Zoran Ren

• Dipl. Eng. Mech. Eng., M.Sc., Ph.D.
• Professor (Full) at University of Maribor

Strain rate hardening caused by the changed deformation mode is a fascinating phenomenon in cellular metamaterials where the material’s stiffness and energy absorption capabilities increase as the strain rate increases. This unique behaviour is attributed to a combination of micro-inertia effects, base material’s strain rate hardening and inertia e...

This study investigates how varying cell size affects the mechanical behaviour of photopolymer Triply Periodic Minimal Surfaces (TPMS) under different deformation rates. Diamond, Gyroid, and Primitive TPMS structures with spatially graded cell sizes were tested. Quasi-static experiments measured boundary forces, representing material behaviour, ine...

This paper proposes an innovative multi-material approach for introducing auxetic behaviour to syntactic foams (SFs). By carefully designing the size, shape, and orientation of the SFs, auxetic deformation behaviour was induced. Re-entrant hexagon-shaped SF elements were fabricated using expanded perlite (EP) particles and a plaster of Paris slurry...

The script entitled „Introduction to the computer simulations“ in the field of Engineering Computer Simulations is intended as a study aid in the lectures of the courses Engineering Computer Simulations for foreign students at the University of Maribor and for students at Kumamoto University, Japan. It explains all the material that students must m...

For today's requirements, the material itself is often not sufficient anymore. This leads to structural cellular materials and metamaterials, which allow for more degrees of freedom by a strong structure–property relationship. Auxetic structures investigated in this contribution belong to metamaterials. Auxetic metamaterials show huge advantages in...

This study investigated the high-strain rate mechanical properties of open-cell aluminium foam M-pore®. While previous research has examined the response of this type of foam under quasi-static and transitional dynamic loading conditions, there is a lack of knowledge about its behaviour under higher strain rates (transitional and shock loading regi...

Cellular structures (in general) and auxetic topologies (in particular) have excellent energy-dissipation characteristics and can be used as lightweight impact-energy absorbers. This paper aims to experimentally and computationally examine the behaviour of novel re-entrant auxetic graded aluminium panels filled with polyurethane foam in an auxetic...

A recently introduced novel three‐dimensional (3D) auxetic axisymmetric chiral structure (ACS) with a periodic structure exhibits nonuniform stress distribution, with stresses concentrated primarily on the inner part of the structure. The structural optimisation of one unit cell of the validated computational model was conducted to determine the op...

One of the most promising options for future crashworthiness applications is thin-walled tubes filled with various cellular materials (e.g. metal foam). Of higher interest are the shell-based lattices, which have lately gained popularity due to their superior qualities over strut-based lattices. In this work, we investigate the mechanical response...

The impact and blast resistance of sandwich panels with uniform and functionally graded Triply Periodical Minimal Surface (TPMS) cellular cores were analysed in this study. The computational models were validated with available experimental compression tests of four different types of TPMS cellular structures (Diamond, Gyroid, IWP and Primitive). T...

Foam-filled tubes represent one of the best energy-absorbing components for future crashworthiness applications. Triply Periodic Minimal Surface (TPMS)-filled tubes were investigated under quasi-static and dynamic three-point bending loading conditions. The TPMS-filled tubes enhanced specific energy absorption up to 46% compared to the sum of empty...

Novel three-dimensional (3D) axisymmetric chiral structures with negative and zero Poisson's ratios are presented based on the existing 3D conventional chiral unit cell. The conventional tetra-chiral unit cell is mapped to the axisymmetric space to form the new 3D axisymmetric chiral structure. Two different structure designs are characterised depe...

A conventional compound fabric was used to develop a modern, multifunctional material with an auxetic behaviour and a tailored open area for particle filtration. Such material was produced using traditional textile technology and laser cutting, to induce a rotating squares unit geometry. The behaviour was investigated of three different rotating un...

