Investigating the Effects of Geometrical Parameters of Re-Entrant Cells of Aluminum 7075-T651 Auxetic Structures on Fatigue Life

Abstract

In this study, the effects of two geometrical parameters of the re-entrant auxetic cells, namely, internal cell angle (θ) and H/L ratio in which H is the cell height, and L is the cell length, have been studied on the variations of Poisson’s ratio and fatigue life of Aluminum 7075-T6 auxetic structures. Five different values of both the H/L ratio and angle θ were selected. Numerical simulations and fatigue life predictions have been conducted through the use of ABAQUS (version 2022) and MSC Fatigue (version 11.0) software. Results revealed that increases in both the H/L ratio and angle θ improved the average value of Poisson’s ratio. Increasing the H/L ratio from 1 to 1.4 and θ from 50° to 70° increased the values of Poisson’s ratio, respectively, 7.7% and 80%. In all angles, increasing the H/L values decreased the fatigue life of the structures significantly. Furthermore, in all H/L values, an increment in θ caused a reduction in fatigue life. The effects of H/L and θ parameters on fatigue life were dominant in the low cycle fatigue regime. Results also showed that the H/L ratio parameter had greater influence as compared to the θ angle, and the structures with higher auxeticity experienced higher fatigue resistance. It was found that the auxetic property of the structure has a direct relationship with the fatigue resistance of the structure. In all samples, structures with greater auxetic property had higher fatigue resistance.

Fatigue response of auxetic structures: A review

“…In terms of θ-angle, and regardless of the H/L ratio, higher values are responsible for shorter fatigue lives. 40 However, the effect described above for θ-angle and H/L ratio is more significant for LCF than for HCF because higher stress concentrations are combined with higher stress levels and, consequently, a greater probability of crack initiation.…”

“…For example, regardless of the θ-angle, higher H/L ratios decrease the fatigue life, but this effect is more relevant for the LCF regime than for the high cycle fatigue (HCF) regime. This can be explained by the load transfer that occurs at the nodes that connect the longitudinal and transverse arms of the cell, which are regions of stress concentration and, consequently, more prone to fatigue F I G U R E 5 Representation of the geometric parameters analyzed for the re-entrant auxetic cell (adapted from Ghiasvand et al 40 ). [Colour figure can be viewed at wileyonlinelibrary.com] failures.…”

“…30 The angle of the auxetic cell geometry, for example, is considered one of the most important parameters for maximizing the mechanical performance of auxetic structures. [37][38][39][40] According to Subramani et al, 37 it is possible to obtain a 54% increase in the Poisson's ratio by changing the angle of the re-entrant structure from 74.5 to 85 . Zhang et al, 38 using the finite element method, studied the effect of re-entrant cell angle on the impact performance and that its increase leads to higher energy absorption values.…”

“…However, in addition to the geometries of the unit cells, the literature also mentions other parameters that can affect the structural performance of an auxetic structure 30 . The angle of the auxetic cell geometry, for example, is considered one of the most important parameters for maximizing the mechanical performance of auxetic structures 37–40 . According to Subramani et al, 37 it is possible to obtain a 54% increase in the Poisson's ratio by changing the angle of the re‐entrant structure from 74.5° to 85°.…”

“…Finally, Ghiasvand et al 40 studied the effect of two geometric parameters on the Poisson's ratio values and fatigue response of Aluminum 7075‐T6 re‐entrant auxetic structures: the internal angle ( θ ) and the cell height/cell length (H/L) ratio (see Figure 5). This study was based on numerical simulations duly validated by data available in the literature, and, for this purpose, ABAQUS software was used to obtain the Poisson's ratios and the stress analysis required for the MSC Fatigue software used to predict the fatigue lives.…”