Low Cycle Fatigue Behaviour of API 5L X65 Pipeline Steel at Room Temperature

In this study, the low-cycle fatigue (LCF) behaviour of API 5L X65 pipeline steel was investigated under fully-reversed strain- controlled conditions at room temperature. The companion test method was used to obtain the cyclic stress-strain response curve with strain amplitudes ranging between 0.3% and 1.2%. Results showed that the steel undergoes both cyclic hardening and softening behaviour depending on the strain amplitude. At relatively lower strain amplitudes (0.3 - 0.8%), only cyclic softening was observed whereas at higher strain amplitudes, softening behaviour was preceded by significant hardening in the first few cycles. Based on the cyclic half-life results, the LCF life decreased while stress amplitude and plastic strain amplitude increased with total strain amplitude. Furthermore, analysis of stabilised hysteresis loops showed that the steel exhibits non-Masing behaviour. Complimentary scanning electron microscopy examinations were also carried out on fracture surfaces to reveal dominant damage mechanisms during crack initiation, propagation and fracture.