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Öğe Foams of gray cast iron as efficient energy absorption structures: a feasibility study(Wiley-V C H Verlag Gmbh, 2019) Kaya, Ali Can; Zaslansky, Paul; Rack, Alexander; Fischer, Sebastian F.; Fleck, ClaudiaGray cast iron foams, were produced using a reticulated polyurethane by using a modified investment casting process. To evaluate the attributes of the produced micro-geometries, foam segments and single struts are investigated by light and electron microscopy in 2D, and synchrotron micro-computed tomography in 3D. Mechanical properties are determined by macro/micro-mechanical testing and nanoindentation. In the microstructure of the gray cast iron struts, both flake-like coarse type A and locally fine undercooled type D graphite particles are observed. The distribution of the two graphite types is heterogeneous and is the likely cause for the large scatter of the mechanical properties of the single struts. The high graphite content and the resulting brittle behavior of the struts lead to strong serrations in the stress-strain curve of the foams with a negative effect on energy absorption. We found a relatively low energy absorption efficiency of below 50% as compared to 75% in 316L austenitic steel struts. The small specimen size results in scale effects which strongly influence the mechanical properties of the foams. Further improvement in the fabrication of gray cast iron foams is needed to tailor graphite distributions and optimize performance of cast iron-based foams.Öğe Modeling of complex gray cast iron open-cell foams revealing insights on failure and deformation on different hierarchical length-scales(Wiley-VCH, 2021) Kaya, Ali Can; Zaslansky, Paul; Fleck, ClaudiaOpen-cell gray cast iron foams are a class of porous materials of increasing interest, with great potential to be used in energy damping applications and for sound isolation. We created finite element (FE) models from 3D reconstructed data of foams and of isolated struts imaged with resolutions spanning from 0.65 to 10.5 mu m. Representative volume elements (RVE) are loaded in tension to simulate and analyze foam mechanical behavior. A ductile damage model is used for tensile testing of single struts and compression testing of foams. RVEs loaded along three axes demonstrate important effects of orientation of graphite particles in the microstructure. Only simulation results that take failure into consideration are consistent with experimental findings. Strut fracture initiation strongly depends on the cross-sectional area and its circularity and foam simulation results are heavily influenced by damage modeling. The complexity of the open-cell foam behavior is revealed at several hierarchical levels. Simulations in which material properties are assigned excluding damage overestimate the experimental results of the foam; conversely, a very good agreement is observed if damage is considered. The models exhibiting anisotropic mechanical properties fully reproducing large fluctuations in the mechanical properties of the struts are observed in in situ experiments.Öğe Single steel strut mechanical testing: challenges and future research directions(Taylor & Francis Ltd, 2019) Kaya, Ali Can; Zaslansky, Paul; Fischer, Sebastian; Fleck, ClaudiaOpen-cell stainless steel foams, composed of hollow struts, are excellent candidates for energy absorption and thermo-mechanical applications. The basic mechanical element responsible for the function of these foams is the single strut. However, testing and characterisation of single foam struts to predict the foam strength have stirred up a new debate about approaches to micro-tensile testing of such elements. In this paper, we present a protocol for in-situ micro-tensile testing of hollow steel struts using a custom-made grip system. The adapted grips make it possible to perform analysis of the deformation of multiple sintered struts. Here we present and discuss challenges encountered during such micro-tensile testing of hollow steel struts.Öğe Strain hardening reduces energy absorption efficiency of austenitic stainless steel foams while porosity does not(Elsevier Sci Ltd, 2018) Kaya, Ali Can; Zaslansky, Paul; İpekoğlu, Mehmet; Fleck, ClaudiaStrain hardening significantly affects the mechanical function of steel foams. We compare hardening and failure of two commercially available austenitic stainless steel foams (316L and 310) spanning strut porosities of 9.4 to 144%. Damage is correlated with strut microstructure and geometry, combining in-situ quasi-static compression testing in the SEM, 3D-evaluation by synchrotron mu CT and bending simulations. We provide an analytical model for the experimentally observed strain hardening. Upon compression, 316L steel foams exhibit a plateau regime of continuously increasing stress due to the hardening effect, whereas 310 steel foams show almost constant plateau stress This is explained by the much less ductile behaviour of the 310 steel foam struts as compared to 316L steel foam struts. Finite element modelling suggests that significant stress concentrations develop around micro porosities in the 310 struts. Due to its finer and less porous microstructure, the 316L foam exhibits a larger energy absorption capacity than the 310 foams. This results in distinctly different efficiency-strain curves. However, up to about 25% strain, the efficiency values are surprisingly similar. Thus, modification of microstructure and/or pore micro-geometry can be used to optimise the stress-strain response to achieve the desired energy absorption property of steel foams. (C) 2018 Elsevier Ltd. All rights reserved.