Metallic Hollow Sphere Structures Research Articles

Energy-absorption behavior of metallic hollow sphere structures under impact loading

An integrated experimental and computation study is presented of the energy-absorption behavior of metallic hollow sphere structures (MHSS) as an innovative anti-collision material to protect bridge piers from vehicle impact loading. Quasi-static compression and dynamic split Hopkinson pressure bar impact testing were first conducted to compare different deformation and energy-absorption characteristics of MHSS subjected to different loading modes, followed by more realistic laboratory downscaled model impact testing and finite element modeling (FEM) of collisions between real-scale bridge piers and vehicle to further examine the energy-absorption capability of MHSS under dynamic loading. Results show that the MHSS exhibits an increased structural stiffness (e.g., by ~20 times), yield strength (by 4–5 times), and toughness (by 200%) with increasing loading rate (i.e., from static to impact loading). When used as a protective layer on bridge piers, the downscaled model tests and FEM simulations validated the MHSS can provide remarkable anti-collision function to concrete structures: MHSS can substantially reduce the horizontal accelerations (by 63–68%), displacements (by 56.6%), and surface strains of bridge piers, and there reductions further increases with impact velocity. The high energy-absorption capability of MHSS may originates from the exceptionally high, multiscale porosity as well as the excellent mechanical properties of the MHSS material. The findings suggest that MHSS can be an effective energy-absorptive material for anti-collision protection of bridge structures.

Bending performance evaluation of aluminium alloy tubes filled with different cellular metal cores

A comprehensive bending performance and energy absorption capability of aluminium alloy tubes filled with different cost-effective cellular metal cores were experimentally evaluated for the first time. The following cellular metal cores were evaluated: i) Advanced Pore Morphology (APM) foam, ii) hybrid APM foam and iii) Metallic Hollow Sphere Structures (MHSS). The results have been compared also with the performance of aluminium alloy tubes filled with (ex-situ and in-situ) closed-cell aluminium alloy foam. The three-point bending tests have been performed at two loading rates (quasi-static and dynamic) and supported by infrared thermography to evaluate the deformation mechanism, damage progress and failure modes. A thorough heat treatment sensitivity (due to the fabrication procedures of composite structures) study on the aluminium tubes has been performed as well. The results show that a reliable and predictable mechanical behaviour and failure can be achieved with proper combination of tubes and cellular metal core. A low scatter of bending properties and energy absorption capability has been observed. The hybrid APM and the ex-situ foam filled tubes achieved the highest peak load. However, they also exhibit a rapid load drop and abrupt failure once the structure has reached the peak load. The APM, MHSS and in-situ foam filled tubes show more ductile behaviour with a predictable failure mode.

Estimating the base material properties of sintered metallic hollow spheres by inverse engineering procedure

The base material properties of single sintered metallic hollow spheres cannot be determined directly because of their small size, with the outer diameter ranging from 1 to 15mm. The paper proposes an inverse engineering procedure for their determination by applying the computational optimisation based on an extensive experimental testing data. Initially, the compressive response of single iron hollow spheres of selected sizes of 2 and 3mm in diameter has been determined by quasi-static and dynamic experimental testing. The exact geometrical data of the analyzed spheres was obtained by μCT scanning and then used for creation of the corresponding FE models. These were subjected to the same loading conditions in computational simulations as those used in experimental testing. The non-gradient optimization process based on genetic algorithm was applied to define appropriate base material properties of analyzed spheres, which returned the same response of computational models as the one recorded in experiments. This way the base material properties of sintered iron hollow spheres were obtained for the first time, which can be used for computational studies of the metallic hollow sphere structures behavior.

