Matej Vesenjak

Mechanical Properties of Advanced Pore Morphology Foam Elements

Advanced pore morphology (APM) foam, consisting of sphere-like metallic foam elements, exhibits some particular mechanical properties with unique application possibilities. The article presents the results of experimental and computational testing of APM foam elements to determine their mechanical behavior under quasi-static and dynamic compressive loading conditions. Additionally, an infrared thermal imaging camera has been used during experimental testing. Evaluated mechanical properties give better insight into the behavior of single APM foam elements under different types of loading and provide a good base for further studies of the topology and morphology influence on the global behavior of composite structures, based on APM foam elements.

Behavior of composite advanced pore morphology foam

The mechanical characterization of advanced pore morphology (APM) foam, consisting of sphere-like metallic foam elements, is very limited since APM foam has been developed only recently. The purpose of this research was thus to determine the behavior of APM spheres and its composites when subjected to compressive loading. Single metallic APM spheres have been characterized with experimental testing and computational simulations, providing the basic properties and knowledge for an efficient composition of composite APM foam structures. Then, the APM foam elements were molded with epoxy matrix resulting in new composite structures. These composites have been adhered together with the epoxy resin achieving partial and syntactic morphology. The mechanical characterization of composite APM foam structures was based on experimental testing results with free and confined boundaries. The results of the performed research have shown valuable mechanical properties of the composite APM foam structures. Furthermore, they offer new possibilities for their use in general engineering applications.

Impact Behavior of Composite Hollow Sphere Structures

Porous composite materials constitute an innovative group of lightweight materials which combine high specific stiffness, good damping properties, and thermal insulation with the ability to absorb large amounts of energy at a low constant stress level. In the scope of this study, adhesively bonded metallic hollow sphere structures (MHSS) fully embedded within the adhesive matrix are considered with aim to determine their macroscopic behavior under uniaxial impact loading conditions by means of parametric computational simulations. The base material properties have been determined by quasi-static and dynamic experiments. Three topologies of syntactic hollow sphere structures of various dimensions are considered, namely the cubic primitive, the body centered cubic and the face centered cubic topology. Results of computational simulations show significant influence of topology and strain rate sensitivity on the composite structure behavior, while the influence of metallic hollow sphere wall thickness is less pronounced. Computational simulations show that it is possible to combine the MHSS topology, metallic hollow sphere wall thickness and strain rate sensitivity to achieve any desired dynamic response of MHSS adapted to a given engineering problem.

Behaviour of cellular structures with fluid fillers under impact loading

The paper describes experimental and computational testing of regular open-cell cellular structures behaviour under impact loading. Open-cell cellular specimens made of aluminium alloy and polymer were experimentally tested under quasi-static and dynamic compressive loading in order to evaluate the failure conditions and the strain rate sensitivity. Additionally, specimens with viscous fillers have been tested to determine the increase of the energy absorption due to filler effects. The tests have shown that brittle behaviour of the cellular structure due to sudden rupture of intercellular walls observed in quasi-static and dynamic tests is reduced by introduction of viscous filler, while at the same time the energy absorption is increased. The influence of fluid filler on open-cell cellular material behaviour under impact loading was further investigated with parametric computational simulations, where fully coupled interaction between the base material and the pore filler was considered. The explicit nonlinear finite element code ls-dyna was used for this purpose. Different failure criteria were evaluated to simulate the collapsing of intercellular walls and the failure mechanism of cellular structures in general. The new computational models and presented procedures enable determination of the optimal geometric and material parameters of cellular materials with viscous fillers for individual application demands. For example, the cellular structure stiffness and impact energy absorption through controlled deformation can be easily adapted to improve the efficiency of crash absorbers

From Stochastic Foam to Designed Structure: Balancing Cost and Performance of Cellular Metals

Over the past two decades, a large number of metallic foams have been developed. In recent years research on this multi-functional material class has further intensified. However, despite their unique properties only a limited number of large-scale applications have emerged. One important reason for this sluggish uptake is their high cost. Many cellular metals require expensive raw materials, complex manufacturing procedures, or a combination thereof. Some attempts have been made to decrease costs by introducing novel foams based on cheaper components and new manufacturing procedures. However, this has often yielded materials with unreliable properties that inhibit utilization of their full potential. The resulting balance between cost and performance of cellular metals is probed in this editorial, which attempts to consider cost not in absolute figures, but in relation to performance. To approach such a distinction, an alternative classification of cellular metals is suggested which centers on structural aspects and the effort of realizing them. The range thus covered extends from fully stochastic foams to cellular structures designed-to-purpose.

