PSI - Issue 13

Letícia dos Santos Pereira et al. / Procedia Structural Integrity 13 (2018) 1985–1992 Author name / Structural Integrity Procedia 00 (2018) 000–000

1991

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Considering Charpy and X80 steel modeled using GTN, the total absorbed energy was ≈ 350 J , while the energy associated with steady state propagation was only ≈ 14 J , which means only ≈ 4 %. This means that only 4 % of the energy obtained from the test data represents steady state ductile fracture and would be suitable to be considered in crack arrest methods. The same analysis based on XFEM results provided 3.7 % of total energy, indicating that the methodologies considering both damage models are comparable. Considering DWTT and X80 steel modeled using GTN, the total absorbed energy was ≈ 25300 J , while the energy associated with steady state propagation was ≈ 4500 J , which means ≈ 18 %. This means that 18 % of the energy obtained from the test represents steady state ductile fracture and would be suitable to be considered in crack arrest methods. The same analysis based on XFEM results provided 16.6 % of total energy. One can realize that the percentage of energy related to ductile fracture for the case of DWTT is much larger than Charpy, but it is still reduced if compared to the total absorbed energy – less than one quarter of the total is in fact phenomenologically associated with the propagation of the ductile fracture that could be related and useful to existing methods for predicting arrestability, which support structural integrity assessments of real gas pipelines. Such data deserve experimental validation, but indicates the potential and limitations of these widely employed specimen geometries and loading modes. In addition, call the attention to the need for a better comprehension of such stresses and energies and how respective quantities and fields can be compared to analogous running cracks in gas pipelines made of modern high-toughness steels. 5. Concluding remarks It was possible to calibrate the damage parameters for both GTN and XFEMmodels. Load-displacement evolutions presented very good experimental agreement. The results for both models in terms of stress states and energies were close, indicating that both methodologies can be considered comparable for the desired analyses. It is interesting to mention that GTN is more phenomenological, while XFEM proved to be simpler and more pragmatic. The steady state crack propagation analyzes for X80 steel indicate that it is limited or does not take place in the case of Charpy geometry; in contrast, it is more characteristic and pronounced in DWTT. One of the limitations in using the Charpy specimen is the size of the remaining ligament; low stress triaxiality is an additional concern. Such limitations lead to only approximately 4 % of the total absorbed energy associated to stable crack propagation in this geometry. The percentage for the DWTT war larger, around 17 %, but it is still considered small if compared to total absorbed energy. The results demonstrate that DWTT potentially presents some benefits if compared to Charpy, but both cases present limitations to characterize the crack propagation resistance for advanced steels presenting high toughness and large amounts of plasticity. Finally, the selected results presented here calls the attention to the need for a better comprehension of such stresses and energies and how respective quantities and fields can be compared to analogous running cracks in gas pipelines made of modern high-toughness steels. Acknowledgements This investigation is supported by the Brazilian Metallurgy and Mining Company (CBMM) and by Centro Universitário FEI, Brazil. References ASTM, 2013. ASTM E23-12c. Standard Test Method for Drop-Weight Tear Test of Ferritic Steels. ASTM International, West Conshohocken, PA. Fries, T.; Belytschko, T. (2010), The extended/generalized finite element method: An overview of the method and its applications. International Journal for Numerical Methods in Engineering 84, 253-304. Campilho, R. D. S. G. et al., 2011. Strength prediction of single- and double-lap joints by standard and extended finite element modelling. International Journal of Adhesion and Adhesives 31, 362–372. Gurson, A. L., 1977. Continuum theory of ductile rupture by void nucleation and growth: part I - yield criteria and flow rules for porous ductile media. Journal of Engineering Materials Technology 99, 2–15. ISO, 2011. EM ISO 148-1. Metallic metals - Charpy pendulum impact test - Part:1. ISO. Geneva. ASTM, 2014. ASTM E23-12c. Standard Test Method for Notched Bar Impact Testing of Metallic Materials. ASTM International, West Conshohocken, PA.