Mechanical Approach to Fracture Toughness Improvement for High Strength Steel and Its Weld HAZ by Controlling Morphology of Microstructure

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This study discussed the transfer of local stress fields in microstructures to fracture property evaluation of structural steels. Initiation behavior of brittle microcrack in dual-phase steels was mainly focused. The Weibull stress employed in the local approach is available for the evaluation of fracture driving force of each microstructure in dual-phase steels. The idea in this procedure is the integration of local stress in the microstructure responsible for fracture initiation. The local stress in dual-phase steels can be different considerably from a macroscopic stress obtained from the global analysis of specimen. Generally, the high-strength phase bears larger stress than the low-strength phase, which means that fracture property evaluation based on the global stress field is not always reliable. The local stress in the microstructure is affected by the morphology of second phase and matrix. Increase in strength and slenderness of hard phase leads to elevation of fracture driving force of the hard phase. The stresses in low-strength phase can be relaxd to a large extent, provided that the low-strength phase is enclosed by the high-strength phase. These microscopic characteristics of stress fields are important in the fracture control design of structural steels. The Weibull stress is expected as a measure to link between microscopic and macroscopic aspects of fracture.