Investigation of the Mechanical Behavior of Open‐Cell Ni Foams by Experimental and FEM Procedures

Abstract

Cellular materials such as wood, bone and other living tissues are frequently encountered in nature. Metal foams are a relatively new class of recyclable materials and various new methods and manufacturing technologies developed recently for their production. Metal foams are increasingly used in many engineering applications due to their low weight in combination with their increased heat transfer potential and mechanical strength. The mechanical behavior of cellular structures is governed by their internal architecture. Given the wide range of potential applications, in the automotive, building and other industries, the study of the mechanicalbehavior of metal foams is considered imperative. Several analytical and numerical approaches can be found in the literature. Analytical models based on beam theory have been derived by Gibson and Ashby giving the effective mechanical properties as functions of the structure’s relative density. Analytical methods and Finite Element simulations based on beam elements are used for analyzing the effective stiffness of open cell metallic foams with tetrakaidecahedral unit cells. In Gong and Kyriakides, and Gong et al., the linear and nonlinear compressive response of polyester urethane open cell foams is modeled using idealized periodic structures consisting of tetrakaidecahedral cells. Analytical and numerical analyses of the effective stress–strain behavior of a two-dimensional foam model in the nonlinear elastic regime are presented by Hohe and Becker. Various three-dimensional open cell structures are studied in Luxner et al., [13] and Luxner et al., using a periodic microfield approach. X-ray tomography has recently proved to be a very powerful tool allowing the characterization of the microstructure of materials or the architecture of cellular materials. X-ray tomography allows large deformations of cellular solids to be imaged in a non destructive way: thus the important buckling, bending or fracture events appearing during the deformation can be visualized. The majority of theoretical simulations which relate the structure of cellular materials with their properties refer to periodic models, while most cellular solids are random materials. There are cases where the theoretical results for such models are in good agreement with the properties of real materials. However, as most real cellular solids are not periodic, there is a need to develop approaches taking into consideration this aspect. In this paper, the elastoplastic behavior of a Nickel opencell foam is investigated under tension loads. A scanning electron microscope equipped with an apparatus of microtension was used. Ex-situ and in-situ micro-tension tests were conducted. The latter allows determination of the mechanical properties through real-time observations of the local deformations of the struts, while correlating them with the imposed. High resolution X-ray tomographies, carried out to capture the exact 3D geometry of the foams were used in turn to simulate mechanical behavior of the foam with the aid of a computational procedure based on Finite Element Modeling (FEM). The porous microstructure of metallic foams causes microscopic stress localization during loading, which reduces the overall strength and therefore limits the application of such materials. The results of the present study also allow the visualization and determination of the local stresses developed in the metal foam.