Corrosion fatigue crack growth of serviced API 5L X56 submarine pipeline

Corrosion fatigue tests on standard compact tensile (CT) specimens extracted from API 5L X56 pipe-in-pipe pipeline after servicing for 15 years have been conducted in air and in seawater environment (corrosion fatigue) respectively. Effects of maximum load, stress ratio and loading frequency on the fatigue crack growth (FCG) rate are considered in the fatigue tests, and such effects were investigated and discussed based on the tested results. The maximum load has a remarkable influence on the fatigue life of CT specimens. However, it has a slight effect on the two constants in Paris' law. Stress ratio has a significant influence on the FCG in middle stage, and FCG rate decreases with the stress ratio increases. With the increase of loading frequency, FCG rates decrease and fatigue life increase. Using scanning electron microscope, the fracture surface of the fatigue crack is observed and analyzed, and transgranular cleavage fracture is found on the fracture surface. The effects of the maximum load, the stress ratio and the loading frequency on the crystal surface and on the FCG mode were analyzed and compared based on such observation on the fracture surface of the fatigue crack.

Automated summary

Corrosion fatigue tests were conducted on standard compact tensile specimens extracted from API 5L X56 pipe-in-pipe pipeline after 15 years of service. The effects of maximum load, stress ratio, and loading frequency on fatigue crack growth rate were investigated. Results showed that maximum load significantly influenced the fatigue life of the specimens, while it only had a slight effect on the constants in Paris' law. Stress ratio had a significant influence on fatigue crack growth rate, with a decrease observed as the stress ratio increased. Additionally, increasing the loading frequency resulted in decreased crack growth rates and increased fatigue life. Fracture surfaces of fatigue cracks were observed and analyzed using a scanning electron microscope, revealing transgranular cleavage fracture.