PSI - Issue 56

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Estera Vălean et al. / Structural Integrity Procedia 00 (2023) 000 – 000

Estera Vălean et al. / Procedia Structural Integrity 56 (2024) 58 – 64 © 2023 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the SIRAMM23 organizers Keywords: Additive manufacturing; Fracture; Fused Deposition Modelling; local approaches. 1. Introduction Fused Deposition Modeling (FDM) is an additive manufacturing (AM) process that involves the extrusion of material from a nozzle to realize 3D parts from plastic materials. Ease of use, prototyping accuracy, and low cost make it a widely used AM technique. In FDM, a filament of thermoplastic printing material is melted through a printer extrusion die and deposited line by line and layer by layer on a heated printing plate. Producing a printed structure involves generating a digital design of the model via 3D design software and running it through the printer until the complete model is reproduced (Vaezi et al. 2013; Song et al 2017; García Plaza et al. 2019). Despite the advantages of this process, if the process parameters are not optimized problems may arise such as poor adhesion or large process-induced defects leading to detrimental effects on the mechanical performance with failures facilitated by the delamination between different layers. Nowadays, FDM is mainly used in dealing with thermoplastic materials such as polylactic acid (PLA). Due to its excellent properties and low cost, PLA has evolved as perhaps the most widely used material in 3D printing. It is a biodegradable, recyclable and compostable thermoplastic polymer with high strength and high modulus, which has proven its potential to replace many conventional polymers in various industrial applications (Farah et al. 2016; Savioli Lopes et al. 2012). It is worth noting that FDM can be exploited also to build components made of materials such as metals, ceramics or cermets employing high-filled polymers during the FDM process followed by post-processing phases to achieve the final component, (Spiller et al. 2022 1 ; Spiller et al. 2022 2 ). Other materials of high interest for FDM process are fiber reinforced polymers. However, such reinforced materials are still a field of research due to the high variability of mechanical performance that can be achieved depending on the process parameters (Ajay Kumar et al. 2020; Durga et al. 2019; Marsavina et al. 2022). Besides the variety of materials that can be used through AM and the flexibility of AM processes in production of highly complex components, proper tools are still required to assess fracture behavior and fatigue life, (Mohammadizadeh et al. 2019; Foti et al. 2023). The so-called local approaches provide a suitable solution being their critical values independent of both the overall geometry of the component and the loading conditions. With this purpose, the present work investigates through an energetic local approach the fracture behavior of AM notched specimens made of PLA and carbon fiber reinforced PLA. 2. Fundamentals of the strain energy density Among local approaches, the strain energy density (SED) method, as proposed by Lazzarin et al. (2001, 2002), is an energy-based local approach widely validated to assess fracture both in static and dynamic conditions (Foti et al. 2020). The SED method is founded on the idea that brittle fracture happens when the total elastic energy, averaged over a given control volume, reaches a critical value. According to the method hypothesis, both the critical SED value and the characteristic length, , defining the control volume represent material properties; they are indeed independent on both the local geometrical conditions and on the loadings mode, even if the shape and the location of the control volume depends on them (for further details, see Foti et al. 2021). It is worth highlighting that, under the assumption of ideally brittle material, the critical SED value, , can be estimated through the conventional ultimate tensile strength, , of a smooth specimen: 59

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σ

2E UTS

C W =

(1)

Thanks to the absence of stress gradients, in the case of smooth specimens no information are needed regarding the control volume characteristic length. On the other hand, when geometrical discontinuities are present in the