On the isotropic and anisotropic thermal conductivity of metallic hollow sphere structures

Metallic hollow sphere structures (MHSS) combine the well-known advantages of cellular metals with the excellent material properties of solid base metals. The basic cell is a hollow sphere. Metallic hollow sphere structures can be assembled by sintering, soldering, or adhesive bonding. The macroscopic thermal property and, more specifically, the thermal conductivity is an essential material property. The objective of this work is to address the thermal conductivity of MHSS in dependence of the geometrical parameters of a single sphere and the ordering scheme of the assembled structure with a special focus on isotropy and anisotropy. From a macroscopic point of view, which is often used within the engineering design process, the characterization of the properties of MHSS demand an approximative description coming out of a homogenization. Within this paper the unit cell approach is used. The numerical analysis is done on the basis of a parameterized 3D-CAD model enabling the variation of the sphere size and wall thickness as well as the geometry of the interconnection between spheres. Periodic boundary conditions are applied. Results for the macroscopic thermal conductivity are presented for regular-packed (CP, BCC, FCC, and HCP) as well as randomly packed unit cells representing sintered or soldered MHSS. The anisotropy is discussed by the 3 × 3 thermal conductivity tensor.

Influence of Interphase Imperfections on the Macroscopic Stiffness of Hollow-Sphere Reinforced Composites

Metallic hollow-sphere structures are a new type of advanced materials with promising mechanical properties for multifunctional applications. A simple computational model to consider damage in the sphere-matrix interphase is presented. Two types of damage distributions, i.e., localized and randomized, are investigated. The procedure is applied to steel hollow spheres embedded in an epoxy matrix. It is shown that the interface damage lowers the mechanical properties and localized damage is more critical than randomized at higher damage values.

Laser drilling of hollow sphere structures

A special composite and cellular material – the metallic hollow sphere structure (MHSS) – represents an advanced material. The core element of the structure is a hollow sphere with a wall out of porous steel. This shell allows a high geometric reproducibility. It is an interesting base material for lightweight design. However, processing technologies for further manufacturing of metallic hollow sphere structure are necessary. Laser beam drilling is a highly efficient technique with heat input concentrated locally at the structure. Laser drilling is applied to metallic hollow sphere structures. The influence of different geometries and joining technologies of the spheres on the drilling behaviour is investigated.Percussion drilling is used to form holes with a CO2‐laser into the metallic hollow sphere structure. The investigated mean laser power is 400 W. Two different focal lenses with focal lengths of 5″ and 7.5″ allow investigations with different focal diameters and Rayleigh lengths. Diameter and roundness of the laser drilled holes are measured in five layers of different drill depths z. The maximum drilled depth is about 40 mm with a 5″ lense and about 50 mm with the f = 7.5″ lens. The relative roundness is determined and decreases with increasing drill depth. The diameter d(z) as a function of the drill depth follows the shape of an isophote, a line of constant intensity inside the laser beam. The brazed hollow sphere structures have considerably less drill depth than the sintered.

Functionalized Metallic Hollow Sphere Structures

Metal hollow spheres (MHS) and metal hollow sphere structures are special types of cellular metals with an enormous application potential in structural and functional applications. A lot of effort has been made during the last two decades in order to develop the manufacturing process and to investigate the properties of this type of structure. Additional functionalizing of MHS is possible by filling the spheres with ceramic powders or phase change materials. Furthermore, by coating the spheres with different ceramic layers new properties and functions are added. Thus, the structures show excellent mechanical damping properties as well as high heat capacity for thermal storage and fast heat loading and unloading. Grinding or ceramic coating bring more functions to the surface of the spheres. The manufacture and properties of different types of functionalized MHS and their possible applications are highlighted here.

On the numerical simulation of laser beam cutting of hollow sphere structures

AbstractLaser beam cutting is a highly efficient technique to cut materials. The relatively small amount of energy being involved during the process results in a small heat affected zone. This characteristic makes laser beam cutting interesting for composite materials. Within this paper the cutting process is applied to metallic hollow sphere structures (MHSS), a relatively new group of advanced composite materials. The metallic hollow sphere structures combine the well‐known advantages of cellular metals without major scattering of their material parameters. They are characterised by high geometry reproduction leading to relatively constant mechanical and physical properties. Numerical simulation is used in order to define proper process parameters for a large variety of metallic hollow sphere structures. The finite element method based simulation covers material parameters as well as process parameters like the cutting velocity. Heat conduction, convection and heat radiation is taken into account. The temperature distribution is our fundamental result. It should be mentioned that phase change from solid to liquid state is also included in the computational procedure.

