Limit state condition in the tensile failure for the strain based-design (SBD) currently considering is the point of maximum load which is evaluated by curved wide plate (CWP) testing or full scale pipe tensile testing. Maximum loading point is understood as the onset of instability of the structure. However, the material behavior controlling structural instability is not well understood since it includes many aspects of material response such as local strain concentration, ductile crack initiation and stable crack growth. In order to clearly specify the material property suitable for SBD, it is important to understand the fundamental behavior of the linepipe steels that leads to ductile crack initiation and following ductile tearing. In this paper, critical condition for ductile crack initiation was investigated by both small scale and large scale testing, notched round bar and wide plate testing, by using X80 and X100 linepipe steels and welds. Two different analytical procedures, equivalent plastic strain criterion and damage mechanical analysis, were applied to evaluate the local material conditions for ductile crack initiation. As was already verified by many other researches, the critical equivalent plastic strain can be used as the local criterion for ductile crack initiation which is not affected by specimen geometry. However, equivalent plastic strain is still macroscopic parameter that is not reflected by microscopic feature of the steel. Therefore, the Gurson-Tvergaard damage mechanical model was applied to further understand microscopic material behavior to ductile crack initiation. Material parameters for G-T model were carefully evaluated depending on the microscopic characteristics of each steel. By selecting appropriate material parameters, the critical condition for ductile crack initiation was estimated by the critical void volume fraction, which is independent of specimen geometry. Effect of microstructural characteristics on crack initiation was also investigated in this study.
- International Petroleum Technology Institute and the Pipeline Division
Application of Damage Mechanics Modeling to Strain Based Design With Respect to Ductile Crack Initiation
Ishikawa, N, Sueyoshi, H, & Igi, S. "Application of Damage Mechanics Modeling to Strain Based Design With Respect to Ductile Crack Initiation." Proceedings of the 2010 8th International Pipeline Conference. 2010 8th International Pipeline Conference, Volume 4. Calgary, Alberta, Canada. September 27–October 1, 2010. pp. 279-286. ASME. https://doi.org/10.1115/IPC2010-31506
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