Development of Cleavage Fracture Initiation Model for Bainite Steels Based on Micromechanism
- Itsuki Kawata (University of Tokyo) | Takshi Hiraide (JFE Steel Corporation) | Kazuki Shibanuma (University of Tokyo) | Tomoya Kawabata (University of Tokyo) | Shuji Aihara (University of Tokyo)
- Document ID
- International Society of Offshore and Polar Engineers
- International Journal of Offshore and Polar Engineering
- Publication Date
- September 2016
- Document Type
- Journal Paper
- 278 - 286
- 2016. The International Society of Offshore and Polar Engineers
- cleavage fracture, fracture initiation, fracture toughness, Bainite steel, local approach
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- 37 since 2007
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The authors propose a cleavage fracture initiation model for bainite steels. The authors considered three stages of fracture initiation in the model: stage I, microcrack initiation in the brittle phase; stage II, propagation of the microcrack to a neighbor matrix; and stage III, propagation of the cleavage crack across the packet boundary. The cleavage fracture is assumed to initiate when all three stage conditions are satisfied in any one of the volume elements surrounding a notch tip. The authors applied this assumption to bainite steels and validated this model by comparing results of the analysis and toughness test results.
Recently, the strength of steel plates used for offshore structures and container ships has been increased, satisfying the demand for the increase in structural size scale and for performance in more severe operating conditions. Bainite steels have been used for various structures for achieving higher strength. However, the mechanism of cleavage fracture initiation in bainite steels remains unclear.
Beremin (1983) proposed a stochastic model of brittle fracture, assuming that brittle fracture follows, essentially, a weakest-link mechanism. He assumed multiple volume elements in a material and that each volume element contains one microcrack. In Beremin’s model, the critical stress of the microcrack in a volume element controls the fracture of the whole of the material. Beremin’s model led to Weibull stress, and the Weibull stress parameters can be obtained from multiple fracture toughness tests. These parameters are considered as material properties that control the fracture toughness distribution of the material. However, this model requires many fracture toughness tests to derive these parameters and does not explain the details of the fracture initiation mechanism of complex microstructures like bainite.On the other hand, Martin-Meizoso et al. (1994) proposed a model for predicting the fracture toughness of bainite steels. In that model, the process by which a carbide crack grows into a bainitic packet and propagates across a packet boundary was formulated. Lambert-Perlade et al. (2004) assumed the same process as in the model by Martin-Meizoso et al. (1994) and modeled fracture initiation of the simulated heat-affected zone (HAZ) containing an upper bainite microstructure. However, formulations for critical conditions of the fracture initiation process in the models are too simple to predict the toughness of the bainitic steels.
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