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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Khare et al. (2018), for example, proposed a novel S-hinged auxetic structure, with a Poisson's ratio of -1.5 made of PLA (polylactic acid polymer), which was later compared with a traditional re-entrant geometry. The fatigue tests were carried out in compression and their main goal was to determine whether the proposed new structure would be able to recover its initial strain after several repeated cycles. In addition to confirming the objective of the tests, authors also concluded that the recovery was superior to that of the re-entrant geometry. In this case, the S-shaped hinge design minimizes the stress concentration and, consequently, promotes better distribution of both stress and strain along the length of the hinge. Therefore, it is possible to obtain more flexible structures where repeated elastic compression is allowed for greater strains than those possible for the parent material. Using the 3D printing technology, Xu et al. (2020) produced cementitious cellular composites (CCCs) with auxetic behaviour reinforced by 0%, 1% and 2% in volume of Polyvinyl Alcohol (PVA) fibres. Authors observed that the auxetic behaviour of CCCs is due to the crack bridging effect of the reinforced cementitious material. In fact, during the compression loading, the sections' rotation is accompanied by the fibres pulling out at the joints of each individual cell. In terms of fatigue response, authors observed that the damage occurs in the cementitious matrix during the first 3000 cycles, which was evidenced by a rapid drop of the maximum load. Afterwards, both the maximum load and energy dissipation started to increase, revealing that CCCs are capable of recovering fatigue damage under cyclic loading due to fibrillation of PVA fibres under numerous cyclic loads. Essassi et al. (2020) studied the fatigue bending response of sandwich composites with re-entrant auxetic core under various stress ratios and densities between 8.5% and 33.5 %. From the experimental tests, it was found that sandwiches with low core density were responsible for the maximum fatigue life as well as the maximum load supported by the sandwich was lower. In this context, authors recommend that design criteria should be based on a compromise between maximum load and fatigue life, whose selection of core density is a determining factor for this purpose. In another study by Khawla Essassi (2021), the same authors observed that he auxetic honeycomb core has the highest dissipated energy and the highest damping ratio when compared to the conventional one. Yousuf et al. (2020) analysed the mechanical and shape memory properties of 4D printed multi-cell auxetic honeycomb cellular structures with tunable stiffness (between 0.179 and 0.242 kN/mm) and Poisson’s ratio (between -0.33 and +0.69). It was observed that, under constant cyclic conditions, the induced residual strains affect the mechanical properties, i.e., they degrade the Poisson's ratio and the structural stiffness. Moreover, authors also noted that the magnitude of the induced residual strains depends on the applied strain level and number of cycles applied. Because the auxetic structures are ideal for absorbing and dissipating mechanical, sound and thermal energy, Lvov et al. (2020) studied the low-cycle fatigue response to compression of 3D-printed re-entrant honeycomb auxetic structures made from TPU (Thermoplastic polyurethane) and observed that those based on hexagonal auxetic cells have a fatigue life about 1.75 times longer than that observed in similar non-auxetic structures. Subsequently, these authors used the same cell structures, but now produced by SLM /(selective laser melting) technology and using an AlSi 11 CuMn powdered aluminum alloy for biomedical applications, and observed that the auxetic structure failed for a load of 12 kN and after 2000 cycles, while the non-auxetic structure failed for a load of 8 kN and after 1000 cycles (Lvov et al. (2020)). Furthermore, these studies have also shown that auxetic structures can be used to dissipate mechanical energy due to their ability to absorb less energy than non-auxetic structures. Ulbin et al. (2020) performed computational fatigue studies on optimised auxetic cellular structures produced by SLM (selective laser melting) and using an AlSi 10 Mg powdered aluminum alloy, considering five structures with different negative Poisson’s ratios (from -0.01 to -2.41) and the fillet radius of the cellular struts (from 0.1 to 0.9 mm) chosen as a parameter. It was found that the longest fatigue life was obtained for structures with the minimum auxetic characteristics, i.e., higher Poisson's ratios, and the fillet radius has a significant effect on fatigue life. In this case, the fatigue life decreased for smaller fillet radiuses (less than 0.3 mm), due to high stress concentrations, but also for larger fillet radiuses (more than 0.6 mm) due to the movement of the plastic zone away from the edge of the cell connections (Ulbin et al. (2020)). More recently, the compression-compression high-cycle fatigue performance of additively manufactured auxetic meta-biomaterials produced by commercially pure titanium was studied by Kolken et al. (2021). These authors considered auxetic structures with re-entrant hexagonal honeycomb unit cell with different relative density, re-entrant angle, and Poisson’s ratio. Authors observed that auxetic structures have longer fatigue lives than non-auxetic structures made of the same material, where the majority of the strain versus fatigue life curves are characterized by the three-stage typical of porous metals and the minority exhibit multiple strain jumps indicating, in this case, non-