PSI - Issue 68

Xingling Luo et al. / Procedia Structural Integrity 68 (2025) 694–700

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Xingling Luo et al. / Structural Integrity Procedia 00 (2025) 000–000

Overall, it was found that all crack paths were macroscopically perpendicular to the loading direction, as expected, with some local deviations due to the presence of inclusions. Moreover, cracks first appeared at the graphite surface and then extended into the matrix. In the single-inclusion models, it was observed that graphite morphology had a minimal effect on the debonding of CGI interfaces; however, the presence of nodular inclusions could notably enhance the macroscopic strength of the material and effectively impede the propagation of cracks within the metallic matrix. Also, some Mode-II cracks were observed in unit-cell models containing the flake graphite particle. In the models with two inclusions and randomly distributed ones, it was found that the shorter the distance between the two particles, the higher the stress concentration in that area, with cracks tending to initiate in it (see red arrows in simulations for random morphology in Fig. 3). After comparing the CZM, ductile damage models, and JC damage models, it was concluded that the latter required a relatively low computational cost while offering a good fit to the experimental data. It also demonstrated its strong predictive capabilities for both crack initiation and propagation (see simulations for real and simplified morphology in Fig 3). Finally, in simulations with random morphology, it was found that the resistance to loading of CGI increased with the increase in the nodularity and the volume fraction of graphite particles. 4. Conclusions The effect of graphite morphology on mechanical properties and crack paths of compacted graphite iron was analysed in this study. Statistical data obtained with ImageJ from microstructural characterisation were utilised to create two-dimensional unit cells and RVEs with single or multiple inclusions with the help of Python scripts. A comparison of different types of models incorporating periodic boundary conditions, cohesive-zone elements, and JC damage models was implemented. Both unit cell models and the RVEs approaches could effectively capture the mechanical and fracture behaviours of CGI, validated with the experimental data. The main points of all models are as follows: • The shape of graphite inclusions significantly influenced the mostly Mode-I crack path under uniaxial tension, with Mode-II cracks observed in unit-cell models containing flake graphite particles. • The minimal distance between the graphite particles could significantly accelerate the crack initiation and propagation when the loading is aligned with the main axis of graphite inclusions. • The JC damage model demonstrated good prediction capabilities for crack growth and offered a good fit for experiment data. • As the nodularity and volume fraction of graphite particles increase, the resistance to tensile loading of CGI also rises. Overall, the presented numerical simulations of crack initiation and propagation under tensile loading provide new insights for further research on the fracture behaviours of CGI or metal matrix composites. Acknowledgements The authors gratefully acknowledge the financial support of the China Scholarship Council (CSC) (Contract No. 202208060383). References Andriollo T, Thorborg J, Tiedje N, et al. (2016) A micro-mechanical analysis of thermo-elastic properties and local residual stresses in ductile iron based on a new anisotropic model for the graphite nodules. Modelling and Simulation in Materials Science and Engineering 24(5): 1–19. Bjørheim F, Siriwardane SC and Pavlou D (2022) A review of fatigue damage detection and measurement techniques. International Journal of Fatigue 154: 106556. Cao M, Baxevanakis KP and Silberschmidt VV (2023) Effect of graphite morphology on the thermomechanical performance of compacted graphite iron. Metals 13(3). Chi S-H (2013) Specimen size effects on the compressive strength and Weibull modulus of nuclear graphite of different coke particle size: IG-110 and NBG-18. Journal of Nuclear Materials 436(1): 185–190. Collini L and Pirondi A (2019) Microstructure-based RVE modeling of the failure behavior and LCF resistance of ductile cast iron. Procedia Structural Integrity 24: 324–336. Collini L and Pirondi A (2020) Micromechanical modeling of the effect of stress triaxiality on the strain to failure of ductile cast iron. Engineering Fracture Mechanics 238: 107270.