Relation between the Fatigue and Fracture Ductile-Brittle Transition in S500 Welded Steel Joints

• Published in: Metals
• Main Authors: Finn Sallaba, Franziska Rolof, Sören Ehlers, Carey Leroy Walters, Moritz Braun
• Format: Article in Journal/Newspaper
• Language: English
• Published: MDPI AG 2022
• Subjects: arctic conditions; weldment fatigue; temperature dependence of material fatigue; fatigue and fracture mechanics testing at low temperatures; fatigue and fracture transitions temperatures; direct-current potential drop method; Mining engineering. Metallurgy; TN1-997; Arctic
• Online Access: https://doi.org/10.3390/met12030385; https://doaj.org/article/ce6f328af2204704ae1cfb7f37a7898f

The formation and propagation of cracks occur through irreversible dislocation movements at notches, material defects, and grain boundaries. Since this process is partly thermally controlled, the resistance to dislocation movements at low temperatures increases. This slows both fatigue initiation and fatigue crack propagation. From recent experimental data, it can be seen that fatigue crack growth is accelerated below the fatigue transition temperature (FTT) that correlates with the ductile-brittle transition temperature (DBTT) found by well-known fracture mechanics tests, i.e., Charpy impact, fracture toughness, and CTOD. Hence, this study investigates the relation between FTT and DBTT in S500 high-strength steel base material and welded joints at low temperatures using fatigue crack growth, fracture toughness tests as well as scanning electron microscopy. From the tests, an almost constant decrease in fatigue crack propagation rate is determined with decreasing test temperature even below the DBTT. At −100 °C, the fatigue crack propagation rate is about half of the rate observed at room temperature for both base material and weld metal.