Examining the hysteresis loop of the SS316L U-shaped pipe considering ratcheting behavior

Abstract

The current study examines the effect of ratcheting in U-shaped SS316L pipe under constant internal pressure and cyclic bending force on its fatigue life. For this purpose, empirical tests are performed using Zwick/Roell Amsler HB100 servohydraulic device. The tests have a pre-ratcheting stage of 400 cycles with different internal pressure, amplitude, and average bending force. The force-displacement hysteresis loops were obtained for each test. Notably, the specimens with equal internal pressure, mean force, and a higher force range tolerates more displacements. Besides, the the loops are created with a larger area and the high strain tolerance. In other words, more displacements are created and the increase of force amplitude results in strains in the initial cycles and moves the material towards hardening faster which is associated with the reduction of the displacement length. The specimen with equal internal pressure and force range, increasing the average force causes expansion of hysteresis loops and acceleration of hardening of the material, and the length of displacement zone decreases. The hysteresis diagrams indicate that increasing the force range has a greater effect on the hardening of the material than increasing the average force. It was found that samples with a higher force range and average force in the pre-ratcheting stage will have a higher fatigue life. Also, the effect of increasing the force amplitude on increasing the fatigue life is more than the average force. The obtained results of this study are beneficial to those aming to advance in the mechanical field and ratcheting.