PSI - Issue 68

A.F Perez et al. / Procedia Structural Integrity 68 (2025) 439–445 A. F. Perez et al. / Structural Integrity Procedia 00 (2025) 000–000 7 for describing the stress state near the crack tip, making it difficult to obtain a relevant plane strain fracture toughness parameter like !" . Equations for calculating the J-integral and J-resistance typically do not account for variations in sample size, assuming that the plastically deformed region normalized by sample dimension will be the same in two samples of different sizes. This assumption has been found to be inaccurate in miniature fracture geometries, as evidenced by limited variation in shear lip size and inconsistent effects of side grooves (Calvet, 2024). 5. Conclusions • Three fracture toughness tests were carried out on P91 steel alloy at 600°C on CT samples without side grooves, 50 mm wide and 25 mm thick, to assess the influence of thickness on the material's toughness. The tests were standardised and to monitor crack extension during the experiments, DCPD and DIC methods were used. The average value of the toughness found is 321 kPa·m. However, these predictions based on the models defined in the standards do not correspond to experimental reality. Using the very limited final crack extension, the new J-R curves estimate a toughness of around 800 kPa·m. • Both the tunnelling effect, which characterises the non-rectilinear propagation of the crack front in the material, and the impact that thickness has on the mechanical behaviour of the specimen at high temperature are not considered by the standards. Indeed, the stresses at 600°C are such that the CT samples show significant zones of contraction visible to the naked eye at the crack tip. Other mechanisms may come into play at this temperature, such as strong creep effects. In short, the plasticity of the material is an important parameter to consider at this temperature level. • !" values found on miniature samples were four times lower, at around 80 kPa·m. These values are not in line with the theory that the thinner the material, the greater the toughness. In addition, with these specimen sizes, the plane strain conditions are not respected, and it is rather the plane stress configuration that takes precedence. This may explain the disparities between the results obtained with the two specimen sizes. Acknowledgments The authors wish to thank all the staff in the Department of Mechanical Engineering at Imperial College London for their help, support and expertise. Dr Yiqiang Wang would like to acknowledge the EPSRC grant (EP/W006839/1) and the Department for Energy Security and Net Zero. References Murry G. Aciers résistant au fluage. Techniques de l’ingénieur – Matériaux . 1999. https://doi-org.rp1.ensam.eu/10.51257/a-v2-m329. G, Boccaccini L, Cismondi F, Gasparotto M, Poitevin Y, Ricapito I. An overview of the EU breeding blanket design strategy as an integral part of the DEMO design effort. Fusion Engineering and Design . 2019. https://doi.org/10.1016/j.fusengdes.2019.01.141 Calvet T.F. High temperature fracture of miniature specimens and dissimilar joints for plasma facing nuclear components. PhD Thesis. Department of Mechanical Engineering. Imperial College London. 2024. Zhang J, Guo Z, Liu K. Mechanical properties evaluation by finite element for P91 high-temperature pipeline with small-scale specimen. Theoretical and Applied Fracture Mechanics . 2022. https://doi.org/10.1016/j.tafmec.2022.103613. Fournier B. Fatigue-Fluage des aciers Martensitiques A9-12%Cr : Comportement et Endommagement . PhD Thesis. École des Mines de Paris. 2007. Touboul M . Étude du comportement mécanique à chaud de l’acier P91 Vers la compréhension des mécanismes intra/intergranulaires sur la tenue en fluage : Application aux structures soudées . PhD Thesis. École des Mines de Paris. 2012. ASTM International. E 1820 – 23a. Standard Test Method for Measurement of Fracture Toughness . United States. IHS. 2023. Poirier J.P. Creep of Crystals. High temperature deformation processes in metals, ceramics and minerals. Cambridge University Press. 1985. https://doi.org/10.1002 /crat.2170211021 Tvergaard V, Needleman A. Analysis of the Cup-Cone Fracture in a Round Tensile Bar. Acta Metallurgica . 1984. https://doi.org/10.1016/0001 6160(84)90213-X James M.A, Newman Jr. J.C. The effect of crack tunnelling on crack growth: experiments and CTOA analyses. Engineering Fracture Mechanics . 2003. https://doi.org/10.1016/S0013-7944(02)00131-5 445