Dynamic behaviour of regular closed-cell porous metals – computational study

The paper describes computational modelling of regular closed-cell cellular materials behaviour when subjected to impact loading conditions. Parametric computational simulations have been carried out to evaluate influences of the relative density, strain rate, pore gas and gas type on the macroscopic dynamic behaviour of cellular materials. The behaviour of the model under uniaxial impact loading conditions and large deformations has been analysed with the LS-DYNA code, which is based on the finite element method. This study helps to clarify which effects are indeed important and would have to be considered in developing new homogenised constitutive relationships for analysing impact problems with use of general computational codes. Additionally, the detailed computational models provide an insight into behaviour of cellular material accounting for pore filler and basic constitutive relations for further development of homogenised models under impact conditions and large deformations. Furthermore, they allow for determination of most appropriate geometrical and material parameters of cellular materials in regard to individual engineering application demands.

Behaviour of Cellular Structures under Impact Loading a Computational Study

New multiphysical computational models for simulation of regular open and closed-cell cellular structures behaviour under compressive impact loading are presented. The behaviour of cellular structures with fluid fillers under uniaxial impact loading and large deformations has been analyzed with the explicit nonlinear finite element code LS-DYNA. The behaviour of closed-cell cellular structure has been evaluated with the use of the representative volume element, where the influence of residual gas inside the closed pores has been studied. Open-cell cellular structure was modelled as a whole to properly account for considered fluid flow through the cells, which significantly influences macroscopic behaviour of cellular structure. The fluid has been modelled by applying a Smoothed Particle Hydrodynamics (SPH) method. Computational simulations showed that the base material has the highest influence on the behaviour of cellular structures under impact conditions. The increase of the relative density and strain rate results in increase of the cellular structure stiffness. Parametrical numerical simulations have also confirmed that filler influences the macroscopic behaviour of the cellular structures which depends on the loading type and the size of the cellular structure. In open-cell cellular structures with higher filler viscosity and higher relative density, increased impact energy absorption has been observed.

Dynamic Behaviour of Metallic Hollow Sphere Structures

The chapter focuses on the dynamic behaviour of metallic hollow sphere structures that constitute an innovative group of lightweight materials, combining high specific stiffness, good damping properties and the ability to absorb large amounts of energy at a constant low stress level. The chapter explains the methodology and results of computational experimenting to clarify and determine the individual influences on the macroscopic behaviour of MHSS, especially under dynamic loading conditions. In the beginning, the material strain rate dependency is described and formulated with several constitutive models. A very important factor at impact loading is material deformation capability and impact energy absorption, which directly influences the deceleration of impacting objects. The impact energy absorption of hollow sphere structures due to their plastic deformation under impact loading is emphasized. The second part of this chapter presents the computational results of metallic hollow sphere structures and their macroscopic behaviour under uniaxial dynamic loading conditions with additional material characterisation considering large strains. Furthermore, the influence of gas inside the metallic spheres on behaviour of metallic hollow sphere structures and their capability of impact energy absorption is addressed. Computational simulations show that it is possible to achieve different dynamic response of metallic hollow sphere structures when subjected to dynamic loading. The topology, wall thickness of spheres and strain rate sensitivity can be combined in a way that the structure response is adapted to a given engineering problem. The chapter concludes with a discussion of the advantages, disadvantages and limitations of dynamically loaded metallic hollow sphere structures and their computational models.

Fluid Models in LS-DYNA and Their Interaction With a Structure in Dynamic Simulations

The paper outlines different approaches to fluid-structure interaction modelling in LS-DYNA. Different formulations (Lagrange, Euler, ALE and SPH) are evaluated and compared with experimental observations of a fluid sloshing problem in a simple container box. Computational simulations have shown that the motion of the fluid can be best described also with the ALE and SPH methods in LS-DYNA. Additionally, such methods are very economical and suitable for analyses of large and more complex models.Copyright

Experimental and Numerical Evaluation of the Mechanical Behavior of Strongly Anisotropic Light-Weight Metallic Fiber Structures under Static and Dynamic Compressive Loading

Rigid metallic fiber structures made from a variety of different metals and alloys have been investigated mainly with regard to their functional properties such as heat transfer, pressure drop, or filtration characteristics. With the recent advent of aluminum and magnesium-based fiber structures, the application of such structures in light-weight crash absorbers has become conceivable. The present paper therefore elucidates the mechanical behavior of rigid sintered fiber structures under quasi-static and dynamic loading. Special attention is paid to the strongly anisotropic properties observed for different directions of loading in relation to the main fiber orientation. Basically, the structures show an orthotropic behavior; however, a finite thickness of the fiber slabs results in moderate deviations from a purely orthotropic behavior. The morphology of the tested specimens is examined by computed tomography, and experimental results for different directions of loading as well as different relative densities are presented. Numerical calculations were carried out using real structural data derived from the computed tomography data. Depending on the direction of loading, the fiber structures show a distinctively different deformation behavior both experimentally and numerically. Based on these results, the prevalent modes of deformation are discussed and a first comparison with an established polymer foam and an assessment of the applicability of aluminum fiber structures in crash protection devices is attempted.