Heat Conductivity of Metallic Hollow Sphere Structures: Numerical Simulation and Experimental Validation

Heat conductivity is a well-known energy transfer method and it is here applied to the study of metal foams and laser processing. Metallic hollow sphere structures (MHSS), a relatively new group of advanced composite materials, combine the advantages of cellular metals without major scattering of their material parameters. They are characterised by high geometry reproduction leading to relatively constant mechanical and physical properties.The laser processing technology provides not only a laser cutting but also a laser soldering procedure. Within this work a laser cutting process is applied to MHSS. Laser beam cutting is a highly efficient technique to cut materials, because the relatively small amount of heat affects only a small heating zone.Numerical simulation is used in order to define proper process parameters for a large variety of MHSS. The finite element method based simulation covers material parameters as well as process parameters like the cutting velocity. Heat conduction and convection are taken into account and the phase change from solid to liquid state as well. Within the simulations the concept of representative volume element (RVE) is applied. The temperature distribution is the fundamental result.

Experimental analysis on the thermal anisotropy of syntactic hollow sphere structures

This research addresses the measurement of the effective thermal conductivity of metallic hollow sphere structures. These structures are fabricated by embedding sintered hollow steel spheres in a polymer matrix. The resulting syntactic foam combines attractive properties such as high specific strength, structural damping and controlled energy absorption with versatile thermal properties. In the present paper, the thermal properties of this material are determined. A novel experimental setup is used that allows the measurement of the directional conductivities in order to investigate thermal anisotropy. The experiment is based on the accurate time-dependent temperature control of a heater plate inside a thermally controlled environment. The effective thermal conductivities are obtained by fitting finite element data to measured transient temperature profiles. By interchanging the alignment of the specimen, thermal anisotropy can be analysed. In addition, the heat capacity of syntactic hollow sphere structures is estimated based on computed tomography data. Results indicate a slight anisotropy of the thermal conductivity that is most likely caused by deformation of the hollow spheres during manufacturing.

Plasticity and damage analysis of metal hollow sphere structures under dynamic compressive loading

PurposeAs compared with homogeneous metals and alloys, cellular metals provide low density, high specific stiffness, high energy absorption and good damping, thus being interesting alternatives to employ as protection against shock and impact. Impact energy is dissipated through cell bending, buckling or fracture. The knowledge and computational modelling of the mechanical behaviour of metal foams structures is thus of great importance for real life applications. The purpose of this paper is to increase the knowledge of the differences in metallic hollow sphere structures' (MHSS) behaviour under dynamic loading, as compared with the corresponding behaviour under static loading and to determine the influence of inertia and loading rate.Design/methodology/approachComputational dynamical finite element analyses of representative volume elements (RVE) of MHSS have been performed considering varying loading rates. Partially bonded geometries are considered and the effect of the spheres' distribution is also taken into account.FindingsThe results of the numerical examples presented show that inertia plays an important role in the dynamic behaviour of this kind of energy‐absorbing structure. When compared with the corresponding values in the quasi‐static case, the effect of inertia makes the peak load higher. If the deformation rate is higher (greater than 1.39 m/s in the studied cases), the characteristic plateau usually present in compressed metal foams can vanish. For the geometries analysed, damage has a small influence on load‐deformation relations.Originality/valueThis paper presents and discusses differences between static and dynamic behaviour of partially bonded MHSS. There are few references in the literature covering this issue by means of numerical analysis.

Laser beam cutting of metallic hollow sphere structures

AbstractMetallic hollow sphere structures (MHSS) represent a relatively new group of advanced composite materials characterised by high geometry reproduction leading to relatively constant mechanical and physical properties. The MHSS combine the well‐known advantages of cellular metals without major scattering of their material parameters. Various joining technologies such as sintering, soldering and adhering can be used to assemble single metallic hollow spheres to interdependent structures and allow adjusting different macroscopic properties. A machining process for MHSS has to reflect the special characteristic of the composite material. Classical mechanical cutting techniques like sawing or milling often fail. Spheres or large pieces are separated from the bulk structure and cause a rough surface at the cutting zone. Other cutting techniques like water jet cutting cannot be used for oxidising material. This paper presents laser beam cutting as a proper technique to cut composite materials like MHSS. The quality of the cutting zone strongly depends on the process parameter e. g. focal length, tool velocity and the structural characteristics of MHSS, e. g. density, sphere diameter and density. The experimental work was done using a CO2‐laser with a power of 1.5 kW. All investigations were done with sintered hollow sphere structures having different macroscopic properties. The maximum depth of a cut kerf was determined for four configurations of MHSS as function of laser velocities, gas pressure of the process gas, focal length of the lense. The depth was evaluated using a power balance of the laser input power on one side and the power needed to warm and melt the MHSS on the other side. A comparison between simulation and experiment of the width and the depth of the cut kerf shows, that the laser beam gets multiple reflections on the surfaces of the hollow spheres, enlarging the width of the cut kerf. The functional dependency between laser velocity and cut kerf depth can be estimated well. Theoretical and experimental results fit well together.

