PSI - Issue 53

J.M. Parente et al. / Procedia Structural Integrity 53 (2024) 221–226 Author name / Structural Integrity Procedia 00 (2019) 000–000

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fracture toughness, and sound absorption (Ali et al. (2015), Farrugia et al. (2021)). In this context, as shown in Figure 1, a significant variety of cell designs have been proposed, including re-entrant, chiral, rigid rotating structure and star shaped structures, among others (Joseph et al. (2021), Kim et al. (2021), Lira et al. (2011), Photiou et al. (2021)). a) b) c) d)

Fig. 1. Auxetic cell designs. a) re-entrant, b) chiral c) rigid rotating structure and d) star-shaped

Although it is not difficult to develop the material itself, it can be challenging to manufacture complex structures, especially when they are used in complex 3D combinations. In this context, using various additive manufacturing processes, such as FFF (fused filament fabrication) for polymers and SLM (selective laser melting) to produce these auxetic materials, are an excellent alternative. In the first case, thermoplastic polymers can be used with the FFF technique because they can be heated and extruded via a nozzle to produce 3D parts directly from a CAD file created by computer software (Padmakumar (2020)). Metals are often printed using 3D technology by selectively fusing metal powder, typically with the aid of a laser. However, both additive manufacturing technologies of additive manufacturing have the benefit of being inexpensive and easy to develop/create (Chow et al. (2022)). Auxetic materials have been the focus of many studies over the last few years, but literature is not abundant on the fatigue behaviour of these materials, which limits the consolidated knowledge on this subject. This phenomenon is particularly significant because it is the most important mechanism of collapse and can result in catastrophic component/structure damage. Therefore, a more consolidated knowledge on this subject allows for improving design codes or even creating new materials with better mechanical performance. In this context, the purpose of this study is to present an overview of the studies that are currently available in the literature on fatigue behaviour in auxetic materials, with a special focus on 3d printing, in order to understand and consolidate the existing knowledge. For example, the first studies on auxetic materials were only published in 1987, but it was only in 2007 that they began to address fatigue behaviour. Of a total of nineteen papers published, nine are studies of fatigue in 3D printed materials and the vast majority were published after 2018, which reveals the incipient nature of this topic. 2. Fatigue performance of auxetic structures The first paper that studied the fatigue response of auxetic materials was carried out by Bezazi et al. (2007), which focused on a structure produced by polyurethane (PU) foam with a negative Poisson's ratio of -0.185. The compressive fatigue response of this foam was then compared with that of a conventional PU foam and another non-auxetic iso density PU foam. The results obtained by the authors showed that the fatigue response occurs in two phases, which depend significantly on the load level. According to the authors, the auxetic foam dissipated energy up to 16 times more than the conventional foam and 3 times more than non-auxetic iso-density PU foam. In another similar study (Bezazi et al. (2009)), the same materials were subjected to tensile fatigue tests and the authors observed that, in this case, there are three phases equally dependent on the load level. Furthermore, significant benefits of auxetic foams over the others in terms of stiffness degradation and energy dissipation have been observed. Later, Ne č emer et al. (2019) studied the fatigue crack initiation and propagation of an aluminum alloy re-entrant auxetic material. This study analysed auxetic cells with two orientations (vertical and horizontal), both with a Poisson's ratio of -0.2. The experimental results were compared with those obtained by computer simulations, which showed good agreement. In both cases, it was found that the crack propagation path depends on the orientation of the cell, but