Cellular metals exhibit diverse properties, depending on their geometries and base materials. This study investigated the mechanism of high-pressure generation during the high-velocity impact of unidirectional cellular (UniPore) materials. Cubic UniPore copper samples were mounted on a projectile and subjected to impact loading using a powder gun t...

Auxetic structures can be used as protective sacrificial solutions for impact protection with lightweight and excellent energy-dissipation characteristics. A recently published and patented shock-absorbing system, namely, Uniaxial Graded Auxetic Damper (UGAD), proved its efficiency through comprehensive analytical and computational analyses. Howeve...

Modern metamaterials are among the most promising solutions for future applications in engineering, transportation, medicine, sports and many other fields of application. An overview of the development, design, fabrication, computational modelling and characterisation of metamaterials with primitive and advanced cellular structures are described an...

Purpose: Recent advances in professional software and computer hardware allow for reliable computational analyses of new engineering materials as well as biological tissues. Therefore, the purpose of this paper is to describe the procedures allowing detailed reconstruction and virtual testing of such materials. Methods: This paper describes the pro...

Uniform lattices composed of one type of lattice structure repeated periodically have been extensively investigated in literature for their mechanical and physical properties. Their promising properties, which include a desirable combination of high strength, stiffness and toughness, suggest that hybrid structures made of two or more lattice types...

The tensile response of chiral auxetic cellular structures was experimentally and computationally studied. The auxetic cellular structures with two different chiral geometries (tetrachiral and hexachiral) were fabricated using laser powder bed fusion (LPBF) technology from two different materials (aluminium alloy AlSi10Mg and stainless steel AISI 3...

The quasi-static and dynamic compressive behaviour of Triply Periodical Minimal Surface (TPMS) sheet-based cellular structures were evaluated in this research. TPMS cellular structures are novel sheet-based metamaterials, which can offer enhancement in mechanical and other engineering properties compared to strut-based cellular structures. Four dif...

This work analyses shear moduli of conventional and auxetic open-cell polymer foams. Shear moduli are i) measured directly and ii) calculated by applying elasticity theory for isotropic solid materials, using Young's moduli and Poisson's ratios from compression tests. Zero and negative Poisson's ratio foams are fabricated from conventional foams us...

This chapter discusses the experimental and computational analysis of the fabrication process of unidirectional porous (UniPore) structures, a type of cellar material, by using industrial pure copper and industrial pure aluminum as base materials. One outer pipe and many paraffin-filled inner pipes were compressed and then welded together to fabric...

Cellular materials are one of the most promising materials for lightweight, efficient, and superior future design of engineering structures, where they can be used as stand-alone material or in combination with other materials as composites. For that purpose, there is a necessity to evaluate the mechanical response of these materials under differen...

The mechanical behaviour of three different auxetic cellular structures, hexa-chiral 2D, tetra-chiral 2D and tetra-chiral 3D, was experimentally investigated in this study. The structures were produced with the powder bed fusion method (PBF) from an austenitic stainless steel alloy. The fundamental material mechanical properties of the sample struc...

The fatigue life of a high-pressure die-cast alloy, AlSi9Cu3, with significant porosity is investigated. Standard specimens were cyclically loaded (R= –1) until fatigue rupture occurred. The specimens were μ-CT scanned to obtain a 3D representation of the pores. Models were used to computationally predict the fatigue life using two different method...

The collection contains contributions by authors who were presented at a workshop within the Erasmus + project MARDS in Maribor, at the University of Maribor.

This study is concerned with the development of a new unidirectional cellular (UniPore) copper structure with multiple concentric pipe layers. The investigated UniPore structures were grouped into three main types, each having a different number of pipes (3, 4, and 5 pipes per transversal cross-section) and different pore arrangements. The specimen...

The advanced composites – laminates, composed of auxetic and non-auxetic layers, were developed and subjected to tensile loading tests to characterize their deformation behaviour. The two-layer laminates were composed of the polymer foam with auxetic cellular structure and the conventional knitted fabric bonded together by the rubber-based adhesive...

