Computer Modeling in Engineering & Sciences |
DOI: 10.32604/cmes.2021.014828
ARTICLE
In-Plane Impact Dynamics Analysis of Re-Entrant Honeycomb with Variable Cross-Section
1Department of Engineering Structure and Mechanics, Wuhan University of Technology, Wuhan, 430070, China
2Hubei Key Laboratory of Theory and Application of Advanced Materials Mechanics, Wuhan, 430070, China
*Corresponding Author: Pin Wen. Email: wenpin@whut.edu.cn
Received: 02 November 2020; Accepted: 11 December 2020
Abstract: Due to the unique deformation characteristics of auxetic materials (Poisson’s ratio
Keywords: Auxetic re-entrant honeycombs; variable cross-section design; in-plane impact; finite element simulation
Different from traditional materials, auxetic materials (Poisson’s ratio
Significant efforts have been made by researchers to study the relationship between the geometric structure and mechanical properties of auxetic material. Xiao et al. [9] predicted the crashworthiness of re-entrant auxetic honeycomb under quasi-static loads and dynamic loads of different impact velocities by using finite element method. The results showed that different load conditions have great influence on the load-bearing, energy absorption and deformation modes of re-entrant auxetic honeycomb. Hu et al. [10] and Hou et al. [11] analyzed the cell structure parameters of re-entrant auxetic honeycomb, including cell wall angle, ratio of wall thickness to wall length, etc. Tan et al. [12] designed two levels of re-entrant auxetic honeycomb based on hexagon substructure and equilateral triangle substructure. It was found that the designed re-entrant auxetic honeycomb has higher energy absorption capacity. Dong et al. [13] studied the influence of wall thickness on re-entrant auxetic honeycomb deformation mode and the effect of negative Poisson’s ratio on crushing stress through experimental and numerical methods. They found that there are great differences in deformation modes and energy absorption between re-entrant auxetic honeycomb with thin-wall and thick-wall. Sun et al. [14] proposed a multi-functional layered honeycomb structure, whose mechanical properties such as Young’s modulus were derived based on the Euler beam theory. Lu et al. [15] designed a new honeycomb structure with a narrow rib in the inner concave honeycomb structure. Fu et al. [16] derived the analytical solutions of equivalent Young’s modulus and Poisson’s ratio for a new chiral three-dimensional auxetic material by using beam theory. Li et al. [17] designed a two-dimensional multi-level concave honeycomb structure, and studied its energy absorption effect under different levels by finite element method. Hou et al. [18] improved the two-dimensional multi-level concave honeycomb structure. Then, they used the finite element method to analyze the dynamics of the improved honeycomb structure in order to further improve the energy absorption effect. Zhang et al. [19] proposed a bio-inspired re-entrant arc-shaped honeycomb, and studied the influence of cellular microstructure on its impact dynamic response characteristics. The above literature review shows the mechanical behavior of auxetic re-entrant honeycombs has been widely studied, but most of the previous studies assume that the cross-section of the structure remains unchanged. There are few studies on auxetic re-entrant honeycomb with variable cross-section.
Recently, some researchers have introduced the idea of variable cross-section, which can improve the impact resistance and energy absorption abilities of structures. In order to improve the crashworthiness of the front longitudinal beam (S-shaped thin-walled beam), Xu et al. [20] designed a variable cross-section S-shaped thin-walled beam. Zhang et al. [21] proposed a kind of multi-cell thin-walled structure with variable cross-section, and studied the influence of wall thickness on its energy absorption abilities. Xiaofei et al. [22] proposed a new rhombic dodecahedron lattice structure with variable cross section, which has better mechanical properties and energy absorption than the original one.
In this paper, based on the variable cross-section design concept and the traditional re-entrant honeycomb structure, a re-entrant honeycomb with variable cross-section (VCRH) was proposed. Different cell wall structures were designed to improve the impact resistance and energy absorption abilities. Then, the dynamic mechanical behavior of VCRH was compared to that of traditional re-entrant honeycomb (RH). Finally, the effects of impact velocity and cell microstructure on the impact deformation characteristics, plateau stress and specific energy absorption (SEA) of VCRH were studied by numerical simulation. This study provides a new way to improve the mechanical properties and impact resistance of cellular materials.
The cell structures of RH and VCRH are shown in Fig. 1. The geometric dimensions of inclined cell wall length l, horizontal cell wall length a and inclined cell wall angle
Different from other materials, the most important characteristic of a cellular material is its relative density. The purpose of using the relative density is to eliminate the influence of the mass of porous structure on the mechanical properties. The relative density is defined as follows:
where
The VCRH cell is composed of two kinds of cell walls, including four inclined cell walls with uniform cross-section and two horizontal cell walls with variable cross-section. The relative density of honeycomb with variable cross-section can be given by the following formula:
where S1 and S2 represent the areas of inclined and horizontal cell walls:
In order to quantify the change in horizontal cell wall cross-section, the cross-section change rate
The cross-section change rate
3 Finite Element Simulation Analysis
In order to analyze the influence of cross-section change rate
3.2 Key Performance Indicators
The nominal stress–strain curve (solid line) of the VCRH is shown in Fig. 3. As seen from the figure, the process of deformation can be divided into three stages: the initial elastic deformation of honeycomb, progressive plastic yield of honeycomb cell, and significant amount of cell yield which leads to densification. Plateau stress (
Here,
As shown in Fig. 3, there are many local peaks in the energy efficiency curve. The nominal strain corresponding to the final peak value (i.e., the point at which the energy efficiency curve begins to decline rapidly) is regarded as the densification strain. In order to study the impact resistance and energy absorption abilities of VCRH, the following key performance indicators based on densification strain are proposed.