Structure borne sound in metallic hollow sphere structures

AbstractMetallic hollow sphere structures (MHSS) form a relatively new group of advanced composite materials characterised by high geometry reproduction leading to stable mechanical and physical properties. The MHSS combine the well‐known advantages of cellular metals in terms of their high ability for energy adsorption, good damping behaviour, excellent heat insulation and high specific stiffness without major scattering of their material parameters. A combination of these properties opens a wide field of potential applications, e. g. in automotive (crash absorber) and aerospace industry (sandwich panels). Various joining technologies such as sintering, soldering and adhering can be used to assemble single metallic hollow spheres to interdependent structures and allow different macroscopic properties. In this study the behaviour of Metallic Hollow Spheres Structures due to structure‐borne sound is investigated. Longitudinal vibrations within a rod serve as an example. Experimental data are compared with analytical solutions and results computed by the finite element method.

Plasticity and damage analysis of metal foams under dynamic loading

AbstractMetallic foams provide low density, high specific stiffness, high energy absorption and good damping and are thus interesting alternatives for structural applications. The impact energy is dissipated through cell bending, buckling or fracture. On the other hand, results of the strain‐rate and inertia effects during dynamic deformation of cellular metals are apparently conflicting. A deeper understanding is found in studies that show that the relative importance of the strain‐rate and inertia effects depends on the impact velocity and the geometry of the structure, but still no particular formulation is generally accepted. In the present paper, computational dynamical analyses of Representative Volume Elements (RVE) of Metallic Hollow Sphere Structures (MHSS) and RVE′s sets are performed considering various geometries, material properties and loading rates.

A lattice Monte Carlo analysis on thermal diffusion in syntactic hollow‐sphere structures

AbstractThe Lattice Monte Carlo method [1] is a computationally intensive approach towards the study of thermal material properties. The Lattice Monte Carlo method has previously been used in the area of mass diffusion, and has more recently been applied towards the analysis of thermal diffusion in materials. Whilst previous studies have focused on two‐phase material consisting of component material and pore geometry only, this study applies the Lattice Monte Carlo method towards the investigation of three‐phase Metallic Hollow Sphere Structures (MHSS). MHSS consist of high‐conducting metallic shells, low‐conducting adhesive and non‐conducting pores. In the present study, three‐dimensional computerised models of the MHSS are built up from analysis of computed tomography (CT) scans. The results of this numerical analysis indicate a spatial variation of the thermal conductivity. However, MHSS can be considered as being isotropic to a good approximation.Die ‘Lattice Monte Carlo’ Methode [1] ist ein rechenintensiver Ansatz zur Bestimmung thermischer Materialeigenschaften. Die ‘Lattice Monte Carlo’ Methode hat sich für die Simulation der Diffusion in Festkörpern etabliert und wird seit kurzem auch für die Analyse thermischer Diffusion in Werkstoffen eingesetzt. Vorhergehende Studien konzentrierten sich auf Zwei‐Phasen‐Materialien bestehend aus Grundmaterial und Pooren. Die jetzige Analyse nutzt die ‘Lattice Monte Carlo’ Methode für die Untersuchung drei‐phasiger metallischer Hohlkugelstrukturen (abgekürzt aus dem Englischen ‘Metallic Hollow Sphere Structures’ als MHSS). MHSS bestehen aus hoch‐leitfähigen metallischen Schalen, thermisch‐isolierenden Kunststoff und nicht‐leitfähigen Poren. Drei‐dimensionale Rechenmodelle wurden in der vorliegenden Studie von computer‐tomographischen Daten abgeleitet. Die Ergebnisse der numerischen Analyse deuten eine räumliche Streuung der thermsichen Leitfähigkeit an. Die lokalen thermischen Leitfähigkeiten sind jedoch isotrop.