Light-weight cellular solids, such as aluminium foams, are promising materials for use in ballistic impact mitigation applications for their high specific deformation energy absorption capabilities. In this study, three different types of aluminium alloy based in-house fabricated cellular materials were subjected to dynamic penetration using the in...

The behaviour of the auxetic structure under external load was regarded as the behaviour of the compliant (flexible) mechanism. The multi-objective topological optimization, based on genetic algorithms and the finite element method, was used to find the optimal shape of such two-dimensional compliant mechanism in this study. The optimization was pe...

The paper focuses on the fabrication of novel aluminium cellular structures and their metallographic and mechanical characterisation. The aluminium UniPore specimens have been manufactured by rolling a thin aluminium foil with acrylic spacers for the first time. The novel approach allows for the cheaper and faster fabrication of the UniPore specime...

Graded chiral auxetic cellular metal structures were produced from copper alloy powder using Selective Electron Beam Melting (SEBM) technique and tested under compressive and shear loading conditions. The predesigned geometry of chiral structures has a variable chiral amplitude through the length of the specimens, which results in graded porosity o...

The study presents the results of an experimental and computational study of the high-velocity impact of low-density aluminum foam into a rigid wall. It is shown that the aluminum foam samples deformed before hitting the rigid wall because of the high inertial forces during the acceleration. During the impact, the samples deformed only in the regio...

The EU has launched EuroHPC Joint Undertaking initiative plan to build an exascale HPC by 2025. A petascale HPC will be built in Slovenia in a concerted effort by 2020. The aim is to establish a national HPC system by own design with low maintenance and power consumption costs of the system. The HPC architecture will be unique, built from the off-t...

The quasi-static and high strain rate response of chiral auxetic cellular structures were evaluated in this work. Samples of the chiral auxetic structures were fabricated with the Selective Electron Beam Melting (SEBM) technique from copper powder. Uniaxial quasi-static and low-speed dynamic compression tests were performed on a universal testing m...

Hybrid metamaterial with auxetic cellular structure and silicon filler was manufactured and mechanically tested for the first time with aim to study its enhanced mechanical properties. The uniform and graded base specimens with auxetic chiral cellular structure were fabricated from copper alloy using the Selective Electron Beam Melting (SEBM) metho...

Development of a small and highly efficient heat exchanger is an important issue for energy saving. In this study, the fabrication method of unidirectional (UniPore) composite cellular structure with long and uniform unidirectional cells was investigated to be applied as a heat exchanger. The composite UniPore structure was achieved by the unique f...

Closed-cell aluminium foams were fabricated and characterised at different strain rates. Quasi-static and high strain rate experimental compression testing was performed using a universal servo-hydraulic testing machine and powder gun. The experimental results show a large influence of strain rate hardening on mechanical properties, which contribut...

The paper describes determination of the material parameters of the Johnson-Cook constitutive model of steel S235 JR sample material by applying the inverse computational methodology using the digital twin model of the SHPB. A quasi-static tensile testing of bulk material was conducted first to determine the base material parameters. This was follow...

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 Struc...

This study analysed the dynamic mechanical properties and unique phenomena of the cubic unidirectional porous (UniPore) copper sample at high-velocity impact. The cubic UniPore copper sample was mounted on a projectile and subjected to impact loading by using a powder gun. The deformation process of the specimens, which were oriented perpendicular...

This paper reports the auxetic behavior of modified conventional non-woven fabric. The auxetic behavior of fabric was achieved by forming rotating square unit geometry with a highly ordered pattern of slits by laser cutting. Two commercial needle-punched non-woven fabric used as lining and the reinforcement fabric for the footwear industry were inv...

The chiral auxetic cellular structures are fabricated using SEBM (selective electron beam melting) method and tested under compressive loading. The results of experimental testing are used for validation of the computational model in LS‐DYNA. Furthermore, ballistic velocity and deformation behaviour of monolithic aluminium (Al 7075‐T651) and titani...