A cellular material with good impact behavior should maintain impact load uniformity during impact. In other words, the maximum stress peak value should be less than the damage critical value, and the fluctuation of stress should be as small as possible. The impact load efficiency (ILE) represents the impact load uniformity of cellular material, which is expressed as:
Here,
Here, Cmin represents the energy absorption efficiency of honeycomb structure under the impact load. The impact resistance of honeycomb structure is negatively correlated with the value of Cmin.
In order to evaluate the energy absorption abilities of honeycomb structure, specific energy absorption (SEA) is defined as the ratio of total energy absorption to mass:
where m, V and
3.3 Analysis of Mesh Sensitivity
In this part, the accuracy and reliability of numerical simulation are verified. In order to verify the accuracy and reliability of the finite element model, the deformation mode of traditional re-entrant honeycomb under impact load is simulated and compared with the literature results [24]. Fig. 4 shows the deformation comparison between the simulation results and the literature results under dynamic impact load (v = 20 m/s). When the impact velocity, material properties and geometric parameters are the same, the local deformation and global deformation of the simulation results are in good agreement with the literature results [24].
Before the simulation, the sensitivity of the numerical results to the mesh size was analyzed. Fig. 5 shows the effect of element size on the plateau stress and the calculation time. The results show that when the element size decreases to 1 mm, the platform stress tends to be stable and converges gradually. However, with the decrease in element size, the calculation time increases rapidly. Considering the accuracy and efficiency of finite element simulation, the mesh size of honeycomb structure is set as 0.5 mm. A large number of convergence tests show that the existing finite element model is accurate and effective, and is suitable to analyze the dynamic characteristics and energy absorption performance of honeycomb structures under impact load. Therefore, the finite element model described above is reliable and can be used for subsequent research.
In order to study the influence of the cross-section change rate
It can be seen from Fig. 6 that the VCRH can enter the deformation mode of layer-by-layer collapse more quickly, which means the cells nearest to the impact end are crushed more quickly. This is mainly because the two ends of horizontal cell wall of VCRH cell are thicker, and the strain value required for cell collapse is smaller. Fig. 7 shows the comparison of the deformation modes of VCRH unit cells at crushing strain. Tab. 2 shows the strain of re-entrant cellular structure under different cross-section change rates. The larger the cross-section change rate
4.2 Mechanical Characteristics of VCRH
In order to obtain the elastic modulus of the structure, the quasi-static compression simulation of the model was carried out. The total displacement of top surface of VRCH is assumed to be 1 mm under the loading rate of 0.6 mm/min. In this case, the elastic moduli of five models with different values of cross-section change rate
According to Eq. (7), the effects of cross-section change rate
As a critical indicator for good impact resistant structure, the impact load uniformity of VCRH was also investigated. The dynamic response curves of VCRH structure with cross-section change rate
The impact load uniformity is quantified as ILE for better understanding the influence of cross-section change rate
4.3 Energy Absorption Characteristics
In order to evaluate the energy absorption efficiency of honeycomb structure, the minimum dynamic cushioning coefficient (Cmin) is proposed. Cmin is inversely proportional to the cushioning performance of honeycomb. Fig. 11 shows the effect of cross-section change rate
Fig. 12 shows the effect of cross-section change rate
In this study, a re-entrant honeycomb with variable cross-section (VCRH) is proposed based on the concept of variable cross-section design. Compared with the traditional re-entrant honeycomb (RH), the impact resistance and energy absorption of VCRH are significantly improved. The dynamic impact response and energy absorption characteristics of VCRH under different in-plane impact velocities are evaluated by studying the micro-structure parameters. The main conclusions of this paper are obtained by simulation analysis.
The deformation mode of honeycomb structure is determined by impact velocity and cell microstructure. The introduction of variable cross-section into RH leads to the change in macro/micro deformation characteristics during impact. Compared with RH, VCRH can enter the complete collapse stage earlier. Moreover, the crushing strain of VCRH is negatively correlated with the cross-section change rate
Compared with RH, the elastic modulus of the VCRH structure is significantly increased. In addition, at the same impact velocity, the VCRH has higher dynamic platform pressure and SEA than the RH. It is found that the RH has better impact load uniformity and energy absorption efficiency at low speed, while the VCRH structure has better impact load uniformity and energy absorption efficiency at medium and high speed. The results also show that the dynamic platform pressure and SEA are positively correlated with the cross-section change rate. At medium and high impact velocities, the impact load uniformity and energy absorption efficiency are positively correlated with the cross-section change rate. These results can provide a reference for designing improved auxetic re-entrant honeycomb structures.
Funding Statement: This research is supported by the National Natural Science Foundation of China (No. 11902232).
Conflicts of Interest: The authors declare that they have no conflicts of interest to report regarding the present study.
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