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The compatibility of nuclear materials in contact with Generation IV reactor coolants such as lead‑bismuth eutectic (LBE) still requires experimental justification. One of the key mechanical properties is fatigue life, both for reactor design and for life extension programmes. In this work, the characteristics of surface cracks in austenitic stainless steel 316 L was analysed after low cycle fatigue tests at 350 °C in three different environments: air, vacuum, and LBE. The density and length of nucleated fatigue microcracks was characterized for the different environments. Compared to tests in vacuum, a large number of shallow microcracks were found in the LBE environment, while fewer but significantly deeper cracks were observed in air. Finite element calculations were performed to estimate the impact of the different crack depths and densities on the stress behaviour of the material. The large crack density found in LBE had negligible impact on fatigue life when compared to air, and fatigue lives were comparable. Furthermore, due to their shallow nature, the cracks in LBE resulted in a stress response behaviour closer to the absence of environment than the deeper air cracks, which produce an apparent softening of the material during cyclic loading. ispartof: Science Talks vol:6 ispartof: Nuclear Materials Conference 2023 location:Gent, Belgium date:24 Oct - 28 Oct 2022 status: published

Spiral forming is a widely used industrial method for the manufacture of large diameter welded pipes from levelled steel strip. However, the multi-step helical forming process and post-treatment of the pipe influence the material behavior and alter the final mechanical anisotropy of the product. In addition, the complex microstructures of modern pipeline steels contain various sources of anisotropy including residual stresses, crystallographic and morphologic textures, and directional dislocation substructures. They do not only affect the local and global pipe strength, ductility and toughness during monotonic loading but also cause strong strain path change effects, e.g. a pronounced Bauschinger effect. Anisotropy thus poses a true challenge to pipeline designers making it difficult to accurately predict the final pipe behavior from the known properties of the hot-rolled high-strength coil, the starting point of the forming process. Still, such predictive power is vital to guarantee the structural integrity of pipelines without failure in case of in-service loads beyond the elastic range. The pipe manufacturing process can be simplified into two steps: decoiling (incl. levelling) and spiral forming. For subsequent quality control, samples are extracted from the pipe, per standard flattened and tensile tested in hoop direction. With regard to developing a computational twin of the complete manufacturing and testing process, we represent each step by a separate finite element (FE) model. The full through-process simulation workflow thus necessitates tools to transfer the material state between individual FE models. Here, we present such a workflow for the dislocation substructural hardening model by [Peeters et al., Acta Mater. (2001) 49:1607- 1619]. For each material point, macroscopic and microscopic state variables, including residual stress, crystallographic texture and dislocation densities, are first interpolated and then transferred to the next model. In this way, the evolution of different anisotropy sources can be studied starting from coil via pipe and ending with tensile testing

Acoustic emission (AE) was monitored in situ during in-plane tensile deformation of a commercially pure titanium plate. Different load orientations were considered: parallel to the rolling direction (RD), transversal direction (TD) and under an angle of 45 degrees (45D) with RD. The AE event rate, peak amplitude, signal duration, rise time and ring-down counts were monitored during deformation. Orientation dependent mechanical behavior, AE signal characteristics and microstructural evolution was observed and correlated to the difference in texture and twinning phenomena. The intensity and characteristics of AE signals in RD and 45D do not change considerably during deformation, which was attributed to the continuous nucleation and growth of compression twins. Anomalous AE behavior in TD was observed, showing two distinct peaks. All signal parameters show significantly decreased values during a first peak, which was correlated to massive nucleation of small tensile twins. These tensile twins disappear with increasing strain, indicating the end of the first peak. The second peak is comparable to the plateau behavior in RD and 45D and occurs during necking. Compression twinning is dominant, but limited to the necking zone. An unfavorable textural effect in TD has been identified, which hinders the formation of compression twins during the homogeneous deformation, as opposed to RD and 45D, resulting in anomalous AE behavior. The acoustic emission technique proves to be a complementary technique to electron diffraction as it allows monitoring and identifying the twinning modes of commercially pure titanium in situ. ispartof: Proc. EWGAE 2018, 33rd European Conference on Acoustic Emission Testing ispartof: 33rd European Conference on Acoustic Emission Testing location:Senlis, France date:12 Sep - 14 Sep 2018 status: Published online