The computational and experimental investigation of the fatigue behaviour of AlSi7 aluminium foam is presented in this study. The internal structure of a highly porous specimen was recorded using CT images to enable the subsequent detailed numerical modelling using the finite element method. A new approach for determination of the fatigue life of f...

The mechanical properties of Advanced Pore Morphology (APM) foam elements depend strongly upon their internal porous and external structural geometry. This paper reports on a detailed investigation of external (e.g. shape and size) and internal (e.g. distribution, size, number of pores) geometry and porosity changes of APM foam elements, during com...

The compressive behavior of porous A6061 alloy with aligned unidirectional pores was investigated. Porous specimens with various sizes and relative thicknesses (thickness t/length l) of cell walls were prepared via machining after various heat treatments. Compression tests were conducted on porous specimens in the direction perpendicular to the por...

Chiral auxetic cellular structures were fabricated from Ti6Al4V alloy using the Selective Electron Beam Melting method, and tested experimentally under quasi-static and dynamic compression loading conditions. The experimental results were used to validate built computational models of auxetic cellular structure in LS-DYNA. The models were used to s...

In this study, the crushing behavior of auxetic structures built from inverted tetrapods with linearly graded porosity under impact is analyzed both experimentally and computationally. The auxetic samples with linearly graded porosity are fabricated from Ti‐6Al‐4V alloy powder by the selective electron‐beam melting method. A detailed geometrical an...

Adhesive bonds have very strong influence on mechanical properties of composite particulate metal foams. This study experimentally investigates for the first time the geometrical and mechanical properties of PA 12 adhesive bonds between spherical advanced pore morphology (APM) elements made of AlSi10 foam. A new experimentation setup for mechanical...

Auxetic cellular structures build from inverted tetrapods were experimentally tested at high strain rate compression loading for the first time. The strain rates up to 10,000 s-1 were achieved with gas powder gun, where the shock deformation mode is predominant. The deformation localizes in the deformation front between the impacting specimen and t...

The paper is concerned with mechanical behaviour characterisation of auxetic cellular structures built from inverted tetrapods. The auxetic cellular structures were fabricated from the Ti-6Al-4V alloy powder using the selective electron-beam melting method (SEBM). The fabricated specimens were experimentally subjected to different loading condition...

This paper proposes the new methodology for geometrical properties identification of step‐wise deformed closed‐cell aluminum alloy foam. The change of internal structure of cylindrical foam specimens during deformation is ex situ recorded by a micro computed tomography scanner. The geometry of five specimens is analyzed in un‐deformed and several d...

The explosive compaction fabrication process of cylindrical uni-directional porous copper, consisting of an outer pipe completely filled with smaller inner pipes, was investigated by experimental and computational simulations to analyze and optimize the fabrication process conditions. The computational simulations were carried out using a two-dimen...

Multifunctional hybrid foams were developed and tested by combining aluminium alloy open-cell (OC) foam specimens with polymers, epoxy resin and silicone rubber. The rectangular OC foam specimens were impregnated with polymer, completely filling the voids. The aim of this work was to evaluate the effect of the polymer presence in the voids of alumi...

This paper addresses the problem of reconstructing realistic, irregular pore geometries of lotus-type porous iron for computer models that allow for simple porosity and pore size variation in computational characterization of their mechanical properties. The presented methodology uses image-recognition algorithms for the statistical analysis of por...

The National Institute of Technology, Okinawa College (OkNCT) has developed a food processing machine that generates underwater shock waves through wire electrical discharge. The machine can used for sterilization, milling flour, softening, and extraction among others. In this study, we experimentally examined the conditions for food softening usin...

The auxetic cellular structures build from inverted tetrapods were fabricated and experimentally tested using uniaxial compression tests in two orthogonal directions. Based on experimental results, the computational models using homogenised material model were developed and validatedin LS-DYNA. Furthermore, the computational models based on Smooth...

