capacity, indi cating greater energy dissipation and strain accommodation. The hysteresis loops initially expand and subse quently contract as a result of progressive material hardening. For a fixed internal pressure and force range, an increase in mean bending force also causes loop expansion and accelerates material hardening, which in turn reduces the stable displacement range. A comparative assessment of the hysteresis responses demonstrates that variations in force range have a more pronounced effect on material hardening than comparable changes in mean force. Moreover, specimens subjected to higher force ranges and mean forces during the pre-ratcheting stage exhibited improved fatigue life. Overall, the findings indicate that the cyclic force amplitude plays a more dominant role than the mean force level in enhancing fatigue resistance. These results provide valuable insight into ratcheting-induced fatigue behavior in U-shaped piping components and support the safe and reliable design of piping systems operating under combined internal pressure and cyclic bending loads.
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