Large deformation of core materials for sandwich panels

AbstractSandwich panels represent a basic design unit for light weight structures. Sandwich panels typically consist of at least two different materials for core and skin. Within the scope of this paper, the large deformation behaviour of different cellular core materials for sandwich panels is investigated. The large deformation behaviour for different core materials is analysed experimentally by compression tests. We investigate metallic hollow sphere structures (MHSS) and synthetic polyvinyl chloride (PVC) foams. In addition, appropriate computational models are presented to account for the deformation mechanisms and to compare computational to experimental results. With these models we will be able to already validate the future experimental test set‐ups with numerical simulations before the test set‐up is build.Sandwichelemente gelten als Grundstruktur für Leichtbau‐Konstruktionen. Sie bestehen normalerweise aus mindestens zwei verschiedenen Materialien für den Kern und die Deckschichten. Dieser Beitrag befasst sich mit dem Verhalten bei großer Verformung von verschiedenen Kernmaterialien. Dieses Verhalten wird mit einem Druckversuch experimentell für zwei solcher Kernmaterialien untersucht. Die beiden Materialien sind zum einen metallische Hohlkugelstrukturen (MHKS) und zum anderen ein Polyvinylchlorid‐Schaum. Zusätzlich werden dazu passende Simulationsmodelle erstellt, um die experimentell gewonnen Ergebnisse mit solchen aus der FE‐Analyse zu vergleichen. Mit diesen Modellen sind wir in der Lage, zukünftige Test‐Aufbauten bereits vor dem Bau mit Hilfe von numerischer Simulation zu überprüfen.

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.

Experimental and numerical analyses of diffusion bonded hollow sphere structures

AbstractThe reduction of N&V (noise and vibration) is a progressively more important objective for designers of various engineering applications. When a series of physical and mechanical requirements are touched, classic materials reach their limits. Cellular metals with their great diversity of properties open up new vistas. Metallic hollow sphere structures (MHSS) feature a new group of advanced composite materials characterised by high geometrical reproducibility leading to stable properties. In the scope of this study, vibrational analysis of sintered metallic hollow sphere structures is performed. The objective is a description of the dynamic response by numerical simulation. A detailed finite element model would exceed available resources. Therefore, a proper model with special descriptions for hollow spheres and interconnections has been developed and is presented in detail. Numerical results are compared with vibrations measured by laser scanning vibrometer and analytical analysis.

Hollow sphere structures: a study of mechanical behaviour using numerical simulation

AbstractOne way to obtain a cellular metal with a uniform microstructure is to employ hollow metallic spheres welded or bonded. Such materials are known as MHSS (Metallic Hollow Sphere Structures). Cellular metals can be applied as crash absorbers that act in compression and can be severely deformed. Thus, finite deformations and damage must be considered in the numerical simulation. In this work, the numerical simulation via finite elements is employed to study the mechanical behaviour of MHSS. The study employs the concept of RVE (Representative Volume Elements) where a cell is used to represent the whole material behaviour. Numerical results are compared with experimental data.

Metallic Hollow Sphere Structures Manufacturing Process

AbstractThis paper focuses on manufacturing process of regular Metallic Hollow Sphere Structures (MHSS) through brazing technique. As a large stress level is generally confined into the necks formed by brazed spheres, the influence of the filler material on mechanical behavior of cellular metal has been studied. The microstructures of joints resulting from nickel hollow spheres brazing with different commercial fillers “MBF 30” and “MBF 1006” were compared by Scanning Electron Microscopy (SEM) and microhardness testing. These studies revealed a wide boron diffusion into nickel shells through grain boundaries for “MBF 30” brazing, with the formation of borides in a fine brittle eutectic structure. Conversely it was observed that the eutectic structure concentrates at the necks for “MBF 1006” and can be completely eliminated by diffusion-brazing, despite of the shells thinness. The uniaxial compressive tests of HSP specimens have shown two different strain mechanisms depending on brazing process.