Auxetic cellular materials show great potential for improved properties in case of impact loading. Due to this potential, auxetic structures made from inverted tetrapods are extensively analyzed experimentally and computationally. The built samples are compressed in two orthogonal directions to determine their base mechanical properties and deforma...

A method for fabricating an unidirectional porous (UniPore) aluminum structure employing a cylindrical explosive compaction assembly is proposed and the condition for successful recovery is discussed. The microstructure of the compacted samples is demonstrated to show tight bonding between the walls with uniform pores oriented in one direction. The...

The paper reports on findings of an experimental investigation of the low cycle fatigue behaviour of high performance Cr-Mo-V hot working steel, which is used for the forging die inserts in hot forging operations. Fatigue tests of standardised uniform-gauge test specimens with duplex coating (plasma nitrided and hard PVD coating) were carried out i...

A method for synthesizing a unidirectional porous （UniPore） aluminum structure employing a cylindrical explosive compaction assembly is proposed and the condition for successful recovery is discussed. The microstructure of the compacted samples is demonstrated to show tight bonding between the walls with uniform pores oriented in one direction. The...

Auxetic cellular materials are modern materials which have some unique and superior mechanical properties. As a consequence of the structural deformation of their internal cellular structure they exhibit a negative Poisson's ratio, i.e. they significantly increase in volume when stretched and vice versa. The effect of negative Poisson's ratio is us...

The multi-material concept in design using lightweight components (e.g. light metals and cellular materials) provides for several advantages in various industries with the aim to develop cost-effective energy absorbing lightweight structures with reliable and predictable mechanical behaviour. This approach allows designing and manufacturing highly...

Porosity is the main characteristic of cellular composite structures. This article presents a simple theory and guidelines for estimating total and partial porosities in multi-phase cellular materials with granular structure, based on mass measurements in different phases of production process. The theory allows for estimation of the total, closed,...

This paper introduces an improved method for the detailed geometrical characterization of perlite-metal syntactic foam. This novel metallic foam is created by infiltrating a packed bed of expanded perlite particles with liquid aluminium alloy. The geometry of the solidified metal is thus defined by the perlite particle shape, size and morphology. T...

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 15 mm. The paper proposes an inverse engineering procedure for their determination by applying the computational optimisation based on an extensive experimental testing data. In...

The paper focuses on characterisation of yielding and post-yield behaviour of metals with closed-cell cellular structure when subjected to multiaxial dynamic loading, considering the influence of the relative density, base material, strain rate and pore gas pressure. Research was conducted by extensive parametric fully-coupled computational simulat...

A new type of porous metal with uni-directional pores (UniPore) has been developed using the explosive compaction fabrication process of rolled copper foil with plastic spacers. The interface bonding in the novel UniPore metal structure has been investigated by metallographic analysis. The mechanical properties of the new rolled UniPore specimens w...

The paper gives a short overview of geometrical characterization, experimental testing, computational modelling and finite element analysis of various cellular metals: Advanced Pore Morphology (APM) foam, open-cell aluminum foam, Metallic Hollow Sphere Structure (MHSS) and cellular metals with uni-directional pores (UniPore). The geometrical analys...

The paper presents the first extensive research on microstructural and mechanical properties of a novel porous copper structure with unidirectional longitudinal pores (UniPore) structure. The unique fabrication method is based on explosive compaction of copper pipes. The high pressure achieved by explosive detonation during compaction causes explos...

Cellular structures are inhomogeneous materials where tracing the yielding process is of particular importance in evaluating the impact energy absorbing capabilities. This paper gives an overview of using Infrared thermography to visualize and record the yielding process including development of plastification zones, slide-lines formation, densific...

The paper reports findings of investigating optimal interface material parameters of product handles to improve its ergonomics by increasing the user performance, comfort, and lower the risk of cumulative trauma disorder development. Optimisation methods in combination with the finite element computer simulations of a human fingertip whilst graspin...

