Elsevier

Composite Structures

Volume 124, June 2015, Pages 409-420
Composite Structures

Mesoscopic investigation of closed-cell aluminum foams on energy absorption capability under impact

https://doi.org/10.1016/j.compstruct.2015.01.001Get rights and content

Abstract

This paper presents a three-dimensional (3D) mesoscopic model of closed-cell aluminum foams. This model is different from the model developed using Voronoi technique. Randomness of pores in shape and distribution is taken into account. The cell-wall thickness varies spatially which is consistent with test observations. Energy analysis approach is presented and validated. Numerical simulations are conducted to investigate the energy absorption capability of the closed-cell aluminum foams under impact loading. Effects of mesoscopic configurations of aluminum foams (cell-wall strength and porosity) on energy absorption capability are investigated numerically. Mesoscopic responses of the cell-walls are discussed to reveal the mechanism of energy absorption capability. It reveals that the energy absorption capability originates from the permanent deformation (plastic strain, collapse and fracture) of cell-walls under impact.

Introduction

Intense dynamic events (such as high-velocity impact, blast and explosion) may occur and lead to extreme damage to structures, vehicles and human beings. The researches to mitigate the damage have long been of interest and remain an active subject.

Aluminum foams are a class of cellular metals and have been used extensively in engineering [1], [2], [3], [4], [5] with excellent performance. It behaves both metallic and foam-like properties, undergoing large strains at relatively constant stresses under compression. Energy absorption capability originates from the process of plateau stress region [6], [7], [8]. The sandwich type structures [9], [10], [11], [12], comprising the aluminum foams in the core and the thin steel cover plates, have been used to withstand higher impulse under intense dynamic events. They are mounted on building structures, protecting shelters and other foam-filled devices, being supposed to alleviate impact and blast damage. In the past few decades, many researches [13], [14], [15], [16], [17], [18], [19], [20], [21] have been conducted to examine the energy absorption capability of metallic foam. Many studies [22], [23], [24], [25], [26], [27], [28], [29], [30] have been conducted to examine the dynamic responses and energy absorption capability of the sandwich type structures of metallic foams, especially under blast loading. It was revealed that the dynamic properties of metallic foams are affected by many factors, such as the mesoscopic configurations of foams (density, cores gradations, cell-wall strength, etc.), panel thickness and loading strain rate.

As is known to all, Aluminum foam is a highly complex porous material. It behaves multiscale characteristics inherently and heterogeneity originated from the randomly distributed cells [31]. At mesoscale, it is considered to be composed of numerous pores randomly distributed in the specimen. Insights into the mesoscopic responses are of high importance to the mechanical behavior of aluminum foams and to the energy absorption capability under impact loading.

With the development of computing technology, numerous approaches considering the microstructure of cellular materials have been proposed to resolve heterogeneous variations in recent years. Li et al. [32], Zhang et al. [33] and Zheng et al. [34] have chosen the Voronoi structures to model the inner geometric structure of closed-cell metallic foams. These models give significant improvement to the researches of the static and dynamic properties of metallic foams. It reveals that the development of the model that can consider the microstructure of cellular materials is critical for further investigations. However, the cell-wall in these models is usually modeled using the shell element, which has uniform cell-wall thickness. The model presented in this paper can take into account the variations in cell-wall thickness spatially.

Not like the models using the Voronoi technique, we present a different method to generate the mesoscopic model of closed-cell metallic foams. This model takes into account the randomness of pores in shape and distribution. Simulations are conducted and compared with test data. The fracture and collapse of cell walls under impact loadings are investigated. Furthermore, we perform investigations using the numerical model to study the energy absorption capability, considering the effects of the mesoscopic configurations of aluminum foams under dynamic loadings.

Section snippets

Impact loading

The impulse originated from high-velocity impact covers a wide range of strain rate, approximately from 1 s−1 to 1 × 106 s−1. In the impact events, the impact velocity is an important factor affecting the material response [10], [35]. The impulse is produced by impact and imparted to the foam-filled device. The magnitude of the impulse is apparently influenced by the velocity. The transmission process is affected by the mesoscopic configurations of the foams. The sandwich type structure, as shown

Mesoscopic model set-up

The mesoscopic structure of closed-cell aluminum foams has numerous closed cells distributed randomly in the specimen. In this section we take into account the cells in the foam, giving the algorithms for the generation of the 3D models of closed-cell aluminum foams.

The algorithms are composed of three steps. The first is the generation of the convex polyhedron, modeling the pores in the specimen. The second is to put the generated pores into the specimen, forming the model of the aluminum

Analysis approach

The hydrocode is a powerful and reliable approach and has been used extensively in modeling intense dynamic events, such as impact and blast. In a hydrocode, material models which properly describe material behavior under high strain rate are necessary. The material model is usually composed of two parts. The first part is the strength criterion that controls the yield strength according to stress invariants. The second part is the equation of state (EOS) that determines the hydro-pressure in

Impact analysis

The above validated analysis approaches and model are used to conduct a series of impact investigation. The effects of the cell-wall strength, porosity and impact velocities on energy absorption capability are studied. For insights into the mechanism of energy absorption, the failure mode of cell-wall is investigated. And the process of stress concentration and strain localization are shown to reveal the energy absorption mechanism.

Discussion

For further insights into the mechanism of aluminum foams energy absorption, mesoscopic investigations are conducted under impact. Fig. 18 gives the mesoscopic collapse and fracture of cell-wall under impact loading. The impulse originated from impact covers a wide range of strain rates, causing the cell-wall exceeding the yield stress. Collapse and fracture occur to cell-wall. It consumes large amounts of energy, kinetic energy. Fig. 18(a)–(d) show that stress concentration occurs mainly to

Conclusions

This paper is composed of three parts: (1) development of the 3D mesoscopic model used for the finite element analysis of aluminum foams, (2) presentation and validation of the analysis approach, (3) analysis of energy absorption capability of aluminum foams under impact loadings. Dynamic compaction of aluminum foams subjected to impact loading is investigated considering the influence of impact velocity and mesoscopic configurations (porosity and cell-wall strength) based on the proposed 3D

Acknowledgments

This work is supported by National Natural Science Foundations of China (51321064, 51478464), by National Key Scientific Instrument and Equipment Development Project of China (2014YQ240445) and by Program for New Century Excellent Talents in University (NCET-12-1008).

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