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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the fatigue strength is not affected. These authors have deepened this research line and published other studies on the subject. In this context, they developed a computational study with the aim of comparing the fatigue life of two typical 2D auxetic cellular structures, i.e., a chiral auxetic structure and a re-entrant auxetic structure (Ne č emer et al. (2020)). The damage initiation and evolution law were used in the low-cycle fatigue analysis, whose necessary parameters were obtained experimentally. Authors found that the highest fatigue strength was obtained for the chiral auxetic structure, because the stresses are distributed more uniformly along the individual cells than in the re-entrant auxetic structure. They also noted that the fatigue damage occurred at the intercellular connections, while in the re-entrant geometry the fatigue damage occurred at the corner of the cells. More recently, Ne č emer et al. (2022) studied how the unit cell's orientation affects the crack's path and fatigue life based in experimental and numerical approaches. For this purpose, re-entrant and rotated re-entrant auxetic specimens made of Al-alloy 7075-T651 were considered. These authors found similar fatigue lives, but the crack paths were different and are influenced by the orientation of the unit cell. In terms of damage, the fracture cell struts initially occur at the corner of the individual unit cell and subsequently expand in the direction of the maximum stress concentration. In a similar study Ne č emer et al. (2022), studied the fatigue response of chiral and re-entrant auxetic structures produced in a 5083-H111 aluminium alloy. Compared to the chiral auxetic specimen, the stiffness of the re-entrant auxetic specimen was almost ten times higher, considering a very similar relative porosity. For the same average strain amplitude, the chiral structure showed longer fatigue life than the re-entrant structure, while the re-entrant one (more rigid) had higher amplitude forces for the same fatigue life. It was also observed that, although the dispersion band of the chiral structure is shorter than that of the re-entrant structure, the slopes of the fatigue curves for both structures are very similar. This was explained by the fact that the chiral structure has more rounded intercellular links than the re-entrant one. In fact, the fractographic analysis revealed that the fractures mainly occurred in the longitudinal linkages and at locations with the largest curvatures. In their latest study, Ne č emer et al. (2022) analysed the influence of the unit cell orientation on the crack path and fatigue life. For this purpose, they used re-entrant auxetic structures made from 7075-T651 aluminium alloy and two different auxetic structures were considered (a re-entrant auxetic structure along the -direction and a a re-entrant auxetic structure along the x -direction). From the quasi-static tests, it was possible to conclude that the re-entrant auxetic structure along the -direction is stiffer than the re-entrant auxetic structure along the x -direction and, consequently, supports higher loads. In terms of low cycle fatigue (LCF) tests, both auxetic structures have very close lives, but the cell fracture typically starts in the corner of the individual unit cell links that are oriented at 45ºin relation to the loading direction, and it subsequently propagates in the direction of maximum stress concentration. Therefore, as the authors have already mentioned in another study (Ne č emer et al. (2019)), the unit cell’s orientation has a minor influence on the fatigue life, but significantly affects the direction of the fatigue failure path (Ne č emer et al. (2022)). Experimental and numerical studies were developed by Tomažin č i č et al. (2019) to predict the fatigue lives of standard and re-entrant auxetic cellular specimens, concluding that the Brown-Miller criterion should be used to predict the fatigue life of complex structures made from the Al7075-T651 alloy. Furthermore, it was found by the authors that complex shapes should be carefully machined to prevent geometric heterogeneities that drastically reduce the fatigue life. Finally, in a similar study, Francesconi et al. (2019) developed experimental and numerical studies on fatigue life and crack propagation in two-dimensional perforated aluminum structures, which involved geometries with a positive Poisson’s ratio (PPR), using circular holes, and geometries with auxetic stop-hole and straight-groove hole. These authors found that auxetic structures have a fatigue life more than 20% longer than porous PPR structures with the same porosity, despite having holes with higher stress concentrations. Furthermore, numerical studies have shown that auxetic structures maintain their negative Poisson's ratio even in the presence of cracks, as well as that the auxetic structure delays the crack initiation and spread the damage over a larger area. 3. Fatigue performance of 3D printed auxetic structures Due to its low cost, simplicity of production, and capacity to produce complex geometries, 3D printing is a new technology that has been gaining popularity recently. In this context, it becomes ideal for creating auxetic structures, because they require cells with shapes that can be very complex (AlMahri et al. (2022)). Although there are several studies on the mechanical performance of these 3D-printed materials/structures in the literature, only nine articles focus on their response to fatigue.