PSI - Issue 42

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Author name / Structural Integrity Procedia 00 (2019) 000 – 000

Marouene Zouaoui et al. / Procedia Structural Integrity 42 (2022) 680–686 © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of 23 European Conference on Fracture - ECF23 Keywords: Auxetic cell pattern, metal 3D printing; fracture toughness; 17-4PH steel; extrusion additive manufacturing (ADAM).

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1. Introduction The Design for Additive Manufacturing (DfAM) is a rising research area where components are designed to minimize costs whilst maximizing their functionality and quality. Complex geometries and shapes can therefore be manufactured with minimum material usage and enhanced properties, Gardan (2019). Metallic extrusion additive manufacturing or Fused Deposition Metallic (FDMe) by 3D printing is a rising technology that uses a spooled filament of metallic powder embedded in a plastic matrix. This method avoids handling the metallic powder that requires many safety measures. Therefore, more interest is attributed to this technology. First introduced by Wu et al. (2002) under the name of fused deposition of metals. The method uses a hot head to extrude a filament and shape a geometry through a layer-by-layer process. The printed part, also called the green part, undergoes two separate post processes, (i) a debinding phase using a solvent and (ii) a sintering phase to get the solid part (ie. final specimen). The company Markforged developed a process called Atomic Diffusion Additive Manufacturing (ADAM) which is used in this study, Markforged Metal 3D Printer (2021). Various works recently studied the properties of the manufactured parts using 17-4PH stainless steel. The mechanical behavior was studied at the microscale by Bouaziz et al. (2020), in which the influence of the layer thickness has been also investigated. In the same manner, Henry et al. (2021) explores the effect of printing orientations on the mechanical response under various loadings conditions. The density, the surface roughness and the dimensional accuracy of 17-4PH samples were analyzed by Galati et al. (2019). Using the ADAM process gives more freedom for part consolidation and manufacturing complex structures. Consequently, introducing cell structures in the printed parts will not only reduce the material usage but also provides specific characteristics to the structure. Auxetic cells are well characterized by their negative Poi sson’s ratio , Kolken et al. (2017). They were widely developed thanks to the AM process, Meena et al. (2019), Beharic et al. (2018). The proposed concept herein is using the auxetic macroscopic effect to change the behavior near a V notch in order to delay the crack initiation. The effect of integrating honeycomb and auxetic cells on fracture toughness will be examined to compare both. Single Edge Notch Bending (SENB) specimens were designed with surface cell patterns. Auxetic cells with parallel and perpendicular patterns to the notch direction will be compared to a conventional honeycomb pattern. The fracture behavior assessment is accomplished through the calculation of the energy to fracture.

2. Specimens and experimental setups 2.1. Fabrication process and material

The specimens were manufactured using Markforged’s Atomic Diffusion Additive Manufacturing (ADAM) process. After designing the specimen geometry and creating the CAD file, the software Eiger developed by Markforged is employed for slicing, Eiger 3D Printing Software (2021). It also scales the part to make up for the shrinkage during the sintering phase. The printing phase is then launched. It is similar to a classic Fused Filament Fabrication FFF process with the particularity of using metal powder of 17-4PH Markforged stainless steel bound in a plastic matrix as a filament. The heated extrusion head deposits the raster at 230 °C following the trajectories generated while slicing. The nozzle diameter is 250 µm and the layer height resolution is 125 µm. The part is printed with 100% infill without raft