Keywords. Aluminium foam; In-situ foam-filled tubes; Three-point bending; Uniaxial compression behaviour; Deformation modes.

Most authors have focused on the sizes and the shapes of the product handles, but neglected those interface materials of the handles, which could fur-ther improve the ergonomics of the product. Therefore we utilized optimisation method to determine optimal interface material properties of a product for opti-mal mechanical response of the system usi...

The aim of this paper is to study the uniaxial-compressive crush performance of thin-walled structures filled with Advanced Pore Morphology (APM) foam elements, exploring their deformation and failure mechanisms. The APM-foam elements are integral-skin closed-cell foams of near spherical shape fabricated through the powder-Compacting-Foaming method...

The manuscript discusses mechanical properties and microstructure of copper before and after the explosive treatment. The aim of this research was to characterize and evaluate the influence of explosive treatment which is present during the manufacturing procedure of a new type of unidirectional cellular structure. For this purpose testing of tensi...

The forging die used in a hot forming operation on a hydraulic drop hammer is often exposed to high thermal and mechanical loadings where the service life of the die insert is mainly limited by wear and surface cracking. The surface of the die should thus be appropriately hardened through surface preparation and treatment to reach the required mech...

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...

The paper presents a detailed geometrical characterization of APM foam elements with sphere-like interconnected closed-cell porous structure and integral skin. The influence of the alloying element Si on internal structure (pore size, spatial distribution, sphericity and orientation) for different APM foam element sizes (made of AlSi7 and AlSi10) h...

A new method to fabricate uni-directional porous (UniPore) metal employing the explosive compaction process of cylindrical pipe assembly is proposed. The proposed fabrication method enables production of long rods with uniform longitudinal pores along the whole rod length, exhibiting typical porous material behavior in the transverse direction whil...

The prediction accuracy of computational simulations of various biomechanical problems of human bones depends on proper modelling of the problem geometry and boundary conditions but it is also essentially dependent on proper description of the mechanical properties of the all constitutive elements. As the human mandibular bone is a very important l...

The paper presents the results of studying the influence of silicone polymer pore filler on the macroscopic quasi-static and dynamic compressive behaviour of aluminium foam with irregular open-cell structure. The study is based on a mechanical experimental testing programme, where the deformation mechanism and mechanical energy absorption capacity...

During the development process of a new type of steel reinforced wooden road safety barrier parametric computational simulations were used to simulate the experimental vehicle impact certification tests as prescribed by the standard EN 1317. First a detailed study of pre-stressed bolt connection behavior between the guardrail and the guardrail conn...

The paper addresses the structural characterisation of advanced pore morphology (APM) foam elements. APM foam elements have a sphere-like shape with solid skin and a complex internal porous structure. The understanding of structure-property relations depends on accurate characterisation of this complex porous geometry. To this end, micro-computed t...

This chapter presents the cellular materials and their composites in general and then mostly concentrates on the particularities of advanced pore morphology (APM) foam. Single metallic APM spheres and their mechanical properties are characterized with experimental testing and computational simulations, providing the basic properties and knowledge f...

Lotus-type porous materials exhibit some unique anisotropic mechanical and thermal properties which are very useful for a number of industrial applications. This paper evaluates several computational models for determining the compressive engineering elastic modulus and engineering yield stress of lotus-type iron in transversal and longitudinal dir...

In this manuscript, the behavior of metallic foam under impact loading and shock wave propagation has been observed. The goal of this research was to investigate the material and structural properties of submerged open-cell aluminum foam under impact loading conditions with particular interest in shock wave propagation and its effects on cellular m...

Cellular structures are increasingly being used in modern engineering due to their advantageous properties. However, the behaviour of cellular structures made of brittle materials under mechanical loading is often very unpredictable. This paper reports on an experimental study using different silicone rubber mixtures as pore filler material in an o...