PSI - Issue 39

Branko Nečemer et al. / Procedia Structural Integrity 39 (2022) 34 – 40 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Generally, cellular structures are a relatively new class of meta-materials which present a unique opportunity for adoption in light-weight structures, which are useful in modern engineering practice (X. Ren (2018), Hou (2015), Grima (2006), Meena (2019) and Zhao (20018)). Auxetic structures are a new type of porous material that exhibit a negative Poisson's ratio. The negative Poisson's ratio is a consequence of rotating cells in the geometry of the auxetic structure when an external load is applied (Nečemer (2019)) . The advanced geometrical possibilities of auxetic cellular materials provide many opportunities for their wide application, due to their particular and unique mechanical properties. Auxetic materials have a huge potential in different applications like in aerospace engineering, the automobile industry, and construction etc. (Milton (1992), Shan (2015)). Although considerable progress has been made in the field of auxetic structures, more research on utilising such structures in practical applications is still needed. In the last decades, the numerous researches have been performed in the field of the fatigue and fracture behaviour of cellular structures. In the review article presented by Nečemer et al (2019), different cellular structures are analysed in regard to their mechanical response under static and dynamic loading. However, only a few researches have been focused on the fatigue and fracture behaviour of the auxetic cellular structures. Nečemer et al. (2020) investigated the geometrical effect of the unit cell on the deformation behaviour of chiral auxetic cellular structures subjected to the multiaxial loading conditions. Kramberger et al. (2019) investigated the shape effect and distribution of unit cells on the fracture behaviour of 2D honeycomb and auxetic cellular structures under quasi-static loading conditions. Nečemer et a l. (2019) investigated the orientation of the unit cell in the auxetic structure on its fatigue behaviour. The same authors (Nečemer (2020)) investigated the fatigue behaviour of the chiral and re-entrant auxetic structures by using the inelastic energy approach. In this study, the influence of the orientation of the unit cell on the crack path and fatigue life was studied by using both experimental and computational approach. In this study, the two geometric layouts of the base cell, i.e. the re entrant and rotated re-entrant structure were analysed. The relative porosity of analysed auxetic specimens were approximately 60 %. The difference between these two structures is in the orientation of the based unit cell. In the computational analyses, the strain life approach based on the Coffin-Manson model, with a Morrow mean stress correction was applied. 2. Experimental tests In this section, the experimental tests of the analysed auxetic specimens are presented. For this purpose, a Compact Tension (CT) specimen with two different orientations of base unit cells was selected (see Fig. 1 ). It was assumed that the initial macroscopic notch extends through two (for re-entrant auxetic structure) or three (for rotated re-entrant auxetic structure) base cells.

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Fig. 1. (a) Re-entrant auxetic structure; (b) Rotated re-entrant auxetic structure